TEMPRATURE MEASUREMENT INSTRUMENTS

In this presentation we have discussed about temperature measuring instruments used in industry. Like Mechanical , electrical and non contact types instruments for measuring temperature

1. temperature measurement instruments Name- Anjani Kishor Rishabh Enrollment Number-21546001 M.Tech Pulp and Paper Technology (2021-23 ), IIT ROORKEE

2. Basics Temperature is as fundamental a physical concept as the three basic quantities of mechanics: mass, length, and time. Temperature is an expression that denotes a physical condition of matter. Temperature is a measure of the thermal energy in a body, which is the relative hotness or coldness of a medium and is normally measured in degrees using one of the following scales; Fahrenheit (F) Celsius or Centigrade (C) Rankine (R) Kelvin (K) Absolute zero is the temperature at which all molecular motion ceases or the energy of the molecule is zero.

3. Comparison of temperature scale

4. HEAT Heat is a form of energy; as energy is supplied to a system the vibration amplitude of its molecules and its temperature increases. The temperature increase is directly proportional to the heat energy in the system

5. Linear thermal expansion Linear thermal expansion is the change in dimensions of a material due to temperature changes. The change in dimensions of a material is due to its coefficient of thermal expansion that is expressed as the change in linear dimension per degree temperature change. Where L2 = final length L1 = initial length α = coefficient of linear thermal expansion T2 = final temperature T1 = initial temperature L2 = L1 [1 + α ( T2 − T1 )]

6. Volume thermal expansion Volume thermal expansion is the change in the volume per degree temperature change due to the linear coefficient of expansion Where V2 = final volume V1 = initial volume β = coefficient of volumetric thermal expansion T2 = final temperature T1 = initial temperature V2 = V1 [1 + β ( T2 − T1 )]

7. Temperature Measuring Devices

8. Expansion Thermometer ->Mercury Thermometers The most common direct visual reading thermometer The operating range of the mercury thermometer is from −30 to 800°F (−35 to 450°C ) (freezing point of mercury − 38°F [− 38°C ]).

9. Construction and Working The device consisted of a small bore graduated glass tube with a small bulb containing a reservoir of mercury. The coefficient of expansion of mercury is several times greater than the coefficient of expansion of glass, so that as the temperature increases the mercury rises up the tube giving a relatively low cost and accurate method of measuring temperature.

10. Advantages Disadvantages Simple in construction, Relatively inexpensive, Easy to use and portable, The most widely used method of temperature measurement having industrial, chemical, clinical and meteorological applications . Fragile and hence easily broken, Can only be used where the liquid column is visible, Cannot be used for surface temperature measurements, Cannot be read from a distance and are Unsuitable for high temperature measurements.

11. Liquids in glass devices Liquids in glass devices operate on the same principle as the mercury thermometer. The liquids used have similar properties to mercury, i.e., high linear coefficient of expansion, clearly visible, non-wetting, but are nontoxic.

12. The liquid in glass thermometers is used to replace the mercury thermometer and to extend its operating range. These thermometers are accurate and with different liquids can have an operating range of from −300 to 600°F (−170 to 330°C ).

13. Bimetallic strip It is relatively inaccurate, slow to respond, not normally used in analog applications to give remote indication, and has hystersis . The bimetallic strip is extensively used in ON/OFF applications not requiring high accuracy, as it is rugged and cost effective. Their operating range is from −180 to 430°C and can be used in applications from oven thermometers to home and industrial control thermostats.

14. Principle Bimetal thermometers work on the principle that different metals expand at different rates as they are heated. By using two strips of different metals in a thermometer, the movement of the strips correlates to temperature and can be indicated along a scale.

15. Bimetallic Thermometers working A bimetallic thermometer is a temperature measurement device. It converts the media’s temperature into mechanical displacement using a bimetallic strip. The bimetallic strip consists of two different metals having different coefficients of thermal expansion. Bimetallic thermometers are used in residential devices like air conditioners, ovens, and industrial devices like heaters, hot wires, refineries, etc.

