Instrumentation Interview Q&A EMERSON EDUARDO RODRIGUES

Interview questions - Basic instrumentation
Define physical quantity ?
Any quantity which can be measured is known as physical quantity.
Define Measurend ?
A physical quantity which is being measured.
Define sensors?
Sensors are devices which transform one physical quantity to another physical quantity.
Define Transducers ?
A device which provides a usable output in response to a specified measurend.
Define Active Instruments ?
An active instrument uses an auxiliary energy source to produce the desired output signal.
Define Passive instruments ?
A passive instrument can convert physical or chemical properties into another property without the
aid of an auxiliary source.
Define Primary Instrument ?
A primary instrument is a scientific instrument,which by its physical characteristics is accurate and is
not calibrated against anything else.
Define Secondary Instrument ?
Secondary instruments must be calibrated against a primary standard.
What is the difference between measured value and true value?
Measured Value - Any value or any reading calculated from measurement system or measuring
instrument is known as measured value.
True value - Any value calculated from rated values is known as true value.
Interview Questions and Answers - Control Loop
What is a Primary element ?
Primary element is the measuring element that quantitatively converts the measured variable energy
into a form suitable for measurement.

What is a transmitter ?
A transmitter is device which converts a non standard signal into a standard signal. 3-15 psi,4-20 ma
etc.
What is meant by controlled variable?
A controlled variable is the value which is sensed to originate a feedback signal. Controlled variable
is also known as process variable.
What is a controller?
A controller is a device which operates automatically to regulate a controlled variable.
What is Controller algorithm?
Controller algorithm is a mathematical representation of the control action to be performed.
What is set point?
Set point is an input variable which sets the desired value of the controlled variable.
Question - Draw a closed control loop?

what is error in a closed loop ?
The algebraic difference between the real value and ideal value of the measured signal.
What is manipulated variable?

A quantity or condition which is varied as a function of the algebraic error signal so as to cause a
change to the value of the directly controlled variable.
What is feedback control?
Control action in which a measured variable is compared to its desired value to produce an actuating
error signal which is acted upon in such a way as to reduce the magnitude of the error.
What Cascade control ?
Control in which the output of one controller is introduced as the set point for another controller.
Saudi Aramco Interview questions:
Instrument/DCS Engineer
what is cascade loop and how it work and why it used instead of PID single loop? In cascade, Which
loop is faster and slower? Please explain with loop drawing and example…
A.1) In cascade loop the parameter to be controlled is in series with another parameter which is to
be measured for normal operating conditions i.e 2 manipulated variables are in series
it is used in fractional distillation system , boiler operation in power plants etc In pid
only one variable is measured and controlled at a time .it is used for temperature , pressure , flow
control system
A.2) cascade loop is the combination of two pid control.where 1st controller output ie mv is the input
of 2nd controller.1st contrroler more fater than second controoler.
it’s used in boiler three element.it’s control feed water flow,drum level and steam flow.
Difference between PLC and DCS
Cold Loop checking & Hot Loop Checking?
plc is a programmable logic controller which can handle the binary input and output of the logic
statement which are stored in its memory.its a digitally operating system which are logic stored in its
memory by programming system.
DCS:-Distributed control system its control all types of process variable at a time through this we can
control the whole plant,we can see the status of plant,and handling yhe unit.
A2)In PLC the Scan time is less.but the scan time of DCS is high.Because it Control the whole plant.
So in safety loop we used PLC
A 3)First let’s take a quick brief history about controllers:
1 - For analog control loops we have a stand alone PID controller.
2 - Then we get a networked PID controllers through a hub and HMI for configuring and
monitoring.The PID itself contain a network port.SIEMENS introduced PID 353 & PROCIDIA
software for it.
3 - Then we get DDC - DIRECT DIGITAL CONTROL, single CPU, I/O’S for analog instruments only
& networked PC is an option.SIEMENS introduced APOGEE, FISHER introduced ROC.
4 - The next phase is DCS - Distributed Control System, and this is first introduced by HONEYWELL
& YOKOGAWA.Ability for handling large scale of I/O’S, discrete and analog, complex control loops,
PID algorithms calculations and executions.
5 - PLC is basically designed and produced for discrete on/off control, limited I/O’S.
but modern PLC with great progress in h/w technology introduced itself as a less expensive option

for PID, with it’s complicated calculations and algorithms.
6 - by the time with great achievements in H/W tech. and networking capabilities the single PLC can
handle discrete and analog instruments with wide range.
7 - Now the man can make his own DCS with multible networked PLC’S, but it will need a great
engineering and effortsto get near from DCS features and functionality.
Why ues 250 ohms Resistor during hart type instruments calibration with hart protocol in details ?
The 250 ohm resiatance is necessary to support the communication between transmitter and HART
because it equalizes
1. loop resistance
2. barrier resistance
3.wire resistance
3. receivers resistance
Wht is Dry Leg Calibration and Wet Leg Calibration?
Dry leg calibration and Wet Leg Calibration are used for
closed and pressurized vessels level measurement.
Difference:
Wet leg calibration is used for vapourised liquid(which
will be condensated by atmosphere)level.In LP side
Tapping ,liquid filled .
Dry leg calibration for the liquid under pressurised
condition which will not be condensated.
Why we use 4 - 20 ma?
To operate a Digital signal we need 1-5Vdc. For Analog
Instruments we are using 250 ohms as load resistance. When
we converting this signal into current By Ohm’s
Law V=IR
I=V/R
Put, V=1V Then I=1/250=4mA
Put, V=2V Then I=2/250=8mA
Put, V=3V Then I=3/250=12mA
Put, V=4V Then I=4/250=16mA
Put, V=5V Then I=5/250=20mA
*Also, since this is linear region
*Not possible to cause spark
*Since it is a current signal we can transmit it into long distance.
*Suppose if the instrument is dead we can identify it.

in earlier days we had used BJT transister in signal
conditioning circuits for amplification purposes so in BJT
transfer characteristics 4 to 20 milli amps is a linear
region,…but now-a-days we re using cmos only… as a
convention only we are following this 4-20 range

2}///another reason…for not using 0 milli amp
coz if we use like that we cannot distinguish between true
signal zero and dead zero(break in wires)
A temperature sensor (RTD) connected via a temperature transmitter is adjusted to give an output of
4ma at 60 deg and 20 ma at 600 deg. This output (4-20ma) from the transmitter is connected to an
IGBT-inverter to control a high speed (-12,000 to +12,000 RPM) DC motor. And in the inverter it is
programmed as, for 4ma the motor speed will be -3,000 RPM and for 20ma the motor speed will be
+10,000 RPM. The question is : If the motor speed is +5,500 (RPM), what will be the temperature? (-
sign indicates that the motion is in reverse direction).
413 deg and mA will be 14.64
difference between null and deflecting instrument.
The accuracy of these two instruments depends on different
things. For the Null type ins it depends on the linearity
and calibration of the spring, while for the second it
relies on the calibration of the weights. As calibration of weights is much easier than careful choice
and calibration
of a linear-characteristic spring, this means that the
second type of instrument will normally be the more
accurate. This is in accordance with the general rule that
null-type instruments are more accurate than deflection
types.
what is bevel? why bevel required in orifice plate?
Orifice plates which are used in flow measurement are mostly concentric “square-edge” type. The
bore of the orifice is not constant when one moves from the upstream side to the downstream side
along the cross section. The bore size is constant for a few mm only and then expands till it reaches
the downstream face. This inclined cut is called the bevel in an orifice.
The main reason to have a bevel in the orifice is to avoid turbulence thereby providing more
accuracy to the measurement. Restriction Orifice plates are not beveled since their intent is to
reduce the pressure only.
what is pyrometer? which purpose it has to be used?
Working definition of process ?
A process is anything that changes

A Pyrometer is a type of remote-sensing thermometer used to measure the temperature of a
surface, Various forms of Pyrometers have historically existed.
Cold loop checking is about to identify and ensure the not powered / not connected loop.
Hot loop checking is done once the instruments is connected with.
The core reason for the development of 4-20 mA was for 2 wire, loop powered instrumentation,
where 3.5-3.6 mA is used for powering the instrument. Without a minimum 3.6 mA field instruments

would not have any energy or power to run themselves! They’d be 3 or 4 wire devices with separate
wiring for DC power.
You can’t have 2 wire, loop powered 0-20 mA or zero to anything mA because there is no energy to
power the transmitter at zero mA. There is energy to power the transmitter with an ‘elevated zero’ at
4 mA.
3.8 mA has become a range for low failsafe; 3.9 mA is a range for underflow leaving 4.0 mA for a
true engineering ‘elevated’ zero.
There are multiple advantages of 2 wire loop power 4-20 mA, as the accepted conventional wisdom
lists:
 live zero is handy to indicate a broken circuit or a dead transmitter
 the loop can be made intrinsically safe
 it has the advantages of less susceptibility to noise that is inherent in current vs voltage signals
 a current loop adjusts for minor resistance changes in loop wiring (rusty terminals)
 it does create 1-5 volts when shunted through a 250 ohm resistor, but there neither was (historically)
nor is anything particularly superior about 1-5Vdc. The 1:5 ratio of 4-20mA is the same as its
predecessor, 3-15 psi (20-100kPa) pneumatics, which is nice, but again, there’s nothing inherently
superior about one ratio vs another.
But those are advantages only, none are the real primary reason for an elevated live zero: 2 wire
loop powering of the instrument.
Don’t fail your interview because the interviewer doesn’t know either, but if you’re a professional in
the field, you should know the real reason for the 4-20mA standard.

i got 14.46 ma for the current?
Thanks Sir for your information sharing
Kindly explain how does it make equalization and what the benefits from that equal.?
and thanks again…
Please send me how get the answer ? if any equation…
Proportional-Integral-Derivative (PID) control is the most common control algorithm used in industry
and has been universally accepted in industrial control. The popularity of PID controllers can be
attributed partly to their robust performance in a wide range of operating conditions and partly to
their functional simplicity, which allows engineers to operate them in a simple, straightforward
manner.As the name suggests, PID algorithm consists of three basic coefficients; proportional,
integral and derivative which are varied to get optimal response. Closed loop systems, the theory of
classical PID and the effects of tuning a closed loop control system are discussed in this paper.

