Metrology instrumentation measurements.pptx

Instrumentation, pressure temperature flow measurements

1. UNIT-I INSTRUMENTATION

2. What is measurement? In order to compare or determine the value of a physical variable, some kind of measurement is to be carried out. Fundamentally, measurement of a quantity is the act or the result of a quantitative comparison between predefined standard and an unknown magnitude. (OR) Measurement is required to test if the elements that constitute the system function as per the design expectation and finally to evaluate the functioning of the system itself.

3. If the result is to be meaningful, the act of measurement must satisfy the following requirements: (1) the standard must be accurately known and internationally accepted, and (2) the apparatus and the experimental procedure adopted for comparison must be provable. Methods of measurements The two basic methods of measurement are: a DIRECT COMPARISON with the primary or the secondary standard and an INDIRECT COMPARISON with a standard, through a calibrated system.

4. Calibration A measuring instrument is to be checked for accuracy at frequent intervals with a known standard, and any discrepancy between the measured value Calibration procedures thus involve a comparison of a particular instrument with either ( i ) A PRIMARY STANDARD, (ii) A SECONDARY STANDARD, (iii) A KNOWN INPUT SOURCE.

5. (a) PRIMARY SENSING ELEMENT (3) This is an element which first receives energy from the medium to be measured and produces a proportional output . The output signal of the primary sensing element is a physical variable such as displacement or voltage. The primary sensing element, therefore, is a primary transducer(sensor) that converts ( transduces ) one physical variable or effect into another.

6. VARIABLE CONVERSION ELEMENT(4): the output signals of the primary sensing element may require to be converted to a more suitable variable while preserving its information contents EX: conversion of displacement to voltage in a pressure transducer VARIABLE MANIPULATION ELEMENT(5): It modifies the direct signal by amplification filtering etc .., DATA TRANSMISSION ELEMENT(6): It is necessary to transmit signals from one element to another . DATA PRESENTATION ELEMENT(7): The quantity being measured is to be communicated to a human observer for monitoring control or analysis purposes . This is to presented in a form recognizable by human.

7. The input to the instrument could be either constant or rapidly varying with time. Therefore the performance of an instrument is under the following two heads: (1) STATIC PERFORMANCE CHARACTERISTICS, and (2) DYNAMIC PERFORMANCE CHARACTERISTICS . (1) STATIC PERFORMANCE CHARACTERISTICS An instrument may be used to measure quantities which are either constant or vary very slowly with time.

8. STATIC CALIBRATION: Static calibration refers to a procedure where an input, either constant or slowly time varying, is applied to an instrument and corresponding output measured, while all other inputs (desired, interfering, modifying) are kept constant at some value. The functional relationship between the output quantity q , and the input quantity q 1 , is referred to as static calibration valid under the stated constant conditions of all other inputs

9. STATIC PERFORMANCE CHARACTERISTICS LINEARITY If the relationship between the output and input can be expressed by an equation of the form q = a + kq i where a and k are constants, the instrument is said to possess linearity. Linearity, in practice, is never completely achieved, and the Deviations from the ideal are termed as linearity tolerance INDEPENDENT LINEARITY and the PROPORTIONAL LINEARITY are the two forms of specifying linearity

10. As an example, ±3% independent linearity means that the output will remain within values set by two parallel lines spaced ±3% of the full scale output from the leased line. The ideal value is never more than +3% away from the recorded value regardless of the magnitude of the input It may, however, be noted that an instrument which does not possess linearity can still be highly accurate

11. STATIC SENSITIVITY The static sensitivity is defined as the slope of the calibration curve i.e. If the input-output relation is linear, the sensitivity is constant for value of input . The sensitivity of an instrument having a non-linear static characteristic depends on the value of the input quantity and should be specified as Sensitivity=   Sensitivity=  

12. REPEATABILITY If an instrument is used to measure same or an identical input many times and at different time intervals, the output is not the same but shows scatter . This scatter or deviation from the ideal static characteristics.in absolute units or a fraction of the full scale, is called repeatability error

13. HYSTERESIS-THRESHOLD RESOLUTION While testing an instrument for repeatability, it is often seen that input-output graphs do not coincide for continuously ascending and then descending values of the input . This non-coincidence of input-output graphs for increasing and decreasing inputs arises due to the phenomenon of hysteresis

