Instrument Classification : Active & Passive Instrument

Instruments

1. Electronic Instrumentation
Lecturer Touseef Yaqoob
1
Instrument Classification
1)Active Instrument
2)Passive Instruments
Instruments are either active or passive
according to whether the instrument
output is entirely produced by the
quantity being measured or whether the
quantity being measured simply
modulates the magnitude of some
external power source.

2. Electronic Instrumentation
Lecturer Touseef Yaqoob
2
Examples
An example of passive instrument is
the pressure measuring device in
which the pressure of the fluid is
translated into movement of a
pointer against a scale.
An example of a active instrument is a
float-type petrol-tank level indicator.

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Lecturer Touseef Yaqoob
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Points to remember
In active instruments the external power source
is usually electrical in form, but in some cases it
can be pneumatic or hydraulic.
One important difference between active and
passive instruments is the level of measurement
resolution which can be obtained.
Passive instruments are normally cheaper to
manufacture then active instruments.

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Static characteristics of
instruments
Accuracy
Precision/repeatability
Tolerance
Range or span
Bias
Linearity
Sensitivity of measurement
Sensitivity to disturbance
Hysteresis
Dead space
Threshold
Resolution

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Accuracy:
Accuracy is the extent to which a reading
might be wrong, and is often quoted as a
percentage of full-scale reading of an
instrument.
Precision:
Precision is a term which describes an
instrument’s degree of freedom from random
errors.

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Tolerance:
Tolerance is a term which is closely related to
accuracy and defines the maximum error
which is to be expected in some value.
Range or Span:
The range or span of an instrument defines
the minimum and maximum values of a
quantity that the instrument is designed to
measure.

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Bias:
Bias describes a constant error which exists
over the full range of measurement of an
instrument.
Linearity:
It is normally describes that the output reading of
an instrument is linearly proportional to the
quantity being measured.
Sensitivity:
Sensitivity is a measure of the change in
instrument output which occurs when the
quantity being measured changes by a given
amount.

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Sensitivity to disturbance:
Sensitivity to disturbance is a measure of
the magnitude of change in static
characteristic of a instrument due to
environmental changes.
Such changes effect instrument in two main
ways.
1)Zero drift
2)Sensitivity drift (Scale factor drift)

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Hysteresis:
The non-coincidence
between loading and
unloading curves is
known as hysteresis.
Dead space:
Dead space is defined
as the range of
different input values
over which there is no
change in output
value.

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Lecturer Touseef Yaqoob
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Threshold:
The minimum level of input before the change
in the instrument output reading is of large
enough magnitude to be detectable is known
as the threshold of the instrument.
Resolution:
The minimum reading that can be taken from
instruments.

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The Dynamic Response of
Measuring Instruments
The dynamic responseof a measuring
instrument is the change in the output y
caused by a change in the input x. Both x and
y are functions of time t .
Classes of Linear Instruments
Zero Order Instruments
First Order Instruments
Second Order Instruments

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Zero Order Instruments
A zero order linear instrument has an output
which is proportional to the input at all times in
accordance with the equation
y(t) = Kx(t), where K is a constant called the
static gain of the instrument.
The static gain is a measure of the sensitivity of
the instrument.
An example of a zero order linear instrument is a
wire strain gauge in which the change in the
electrical resistance of the wire is proportional
to the strain in the wire.

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First Order Instruments
A first order linear instrument has an output
which is given by a non-homogeneous first
order linear differential equation
tau.dy(t)/dt + y(t) = K.x(t), where tau is a constant,
called the time constant of the instrument.
In these instruments there is a time delay in their
response to changes of input. The time constant
tau is a measure of the time delay.
Thermometers for measuring temperature are
first-order instruments.

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Second Order Instruments
A second order linear instrumenthas an
output which is given by a non-homogeneous
second order linear differential equation
d2y(t)/dt2 + 2.rho.omega.dy(t)/dt + omega2.y(t) = K.omega2.x(t),
where rhois a constant, called the damping
factorof the instrument, and omegais a
constant called the natural frequencyof the
instrument.

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Under a static input a second order linear
instrument tends to oscillate about its position
of equilibrium. The natural frequency of the
instrument is the frequency of these
oscillations
Friction in the instrument opposes these
oscillations with a strength proportional to the
rate of change of the output. The damping
factor is a measure of this opposition to the
oscillations

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An example of a second order linear instrument is a
galvanometer which measures an electrical current by the
torque on a coil carrying the current in a magnetic field. The
rotation of the coil is opposed by a spring. The strength of
the spring and the moment of inertia of the coil determine
the natural frequency of the instrument. The damping of the
oscillations is by mechanical friction and electrical eddy
currents.
Another example of a second order linear instrument is a U-
tube manometer for measuring pressure differences. The
liquid in the U-tube tends to oscillate from side to side in the
tube with a frequency determined by the weight of the liquid.
The damping factor is determined by viscosity in the liquid
and friction between the liquid and the sides of the tube.