The HART standard specifies “master” devices in a HART network transmit AC voltage signals,
while “slave”
devices transmit AC current signals.
I got 419.1deg and 14.64 mA
…………………………………………………………………………………
Vortex meters are commonly used in following services
1.Steam
2.Cooling water
3.Process water
4.Light hydrocarbons where large turndown is required.
5.Gas flow where large turndown is required

There are different common problems in control valves. Here we are going to discuss such
problems:
1. Mechanical friction:
Moving parts of control valves can be subjected to friction, primarily between the valve stem and the
stem packing. Some degree of friction is inevitable in valve packing and in some types of trim where
components must move past each other throughout the full range of valve stem travel (e.g. cage-
guided globe valves, rotary ball valves), and the goal is to minimize friction to a bare minimum while
still maintaining a pressure-tight seal.
The presence of friction in a control valve increases the force necessary for the actuator to cause
valve movement. If the actuator is electric or hydraulic, the only real problem with increased force is
the additional energy required from the actuator to move the valve (recall that mechanical work is the
product of force and parallel displacement). If the actuator is pneumatic, however, a more serious
problem arises from the combined effects of static and dynamic friction.

2. Flashing:
When a fluid passes through the constrictive passageways of a control valve, its average velocity
increases. As fluid velocity increases through the constrictive passages of a control valve, the fluid
molecules kinetic energy increases. In accordance with the Law of Energy Conservation, potential
energy in the form of fluid pressure must decrease correspondingly. Thus, fluid pressure decreases
within the constriction of a control valve’s trim as it throttles the flow, then increases after leaving the
constrictive passageways of the trim and entering the wider areas of the valve body.
If the fluid being throttled by the valve is a liquid, and its absolute pressure ever falls below the
vapour pressure of that substance, the liquid will begin to boil. This phenomenon, when it happens
inside a control valve, is called flashing.
3. Cavitation:
Fluid passing through a control valve experiences changes in velocity as it enters the narrow
constriction of the valve trim (increasing velocity) then enters the widening area of the valve body
downstream of the trim (decreasing velocity). These changes in velocity result in the fluid molecules’
kinetic energies changing.
If fluid being throttled is a liquid, and the pressure at the constriction is less than the vapor pressure
of that liquid at the flowing temperature, the liquid will spontaneously boil. This is the phenomenon of
flashing previously described. If, however, the pressure recovers to a point greater than the vapor
pressure of the liquid, the vapor will re-condense back into liquid again. This is called cavitation
4. Choked flow:
Both gas and liquid control valves may experience what is generally known as choked flow. Simply
put, “choked flow” is a condition where the rate of flow through a valve does not change substantially
as downstream pressure is reduced.
In a gas control valve, choking occurs when the velocity of the gas reaches the speed of sound for
that gas. This is often referred to as critical or sonic flow.
In a liquid control valve, choking occurs with the onset of flashing52. The reason sonic velocity is
relevant to flow capacity for a control valve has to do with the propagation of pressure changes in
fluids.
5. Valve noise:
A troublesome phenomenon in severe services is the audible noise produced by turbulence as the
fluid moves through a control valve. Noise output is worse for gas services experiencing sonic
(critical) flow and for liquid services experiencing cavitation, although it is possible for a control valve
to produce substantial noise even when avoiding these operating conditions.
Noise produced by a control valve also translates into vibration imposed on the piping, which may
cause problems such as loosening of threaded fasteners over time. One way to reduce noise output
is to use special valve trim resembling the trim used to mitigate cavitation.

6. Erosion:
A problem common to control valves used in slurry service (where the process fluid is a liquid
containing a substantial quantity of hard, solid particles) is erosion, where the valve trim and body
are worn by the passage of solid particles.Another cause of erosion in control valves is wet steam,
where steam contains droplets of liquid water propelled at high velocity by the steam flow
Why Is Reduced Trim Required In Control Valves?
What Is The Trim In A Control Valve?
The trim of the control valve is the parts that are in direct contact with the fluid that is controlled. The
edge of the valve consists of all the parts that will get wet, apart from the body and the bonnet.

• The compensation is the heart of the valve with a primary function to proportional the valve orifice
in such a way that there is an established relationship between the capacity of flow and the elevation
of the plug of the valve.
• The secondary function can be closed hermetically.
• Fit includes seat, plug, stem, gland follower, gland nut, plug, guide bushings, and cage
• The components of the packing box considered as trim are the packing, the follower, the spring,
the ring of the flashlight and the packing retainer.
• The secondary molding parts are derived from the sumps, the seat retaining ring, the seals of the
seat body and the spacers.

Why Is Reduced Trim Required In Control Valves?
• The control valves are sized according to the requirements of the application and must meet both
the Cv and speed criteria.
• The reduced setting is used when it is necessary that the valve has a Cv capacity lower than the
maximum possible in that valve size.
• The most common reason for a reduced cut is that the flow rate is low for the required valve size,
particularly when the 25 mm valves have been specified as the smallest size to be used. Some
plants stipulate that no control valve should have less than two sizes smaller than the size of the line
and that the valve should not be less than half the size of the line.
• The second reason is that in gas or high-pressure steam applications, the valve is invariably sized
at the speed limits of the output port and the required CV is much smaller than the total internal
diameter Cv.
3 types of control valve trim
The figure below shows the trim characteristics and illustrates the relationship between the
percentage of flow and valve stem travel from 0% to 100%. There are 3 types of control valve trim:

• **Quick setting (quick opening): the ** quick release of the valve opens quickly and is used for
on/off service. The main applications include liquid discharge, pressure relief and measurement.
• Normal (Linear): the nominal configuration of the valve is used to throttle liquids, control the fluid
level and in applications where water hammer has been a problem.
• Equal%: used to control the pressure or flow of gases and vapors in throttling applications.
Instrumentation Interview question : What is
the difference between Transducer and
Transmitter?

Transducer - Transducer converts one form of energy into another form.
For example : Converts mechanical energy into electrical energy.
Thermocouple. Converts temperature into electrical form.
Transmitter : Transmitter converts a non standard signal into a standard signal.
Example : Electronic pressure transmitter. Standard signals - 4-20ma 3-5psi
What is a valve?
A valve is a device used to control the flow of fluid in a stream.
What is a control valve ?
A control valve changes the flow rate in the stream as a response to the controller output to change
the manipulated variable. The controller can adjust the valve opening at any stage between ON &
OFF.
What are check valves ?
Check valves allow flow only in one direction. If there is a higher pressure opposite to the allowed
flow direction then the valve closes.
What is a Pressure relief valve?
Pressure relief valves are safety valves. Pressure relief valves are connected parallel to the
mainstream of flow, when there is an excesses pressure above setpoint in the line, the relief valve
opens and feedback the fluid to the compressor or tank.
What are Gate valves?
Gate valves are used as shut off valves. There is only two action for Gate valves, ON & OFF. Mostly
Gate valves are manually operated in industries.