14. Hysteresis effects are best eliminated by taking readings corresponding to ascending and descending values of the input and then taking their arithmetic average. Threshold and resolution are the other two characteristics of measuring instrument. READABILITY AND SPAN The readability depends both on the instruments and observer and often is not stated. The span refers to the range of the instrument

15. MEASUREMENT OF FORCE AND TORQUE The measurement of force involves the determination of its magnitude as well as its direction. The measurement of force may be done by any of the two methods: Direct methods: These involve a direct comparison with a known gravitational force on a standard mass, say by a balance. Indirect methods. These involve in measurement of the effect of force on a body

16. DIRECT METHODS A body of mass ‘m’ in the earth's gravitational field experiences a force that is given by W=mg where W is the weight of the body. Any unknown force may be compared with the gravitational force (mg) on the standard mass m

17. ANALYTICAL BALANCE The constructional details of an analytical balance show a schematically in Fig. the balance arm rotates about the knife edge at point o the two forces w1 and w2 are applied at the ends is an unknown force w1 is the known force due to standard mass.

18. Point G is the centre of gravity of the arm , and W B is the weight of the balance arm . Fig shows the balance arm in an unbalanced position when the force W 1 and W 2 are unequal. This unbalance is indicated by the angle ‘ Ө ’ which the pointer makes with the vertical. In the balanced position W 1 = w 2 and hence is zero. Therefore, the weight of the balance arm and the pointer do not influence the measurements

19. INDIRECT METHODS ACCELERATION MEASUREMENT The measurement of force by measuring acceleration of a standard mass M when force is acting on this is based on the principle that F=Ma, where 'a' is the acceleration. The measurement can be carried out by accelerometers. the force determined is the resultant force acting on the mass.

20. PROVING RINGS One of the very useful and important devices is proving ring . This has been the standard for calibrating tensile-testing machines Proving ring can be used over wide range of loads starting from 1500 to 15 x 10 5 N In another variant of proving ring, a differential transformer is used for the measurement of deflection

21. Principle Within elastic limit when a force is applied to material the deformation produced is proportional to applied force . By measuring deformation the force can be measured

22. LVDT is used to  measure the physical quantities such as Force, Tension, Pressure, Weight, etc . These quantities are first converted into displacement using primary transducers and then it is used to convert the displacement to the corresponding Electrical voltage signal.

23. Figure shows a compression-type proving ring. The compressive load deforms the ring a sensitive micrometres is employed for the deflection measurement . Bosses are provided to clamp the ring rigidly to avoid rotation.

24. To obtain a precise measurement one edge of the micrometer is mounted on a vibrating reed device which is plucked to obtain a vibratory motion . The micrometer contact is moved forward until a noticeable damping of the vibration is observed. Deflection measurements may be made within ± 0.50 pm

25. LOAD CELLS USING STRAIN GAUGES Force transducers intended for weighing purposes are called load cells . Instead of using total deflection as a measure of load, strain-gauge load cells measure load in terms of unit strains . For very large loads, Direct tensile- compressive member may be selected . For small loads, strain amplification provided by bending may be employed to advantage. The gauges are so mounted as to give maximum output , and compensation for bending and temperature variations . The sensitivity is 2(1+µ) times that achieved with a single active gauge in the bridge. Compression cells of this kind have been used with a capacity of 15 x 10 6 N .

26. Figure illustrate proving ring strain gauge load cells. In Fig (a) the bridge output is a function of the bending strains only, the axial components being cancelled in the bridge arrangement. The arrangement of Fig(b) provides a somewhat higher sensitivity because the output includes both the bending and axial components sensed by gauges R 1 and R 4 .

27. TEMPERATURE SENSITIVITY OF LOAD CELLS USING STRAIN GAUGES: The careful analysis indicates that the temperature influences the measurements in the following two ways: change of dimensions change in the Young's modulus of the material. In a strain-gauge bridge circuitary, the bridge sensitivity is made to vary with temperature in a direction opposite to that caused by the variation of Young's modulus of the material.

28. if deflection increases, the material becomes more spring and hence collects more for a given load. This results in an increase in the sensitivity of the load cell. This increased sensitivity is set off by reducing the sensitivity of the bridge by the use of a thermally sensitive compensating resistance Rs as shown in Fig(a). In order to carryout calibration, usually two resistances each of nominal value of (Rs/2)are used as shown in Fig (b).