16. Construction and design A bimetallic thermometer works by using two basic properties of metal: The thermal expansion property of the metal The coefficient of thermal expansion of different metals is different for the same temperature. The main component of the bimetallic thermometer is the bimetallic strip. The bimetallic strip consists of two thin strips of different metals, each having different coefficients of thermal expansion. Thermal expansion is the property of a metal to change its shape or volume with a change in temperature. The metal strips are connected along their length by fusing them together or riveting. The strips are fixed at one end and free to move on the other end.

17. Working The two metals typically used are steel and copper, but steel and brass can also be used. Since their thermal expansion is different, the length of these metals changes at different rates for the same temperature. Due to this property, when the temperature changes, the metal strip at one side expands and the other does not, which creates a bending effect. When the temperature rises, the strip will turn in the direction of metal with the lower temperature coefficient. When the temperature decreases, the strip bends in the direction of metal having a higher temperature coefficient. The deflection of the strip indicates the temperature variation. This bending motion is connected to the dial on the thermometer, outputting the media’s temperature .

18. Advantages Disadvantages Simple and robust design Less expensive than other thermometers They are fully mechanical and do not require any power source to operate. Easy installation and maintenance Nearly linear response to temperature change Suitable for wide temperature ranges They are not advised to use for very high temperatures. They may require frequent calibration. May not give an accurate reading for low temperature. Calibration is disturbed if roughly handled

19. Pressure-spring thermometers These thermometers are used where remote indication is required, as opposed to glass and bimetallic devices which give readings at the point of detection. The pressure-spring device has a metal bulb made with a low coefficient of expansion material with a long metal tube, both contain material with a high coefficient of expansion; the bulb is at the monitoring point. The metal tube is terminated with a spiral Bourdon tube pressure gage (scale in degrees) as shown in Fig.

20. The pressure system can be used to drive a chart recorder, actuator, or a potentiometer wiper to obtain an electrical signal. As the temperature in the bulb increases, the pressure in the system rises, the pressure rise being proportional to the temperature change. The change in pressure is sensed by the Bourdon tube and converted to a temperature scale. These devices can be accurate to 0.5 percent and can be used for remote indication up to 100 m but must be calibrated, as the stem and Bourdon tube are temperature sensitive.

21. Thermistors- Basics Thermistors are a class of metal oxide (semiconductor material) which typically have a high negative temperature coefficient of resistance, but can also be positive. Thermistors have high sensitivity which can be up to 10 percent change per degree Celsius, making them the most sensitive temperature elements available, but with very nonlinear characteristics. The typical response times is 0.5 to 5 s with an operating range from −50 to typically 300°C . Devices are available with the temperature range extended to 500°C .

22. Definition: The thermistor is a kind of resistor whose resistivity depends on surrounding temperature. It is a temperature sensitive device. The word thermistor is derived from the word, thermally sensitive resistor. The thermistor is made of the semiconductor material that means their resistance lies between the conductor and the insulator. The variation in the thermistor resistance shows that either conduction or power dissipation occurs in the thermistor. The circuit diagram of thermistor uses the rectangular block which has a diagonal line on it. Symbol of thermistor

23. Types of Thermistor The thermistor is classified into types. They are the negative temperature coefficient and the positive temperature coefficient thermistor. 1 . Negative Temperature Coefficient Thermistor – In this type of thermistor the temperature increases with the decrease of the resistance. The resistance of the negative temperature coefficient thermistor is very large due to which it detects the small variation in temperature . 2 . Positive Temperature Coefficient Thermistor – The resistance of the thermistor increases with the increases in temperature.