What is cavitation?
Cavitation can occur in valves when used in throttling or modulating service. Cavitation is the
sudden vaporization and condensation of a liquid downstream of the valve due to localized low
pressure zones.
When flow passes through a throttled valve, a localized low pressure zone forms immediately
downstream of the valve. If the localized pressure falls below the vapour pressure of the fluid, the
liquid vaporizes (boils) and forms a vapour pocket. As the vapour bubbles flow downstream, the
pressure recovers and the bubbles violently implode causing a popping or rumbling sound similar to
tumbling rocks in a pipe.
What is Flashing?
If the local pressure within the restricted flow area drops below the vapor pressure of the liquid,
which is a condition called the “vena contracta,” vaporization occurs (i.e., vapor bubbles would form
in the liquid). If the downstream pressure remains below the vapor pressure, the process is said to
be a flashing service,
Interview Questions and Answers : Power Plant Instrumentation and Electrical Engineers
1. Name the four major circuits in steam power plant.
• Coal and ash circuit
• Air and flue gas circuit
• Feed water and steam circuit
• Cooling water circuit
2. What consists of cooling water circuit and coal & ash circuit in steam power plant?
The cooling water circuit consists of a pump, condenser and cooling tower. The coal and
ash circuit consists of coal delivery, preparation of coal, handling of coal to the boiler
furnace, ash handling and ash storage.
3. What is the main purpose of the reservoir?
The main purpose of reservoir is to store water received from catchments areas during the
rainy seasons and supply the same during the dry season.
4. What is the main purpose of the dam?
The main purpose of the dam is to increase the height of water level and also to increase
the working head of the hydraulic power plant.
5. What is the use of surge tank?
The surge tank is used to provide better regulation of water pressure in the system. The
surge tank controls the water when the load on the turbine decreases and supplies water
when
the load on the turbine increases. Thus, surge tank controls the pressure variations resulting
from the rapid changes in water flow in penstock and hence prevents water hammer.
6. Explain about penstock?
The pipe between surge tank and prime mover is known as penstock. It is designed t o
withstand high pressure. It is made up of reinforced concrete. In very cold areas, the
penstock
is buried to prevent ice formation and to reduce the expansion joints.
7. What is the use of spill Ways?
Spillway is like a safety valve of the dam. It discharges major flood without damaging the
dam. It keeps the reservoir level below the maximum level allowed.

8. What is the main purpose of high-pressure boilers?
The high-pressure boilers are used to increase the efficiency of the plant and to reduce the
cost of electricity production.
9. State important advantages of high-pressure boilers?
• The amount of scale formation is less, since the velocity of water through pipes are more.
• All parts of the system are heated uniformly, so there is no danger of overheating.
10. Name important high pressure boilers?
• La Mont boiler
• Benson boiler
• Loeffler boiler
• Velox boiler.
11. Name the different types of coal transforming equipments?
A. Belt conveyors
B. Screw conveyors
C. Bucket elevators
D. Grab bucket elevators
E. Skip hoists
F. Flight conveyors.
The coal transfer starts by carrying of coal from-unloading point to the storage site.
12. Define forced draft and induced draft cooling towers
If the fan is located at the bottom of the tower and air is blown by the fan up through
the descending water it is called as forced draft cooling towers If the fan is located at the
top of the tower and air enters through the louvers located on the tower’s side and drawn
up and discharge through the fan casing, it is called as induced draft
13 Name the different components of nuclear reactor?
1. Nuclear fuel
2. Moderator
3. Control rods
4. Reflectors
5. Reactor vessel
6. Biological shielding
7. Coolant
14 Define speed.
Speed is a variable which refers to the revolutions per minute of some piece of rotating equipment.
15 Which is the most frequently used speed measuring instrument?
Tachometer
16 What are the types of tachometer?
a) A.C. tacho generator
b) D.C. tacho generator
17.What are the types of D.C.tacho generator?
a) Permanent magnet type
b) Separately excited field type

18.What are the disadvantages of D.C. tacho generator?
A.C. ripple is present in the output signal. The magnitude of the ripple is 2% of the out[ut
D.C.level.
19.Name the types of rotor used in A.C.tacho generators.
a) Drag cup rotor
b) Squirrel cage rotor
20.What are the advantages of Squirrel cage rotor?
*Cheaper
*Occupies less space
21 Give the formula to calculate the shaft speed.
Shaft spped = (disk speed * No. of openings in the disk) / No.of images
22 Define torque.
Torque is defined as the force which tends to change the linear motion or
rotation of a body.
23 What is the other name for inline stationary torque sensor?
Relative regular twist torque sensor
24 What are the types of torque transducer?
i) Inline rotating torque sensor
ii) Inline stationary torque sensor
iii) Optical torque sensor
iv) Proximity torque sensor
25 What are the advantages of optical torque sensor?
Low cost, small physical size
26 Define load cell.
Load cell are devices that convert force into pressure , which are then
measured.
27 Why magneto elastic load cell is also called pressductor load cell?
The degree of change has a direct relationship with the applied stress or
force. So it is called pressductor load cell.
28.What are the advantages of magneto elastic load cell?
Extremely robust transducer
• Produces relatively high output signal levels
• Overload ratings are as high as 15 times the rated loads
29.Write the abbreviation of API scale.
API - American Petroleum Institute

30 Write the disadvantages of bridge type gas densitometer.
The major disadvantage is that the variations in ambient temperature will
introduce errors.
31 For what purpose accelerometers are used?
• For the measurement of shock & vibration
• For gross measurement of acceleration of vehicles like aircraft,
submarines etc.
32 What are the advantages of LVDT?
is used for steady state and low freequency vibration measurements.
• Smaller amss, so, it is used for the measurement of vibrations of higher frequencies.
33 Define piezo electric effect.
If the dimension of crystal are changed by the application of a mechanical force, an electrical
potential appears across the crystal. This effect is called piezo electric effect.
34 What are the features of piezo electric accelerometers?
Small in size and weight
• It can be used for vibration and shock measurements.
• High output impedence
• Their response is poor at low frequencies
35 Define density
It is defined as the mass per unit volume of a substance under fixed conditions.
36 Define specific gravity.
It is defined as the ratio of density of one substance to the density of another reference substance
both obtained at same temperature & pressure.
37.What is the other name for specific gravity?
Relative density.
38 Define viscosity.
It is the property of the fluid which gives the resistance to the flow.
39.What are the units of density?
Kg/m3 or gm/litre or gm/ml
40 What are the main parts of an electrical pressure transducer?

(a) Pressure sensing element such as a bellow, a diaphl11gm or a bourdon
tube.
(b) Primary conversion element. e. g resistance or voltage.
© Secondary conversion element.
41 What is the principle of operation of a piezoelectric pressure transducer?
When pressure is applied to a piezoelectric crystal such as quartz, an
electrical charge is generated.
42 What are the types of thermal conductivity gauges?
(a) Pirani gauge
(b) Thermocouple gauge
43 What is the purpose of ionization gauge?
Ionization gauge is used to measure the density of a gas.
44 What is the function of a dead weight tester?
Dead weight tester is used to calibrate bourdon gauges .It is used as a measuring device and also
as a calibration method.
45 What are the disadvantages of using thermocouple gauge?
(i) Easily damaged by organic vapours
(ii) The filaments can be coated with a deposit of devapourised vapours
which alters the way the filament transfers heat.
46 Define thermal conductivity
The ability of the material to carry heat by conduction is called as the
thermal conductivity.
47 What is a vacuum pressure?
Pressure which are below the atmospheric pressure are called vacuum pressure.
48.What is the purpose of calibrating a pressure measuring instrument?
It is used to adjust the output signal to a known range of pressure. It includes zero, span and
linearity adjustment.
49. What are the disadvantages of LVDT pressure transducer?
Large core displacement are required for appreciable amount of differential output.
Temperature affects the performance of the transducer.
They are sensitive to stray magnetic fields.

50 What is the principle of operation of a capacitive pressure transducer?
It is based on the principle of the familiar capacitance equation of the
parallel plate capacitor (ie) C=G0GrA/d farad
(ie) capacitance = GA/d
G0=8.85*exp(-12)f/m2
Gr=Dielectric constant
A ->Area of each plate
d->Distance between two plates
51 Define temperature?
The temperature of a substance is a measure of hotness or coldness of that
substance.
51 What is the difference between temperature and heat?
Temperature is defined as “degree of heat”.
Heat is used to mean “quantity of heat”.
52 What are the temperature scales?
Lower fixed point or ice point.
Upper fixed point or steam point.
53 Define triple point?
A particular temperature and pressure at which three different phases of
one substance can exit in equilibrium is known as “triple point”.
54 Write some methods of measurement of temperature?
Expansion Thermometer.
Filled system Thermometer.
Electrical Thermometer.
Pyrometer.
55 What are the different types of filled system Thermometer?
Gas-filled Thermometer.
Liquid-filled Thermometer.
Mercury-filled Thermometer.
Vapour-filled Thermometer.
56 Define Seeback effect?
If two dissimilar metals are joined together to form a closed circuit, there will be two
junction where they meet each other. If one of these junctions is heated, then a current flow in
the circuit which can be detected by a galvanometer. The amount of current depends on the
difference in temperature between the two junctions and on the charactSeristics of the two
metals. This was observed by Seeback & hence known as Seeback effect.
57 Which effect is used in thermocouple?
Seeback effect is used in thermocouple.