29. TORQUE MEASUREMENT Torque is a twisting or turning force that tends to cause rotation around an axis, which might be a center of mass or a fixed point. The measurement of torque is necessitated in order to obtain load information necessary for stress or deflection analysis . The torque T may be computed by measuring the force F at a known radius r from the following relation: T=Fr. Torque measurement is required for the determination of the mechanical power, either power required to operate a machine for power developed by the machine. The power is calculated from the rotation: P=2πNT Where, N is the angular speed in revolutions per second. Torque measuring devices used in this connection are commonly known as dynamometers.

30. There are basically three types of dynamometers: ( i ) ABSORPTION DYNAMOMETERS : They absorb the mechanical energy as torque is measured for measuring power(or) torque developed by power sources such as engines and motors. (ii) DRIVING DYNAMOMETERS : These dynamometers measure power or torque and as well provide energy to operate the devices to be tested . They are, therefore, useful for studying performance characteristics of devices such as pumps and compressors. (iii) TRANSMISSION DYNAMOMETERS . These are passive systems and are placed at an appropriate location within a machine or in between machines to measure torque at that particular location .

31. The first two types can be grouped as mechanical and electrical dynamometers MECHANICAL DYNAMOMETERS: Pony Brake is one of the simplest dynamometers for measuring power output (brake power). It is to attempt to stop the engine using a brake on the flywheel and measure the weight which an arm attached to the brake will support, as it tries to rotate with the flywheel.

32. The torque exerted on the Prony brake is T=FL Where, Force F is measured by conventional force measuring instruments. The power dissipated in the brake is calculated from Where, force F (measured at arm L) is in Newton’s, L is the length of reaction arm in meters, N is the angular speed in revolutions per minute, P in watts.  

33. ELECTRIC DYNAMOMETERS a device that is designed to measure the torque of electric motors. Electric dynamometers are used to determine mechanical or electromechanical characteristics of the motors. Such a dynamometer is an electric machine that operates as a generator and is mechanically coupled to a motor to be tested. DC generators are most often used as electric dynamometers. The torque developed by an electric motor is given by the equation where U is the voltage across the generator terminals in volts, I is the current in the field winding in amperes, n is the rotation rate in rpm, and η is the efficiency of the generator.

34. The electric dynamometers can be grouped into the following two classes: 1. d.c. dynamometers or generators. 2. Eddy-current or inductor dynamometers. D.C. DYNAMOMETERS It is usable both as an absorption and as a transmission dynamometer. it is most widely used for power and torque measurements on ic engines, small steam turbines, pumps etc.

35. The torque is measured by measuring a balancing force (say by a load cell) at a fixed known moment arm extending from the body of the dynamometer When used as a transmission dynamometer, it perform as a d.c. motors. It then measures the torque and power input to the machine ,for example a pump that absorbs power.

36. Two good features of the the d.c. dynamometer are its good performance at low speeds and ease of control. It can be adjusted to provide from zero to the so called base speed.

37. EDDY-CURRENT DYNAMOMETER: The eddy current dynamometer was invented by Martin and Anthony Winther in about 1931. The working principle of eddy current dynamometer is shown in the figure below. It consists of a stator on which are fitted some electromagnets and a rotor disc made of copper or steel and coupled to the output shaft of the engine. When the rotor rotates, eddy currents are produced in the stator due to magnetic flux set up by the passage of field current in the electromagnets . These eddy currents are dissipated in producing heat so that this type of dynamometer requires some  cooling arrangement . The torque is measured exactly as in other types of absorption dynamometers, i.e., with the help of a moment arm. The load in internal combustion engine testing is controlled by regulating the current in the electromagnets

38. ADVANTAGES OF EDDY CURRENT DYNAMOMETERS: High brake power per unit weight of dynamometer. They offer the highest ratio of constant power speed range (up to 5 : 1). Level of field excitation is below 1% of total power being handled by the dynamometer. Thus, they are easy to control and operate. Development of eddy current is smooth hence the torque is also smooth and continuous under all conditions. Relatively higher torque under low-speed conditions. It has no intricate rotating parts except shaft bearing. No natural limit to size, either small or large.

39. TRANSMISSION DYNAMOMETERS Torque can be measured conveniently by means of solid or hollow tubes These elements are twisted due to the application of a torque . There exist both tensile and compressive strains on the surface at 45° to the tube axis when it is twisted by a torque T In some cases, other forms of elastic elements such as bars of rectangular cross-section etc. are used.