24. Construction 1 . The thermistor is made with the sintered mixture of metallic oxides like manganese, cobalt, nickel, cobalt, copper, iron, uranium, etc. 2 . It is available in the form of the bead, rod and disc. 3 . The bead form of the thermistor is smallest in shape, and it is enclosed inside the solid glass rod to form probes. 4 . The disc shape is made by pressing material under high pressure with diameter range from 2.5 mm to 25mm . 5 . Following figures show various types of construction of thermistors:

25. Thermistors are low cost and manufactured in a wide range of shapes, sizes, and values. When in use care has to be taken to minimize the effects of internal heating. Thermistor materials have a temperature coefficient of resistance ( α ) given by where ∆R is the change in resistance due to a temperature change ∆T and RS the material resistance at the reference temperature. The nonlinear characteristics are as shown in Fig. 8.5 and make the device difficult to use as an accurate measuring device without compensation, but its sensitivity and low cost makes it useful in many applications. The device is normally used in a bridge circuit and padded with a resistor to reduce its nonlinearity.

26. WORKING The thermistor act as a temperature sensor and it is placed on a body whose temperature is to be measured. It is also connected to electrical circuit. When the temperature of the body changes, the resistance of the body also changes, which is directly indicated by the circuit as the temperature since the resistance is calibrated against the temperature.

27. Resistance Temperature Characteristic of Thermistor The resistance temperature coefficient of the thermistor is shown in the figure below. The graph below shows that the thermistor has a negative temperature coefficient, i.e., the temperature is inversely proportional to the resistance.

28. The thermistor has fast response over narrow temperature range. It is small in size. Contact and lead resistance problem not occurred due to large resistance. It has good sensitivity in NTC region. Cost is low. The thermistor need of shielding power lines. The excitation current should be low to avoid self heating. It is not suitable for large temperature range. The resistance temperature characteristics are non linear.

29. Resistance temperature devices Resistance temperature devices ( RTD ) are either a metal film deposited on a former or are wire-wound resistors. The devices are then sealed in a glassceramic composite material. The electrical resistance of pure metals is positive, increasing linearly with temperature. Table 8.5 gives the temperature coefficient of resistance of some common metals used in resistance thermometers. These devices are accurate and can be used to measure temperatures from −300 to 1400°F (−170 to 780°C ). In a resistance thermometer the variation of resistance with temperature is given by RT2 = RT1 (1 + Coeff . [ T2 − T1 ]) (8.14) where RT2 is the resistance at temperature T2 and RT1 is the resistance at temperature T1

30. The resistance thermometer or resistance temperature detector ( RTD ) uses the resistance of electrical conductor for measuring the temperature. The resistance of the conductor varies with the time. This property of the conductor is used for measuring the temperature. T he main function of the RTD is to give a positive change in resistance with temperature. The metal has a high-temperature coefficient that means their temperature increases with the increase in temperature. The carbon and germanium have low-temperature coefficient which shows that their resistance is inversely proportional to temperature.

31. Material used in Resistive Thermometer The resistance thermometer uses a sensitive element made of extremely pure metals like platinum, copper or nickel. The resistance of the metal is directly proportional to the temperature. Mostly, platinum is used in resistance thermometer. The platinum has high stability, and it can withstand high temperature. Gold and silver are not used for RTD because they have low resistivity. Tungsten has high resistivity, but it is extremely brittles. The copper is used for making the RTD element. The copper has low resistivity and also it is less expensive. The only disadvantage of the copper is that it has low linearity. The maximum temperature of the copper is about 120ºC . The RTD material is made of platinum, nickel or alloys of nickel. The nickel wires are used for a limited temperature range, but they are quite nonlinear.

32. The following are the requirements of the conductor used in the RTDs . The resistivity of the material is high so that the minimum volume of conductor is used for construction. The change in resistance of the material concerning temperature should be as high as possible. The resistance of the material depends on the temperature .

33. The resistance versus temperature curve is shown in the figure. The curves are nearly linear, and for small temperature range, it is very evident.