58 What is the purpose of protecting tube in a thermocouple?
It is used to protect the thermocouple from harmful atmosphere, corrosive fluids and also to prevent
from mechanical damage.
59 Explain the principle of operation of thermistor?
They have negative temperature coefficient of resistance i.e. with increase in temperature the
resistance decreases and vice-versa.
60 What is the drawback in two wire RTD?
Two wire RTD’s will give a large lead wire & hence it is impossible for accurate measurement of
temperature.
61 Give some examples of the temperature instruments?
Resistance thermometer.
Thermocouple.
Thermistor.
62 Define pyrometry .
Pyrometry is the technic for measuring the body’s temperature by measuring it’s electromagnetic
radiation.
63 What are the two types of pyrometer ?
a. Radiation pyrometer b. optical pyrometer.
64 What is the advantage of optical pyrometer?
It is used to measure high temperature.
65 How the calibration is adjusted in optical pyrometer?
By adjusting the emissivity.
Que. : Draw an electronic two wire system control loop.
Que. : What are Intrinsically safe system ?
Que. : What does a transmitter output start from 3-15 psi or (0.2 - 1 Kg/Cm2) or 4 - 20 ma. etc. ?
Que. : What is force balance and motions balance principle ?
Que. : What is Process
Que. : What is Manipulated variable , Error and controlled variable
Que. : Define control agent, actuating signal, Deviation, Offset, reference input, Setpoint
Que. : Define Accuracy, attenuation, deadtime,drift, error , Span error,Zero error , static gain,
Hysterisis,Interference,
common mode interference,Normal mode interference, Linearity, Rang, Repeatability,
Reproducability,response, Signal to Noise ratio,
time constant, Span, Zeroshift
What is the meaning of calibration?
Calibration is the comparing of device against an equal or better standard.

What is the meaning of DEADTIME?
DEAD TIME, is caused when there is time interval between the initiation of some action and the
detection of the action

Advantages of Cascade control Loop-
-Isolating the slow outer loop from valve problems and some disturbances.
Fast inner loop take care of these.
Disadvantages of cascade control loop.
-Additional measurement required.
-Additional control required.So controller tuning of secondary controller is required.
When to use cascade control loop?
-Inner loop should be faster (3 times) than outer loop.
What is the tuning sequence of cascade control?
First tune the inner loop
Enable cascade control
Tune the outer loop
Measurement errors can be categorized in to two
1.Systematic Error
A difference from the true value that is usually constant or proportional to the true value.
How Systematic error can be eliminated?
Identify the source and eliminate if possible or determine size of error and compensate for it in
calculations.
What are the sources of systematic error?
Poorly calibrated instrument
Changes in the environment which affect the instrument (e.g high temperature or humidity)
Consistent parallax error in reading a meniscus.
2.Random Error
Inherently unpredictable fluctuations in the reading of an instrument or in the experimenters
technique.
How this error can be eliminated?

minimize by taking repeated measurement and averaging values.
What are the sources of random error?
Random fluctuations in instrument.
Temperature Measurement Techniques
A wide variety of methods are employed for temperature measurement.There are broadly split into
tow groups.
Contact Measurement
Thermocouples
Thermistors
Resistance Temperature Detectors
Liquid filled or vapor filled systems
Bimetallic strips
Non Contact Measurement
Radiation pyrometer

1. What are the process Variables?
2. Define all the process Variable and state their unit of measurement. ?
3. What are the primary elements used for flow measurement?
4. What are the different types of orifice plates and state their uses?
5. How do you identify an orifice in the pipe line?
6. Why is the orifice tab provided?
7.What is Bernoulli’s theorem and where it is applicable?
7. How do you identify the H. P. side or inlet of an orifice plate in line?
8. How do you calibrate a D. P. transmitter?
9. What is the seal liquid used for filling impulse lines on crude and viscous liquid ?
10. How do you carry out piping for a Different pressure flow transmitter on liquids, Gas and
steam services Why ?
11. Draw and explain any flow control loop ?
12. An operator tells you that flow indication is more, How would you start checking?
13. How do you do a zero check on a D.P. transmitter ?
14. How would you do Glycol filling or fill seal liquids in seal pots 7 Draw and explain.
15. How do you calculate new factor from new range using old factor and old range?
16. How will you vent air in the D.P. cell? What if seal pots are used?
17. Why flow is measured in square root?
18. What is absolute pressure?
19. What is absolute zero pressure?
20. What is the maximum Vacuum?
21. What is Vacuum?
22. What are the primary elements for measuring pressure?
23. How will you calibrate an absolute pressure transmitter using vacuum manometer. Range 0-
400mm abs?
24. You are given a mercury manometer range 0 -760 mm ? A vacuum gauge reads 60 mm
vacuum. The test manometer reads 50 vacuum ? Which of the two in correct.

25. Why is an inclined manometer used ?
26. What is the principle of a pressure gauge ?
27. Draw and explain a pressure gauge ? What is the used of a Hair spring ?
28. Briefly explain the different methods of level measurement?
29. Explain how you will measure level with a different pressure transmitter.
30. How is D.P. transmitter applied to a close tank?
31. How is D.P. transmitter applied to an open tank?
32. How is D.P transmitter applied to a close tank & open tank with Dry leg?
33. What is purge level system?
34. Explain the working of a leveltrol.
35. How will you reverse an action of the leveltrol?
36. What is interface level? How do you calculate it?
37. How will you calibrate a leveltrol in the field?
38. How will you calibrate on interface level control. ?
39. How will you apply wt. lest calibration to a leveltrol.
40. What will happen if the displacer has fallen down while in line ?
41. What will happen if the displacer has a hole in it while in line?
42. What is the used of Suppression and elevation?
43. How will you commission D.P. transmitter in field in pressurized vessel.
44. How will you check zero of a level D.P. transmitter while is line?
45. Explain the working of an Enraf level gauge?
46. What are the different methods of temperature measurement? Explain
47. What is Pt 100 mean?
48. What is two wire and three wire R.T.D. system?
49. Draw a potentiometer temp. Measuring circuits and explain its?
50. What is the constant voltage unit?
51. Explain the working of a balancing motor.
52. What is burnout feature ? Explain
53. Why are Thermowells used ?
54. What type of sensing element would you use to measure very low temperature ?
55. What are skin temperature thermocouples ?
56. What is the specialty of thermocouples lead wires ?
57. What is the difference the a wheatstone bridge and a potentiometer ?
58. Explain the application of proportional integral and derivative action?
59. Where is on off control used?
60. What is reset-wind up?
61. Why is reset called integral and Rate derivative ?
62. Explain tuning of controllers.
63. Explain the working of an electronic P.I.D. controller.
64. What is an analogue integrator and an analogue differentiator ?
65. What is an anti reset wind up ?
66. What are De-saturators ?
67. Explain the working of Rotameter?
68. Explain the working of a magnetic meter.
69. Explain the working of a turbine meter.
70. Explain the working of a Pitot tube.
71. Where is the integral orifice used ?
72. What are types of taps used for orifices ?
73. What is Reynolds number ?
74. How would you choose differential range ?
75. What is positive Displacement meters ?
76. What is a control valves ?
77. What are the different types of control valves ?

78. What is the use of single seated valve ?
79. What is the use of double seated valve ?
80. What is Cv of a valve ?
81. What are the different types of actuators ?
82. What types of bonnets would you use of high temp. and very low temp. ?
83. How will you work on a control valve while it is line ?
84. What is the use of a valve positioner ?
85. When can a by pass be not used on a positioner ?
86. What is the use of butterfly valves ?
87. What is the use of three way valves ?
88. What are the different types of plugs ?
89. What is a cage valve ?
90. What is the use of link connected to the valve positioner ?
91. What is the use of booster relays ?
92. What is the use of Angle valves ?
93. What are the different valve characteristic ?
94. What is a solenoid valve ? Where it is used ?
95. How will you change the valve characteristics with positioner ?
110.How will you change the action of a control valve ?
96. How will you select the control valve characteristics ?
97. What is the effect of pipe reducers on valve capacity?
98. An operator tells you that a control valve in a stuck ? How will you start checking ?
99. Where is an Air to close and Air to open control valves used ?
100. Why does control valve operate at IS psi ?
101. Instrument transmitters are split in to two broad categories.
1.Loop Powered Instruments.
Loop powered instruments are instrument with two wire connection to controller (PLC /DCS/
Stand Alone controller).
Two wire connection supplying both power and communication.
Power supplied over the current loop from the control room.
Entire transmitter must operate on <3.2ma for a standard transmitter and <2.7 ma for a HART
enabled transmitter.
Only low power available. Total available power <50 mW

2.Non Loop Powered
For Non Loop powered instrument there is a separate power supply from 4-20 ma
communication.
Four wire connection.Two for power and two for communication.
No power consumption limitation for Non loop powered instruments.
Used in situations where sufficient power cannot be derived from the current loop.
Q)What is pressure ? or Define pressure ?
A)Pressure is the amount of force applied over a defined area.
Q)The factors that will influence hydro static pressure ?
A)Level of the liquid
Density of the liquid
Pressure on the surface of the liquid (vapor space)
Q) Density of liquid
A)Density is the mass of a particular substance per unit of
volume
Q) Definition of Specific Gravity
A)Specific gravity is the ratio of the density of a particular liquid
to the density of water at a reference temperature.