40. The strain gauges are so mounted as to respond to twist only. Sometimes optical methods are used as a coupling between driving and driven machines , or between any two portions of a machine. In cases where strain gauges are used as secondary transducers, electrical connections are made through slip rings, with a provision to lift the brushes when they are not in use, thus minimising wear.

41. MEASUREMENT OF STRAIN AND STRESS When a stress is applied to a body, it gets deformed and these deformations are related to the applied stress. The evaluation of stress distribution in the body is known as stress analysis and includes the determination of kind, magnitude and direction of the stress.

42. There are a number of methods available for measuring strain. ( i ) Mechanical methods (ii) Opto -mechanical methods (iii) Electrical strain gauges (a) capacitance gauges (b) inductive gauges (c) piezo-electric gauges (d) resistance gauges (iv) Grid method (v) Moire Fringe technique (vi) Interferometry (vii) Photo-elasticity

43. MECHANICAL METHODS the mechanical gauges were not suitable for the measurement of steep gradients of strain . Various other factors like friction, lost motion, the weight and inertia , and the flexibility of parts hinder accurate measurements by these instruments. Another mechanical gauge is ' microkator ' and makes use of a double helix , oppositely twisted with a pointer attached in the centre. The gauge is sensitive. it can sense displacements in the range of 0.2 µm . It has a fair dynamic response.

44. OPTO-MECHANICAL METHODS Optical levers are used for magnification in the opto -mechanical gauges. Because of high magnification achievable, these gauges are of smaller length typically of 50 mm. One such gauge is the Tuckerman gauge shown in Figure. It consists of two separate sub-systems ( i ) an extensometer (ii) an auto-collimator. Extensometer: The extensometer comprises of a right angle roof prism and a tungsten-carbide rocker called as the Lozenge by the manufacturer.

45. AUTO-COLLIMATOR: The auto-collimator provides a beam of collimated light and receives it back after reflection from the prism-rocker assembly of the extensometer. The extensometer is mounted on the specimen and The right angle roof prism is adjusted such that the beam reflected from prism-rocker assembly makes an image of the graticule at a reference point as viewed through an eye-piece. When the specimen is deformed, say elongated along the length of the gauge, the rocker edge will move outwardly tilting its mirror surface by an angle ф . The reflected beam from prism rocker assembly will be deflected by an angle of 2 ф , which results in the shift of the image by 2f ф . where f is the focal length of the auto-collimator. the shift of the image is calibrated as the strain. An important feature of this instrument is that there need not be a fixed relationship between the positions of the auto-collimator and the extensometer.

46. ELECTRICAL METHODS: The methods which measure the change of some electrical quantities arising due to deformation (strain) in the body fall under electrical methods. These include ( i )capacitive gauges, (ii) inductive gauges, (iii)piezo-electric gauges (iv) resistance gauges. The electrical methods of measuring strain possess the advantage of high sensitivity and ability to respond to dynamic strains . Both the capacitive and inductive type gauges are of large mass and size, and are used only for some special applications. These gauges are sometimes used as load indicators, mounted directly on the machine frame. these gauges are quite rugged and maintain

47. these gauges are quite rugged and maintain the calibration over long period of time. Figure (a) illustrates all inductive gauge. A deformation results in the variation of air gap and hence the inductance. A linear differential transformer can also be used as an inductive gauge. Figure (b) shows a capacitive type strain gauge which is used in a torque meter . Torque carried by an elastic member causes a shift in the relative positions of the teeth, thereby changing the effective area and hence the capacitance . The changes in inductance or capacitance due to strains caused by loading are calibrated in terms of strain

48. Piezo-electric strain gauges are mainly used for studying dynamic inputs . One of the piezo-electric materials that is used for this purpose is barium- titanate . Wafers of barium- titanate of 0.25 mm thickness with suitable electrodes are bonded to the specimen with Duco сеmеnt. Due to their good dynamic response, stability, range of available size, case of data presentation and processing etc., resistance strain gauges are widely used for stress analysis. The resistance strain gauges are of two types: ( i ) unbonded strain gauges (ii) bonded strain gauges. UNBONDED STRAIN GAUGES : The unbonded strain gauges are made of a high-tensile resistance wire of about 0.025 mm diameter and of about 25 mm in length.