34. Construction of Resistive Thermometer The resistance thermometer is placed inside the protective tube for providing the protection against damage . 2. The resistive element is formed by placing the platinum wire on the ceramic bobbin. 3 . This resistance element is placed inside the tube which is made up of stainless steel or copper steel. 4 . The lead wire is used for connecting the resistance element with the external lead. 5 . The lead wire is covered by the insulated tube which protects it from short circuit. 6 . The ceramic material is used as an insulator for high temperature material and for low-temperature fiber or glass is used

35. Operation of Resistive Thermometer The tip of the resistance thermometer is placed near the measurand heat source. The heat is uniformly distributed across the resistive element. The changes in the resistance vary the temperature of the element. The final resistance is measured. The below mention equations measure the variation in temperature : where Rθ – approximation resistance at θºC Rθ0 – approximation resistance at θ0 ºC Δθ – θ – θ0 change in temperature ºC α θ0 – resistance temperature coefficient at θ0 ºC

36. The accuracy of RTD is good. Temperature compensation is not necessary. The RTD available in wide range . The stability maintained over long period of time. It can be used to measure differential temperature. It is suitable for remote indication. It can be easily installed and replaced. 1. The cost of RTD is high. 2. It has large bulb size. 3 . Low sensitivity occurs in RTD . 4 . Affected by shock and vibration. 5 . Possibility of self heating. 6 . Bridge circuit is needed with power supply

37. Thermocouples Thermocouples are formed when two dissimilar metals are joined together to form a junction. An electrical circuit is completed by joining the other ends of the dissimilar metals together to form a second junction. A current will flow in the circuit if the two junctions are at different temperatures as shown in Fig.

38. Thermocouple A thermocouple is comprised of at least two metals joined together to form two junctions. One is connected to the body whose temperature is to be measured; this is the hot or measuring junction. The other junction is connected to a body of known temperature; this is the cold or reference junction. Therefore the thermocouple measures unknown temperature of the body with reference to the known temperature of the other body.

39. Thermocouple: Working Principle The working principle of thermocouple is based on three effects, discovered by Seebeck , Peltier and Thomson. They are as follows: 1 ) Seebeck effect : The Seebeck effect states that when two different or unlike metals are joined together at two junctions, an electromotive force ( emf ) is generated at the two junctions. The amount of emf generated is different for different combinations of the metals. 2 ) Peltier effect : As per the Peltier effect, when two dissimilar metals are joined together to form two junctions, emf is generated within the circuit due to the different temperatures of the two junctions of the circuit.

40. 3) Thomson effect: As per the Thomson effect, when two unlike metals are joined together forming two junctions, the potential exists within the circuit due to temperature gradient along the entire length of the conductors within the circuit. In most of the cases the emf suggested by the Thomson effect is very small and it can be neglected by making proper selection of the metals. The Peltier effect plays a prominent role in the working principle of the thermocouple.

41. Thermocouple: Working It comprises of two dissimilar metals, A and B. These are joined together to form two junctions, p and q, which are maintained at the temperatures T1 and T2 respectively. Remember that the thermocouple cannot be formed if there are not two junctions. Since the two junctions are maintained at different temperatures the Peltier emf is generated within the circuit and it is the function of the temperatures of two junctions .

42. If the temperature of both the junctions is same, equal and opposite emf will be generated at both junctions and the net current flowing through the junction is zero. If the junctions are maintained at different temperatures, the emf’s will not become zero and there will be a net current flowing through the circuit. The total emf flowing through this circuit depends on the metals used within the circuit as well as the temperature of the two junctions. The total emf or the current flowing through the circuit can be measured easily by the suitable device.

43. The device for measuring the current or emf is connected within the circuit of the thermocouple. It measures the amount of emf flowing through the circuit due to the two junctions of the two dissimilar metals maintained at different temperatures. In figure 2 the two junctions of the thermocouple and the device used for measurement of emf (potentiometer) are shown. Now , the temperature of the reference junctions is already known, while the temperature of measuring junction is unknown. The output obtained from the thermocouple circuit is calibrated directly against the unknown temperature. Thus the voltage or current output obtained from thermocouple circuit gives the value of unknown temperature directly.

