Temperature Measurement--Summary / Quiz

Summary

Our review at the start of the section revealed that there are 10 different physical principles used commonly as the basis for temperature-measuring devices. During the course of the section, we then looked at how each of these principles is exploited in various classes of temperature measuring devices.

We started off by looking at the thermoelectric effect and its use in thermocouples and thermopiles, and also the derived devices of digital thermometers and continuous thermocouples.

Thermocouples are the most commonly used devices for industrial applications of temperature measurement. However, despite their relatively simple operating concept of generating an output voltage as a function of the temperature they are exposed to, proper use of thermocouples requires an understanding of two thermoelectric laws. These laws were presented and their application was explained by several examples. We also saw how the output of a thermocouple has to be interpreted by thermocouple tables. We went on to look at the different types of thermocouples available, which range from a number of inexpensive, base metal types to expensive ones based on noble metals. We looked at the typical characteristics of these and discussed typical applications. Moving on, we noted that thermocouples are quite delicate devices that can suffer from both mechanical damage and chemical damage in certain operating environments, and we discussed ways of avoiding such problems. We also looked briefly at how thermocouples are made.

Our next subject of study concerned resistance thermometers and thermistors, both of these being devices that convert a change in temperature into a change in the resistance of the device.

We noted that both of these are also very commonly used measuring devices. We looked at the theoretical principles of each of these and discussed the range of materials used in each class of device. We also looked at the typical device characteristics for each construction material.

Next, we looked at semiconductor devices in the form of diodes and transistors and discussed their characteristics and mode of operation. This discussion revealed that although these devices are less expensive and more linear than both thermocouples and resistance thermometers, their typical measurement range is relatively low. This limits their applicability and means that they are not used as widely as they would be if their measurement range was greater.

Moving on, we looked at the class of devices known as radiation thermometers (alternatively known as radiation pyrometers), which exploit the physical principle that the peak wavelength of radiated energy emission from a body varies with temperature. The instrument is used by pointing it at the body to be measured and analyzing the radiation emitted from the body.

This has the advantage of being a noncontact mode of temperature measurement, which is highly attractive in the food and drug industries and any other application where contamination of the measured quantity has to be avoided. We also observed that different versions of radiation thermometers are capable of measuring temperatures between _100 and +10,000_ C, with measurement inaccuracy as low as _0.05% in the more expensive versions. Despite these obvious merits, careful calibration of the instrument to the type of body being measured is essential, as the characteristics of radiation thermometers are critically dependent on the emissivity of the measured body, which varies widely between different materials.

This stage in the section marked the end of discussion of the four most commonly used types of temperature-measuring devices. The remaining techniques all have niche applications but none of these are "large volume" uses. The first one covered of these "other techniques" was thermography. Also known as thermal imaging, this involves scanning an infrared radiation detector across either a single object or a scene containing several objects. The information gathered is then processed and an output in the form of the temperature distribution across the object is produced. It thus differs from other forms of temperature sensors in providing information on temperature distribution across an object or scene rather than the temperature at a single discrete point. Temperature measurement over the range from _20_ C up to +1500_ C is possible.

Our next subject of study concerned the liquid-in-glass thermometer, bimetallic thermometer, and pressure thermometer. These are all usually classed as thermal expansion-based devices, although this is not strictly true in the case of the last one, which is based on the change in pressure of a fluid inside a fixed-volume stainless-steel bulb. The characteristics and typical applications of each of these were discussed.

The quartz thermometer then formed the next subject of study. This uses the principle that the resonant frequency of a material such as quartz changes with temperature. Such devices have very high specifications in terms of linearity, long life, accuracy, and measurement resolution.

Unfortunately, their relatively high cost severely limits their application.

Moving on, we then looked at fiber-optic temperature sensors. We saw that their main application is measuring temperatures in hard-to-reach locations, although they are also used when very high measurement accuracy is required.

Next, we discussed color indicators. These consist mainly of special paints or crayons that change color at a certain temperature. They are used primarily to determine when the temperature of objects placed in a furnace reach a given temperature. They are relatively inexpensive, and different paints and crayons are available to indicate temperatures between 50 and 1250_ C. In addition, certain liquid crystals that change color at a certain temperature are also used as color indicators. These have better measurement resolution than paints and crayons and, while some versions can indicate low temperatures down to -20_ C, the highest temperature that they can indicate is +100_ C.

Finally, our discussion of the application of different physical principles in temperature measurement brought us to Seger cones. Also known as pyrometric cones, these have a conical shape where the tip melts and bends over at a particular temperature. They are used commonly in the ceramics industry to detect a given temperature is reached in a furnace.

The section then continued with a look at intelligent measuring devices. These are designed for use with various sensors such as thermocouples, thermopiles, resistance thermometers, thermistors, and broad-band radiation pyrometers. Intelligence within the device gives those features such as adjustable damping, noise rejection, self-adjustment for zero and sensitivity drifts, self-fault diagnosis, self-calibration, reduced maintenance requirement, and an expanded measurement range. These features reduce typical measurement inaccuracy down to _0.05% of full scale.