49. Two to twelve loops of the wire are attached to both a stationary frame and a movable platform with the help of pins made of electrically insulating material . Relative motion between the stationary frame and the platform is possible as guided by flexure plates . The resistance wire is preloaded so that it could be used to measure compressive strain . The four resistance wires in this construction are of that the bridge acts as a full bridge as shown in Fig. Unbonded strain gauges are mainly used as elements of force and pressure transducers, and accelerometers rather than for strain measurement.

50. MEASUREMENT OF PRESSURE Pressure is described as force per unit area and is analogous to stress. It is, therefore, expressed as N/m 2 , or in mm of water or mercury

51. DEFINITION OF PRESSURE TERMS ABSOLUTE PRESSURE: Absolute pressure refers to the absolute value of force per unit area exerted by a fluid on the walls of its container . It is the fluid pressure measured above a perfect vacuum. ATMOSPHERIC PRESSURE: The pressure exerted by the earth's atmosphere, as commonly measured by a barometer. At sea level, its value is close to 1.013 X 10 N/m absolute, decreasing with altitude. GAUGE PRESSURE: Gauge pressure indicates the difference between the absolute pressure and local atmospheric pressure. DIFFERENTIAL PRESSURE: This is the difference between two measured pressures . VACUUM : Vacuum represents the amount by which the atmosphere pressure exceeds the absolute pressure .

52. Methods of measuring pressure Pressure measuring techniques can be broadly classified in the following three groups: ( i ) Balancing the pressure exerted by fluid (usually mercury) column like in manometers, McLeod gauge etc. (ii) Measurement of elastic deformations of elements like membrane, diaphragm, Bourdon tube, etc. (iii) Measurement of electrical quantities like in Pirani and Penning gauges, Bridgman gauge, etc.

53. DEAD WEIGHT GAUGE TESTER OPERATION: The dead weight tester is basically a pressure producing and pressure measuring device. The following steps are followed. The valve of the apparatus is closed. A known weight is placed on the platform. Now by operating the plunger, fluid pressure is applied to the other side of the piston until enough force is developed to lift the piston-weight combination. When this happens, the piston weight combination floats freely within the cylinder between limit stops.

54. MANOMETERS U-TUBE MANOMETER: It consists of a glass tube bent like the letter 'U'. In this type of manometer, balancing a column of liquid is done by another column of same or other liquid. One end of the U-tube is attached to the point where pressure is to be measured, while the other end is open to atmospheric pressure. The pressure at point B in the figure is given by: P = ρ 2 g h 2 - ρ 1 g h 1 where, ρ 2 =density of heavy liquid h 2 = height of heavy liquid above reference line ρ 1 = density of light liquid h 1 = height of light liquid above reference line.

55. Fig. 2-1. the manometer is a U-tube about half filled with liquid. With both ends of the tube open, the liquid is at the same height in each leg. Fig. 2-2. When positive pressure is applied to one leg, the liquid is forced down in that leg and up in the other. The difference in height, "h," which is the sum of the readings above and below zero, indicates the pressure. Fig. 2-3. When a vacuum is applied to one leg, the liquid rises in that leg and falls in the other. The difference in height, "h," which is the sum of the readings above and below zero, indicates the amount of vacuum. This device indicates the difference between two pressures (differential pressure), or between a single pressure and atmosphere (gage pressure), when one side is open to atmosphere

56. MICROMANOMETERS Micromanometers are which serve as pressure standards in the range of 0.005 to 500 mm of water . PRANDTL-TYPE MICROMANOMETERS: This U-tube manometer consists of a reservoir of large diameter and an inclined tube with two marks connected through a flexible tube. Fig(a) the reservoir is raised or lowered to restore the liquid level between the two marks Fig(b) the inclined tube is moved vertically.

57. The capillary errors are minimised by bringing the level to a reference null position before the application of the pressure. After the application of pressure either the reservoir or the inclined tube is moved vertically by a lead screw to achieve the null position again.

58. MICROMETER TYPE MANOMETER meniscus and capillary effects are minimised by measuring liquid displacement with micrometer heads fitted with adjustable sharp index points located at or near the centre of large bore transparent tubes that are joined at their bases to form a U as shown in Fig. The contact with the surface of manometer liquid may be sensed optically or electrically.

59. AIR MICROMANOMETER Micromanometers uses air as its working fluid In this device, reference pressure is mechanically amplified by centrifugal action in a rotating disc . The disc speed is adjusted until the amplified reference pressure just balances the unknown pressure P. The null position is obtained by observing the lack of movement of minute oil droplets sprayed into the glass indicator tube located between the unknown and amplified pressures.