44. Types of Thermocouple

45. Base metal Thermocouples – In this category of Thermocouple, the base metal is made from common and inexpensive materials such as Nickel, Iron, Copper. Following types fall under this category- Type K Thermocouple– It consists of Nickel-Chromium or Nickel- Alumel . It is most commonly used. It is inexpensive and has a wide temperature range at the same time it provides accuracy and reliability. It has the temperature range from -330 to 2300⁰F . Its wire colour code is yellow and red. Type J Thermocouple – It consists of Iron/Constantan. It permits smaller temperature range and has a shorter lifespan at a higher temperature. As oxidation problem is associated with Iron, some precautions must be taken when using this type of thermocouple in an oxidising environment. Its temperature range lies in between 32 to 1400⁰F  and wire colour code is white and red.

46. Type T Thermocouple – These consist of  Copper/Constantan . This category of the thermocouple is very stable and is used in very low-temperature applications such as in cryogenics. The temperature range of type T Thermocouple lies in between  -330 to 700⁰F  and its wire colour code is blue and red. Type E Thermocouple – It is composed of  Nickel-Chromium/Constantan . At moderate temperature, it provides higher stability as compared to Type K and Type J. This type of Thermocouple has the highest emf vs temperature values among all commonly used Thermocouples. The temperature range is between  -330 to 1600⁰F  and wire colour code is purple and red. Type N Thermocouple – It is composed of  Nicrosil / Nisil . It is slightly expensive and shares almost similar accuracy and temperature limits as that of K Type. The temperature range lies in between  32 to 2300⁰F  and wire colour code is orange and red.

47. Noble Metal Thermocouple – It has the ability to withstand high temperature and is made up of expensive materials. Following types fall under this category- Type S Thermocouple – It is composed of  Platinum Rhodium- 10%/ platinum . It is mostly used in high-temperature applications. But can be used for lower temperature applications due to high stability. It is mostly used in Biotech industries. Type R Thermocouple – These are composed of  Platinum Rhodium-13%/ Platinum . It is more expensive as it contains more percentage of Rhodium as compared to Type S. Its performance is almost similar to that of Type S. Type B Thermocouple – These are composed of  Platinum Rhodium- 30%/ Platinum Rhodium-6% . It is known to be the highest temperature limit thermocouple among all others. It provides the highest stability and accuracy at high temperature.

48. Advantages Disadvantages 1. The thermocouple is less expensive than RTD . 2. It has wide temperature ranges. 3. It has good reproducibility. 4. The temperature range is 270 to 2700 degree Celsius. 5. It has rugged construction. 6. It does not required bridge circuit. 7. It has good accuracy. 8. It has high speed of response 1. The stray voltage pick up is possible. 2 . As output voltage is very small, it needs amplification. 3 . The cold junction and lead compensation is essential. 4 . It shows non linearity

49. Applications: To monitor temperatures of liquids and gases in storage and flowing pipes and ducts. In industrial furnaces For temperature measurement in cryogenic range .

50. Semiconductors Semiconductors have a number of parameters that vary linearly with temperature. Normally the reference voltage of a zener diode or the junction voltage variations are used for temperature sensing. Semiconductor temperature sensors have a limited operating range from –50 to 150°C but are very linear with accuracies of ± 1°C or better. Other advantages are that electronics can be integrated onto the same die as the sensor giving high sensitivity, easy interfacing to control systems, and making different digital output configurations possible. The thermal time constant varies from 1 to 5 s, internal dissipation can also cause up to 0.5°C offset. Semiconductor devices are also rugged with good longevity and are inexpensive. For the above reasons the semiconductor sensor is used extensively in many applications including the replacement of the mercury in glass thermometer.