This completion of the discussion on all types of intelligent and nonintelligent devices allowed us to go on to consider the mechanisms by which a temperature-measuring device is chosen for a particular application. We reviewed the characteristics of each type of device in turn and looked at the sorts of circumstances in which each might be used.

Our final subject of study in the section was that of calibrating temperature-measuring devices.

We noted first of all that a fundamental difficulty exists in establishing an absolute standard for temperature and that, in the absence of such a standard, fixed reference points for temperature were defined in the form of freezing points and triple points of certain substances. We then went on to look at the calibration instruments and equipment used in workplace calibration. We also established some guidelines about how the frequency of calibration should be set. Finally, we looked in more detail at the appropriate practical procedures for calibrating various types of sensors.

QUIZ

1. Discuss briefly the different physical principles used in temperature-measuring instruments and give examples of instruments that use each of these principles.

2. (a) How are thermocouples manufactured? (b)What are the main differences between base metal and noble metal thermocouples? (c) Give six examples of the materials used to make base metal and noble metal thermocouples. (d) Specify the international code letter used to designate thermocouples made from each pair of materials that you give in your answer to part (c).

3. Explain what each of the following are in relation to thermocouples: (a) extension leads, (b) compensating leads, (c) law of intermediate metals, and (d) law of intermediate temperature.

4. What type of base metal thermocouple would you recommend for each of the following applications? (a) measurement of subzero temperatures (b) measurement in oxidizing atmospheres (c) measurement in reducing atmospheres (d) where high sensitivity measurement is required

5. Why do thermocouples need protection from some operating environments and how is this protection given? Discuss any differences between base metal and noble metal thermocouples in the need for protection.

6. The temperature of a fluid is measured by immersing an iron-constantan thermocouple in it. The reference junction of the thermocouple is maintained at 0_ C in an ice bath and an output EMF of 5.812 mV is measured. What is the indicated fluid temperature?

7. The temperature of a fluid is measured by immersing a type K thermocouple in it. The reference junction of the thermocouple is maintained at 0_ C in an ice bath and an output EMF of 6.435 mV is measured. What is the indicated fluid temperature?

8. The output EMF from a chromel-alumel thermocouple (type K), with its reference junction maintained at 0_ C, is 12.207 mV. What is the measured temperature?

9. The output EMF from a nicrosil-nisil thermocouple (type N), with its reference junction maintained at 0_ C, is 4.21 mV. What is the measured temperature?

10. The output EMF from a chromel-constantan thermocouple whose hot junction is immersed in a fluid is measured as 18.25 mV. The reference junction of the thermocouple is maintained at 0_ C. What is the temperature of the fluid?

11. A copper-constantan thermocouple is connected to copper-constantan extension wires and the reference junction is exposed to a room temperature of 20_ C. If the output voltage measured is 6.537 mV, what is the indicated temperature at the hot junction of the thermocouple?

12. A platinum/10% rhodium-platinum (type S) thermocouple is used to measure the temperature of a furnace. Output EMF, with the reference junction maintained at 50_ C, is 5.975 mV. What is the temperature of the furnace?

13. In a particular industrial situation, a nicrosil-nisil thermocouple with nicrosil-nisil extension wires is used to measure the temperature of a fluid. In connecting up this measurement system, the instrumentation engineer responsible has inadvertently interchanged the extension wires from the thermocouple. The ends of the extension wires are held at a reference temperature of 0_ C and the output EMF measured is 21.0 mV. If the junction between the thermocouple and extension wires is at a temperature of 50_ C, what temperature of fluid is indicated and what is the true fluid temperature?

14. A copper-constantan thermocouple measuring the temperature of a hot fluid is connected by mistake with chromel-constantan extension wires (such that the two constantan wires are connected together and the chromel extension wire is connected to the copper thermocouple wire). If the actual fluid temperature was 150_ C, the junction between the thermocouple and extension wires was at 80_ C, and the reference junction was at 0_ C, calculate the EMF measured at the open ends of the extension wires. What fluid temperature would be deduced from this measured EMF (assuming that the error of using the wrong extension wires was not known)? (Hint: Apply the law of intermediate metals for the thermocouple-extension lead junction.)

15. This question is similar to the last one but involves a chromel-constantan thermocouple rather than a copper-constantan one. In this case, an engineer installed a chromel- constantan thermocouple but used copper-constantan extension leads (such that the two constantan wires were connected together and the copper extension wire was connected to the chromel thermocouple wire). If the thermocouple was measuring a hot fluid whose real temperature is 150_ C, the junction between the thermocouple and the extension leads was at 80_ C, and the reference junction was at 0_ C:

(a) Calculate the EMF (voltage) measured at the open ends of the extension wires.

(b) What fluid temperature would be deduced from this measured EMF, assuming that the error in using the incorrect leads was not known?

