Questions on Thermocouple and Pyrometer

Study the Temperature Sensors Questions and Answers for your interview preparation. This article focus on Thermocouple and Pyrometer Questions.

Thermocouple and Pyrometer

1. Define the Seebeck effect?

When a pair of dissimilar metals are joined together at both ends as shown in the below figure, then emf is induced at the junction of the metals as per the difference in temperature at both junctions, this effect is called the Seebeck effect.

Detailed Explanation:

On heating around a meter long say Pt Rh 13% alloy wire by a small burner millie volts potential difference exists between the ends. The higher the temp difference between the ends the higher the mV. This is called the Seebeck effect.

By measuring the potential difference between the Pt Rh 13% wire ends one can get the hot end temp values by referring to a table that tabulates the hot end developed mV at various temp differences between the ends.

However a wire capable of withstanding the hot end temp say 1200 degrees C or even much more to connect an mV meter across the PtRh 13% wire ends; in addition, it shall develop very low millivolts at different temp differences for max sensitivity.

Scientists by experiment and theoretical analyses have found pure Pt wire meets these requirements. Hence, PtRh 13% Vs. Pt is a thermocouple wire pair.

The other commonly used TC wire pairs are Chromel Vs Alumel, Iron Vs Constantan, Copper Vs Constantan, etc.

2. Define Peltier effect.

When a pair of dissimilar metals are joined together, emf is induced at the junction of the metals.

If the metal is connected to an external circuit some amount of current is drawn, due to which the induced emf will be slightly altered.

3. What is the Thomson effect?

When a pair of dissimilar metals are joined together, emf is induced at the junction of the metals.

If a temperature gradient exists along either/ both of the metals, the junction emf may undergo an additional slight alteration called as Thomson effect.

4. What is a thermocouple?

A temperature measuring device formed by connecting two dissimilar metals and is based on the principle of the Seebeck effect, Peltier effect, and Thomson effect.

5. Say the effects that thermocouple is based on?

• Seebeck effect,
• Peltier effect and
• Thomson effect

6. What are the various types of thermocouple junctions?

• Reference junction (or) cold junction (or) standard junction And
• hot junction (or) measuring junction (or) desired temperature junction

7. What is thermopile?

A series of identical thermocouples connected together in a casing is called a thermopile.

It is also used for measuring average temperature.

8. Define the law of homogeneous circuits?

An emf will not be induced in a circuit with a system consisting of homogeneous metal.

9. Define the law of intermediate metals?

The net emf in the circuit remains unaltered if a third metal is introduced, provided the junctions formed by the third metal are at the same temperature.

10. Define the law of intermediate temperature.

The emf generated at the junction temperature T1& T3 is equal to the algebraic sum of emf generated at the junction in T1-T2 and T2-T3, where T2 lies between T1 &T2

11. Give a few types of Thermocouples with composition and temperature range?

• J-type : Iron and constantan : -200 to 1000
• K type : chromel- alumel : -2000 to 1200
• T type : copper- constantan : -200 to 350

12. What are the compensating techniques used in thermocouples?

• Lead compensation,
• Reference junction compensation(ice bath compensation)
• Isothermal block compensation.

13. What is an isothermal block.

A strip or a block where the uniform temperature is maintained is called an isothermal block.

14. Give some merits and demerits of thermocouple

Merits:

• wide temperature range,
• inexpensive,
• good reproducibility

Demerits:

• nonlinear response,
• need compensating circuits

15. What are the sources of error in thermocouples.

• Junction formed by Faulty soldering technique
• The thermocouple may be used outside their applicable range
• Faulty reference junction compensation may be employed
• Installation faults
• Usage of the wrong type of thermocouple.

16. What are the radiation methods of temperature measurement.

• Total radiation method and
• selective or partial radiation method

17. Define pyrometer.

Technique for measuring temperature without contact. It depends on the relation between the temperature of the heating body and the electromagnetic radiation emitted by the black body.

18. Give the classification of pyrometer?

• Radiation pyrometer,
• Optical pyrometer,
• Selective / partial pyrometer,
• Two color pyrometer

19. What are the uses of radiation pyrometer?

Used for very high-temperature measurement 700 deg c -3500 deg c without the actual contact with the radiating body, Very high response

20. What is the black body?

A body that absorbs all radiation without transmitting any radiation is called a black body.

A black body radiates energy at all spectral wavelengths at a maximum rate corresponding to its temperature.

21. What is the grey body?

An object having emittance less than unity and constant at all wavelengths is called a grey body

22. Give some application of pyrometer?

Used for very high-temperature measurements 700 deg C to 3500 deg C.

23. What are factors that affect the response of thermocouples.

Size or thickness of the thermocouple wires, Sheath material, protection tube, and thermowell, Position /location of the thermocouple with respect to process

24. What is the principle used in optical pyrometer.

Within the visible region, a given wavelength has a fixed color and the energy of radiation is interpreted as intensity or brightness.

Hence the brightness of the light of given color emitted by the hot source indicates the value of temperature.

25. Why protective sheath is used in thermocouple

Protecting the thermocouple metals from contamination and rusting. Preventing against oxidizing, sulphating and in molten baths to withstand thermal and mechanical shocks.

26. Demerits of Optical Pyrometer?

1. It is based on an observer judging 2 colors to be the same.
2. It requires a direct line of sight to the object (or furnace) in which the temperature is to be measured

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