51. Temperature Range and Accuracy of temperature sensors

52. Summary of Sensor Characteristics

53. PYROMETERS All the temperature measuring methods studied earlier require physical contact of thermometer with the body whose temperature is to be measured. But at high temperatures above 1400 °C, the thermometer may melt due to direct physical contact. To solve these problems, a non-contact method of temperature measurement is used. This method is also suitable for moving bodies. Pyrometry is a technique for measuring temperature without physical contact. A pyrometer is a device that is used for the temperature measurement of an object. T he device actually tracks and measures the amount of heat that is radiated from an object. The thermal heat radiates from the object to the optical system present inside the pyrometer. The optical system makes the thermal radiation into a better focus and passes it to the detector. The output of the detector will be related to the input thermal radiation.

54. Pyrometer also is known as an Infrared thermometer or Radiation thermometer or non-contact thermometer used to detect the temperature of an object’s surface temperature, which depends on the radiation (infrared or visible) emitted from the object. Pyrometers act as photodetector because of the property of absorbing energy and measuring of EM wave intensity at any wavelength. These are used to measure high-temperature furnaces. These devices can measure the temperature very accurately, precisely, pure visually and quickly. Pyrometers are available in different spectral ranges ( since metals – short wave ranges and non-metals-long wave ranges).

55. Color pyrometers are used to measure the radiation emitted from the object during the temperature measurement. These can measure the object’s temperature very accurately. Hence the measuring errors are very low with these devices. Color pyrometers are used to determine the ratio of two radiation intensities with two spectral ranges. These are available in series of Metis M3 and H3 and handheld portables Capella C3 in different versions. High-speed pyrometers are used to temperature more fastly and quickly than M3 devices. These are available in combination with 1-color and 2-color pyrometers. These devices can create clear temperature profiles of fast-moving objects and control the adequate temperature level.

56. Working Principle of Pyrometer Pyrometers are the temperature measuring devices used to detect the object’s temperature and electromagnetic radiation emitted from the object. These are available in different spectral ranges. Based on the spectral range, pyrometers are classified into 1-color pyrometers, 2-color pyrometers, and high-speed pyrometers The basic principle of the pyrometer is, it measures the object’s temperature by sensing the heat/radiation emitted from the object without making contact with the object. It records the temperature level depending upon the intensity of radiation emitted. The pyrometer has two basic components like optical system and detectors that are used to measure the surface temperature of the object.

57. Stefan–Boltzmann law According to Stefan Boltzmann law, the amount of radiation emitted per unit time from an area A of a black body at absolute temperature T is directly proportional to the fourth power of the temperature. u/A = σT 4  . . . . . . (1) where  σ is Stefan’s constant = 5.67 × 10 -8  W/ m 2   k 4 A body that is not a black body absorbs and hence emit less radiation, given by equation (1) For such a body, u = e σ AT 4  . . . . . . . (2) Where e = emissivity (which is equal to absorptive power) which lies between 0 to 1. With the surroundings of temperature T , net energy radiated by an area A per unit time. Δu = u – u o  = eσA [ T 4  – T 4 ] . . . . . . (3)

58. Figure - Stefan–Boltzmann law: power radiated by a greybody with different emissivities . 

59. Wien's displacement law Wien's displacement law  states that the black-body radiation curve for different temperatures will peak at different wavelengths that are inversely proportional to the temperature. The shift of that peak is a direct consequence of the Planck radiation law , which describes the spectral brightness of black-body radiation as a function of wavelength at any given temperature . However, it had been discovered by Wilhelm Wien several years before Max Planck developed that more general equation, and describes the entire shift of the spectrum of black-body radiation toward shorter wavelengths as temperature increases .