16. While installing a chromel-constantan thermocouple to measure the temperature of a fluid, it’s connected by mistake with copper-constantan extension leads (such that the two constantan wires are connected together and the copper extension wire is connected to the chromel thermocouple wire). If the fluid temperature was actually 250_ C and the junction between the thermocouple and extension wires was at 80_ C, what EMF would be measured at the open ends of the extension wires if the reference junction is maintained at 0_ C? What fluid temperature would be deduced from this (assuming that the connection mistake was not known)?

17. In connecting extension leads to a chromel-alumel thermocouple, which is measuring the temperature of a fluid, a technician connects the leads the wrong way round (such that the chromel extension lead is connected to the alumel thermocouple lead and vice versa).

The junction between the thermocouple and extension leads is at a temperature of 100_ C and the reference junction is maintained at 0_ C in an ice bath. The technician measures an output EMF of 12.212 mV at the open ends of the extension leads.

(a) What fluid temperature would be deduced from this measured EMF? (b) What is the true fluid temperature?

18. A chromel-constantan thermocouple measuring the temperature of a fluid is connected by mistake with copper-constantan extension leads (such that the two constantan wires are connected together and the copper extension lead wire is connected to the chromel thermocouple wire). If the fluid temperature was actually 250_ C and the junction between the thermocouple and extension leads was at 90_ C, what EMF would be measured at the open ends of the extension leads if the reference junction is maintained at 0_ C? What fluid temperature would be deduced from this (assuming that the connection error was not known)?

19. Extension leads used to measure the output EMF of an iron-constantan thermocouple measuring the temperature of a fluid are connected the wrong way round by mistake (such that the iron extension lead is connected to the constantan thermocouple wire and vice versa). The junction between the thermocouple and extension leads is at a temperature of 120_ C and the reference junction is at a room temperature of 21_ C.

The output EMF measured at the open ends of the extension leads is 27.390 mV.

(a) What fluid temperature would be deduced from this measured EMF assuming that the mistake of connecting the extension leads the wrong way round was not known about? (b) What is the true fluid temperature?

20. The temperature of a hot fluid is measured with a copper-constantan thermocouple but, by mistake, this is connected to chromel-constantan extension wires (such that the two constantan wires are connected together and the chromel extension wire is connected to the copper thermocouple wire). If the actual fluid temperature was 200_ C, the junction between the thermocouple and extension wires was at 50_ C, and the reference junction was at 0_ C, calculate the EMF measured at the open ends of the extension wires. What fluid temperature would be deduced from this measured EMF (assuming that the error of using the wrong extension wires was not known)?

21. In a particular industrial situation, a chromel-alumel thermocouple with chromel- alumel extension wires is used to measure the temperature of a fluid. In connecting up this measurement system, the instrumentation engineer responsible has inadvertently interchanged the extension wires from the thermocouple (such that the chromel thermocouple wire is connected to the alumel extension lead wire, etc.). The open ends of the extension leads are held at a reference temperature of 0_ C and are connected to a voltmeter, which measures an EMF of 18.75 mV. If the junction between the thermocouple and extension wires is at a temperature of 38_ C:

(a) What temperature of fluid is indicated? (b) What is the true fluid temperature?

22. A copper-constantan thermocouple measuring the temperature of a hot fluid is connected by mistake with iron-constantan extension wires (such that the two constantan wires are connected together and the iron extension wire is connected to the copper thermocouple wire). If the actual fluid temperature was 200_ C, the junction between the thermocouple and extension wires was at 160_ C, and the reference junction was at 0_ C, calculate the EMF measured at the open ends of the extension wires. What fluid temperature would be deduced from this measured EMF (assuming that the error of using the wrong extension wires was not known)?

23. In a particular industrial situation, a nicrosil-nisil thermocouple with nicrosil-nisil extension wires is used to measure the temperature of a fluid. In connecting up this measurement system, the instrumentation engineer responsible has inadvertently interchanged the extension wires from the thermocouple (such that the nicrosil thermocouple wire is connected to the nisil extension lead wire, etc.). The open ends of the extension leads are held at a reference temperature of 0_ C and are connected to a voltmeter, which measures an EMF of 17.51 mV. If the junction between the thermocouple and extension wires is at a temperature of 140_ C:

(a) What temperature of fluid is indicated? (b) What is the true fluid temperature?

24. Explain what the following are: thermocouple, continuous thermocouple, thermopile, and digital thermometer.

25. What is the International Practical Temperature Scale? Why is it necessary in temperature sensor calibration and how is it used?

26. Resistance thermometers and thermistors are both temperature-measuring devices that convert the measured temperature into a resistance change. What are the main differences between these two types of devices in respect of the materials used in their constructions, their cost, and their operating characteristics?

27. Discuss the main types of radiation thermometers available. How do they work and what are their main applications?

28. Name three kinds of temperature-measuring devices that work on the principle of thermal expansion. Explain how each works and what its typical characteristics are.

29. Explain how fiber-optic cables can be used as temperature sensors.

30. Discuss the calibration of temperature sensors, mentioning what reference instruments are typically used.

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