Q)Reason for measuring pressure ?
A)Safety
Process efficiency
Cost savings
Inferred measurement of other variables
Q)Three measurable pressure
A)Head (hydrostatic) pressure
Static (line) pressure
Vapour pressure
Q)Pressure-measurement devices can be categorised according to
the reference pressure
A)Absolute
Gauge
Differential
Q) Different type liquid column pressure gauges
A) Barometer
Manometer
Q)What are the basic parts of Mechanical pressure gauges ?
A)Sensing device
Mechanical dial or indicator
Q)Most commonly used pressure sensing device?
A)Bourdon tube
Bellows and capsules
Q)What is pneumatic controller ?
A) A pneumatic controller is a device that, in response to an input pressure, sends a pneumatic
output to a relay
Q)What is pneumatic transmitter ?
A)A pneumatic transmitter is a device that, in response to input pressure, outputs a proportionate,
standardized pneumatic signal.
Flow Measurement - Main Classification
1. Displacement

2. Constriction Type (Differential Head)
3.Velocity Flow Meters
4.Mass flow meters
Different types of displacement type flow meters are
a. Positive displacement meters
b. Metering pumps
Differential head is again classified as Closed Conduit and Open Channel
Closed conduit type flow meters are
a. Orifice plate
b. Venturi tube
c. Flow Nozzle
d. Pitot tube
e. Elbow
f. Target
g. Variable area (Rotameter )
Open channel type flow meters are
a.weir
b.Flume
Different types of Velocity flow meters are
a. Magnetic
b. Turbine
c. Vortex or Swirl
d. Ultrasonic
e. Thermal
Different types of Mass flow meters are
a. Weight types
b.Gyroscope precision types
c.Centrifugal force types
What are the types of Linear displacement
transducers?
 Resistive potentiometer
 LVDT 157
 Strain gauge
 Capacitive transducer
 Variable inductance transducer
 Piezoelectric transducer

 Photo electric transducer
 Hall effect transducer
 Digital transducer
What is SI unit?
What is SI unit?
International system of unit (SI) is world’s most used system of measurement.
International system of unit (abbreviated SI) is used officially in all countries except USA,Myanmar
and Liberia.
It has seven basic units,from which all other units are derived.
Length - Meter
Mass - Kilogram
Time - Second
Electric current - Ampere
Thermodynamic temperature - Kelvin
Luminous intensity - Candela
Amount of substance - Mole
Unit symbols are normal vertical letters and Quantitative symbols are inclined italic letters.All
formulas indicating quantities must be written with italic letters.

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All other units are derived from base units. There are 22 derived units.
In instrumentation we use only 2 base units level and temperature and two derived units flow and
pressure.
Measurement is comparison between the value of process quantity and its unit.

The main advantage of SI system compared to imperial system is that bigger units are obtained from
base units by multiples of ten and smaller units by sub multiples of ten.
For example 1 ton = 1 kg * 1000 (Bigger unit)
1 gram = 1 kg / 1000 (Smaller unit)
SI unit in Instrumentation
2 base units are used in Instrumentation Level and Temperature. Flow and pressure are derived
units.
Pressure = Force / Area.
Unit of pressure is Pascal and it is the force of 1 newton applied to the area of 1 square meter.
Pa = N / m2.
Pressure is the quantity very much used in instrumentation. Pascal is a very small unit of pressure.
So practically a 100000 times greater unit is used and it is called Bar.
1 bar = 100000 Pa.
In instrumentation other than pressure three more quantities are used and they are
Temperature, Level, and Flow.
Temperature is one of the 7 base quantities of SI system of measuring units.
The unit of temperature is kelvin. For practical use another unit is used called Celsius.
Celsius is Kelvin minus 273.16 .
Flow rate is the volume or mass passing through a certain section in a unit of time. Flow is
expressed in kilograms per second or liters per second or their multiples.
Level is the height of a liquid in container.It is measured in meters or percentage of a maximum
level.

Open Loop Control System.
Key point for open loop system is there is no feedback.
That means controllers output doesn’t affect its subsequent input.

Open loop control system can take input from process as long as controller output doesn’t affect its
subsequent input.

Basically the input is used for compensating process load and disturbances.
Example - Hot water temperature control by sensing temperature of supply water.
Here the hot water is produced by mixing steam in to a stream of cold water. Supply of steam is
controlled by the controller according the temperature of incoming cool water supply.
Here the the process variable is hot water temperature. Load is cool water temperature.
Compensator is control valve opening.

Limitations -
Not sensitive to disturbances in compensator’s side. Eg Steam supply
Not sensitive to unmeasured disturbance. Eg Cool Water flow
Set point and measured value cannot generally represent desired result.
Closed Loop
Closed loop control system uses feedback. It means controller’s output affect future controller’s
input.

Here it takes temperature of hot water as controller input and controller calculate the output
according to the hot water temperature.

What are the types of Control Systems?
A control system can be functioned electrically, mechanically, pressure by fluid (gas or liquid), or it
can be combination of these ways. Control systems are used to arrange and manage components in
a way that the required condition or output is obtained. A control system is a system, which controls
other systems.

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There are two types of control systems depending upon the configuration of systems:
1. Open loop control systems
2. Closed loop control systems
Open loop control systems
Open loop control system are systems with no feedback which means those systems in which the
output has no effect on the control action.
An open-loop control system takes input under the consideration and doesn’t react on the feedback
to obtain the output. This is why it is also called a non-feedback control system.

• Input: the excitation applied to the system from an external sources to attain an output is called
input signal.
• Control system: It is an arrangement or a combination of various physical components also called
the sub systems connected in such a manner so as to attain an output.
• Control action: it is quantity that is responsible for activating the system
• Output: the actual system attain from the system is called output signal.

Practical Examples of Open Loop Control System
1. Electric Hand Drier - Hot air (output) comes out as long as you keep your hand under the
machine, irrespective of how much your hand is dried.
2. Automatic Washing Machine - This machine runs according to the pre-set time irrespective
of washing is completed or not.
3. Bread Toaster - This machine runs as per adjusted time irrespective of toasting is
completed or not.
4. Automatic Tea/Coffee Maker - These machines also function for pre adjusted time only.
5. Timer Based Clothes Drier - This machine dries wet clothes for pre-adjusted time, it does
not matter how much the clothes are dried.
6. Light Switch - Lamps glow whenever light switch is on irrespective of light is required or not.
7. Volume on Stereo System - Volume is adjusted manually irrespective of output volume
level.
Closed loop control systems
A closed loop system is also referred as a feedback control system. A system in which the control
action is dependent on the output is called a closed loop system. In closed loop system the output is
constantly monitored and adjusted to the required value by the system.

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The output signal is fed back and compared with the reference input. Depending upon the difference
between the output signal and reference input, corrective actions are taken by the controller to adjust
the output. For example air conditioning unit.
Practical Examples of Closed Loop Control System
1. Automatic Electric Iron - Heating elements are controlled by output temperature of the iron.
2. Servo Voltage Stabilizer - Voltage controller operates depending upon output voltage of the
system.
3. Water Level Controller - Input water is controlled by water level of the reservoir.
4. Missile Launched and Auto Tracked by Radar - The direction of missile is controlled by
comparing the target and position of the missile.

5. An Air Conditioner - An air conditioner functions depending upon the temperature of the
room.
6. Cooling System in Car - It operates depending upon the temperature which it controls.
These are the common interview question for instrumentation designer / draftsman
1. Describe a process flow diagram and a process and instrument drawing.
2. Draw the symbols for a gate, globe, and automatic valve.
3. Draw the symbols for a centrifugal pump and positive displacement pump.
4. Draw the symbols for a blower and a reciprocating compressor.
5. Draw the symbols for a steam turbine and centrifugal compressor.
6. Draw the symbols for a heat exchanger and a cooling tower.
7. Draw the symbols for a packed distillation column and plate distillation column.
8. Draw the symbols for a furnace and a boiler.
9. Draw a simple process flow diagram using the symbols from questions 2–8.
10. What information is obtained from a loop diagram?
11. What information is available on electrical one-line diagrams?
12. What information is contained on a plot plan drawing?
What are the major terminologies related to
Instrument calibration?

Accuracy - The degree of an indicated value to an accepted standard value or true value.
Calibration - correcting or determining the error of an existing scale or of evaluating one quantity in
terms of reading from another.
Factory calibration - adjust all the internal working of the instrument to bring it into agreement with
the standard.
Field calibration - compare one instrument with another instrument.
The other important terms are
Error - Measurement error is the difference between the measured value of a quantity and its true
value.
Precision - Precision refers to how close together a series of measurements are to each other.
Resolution
Repeatability
Reprocucibilty
Traceability
Accuracy - Accuracy refers to how close a measured value is to an accepted value.

What is a control valve?

A control valve is a final control element used to control fluid flow by varying the size of fluid passage
in response to the signals received from controllers.
Control valves are used to control condition such as flow, level, pressure , temperature.
What are the main parts of a control valve?

Basically there are two main sections for valve, the Body and the Actuators
Diaphragm, Actuator steam, yoke, actuator spring, Travel indicator comes under the Actuator part.
Valve body, valve plug, seat ring, guide ring comes under Body part.
What is a valve Cv ?
Valve Cv is the capacity of the valve. In other words, number of gallons of water per minute passing
through the valve in the full option condition with 1PSI pressure drop across the valve at 15 degree
temperature.
What are actuators? what are the types of actuators?
Actuators are the driving unit of a valve, which consist of diaphragm and piston that operates the
valve stem by receiving the control signal.
Actuators are divided into three based on the source of energy used for the movement of the valve
stem .
1. Pneumatic actuators : Compressed air ( 3-15 psi) is used for actuation of diaphragm .
2. Electric actuators : Electric actuator use electric currents.
3. Hydraulic actuators : Hydraulic fluids are used used for actuation.
What is valve glanding?