60. Wien's displacement law states that the spectral radiance of black-body radiation per unit wavelength, peaks at the wavelength  λ peak  given by: where   T  is the absolute temperature.  b  is a constant of proportionality called   Wien's displacement constant , equal to 2.897771955...×10 −3   m⋅ K,or   b  ≈ 2898 μm⋅ K This is an inverse relationship between wavelength and temperature. So the higher the temperature, the shorter or smaller the wavelength of the thermal radiation. The lower the temperature, the longer or larger the wavelength of the thermal radiation . For visible radiation, hot objects emit bluer light than cool objects. If one is considering the peak of black body emission per unit frequency or per proportional bandwidth, one must use a different proportionality constant.

61. However , the form of the law remains the same: the peak wavelength is inversely proportional to temperature, and the peak frequency is directly proportional to temperature. Fig-Black-body radiation as a function of wavelength for various temperatures. Each temperature curve peaks at a different wavelength and Wien's law describes the shift of that peak.

62. When any object is taken whose surface temperature is to be measured with the pyrometer, the optical system will capture the energy emitted from the object. Then the radiation is sent to the detector, which is very sensitive to the waves of radiation. The output of the detector refers to the temperature level of the object due to the radiation. Note that, the temperature of the detector analyzed by using the level of radiation is directly proportional to the object’s temperature .

63. The radiation emitted from every targeted object with its actual temperature goes beyond the absolute temperature ( -273.15 degrees Centigrade ). This emitted radiation is referred to as Infrared, which is above the visible red light in the electromagnetic spectrum. The radiated energy is used for detecting the temperature of the object and it is converted into electrical signals with the help of a detector.

64. Pyrometer Radiation Pyrometer Optical Pyrometer

65. Infrared or Radiation Pyrometers These pyrometers are designed to detect thermal radiation in the infrared region, which is usually at a distance of 2- 14um . It measures the temperature of a targeted object from the emitted radiation. This radiation can be directed to a thermocouple to convert into electrical signals. Because the thermocouple is capable of generating higher current equal to the heat emitted. Infrared pyrometers are made up of pyroelectric materials like polyvinylidene fluoride ( PVDF ), triglycine sulfate (TGS), and lithium tantalate ( LiTaO3 )

66. Infrared or Radiation Pyrometers

67. Advantages Disadvantages It can measure the temperature of the object without any contact with the object. This is called Non-contact measurement. It has a fast response time Good stability while measuring the temperature of the object. It can measure different types of object’s temperature at variable distances. Pyrometers are generally rugged and expensive Accuracy of the device can be affected due to the different conditions like dust, smoke, and thermal radiation.

68. Applications Pyrometers  are used in different applications such as,  To measure the temperature of moving objects or constant objects from a greater distance. In metallurgy industries  In smelting industries Hot air balloons to measure the heat at the top of the ballon Steam boilers to measure steam temperature To measure the temperature of liquid metals and highly heated materials.  To measure furnace temperature.

69. Optical Pyrometers These are one of the types of pyrometers used to detect thermal radiation of the visible spectrum. The temperature of the hot objects measured will depend on the visible light they emit. Optical pyrometers are capable of providing a visual comparison between a calibrated light source and the targeted object’s surface . When the temperature of the filament and the object’s surface is the same, then the thermal radiation intensity caused due to the filament merges and into the targeted object’s surface and becomes invisible. When this process happens, the current passing through the filament is converted into a temperature level.

70. Optical Pyrometers

71. Advantages Disadvantages 1. The optical pyrometer is useful for high temperatures. 2 . It is useful for monitoring the temperature of moving object and distant objects. 3 . It has good accuracy. 4 . No need for contact with target of measurement. 5 . It is light in weight. 1. The accuracy may be affected by dust, smoke and thermal background radiation. 2 . The optical pyrometer is not useful for measuring the temperature of clean burning gases that do not radiate visible energy. 3 . It is more expensive. 4 . It causes human errors.

72. What we have learn today Temperature Measuring Devices

73. Any questions ? Thank You!!