60. MCLEOD GAUGE used mainly for the measurement of vacuum pressures from 1 mm to 10 -6 mm of Hg . It measures a differential pressure , and hence is very sensitive. McLeod gauge is often employed for the calibration of electrical pressure gauges like Pirani and Penning gauges. The main limitations of this gauge are its slow response and extreme care required in its handling .

61. The construction details and procedure of operation are given below: A capillary C of very uniform bore of cross-sectional area is connected with a large bulb B. The vacuum pressure to be measured is connected as shown in Fig. If the capillary contains vacuum, then as the reservoir is raised, the mercury level in tubes 1, 2 and 3 will rise and remain at the same level in all the three tubes till it reaches the end part of the tube 1. This is taken as the reference. The tubes 1 and 2 have the same bore dimensions to avoid surface tension effects.

62. The reservoir is the lowered till the mercury level is below O. the pressure source is thus connected to the capillary C. Therefore, the bulb and capillary are at the pressure of the source, which is to be measured. The reservoir is raised again, thus cutting off the pressure source from the bulb. The gas in the bulb is compressed and confined to the capillary as the reservoir is moved up. When the mercury level in tube 2 has reached the reference level, the height in the capillary is measured and pressure calculated using Boyle's law. Often the capillary readings are directly calibrated in pressure.

63. PRESSURE MEASUREMENT WITH ELASTIC TRANSDUCERS There are different types of elastic transducers C-type Bellows etc.., C-TYPE:

64. bellows:

65. FLOW MEASUREMENT AND FLOW VISUALISATION The flow measurement requires the measurement of both the pressure and temperature. Instruments used in the measurement of flow may be categorised into two main classes: Quantity Meters: total quantity which flows in a given time is measured and an average flow rate is obtained by dividing the total quantity by time. Flow Meters: actual flow rate is measured . meters are used for calibration of the flow meters.

66. QUANITITY METERS: (a) Weight or volume tanks (b) Positive displacement or semi-positive displacement meters. FLOW METERS (a) Obstruction meters ( i ) Orifice (ii) Nozzle (ii) Venturi (iv) Variable-area meters (b) Velocity probes (i) Static pressure probes (ii) Total pressure probes

67. (c) Special methods ( i ) Turbine type meters (ii) Magnetic flow meters (iii) Sonic flow meters (iv) Hot wire/film anemometers (v) Mass flow meters (vi) Vortex shedding phenomenon (d) Flow visualisation methods ( i ) Shadowgraphy (ii) Interferometry. (iii) Schlieren photography

68. QUANTITY METERS They are used for the flow measurement of both liquids and gases. Some of the configurations currently in use include reciprocating piston, reciprocal diaphragm, helical impellers, revolving vane, rotating drum, rotating disc and lobed impellers.

69. Reciprocating piston:

70. Lobbed impellers:

71. Rotatory vane impeller:

72. FLOW METERS OBSTRUCTION METERS OR HEAD METERS When a fluid flows through a pipe with a restriction, the velocity of flow increases due to the decrease in area . When a fluid flow passes in obstruction to it, it suffers a loss in its static pressure. The obstruction may be a restriction, a bend or of any other form. The pressure drop is an indication of the flow rate.

73. Orifice Plate: An orifice plate is a device used for measuring flow rate, for reducing pressure or for restricting flow

74. FLOW NOZZLE The flow nozzle is supported between standard flanges. It allows measurement of flow rates which are about 60 to 65% higher than the maximum flow rate for which an orifice plate can be used. The flow nozzle is mainly used for metering fluids flowing under high pressures through lines of minimum size due to some reason. Another advantage of using nozzle is that it requires smaller straight piping before and after the primary element compared to that of the orifice

75. VENTURI TUBE: It is a device used to measure the flow of liquid in a pipe or to increase the velocity of fluid in a pipe at a particular point PRINCIPLE: the pressure in a fluid moving through a small cross section drops suddenly leading to an increase in flow velocity

76. MEASUREMENT OF TEMPERATURE Temperature is usually defined as a measure of degree of heat Since temperature is a derived quantity no such standard as the standards of mass, length, etc., can be defined. The temperature is thus measured by the measurement of certain properties of matter which are influenced by the degree of heat . The most used are changes in: physical state chemical state dimensions

77. Some important considerations when measuring temperature are: The contact between the sensor and the substance measured should be satisfactory The sensor should be small enough not to disturb the temperature conditions There should be no chemical reactions between the sensor and the substance which will cause heat to be produced or absorbed.