A valve glanding prevents the process fluid from coming out through the valve stem. Glanding is the
part of the valve body.
commonly used gland packing are Teflon, Graphite, Asbestos etc,.
What is a valve positioner ?
A valve positioner relates the input signal and the valve position, and will provide any output
pressure to the actuator to satisfy this relationship, according to the requirements of the valve, and
two ensure actuator stem take a position proportional to the input signal.
What is an Over sized control valve?
If a minimum signal to the valve or minimum opening of the valve brings the process set-point rapidly
then the valve is Over sized for the porpose
How Over sizing can be fixed?
By replacing the valve or changing the valve trim to reduced size.
What are the functions of an Instrument?
The primary function of an instrument is to measure a process variable. While doing so, it performs
several functions such as Transmitting,Signaling,Registering,Indicating,Recording.
Transmitting – Instrument sends the information concerning the measured quantity over some
distance.
Transmitters are used in process control loop.
Signaling – Instruments indicates the general value or range of values of its measured quantity.
Registering – Instrument indicates by numbers or some other symbol of discrete increments, the
value of the quantity being measured.
Indicating – An indicating instrument indicates the instantaneous value of the variable being
measured at that time. Such an instrument normally uses a calibrated scale and pointer.
Recording – Instruments makes a written record of the value of the measured quantity against some
other variable or against time.

An instrumentation system is an assembly of various devices or components to form an integral
whole, for observation, monitoring or control purposes.In other words,it is defined as the design,
construction, and provision of instruments for measurement, control etc.
eg-home security system,weather radar,thermostat,CT-scan,MRI- machine etc
Instrumentation system can be classified into following categories:
On the basis of signal:

1. Analog instrumentation system.
2. Digital instrumentation system.
On the basis of response:
1. Intelligent instrumentation system.
2. Dumb instrumentation system.
Interview Questions : Process control
Engineer
Q)What does PID mean? (Be specific, don’t just define the Three Letter Acronym)
Q)PID is normally used for three-mode control. What does this mean?
Q)What is meant by the term setpoint?
Q)What is a cascade loop?
Q)What is a single loop controller?
Q)Discuss the interrelationship between SP, PV and Output.
Q)What is a marshalling panel?
Q)What is the difference between points and tags?
Q)In process control, what is the function of a transmitter?
Q)What is meant by the term 4 to 20?
Q)What is an RTD?
Q)In process control, what do the abbreviations, AI, AO, DI and DO typically stand for?
Q)What is a closed-loop system?
Q)What is a thermocouple and how does it work?
Q)Discuss the difference between a servo motor and a stepper motor
Q)What is an actuator? Give an example of an actuator:
Q)Describe the function of a controller
Q)What is the difference between a batch process and a continuous process?
Q)What is a controlled variable?
Q)What is an A/D converter
Q)What do we mean when we say a loop needs to be tuned?
Field Instrumentation
Interview Questions and
Answers
1.What are different types of orifice plates? State their uses.
Different orifice plates are: 1. Concentric 2. Segmental 3. Eccentric

Concentric: These plates are used for ideal liquid as well as gases and
steam service. Concentric holes are present in these plates, thats why it is
known as concentric orifice.
Segmental: This plate has hole in the form of segment of the circle. This plate
is used for colloidal and sherry flow measurement.
Eccentric: This plate has the eccentric holes. This plate is used in viscous
and sherry flow measurement.
2.How do you identify an orifice in the pipeline?
An orifice tab is welded on the orifice plate which extends out of the line giving
an indication of the orifice plate.
3.Why is the orifice tab provided?
Following reasons justify for providing orifice tab:
1. Indication of orifice plate in a line
2. The orifice diameter is marked on it.
3. The material of the or ifice plate.
4. The tag number of the orifice plate.
5. To mark the inlet of an orifice.
4.Explain Bernoulli’s theorem. State its application.
Bernoulli’s theorem states that the ‘total energy of a liquid flowing from one
point to another remains constant’. It is applicable for non-compressible
liquids. For different types of liquid flow Bernoulli’s equation changes. There is
direct proportion between speed of fluid and its dynamic pressure and its
kinetic energy. It can be used in various real life situations like measuring
pressure on aircraft wing and calibrating the airspeed indicator. It can also be
used to low pressure in the venturi tubes present in carburetor.
5.How can a D.P. transmitter be calibrated?
D.P. transmitter can be calibrated using following steps:
1. Adjust zero of Transmitter.
2. Perform static pressure test: Give equal pressure on both sides of
transmitter. Zero should not shift either side. If the zero shifts then carry out
static alignment.
3. Perform vacuum test: Apply equal vacuum to both the sides. Zero should
not shift.
4. Calibration procedure: Give 20 psi air supply to the transmitter and vent
L.P. side to atmosphere. Connect output of the instrument to the standard test

gauge. Adjust zero. Apply required pressure to the high pressure side and
adjust the span. Adjust zero gain if necessary.
6.How is flow measured in square root?
Flow varies directly as the square root of pressure. Thus, F=K of square root
of applied pressure. Since this flow varies as the square root of differential
pressure. The pressure pen does not directly indicate flow. Thus flow can be
determined by taking the square root of the pen. Assume the pen reads 50%
of the chart. So, flow can be calculated using the pen measure in the chart.
7.Name different parts of a pressure gauge. Explain the use of hair
spring in the pressure gauge.
Pressure gauge includes following components:
a. ‘C’ type bourdon tube.
b. Connecting link
c. Sector gear
d. Pinion Gear
e. Hair spring
f. Pointer
g. Dial
Use of hair spring: Hair spring is responsible for controlling torque. It is also
used to eliminate any play into linkages..
8.How D.P. transmitter can be applied to close tank?
In closed tank, bottom of the tank is connected to the high pressure side of the
transmitter. Top of tank is connected to the lower pressure side of the
transmitter. In this way vessel pressure can be measured.
9.How D.P. transmitter can be applied to open tank?
In open tank the lower pressure side is vented to the atmosphere. All pressure
is applied to the high pressure side. This vessel pressure is measured through
high pressure side.
10.Explain the working of an electronic level troll?
The variation in level of buoyancy resulting from a change in liquid level varies
the net weight of the displacer increasing or decreasing the load on the torque
arm. This change is directly proportional to change in level and specific gravity
of the liquid. The resulting torque tube movement varies the angular motion of
the rotor in RVDT providing a rotor change proportional to the rotor
displacement, which is converted and amplified to a D.C. current.

11.Explain the working of an enraf level gauge.
Enraf level gauge is based on the ser powered null balance technique. A
displacer serves as continuous level sensing element. A two phase ser motor
controlled by a capacitive balance system winds unwinds the the measuring
wire until the tension in the weight springs is in balance with the weight of the
displaced part immersed in the liquid. The sensing system in balance
measures the two capacitance formed by the moving central sensing rod
provided by the two capacitor plates and the si plates.
12.What is the constant voltage unit?
The constant voltage circuit consists of a rectifier, CR and a filter capacitor. It
is followed by two stages of zener regulation. Abridge configuration is
provided to lamp line voltage zener regulation. Regulation 1 and regulation 2
provides relatively provide constant current. Resistors form a bridge that may
remoment line voltage effects.
13.Explain the burnout feature.
Burnout provides the warnsug feature of driving indicator at the end of scale, if
the input circuit is open. A burnout resistor is provided which develops a
voltage drop between the measuring circuit and amplifier. The polarity of the
signal determines the direction of the servo drive upon an open circuit in the
input.
Upscale burnout: R value 10 M
Downscale burnout: R value 22 M
14.Why thermowells are used? What materials are used in thermo wells?
In numerous applications, it is neither desirable nor practical to expose a
temperature sensor directly to a material. Wells are therefore used to protect
against damage corrosion, arosion, aborsion and high pressure processes. A
thermowell is also useful in protecting a sensor from physical damage during
handling and normal operations.
Materials used in thermowells: Stainless steel, Inconel, Monel, Alloy Steel,
Hastelloy
15.How is automatic reference junction compensation carried out in
temperature recorders?
In automatic reference junction compensation, variable nickel resistor is used.
As the temperature changes, so does its resistance. This reference junction
compensator is located, so that it will be at the temperature of the reference

junction. The reference junction is at the poset where the dissimilar wire of the
thermocouple is rejoined. This joint is invariably at the terminal strip of the
instrument.
16.What are de-saturators?
When, in some processes, e.g. batch processes, long transient responses are
expected during which a sustained deviation is present the controller integral
action continuously drives the output to a minimum or maximum value. This
phenomenon is called ‘integral saturation of the control unit’. When this
condition is met, then this unit is de-saturated.
17.Explain the working of Rotameter.
Variable area meters are special form of head meters. Where in the area of
flow restrictor is varied. So as to hold the differential pressure constant. The
rota meter consists of a vertical tapered tube through which the metered fluid
flows in upward direction. As the flow varies the ‘float’ rises or falls to vary the
area of the passages that the differential across it balances the gravitational
force on the ‘float’. The differential pressure is maintained constant. The
position of the ‘float’ is the measure of the rate of flow.
18.What is the working principle of the magnetic meter?
An electric potential is developed when a conductor is moved across the
magnetic field. In most electrical machinery the conductor is a wire. The
principle is equally applicable to a moving, electrically conductive liquid. The
primary device of commercial magnetic meters consist of straight cylindrical
electrically insulated tube with a pair of electrodes nearly flush with the tube
walls and located at opposite end of a tube diameter. This device is limited to
electrically conducting liquids. The magnetic meter is particularly suited to
measurement of slurries and dirty fluids.
19.Explain the mechanism behind the turbine meter.
Turbine meters consist of straight flow tube within which a turbine or fan is
free to rotate about it s axis which is fixed along g the centre line of the tube.
Mostly, a magnetic pick up system senses the rotation of the rotor through the
tube walls. The turbine meter is a flow rate device, since the rotor speed is
directly proportional to the flow rate. The output is usually in the form of
electric pulses from the magnetic pick up with a frequency proportional to the
flow rate.
20.How would you choose differential range?
The most common range for differential range for liquid measurement is 0-
100. This range is high enough to minimize the errors caused by unequal

heads in the seal chambers. It is also dependent on the differences in the
temperature of the load lines. The 100 range permits an increased in capacity
up to 400. While decrease down up to 20 by merely changing the range tubes
or range adjustments.
21.What is the use of single seated valve?
The single seated valve is used on smaller sizes where an absolute shut off is
required. The use of single seated valve is limited by pressure drop across the
valve in the closed or almost closed position.
22.What is the use of double seated valve?
In double seated valves the upward and downward forces on the plug due to
reduction of fluid pressure are nearly equalized. It is generally used on bigger
size valves and high pressure systems. Actuator forces required are less.
23.What is the use of valve positioner?
Valve positioner can be used for following reasons:
a. Quick action
b. Valve hysterisis
c. Viscous liquids
d. Split range.
e. Line pressure changes on valve
f. Bench set not standard
g. Reverse valve operations
24.What are primary elements of measuring pressure? Which type of
pressure can be measured by these elements?
Primary elements of measuring pressure are:
a. Bourdon Tube
b. Diaphragm
c. Capsule
d. Bellows
e. Pressure springs
These elements are known as elastic deformation pressure elements.
25.Name different types of bourdon tubes.
Types of bourdon tubes:

1. C type
2. Spiral
3. Helix
26.What are different types of control valves?
The commonly used control valves can be defined as follows:
a. Depending on Action: Depending on action there are two types of control
valves
1. Air to close 2. Air to close
b. Depending on body: Depending on body there are 4 types of control
valves
1. Globe valves single or double seated
2. Angle valves
3. Butterfly valves
4. Three way valves
27.What is furnace draft control?
Balanced draft boilers are generally used negative furnace pressure. When
both forced draft and induced draft are used together, at some point in the
system the pressure will be same as that of atmosphere. Therefore the
furnace pressure must be negative to prevent hot gas leakage. Excessive
vacuum in the furnace however produces heat losses through air infiltration.
The most desirable condition is that the one have a very slight negative
pressure of the top of furnace.
28.What is intrinsically safe system?
Intrinsic safety is a technique for designing electrical equipment for safe use in
locations made hazardous by the presence of flammable gas or vapours in
the air. Intrinsically safe circuit is one in which any spark or thermal effect
produce either normally or under specified fault conditions is incapable of
causing ignition of a specified gas or vapour in air mixture at the most ignited
concentration.
29.What is zener diode? What is voltage regulator?
The breakdown region of a p-n diode can be made very sharp and almost
vertical diodes with almost vertical breakdown region are known a s zener
diodes. A zener diode operating in the breakdown region is equivalent to a
battery. Because of this current through zener diode can change but the
voltage remains constant. It is this constant voltage that has made the zener
diode an important device in voltage regulation.

Voltage regulator: The output remains constant despite changes in the input
voltage due to zener effect.
30.What is force balance principle? State some of its’ advantages.
Force balance principle: A controller which generates an output signal by
opposing torque. The input force is applied on the input bellows which moves
the beam. This crackles nozzle back pressure. The nozzle back pressure is
sensed by the balancing bellows which brings the beam to balance. The baffle
movement is very less about 0.002 for full scale output.
Advantages:
a. Moving parts are fewer.
b. Baffle movement is negligible
c. Frictional losses are less
31.What is motion balance principle?
A controller which generates an output signal by motion of its parts. The
increase in the baffle is to move towards the nozzle. The nozzle back
pressure will increase. This increase in the back pressure acting on the
balancing bellows, will expand the bellows. The nozzle is moved upward due
to this. The nozzle will move until motion almost equals the input baffle
motion.
32.How will you test a transistor with a multimeter?
1. Emitter +ve of meter and base -ve output =Low resistance
2. Emitter -ve of meter and base +ve output =High resistance
3. Collector +ve and base -ve output =Low
4. Collector -ve and base +ve output =Low
Emitter: Collector = High resistance
PNP: Opposite Results
33.Explain ratio control system.
A ratio control system is characterized by the fact that variations in the
secondary variable don’t reflect back on the primary variable. A ratio control
system is the system where secondary flow is hold in some proportion to a
primary uncontrollable flow.

If we assume that the output of a primary transmitter is A. and the output of
the secondary transmitter is B, and that the multiplication factor of the ratio
relay is K, then for equilibrium conditions which means set valve is equal to
measured valve, we find the following relation:
KA-B=0 or B/A = K where ‘K’ is the ratio setting off the relay.
34.What is solenoid valve? Where it is used?
A solenoid is electrically operated valve. It consists of solenoid coil in which
magnetic plunger moves. This plunger is connected to the plug and tends to
open or close the valve. There are two types of solenoid valves:
1. Normally Open
2. Normally closed
Use: It is used for safety purpose in different electric work.
What is furnace draft control?
Balanced draft boilers are generally used negative furnace pressure. When both forced draft and
induced draft are used together, at some point in the system the pressure will be same as that of
atmosphere. Therefore the furnace pressure must be negative to prevent hot gas leakage.
Excessive vacuum in the furnace however produces heat losses through air infiltration. The most
desirable condition is that the one have a very slight negative pressure of the top of furnace.
What is intrinsically safe system?
Intrinsic safety is a technique for designing electrical equipment for safe use in locations made
hazardous by the presence of flammable gas or vapours in the air. Intrinsically safe circuit is one in
which any spark or thermal effect produce either normally or under specified fault conditions is
incapable of causing ignition of a specified gas or vapour in air mixture at the most ignited
concentration.
What is zener diode? What is voltage regulator?
The breakdown region of a p-n diode can be made very sharp and almost vertical diodes with almost
vertical breakdown region are known a s zener diodes. A zener diode operating in the breakdown
region is equivalent to a battery. Because of this current through zener diode can change but the
voltage remains constant. It is this constant voltage that has made the zener diode an important
device in voltage regulation.

Voltage regulator: The output remains constant despite changes in the input voltage due to zener
effect.
What is force balance principle? State some of its’ advantages.
Force balance principle: A controller which generates an output signal by opposing torque. The input
force is applied on the input bellows which moves the beam. This crackles nozzle back pressure.
The nozzle back pressure is sensed by the balancing bellows which brings the beam to balance.
The baffle movement is very less about 0.002 for full scale output.
Advantages:
a. Moving parts are fewer.
b. Baffle movement is negligible
c. Frictional losses are less
What is motion balance principle?
A controller which generates an output signal by motion of its parts. The increase in the baffle is to
move towards the nozzle. The nozzle back pressure will increase. This increase in the back
pressure acting on the balancing bellows, will expand the bellows. The nozzle is moved upward due
to this. The nozzle will move until motion almost equals the input baffle motion.
How will you test a transistor with a multimeter?
1. Emitter +ve of meter and base -ve output =Low resistance
2. Emitter -ve of meter and base +ve output =High resistance
3. Collector +ve and base -ve output =Low
4. Collector -ve and base +ve output =Low
Emitter: Collector = High resistance
PNP: Opposite Results
Explain ratio control system?
A ratio control system is characterized by the fact that variations in the secondary variable don’t
reflect back on the primary variable. A ratio control system is the system where secondary flow is
hold in some proportion to a primary uncontrollable flow.
If we assume that the output of a primary transmitter is A. and the output of the secondary
transmitter is B, and that the multiplication factor of the ratio relay is K, then for equilibrium
conditions which means set valve is equal to measured valve, we find the following relation:

KA-B=0 or B/A = K where ‘K’ is the ratio setting off the relay.
What is solenoid valve? Where it is used?
A solenoid is electrically operated valve. It consists of solenoid coil in which magnetic plunger
moves. This plunger is connected to the plug and tends to open or close the valve. There are two
types of solenoid valves:
1. Normally Open
2. Normally closed
Use: It is used for safety purpose in different electric work.
How is automatic reference junction compensation carried out in
temperature recorders?
In automatic reference junction compensation, variable nickel resistor is used. As the temperature
changes, so does its resistance. This reference junction compensator is located, so that it will be at
the temperature of the reference junction. The reference junction is at the poset where the dissimilar
wire of the thermocouple is rejoined. This joint is invariably at the terminal strip of the instrument.
What are de-saturators?
When, in some processes, e.g. batch processes, long transient responses are expected during
which a sustained deviation is present the controller integral action continuously drives the output to
a minimum or maximum value. This phenomenon is called ‘integral saturation of the control unit’.
When this condition is met, then this unit is de-saturated.
Explain the working of Rota meter.
Variable area meters are special form of head meters. Where in the area of flow restrictor is varied.
So as to hold the differential pressure constant. The rota meter consists of a vertical tapered tube
through which the metered fluid flows in upward direction. As the flow varies the ‘float’ rises or falls
to vary the area of the passages that the differential across it balances the gravitational force on the
‘float’. The differential pressure is maintained constant. The position of the ‘float’ is the measure of
the rate of flow.
What is the working principle of the magnetic meter?
An electric potential is developed when a conductor is moved across the magnetic field. In most
electrical machinery the conductor is a wire. The principle is equally applicable to a moving,
electrically conductive liquid. The primary device of commercial magnetic meters consist of straight

cylindrical electrically insulated tube with a pair of electrodes nearly flush with the tube walls and
located at opposite end of a tube diameter. This device is limited to electrically conducting liquids.
The magnetic meter is particularly suited to measurement of slurries and dirty fluids.
Explain the mechanism behind the turbine meter.
Turbine meters consist of straight flow tube within which a turbine or fan is free to rotate about it s
axis which is fixed along g the centre line of the tube. Mostly, a magnetic pick up system senses the
rotation of the rotor through the tube walls. The turbine meter is a flow rate device
, since the rotor speed is directly proportional to the flow rate. The output is usually in the form of
electric pulses from the magnetic pick up with a frequency proportional to the flow rate.
How would you choose differential range?
The most common range for differential range for liquid measurement is 0-100. This range is high
enough to minimize the errors caused by unequal heads in the seal chambers. It is also dependent
on the differences in the temperature of the load lines. The 100 range permits an increased in
capacity up to 400. While decrease down up to 20 by merely changing the range tubes or range
adjustments.
Interview Questions on
Differential Pressure Flow
Transmitter
How do you do a zero check on a differential pressure transmitter?
Zero checking a differential pressure transmitter is an important as it ensures
that the transmitter is not deviating from correct zero differential pressure
reading. It should be done regularly or when one suspects an error in the
readout of the said transmitter.
Before doing anything with a differential pressure transmitter as with other
process instruments, be sure that it is isolated from the system process
especially when the industrial plant is in operation. This can be done resetting
the setpoints of the transmitter, bypassing signal transmissions and putting it
in bypass mode, or putting the transmitter in maintenance mode. It really
depends on the system design of the company. Ensure to inform the control
room personnel before and after the job. Usually jobs like this are done during
plant shutdown and is in for maintenance. Never attempt to adjust, or

manipulate any part of the transmitter either physically or with its parameters
without proper communications with process and production personnel. It
could lead to a plant emergency shutdown which we don’t want to happen.
Now, to zero check a differential presure transmitter, a technician can close
either one of the low pressure side or high pressure side isolation valves.
usually the high pressure valve is the one that is closed to prevent transmitter
damage and must be closed slowly. Open the equalizing valve also slowly.
Both can be closed or open at the same time but do it in a very slow manner.
Also, ensure that the you know the working pressure of the process to have
an insight of how much pressure there would be in case of leakage or any
other undesirable events. The three valves can be found in the valve manifold
or in line with the impulse lines if installed otherwise. Refer to the picture to
see the valves below the transmitter in manifold. The valve in the center
(without the label H for high pressure side and L for low pressure side tapping)
is the equalizing valve.
What to check when differential flow transmitters indication is more?
· First flushing the transmitter. Flush both the impulse lines. Adjust
the zero by equalizing if necessary.
If still the indication is more then.
· Check low pressure (L.P). side for choke. If that is clean then.
· Check the leaks on low pressure (L.P.) side. If not,
· Calibrate the transmitter.
What is Bernoulli’s theorem and where it is applicable?
Bernoulli’s theorem states the “total energy of a liquid flowing from one point
to another remains constant.” It is applicable for non compressible liquids.
Answers.com states that Bernoulli theorem is an expression of the
conservation of energy in the steady flow of an incompressible, inviscid fluid; it
states that the quantity (p/ρ) + gz + (v2/2) is constant along any streamline,
where pis the fluid pressure, v is the fluid velocity, ρ is the mass density of the
fluid, g is the acceleration due to gravity, and zis the vertical height. Also
known as Bernoulli equation; Bernoulli law.
Why is the orifice tab provided? What is its purpose?

The orifice tab is provided due to the following reasons.
• Indication of an orifice plate in a line.
• The orifice diameter is marked on it.
• The material of the orifice plate.
• The tag no. of th e orifice plate.
• The mark the inlet of an orifice.
How do you carry out piping for a differential pressure flow transmitter
on liquids, gas and steam services?

Liquid lines : On liquid lines the transmitter is mounted below the orifice plate
because liquids have a property of self draining

Gas Service :On gas service the transmitter is mounted above the orifice plate
because Gases have a property of self venting and secondly condensate
formation.

Steam Service :On steam service the transmitter is mounted below the orifice
plate with condensate pots. The pots should be at the same level.
What are the different types of orifice plates and their uses ?
The different types of orifice plates are :
• Concentric.
• Segmental.
• Eccentric.
• Quadrant Edge.
Concentric :
The concentric orifice plate is used for ideal liquid as well as gases and steam
service. This orifice plate beta ratio fall between of 0.15 to 0.75 for liquids
and 0.20 to 0.70 for gases, and steam. Best results occur between value of
0.4 and 0.6. beta ratio means ratio of the orifice bore to the internal pipe
diameters.

(45º beveled edges are often used to minimize friction resistance to flowing
fluid )
Eccentric :
The eccentric orifice plate has a hole eccentric. Use full for measuring
containing solids, oil containing water and wet steam. Eccentric plates can
use either flange or vena contracta taps, but the tap must be at 180º or 90º to
the eccentric opening.

Eccentric orifices have the bore offset from center to minimize problems in
services of solids-containing materials.
Segmental :
The segmental orifice place has the hole in the form segment of a circle. This
is used for colloidal and slurry flow measurement. For best accuracy, the tap
location should be 180º from the center of tangency.

Segmental orifices provide another version of plates useful for solids
containing materials.
Quadrant Edge :
It common use in Europe and are particularly useful for pipe sizes less than 2
inchs.

Quadrant edge orifices produce a relatively constant coefficient of
discharge for services with low Reynolds numbers in the range from
100,000 down to 5,000.
What are the primary elements used for flow measurement ?
• Orifice Plate – Orifice plate is used for flow rate measuring in pipe
systems. With orifice plate, pressure drop is created. Based on the magnitude
of pressure drop, flow rate can be calculated. This instrument is very practical
for large tube diameters and for dirty fluid when turbines are not applicable.
• Venturi tube – Due to simplicity and dependability, the Venturi tube
flowmeter is often used in applications where it’s necessary with higher
TurnDown Rates, or lower pressure drops, than the orifice plate can provide.
In the Venturi Tube the fluid flowrate is measured by reducing the cross
sectional flow area in the flow path, generating a pressure difference. After the

constricted area, the fluid is passes through a pressure recovery exit section,
where up to 80% of the differential pressure generated at the constricted area,
is recovered.
• Pitot tube – The pitot tube measures the fluid flow velocity by converting
the kinetic energy of the flow into potential energy.
The use of the pitot tube is restricted to point measuring. With the “annubar”,
or multi-orifice pitot probe, the dynamic pressure can be measured across the
velocity profile, and the annubar obtains an averaging effect.
• Annubars – An annubar consists of several pitot tubes placed across a
pipe to provide an approximation to thevelocity profile, and the total flow can
be determined based on the multiple measurements.
• Flow Nozzle – Flow Nozzle is a differential flowmeter that creates a flow
restriction for, typically, high-velocity, non-viscous, erosive flows.
• Weir & Flumes – A weir is a low dam or overflow structure built across an
open channel. It has a specific size and shape with a unique free-flow, head-
discharge relationship. The edge or surface over which the water flows is
called the crest. Discharge rates are determined by measuring the vertical
distance from the crest to the water surface in the pool upstream from the
crest. Flumes are primary devices that constrict an open channel flow for
measurement. Once the flow is backed up behind the constriction there is a
defined relationship between the depth up stream and the flow through the
constriction. This relationship can either be determined from an equation or a
table.

Instrumentation Interview Q&A EMERSON EDUARDO RODRIGUES

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Instrumentation Interview Q&A EMERSON EDUARDO RODRIGUES