78. EXPANSION THERMOMETERS The thermometers under this category have been subdivided as follows: 1. Expansion of solids (a) Solid-rod thermostats/Thermometers (b) Bimetallic thermostats/thermometers 2 . Expansion of liquids (a) Liquid-in-glass thermometers (b) Liquid-in-metal thermometers 3. Expansion of gases (a) Gas thermometers

79. Expansion thermometers Most solids and liquids expand when they are subjected to an increase in temperature . This is used to indicate temperature in many thermometers. EXPANSION OF SOLIDS This includes: Solid rod thermometers or thermostat. Bimetallic thermometers. Solid rod and bimetallic thermometer are designed using the principle that some metals expand more than others when they are subjected to the same rise in temperature.

80. SOLID-ROD THERMOSTATS/THERMOMETERS: Solid thermostat is based on the differential expansion of materials i.e. a material will expand more than the other when exposed to the same change in temperature. One of the thermostats consists of combination of an invar rod inside a brass tube The other end carries a micro switch, and micrometer etc . to initially adjust the between the free ends of the rod and the tube. When this combination is placed in a temperature bath, the brass tube will expand more than the invar rod

81. so that the position of the free end of the rod changes with respect to the position of the free end of the tube . This differential change may actuate a micro switch to cut off the electric supply when the pre-set temperature is reached . To pre-set the temperature the micro-switch position relative to free end of the invar rod is adjusted by a micrometer . This kind of thermostat is used to cut off the supply the supply to electric heaters and ovens.

82. BIMETALLIC THERMOSTATS: Bimetallic thermostats are used to control higher temperature. It consists of two strips of metal that have different coefficients of linear expansion such as brass and steel. The two pieces are welded and riveted together. When the bimetallic strip is heated, the brass, having a greater value of, expands more than the steel. Since the two strips are bounded together, the bimetallic strip bends into the arc. Thermostats are used for controlling the temperature of laundry irons, hot water storage tanks and for many other purposes. Differential Expansion When metal rod of different metal are heated to the same range of temperature their expansion are different i.e. differential expansion.

83. LIQUID-IN-GLASS THERMOMETER It consists of a very big bulb with very thin walls to hold the temperature sensing liquid. The bulb is connected to a uniform bore capillary which is graduated. On the other end of the capillary is another small bulb called a safety bulb. Figure shows a sketch of a thermometer. Mercury and alcohol are most commonly used liquids. The thermometer works on the principle of differential expansion of liquid.

84. LIQUID IN METAL THERMOMETER:

85. EXPANSION OF GASES CONSTANT-VOLUME GAS THERMOMETER The physical change in this device is the variation of pressure of a fixed volume of gas with temperature. The flask was immersed in an ice bath, and mercury reservoir B was raised or lowered until the top of the mercury in column A was at the zero point on the scale. The height h, the difference between the mercury levels in reservoir B and column A, indicated the pressure in the flask at 0°C.

86. The flask was then immersed in water at the steam point, and reservoir B was readjusted until the top of the mercury in column A was again at zero on the scale; this ensured that the gas’s volume was the same as it was when the flask was in the ice bath This adjustment of reservoir B gave a value for the gas pressure at 100°C.

87. PYROMETRIC CONES Pyrometric cones are used worldwide to monitor ceramic firings in industrial kilns, pottery kilns, and small hobby kilns Orton Pyrometric cones were developed in the late 1800's Cones known as Seger cones are made of minerals in different compositions so that they melt at different temperatures. By varying the composition of the cones, a range of temperature from 600°C to 2000°C can be covered in steps of 25°C to 45°C.

88. To check the temperature, a series of cones of different compositions is placed in the kilo. Cones having a lesser melting temperature will melt, eventually cone will be found which just bends over The maximum sensitivity is +10°C. based on change of colour exhibit a particular colour when a certain temperature is reached

89. ELECTRICAL METHODS The temperature signal is converted into electrical signal either through a change in resistance leading to a change in current or development of emf. The following elements are used to convert temperature into electrical variables: 1. Electrical resistance bulbs. 2. Thermistors. 3. Thermocouples and thermopiles.