Temperature Instruments -20 Most Common Interview Questions with Answers

🔥 Basic & Conceptual Questions (1–15)

1. What is temperature and how is it measured in process industries?
   ➤ Temperature is the measure of thermal energy. It is measured using devices like thermocouples, RTDs, thermistors, infrared sensors, etc.

2. Differentiate between temperature and heat.
   ➤ Temperature is a measure of average kinetic energy of molecules, while heat is the total energy transferred due to temperature difference.

3. Name the common temperature measurement instruments used in industries.
   ➤ RTD, Thermocouple, Thermistor, Infrared (IR) sensor, Bimetallic thermometer.

4. What is the difference between contact and non-contact temperature sensors?
   ➤ Contact sensors (RTD, thermocouple) physically touch the surface; non-contact sensors (IR) measure emitted radiation.

5. Explain the working principle of a Thermocouple.
   ➤ It works on the Seebeck effect—two dissimilar metals generate voltage proportional to the temperature difference between hot and cold junctions.

6. Explain the working principle of an RTD.
   ➤ RTD changes its electrical resistance with temperature, typically using platinum (Pt100).

7. What are the standard types of thermocouples?
   ➤ J, K, T, E, N (base metals); R, S, B (noble metals).

8. What is the Seebeck effect?
   ➤ It is the generation of voltage across two dissimilar conductors when their junctions are at different temperatures.

9. How does a thermocouple generate a millivolt signal?
   ➤ Due to the Seebeck effect; voltage generated is proportional to the temperature difference.

10. What is the typical resistance of a Pt100 RTD at 0°C?
    ➤ 100 ohms.

11. What does Pt1000 mean?
    ➤ An RTD with 1000 ohms resistance at 0°C.

12. How does the resistance of an RTD vary with temperature?
    ➤ Increases linearly with temperature.

13. Compare RTD and thermocouple.
    ➤ RTD: Accurate, stable, costly, limited range.
    ➤ Thermocouple: Wide range, rugged, less accurate.

14. What is a Thermistor?
    ➤ A temperature-sensitive resistor with high sensitivity; NTC and PTC types are used.

15. Which sensors are suitable for high-temperature applications?
    ➤ Thermocouples (Type B, R, S) are suitable up to 1700°C.

---

⚙️ Installation & Application (16–30)

16. What are the standard installation practices for thermowells?
    ➤ Proper immersion length (10x OD), orientation, material selection, and wake frequency calculation.

17. Why is thermal lag a concern in temperature measurement?
    ➤ Delays sensor response to temperature changes; affects control loop performance.

18. What is a thermowell and why is it used?
    ➤ Protective sleeve that isolates the sensor from the process; allows sensor removal without shutdown.

19. What materials are commonly used for thermowells?
    ➤ SS316, Inconel, Monel, Hastelloy, based on process media.

20. How does fluid velocity affect thermowell selection?
    ➤ High velocity causes vibration; use tapered or stepped thermowells and perform wake frequency calculations.

21. What is wake frequency and why is it critical for thermowells?
    ➤ It’s the frequency of vortex shedding; resonance with natural frequency can lead to failure.

22. Explain insertion length and its importance.
    ➤ Should be at least 10 times the diameter; ensures accurate measurement away from wall effects.

23. What is a duplex thermocouple or RTD?
    ➤ Two sensors in one sheath; used for redundancy or backup.

24. Where would you use a surface temperature sensor?
    ➤ On pipelines, motors, or inaccessible areas where immersion is not possible.

25. How are temperature sensors installed in hazardous areas?
    ➤ Using flameproof or intrinsically safe enclosures, with proper certification.

26. What are the temperature class ratings (e.g., T3, T4)?
    ➤ Maximum surface temperature classification for hazardous area equipment (e.g., T4 = ≤135°C).

27. How do you protect a temperature sensor in a corrosive environment?
    ➤ Use coated thermowells (e.g., PTFE), exotic alloys, or isolation barriers.

28. How is an infrared (IR) temperature sensor used?
    ➤ Measures emitted infrared radiation; used for non-contact or moving targets.

29. What precautions are taken when installing sensors in vibrating equipment?
    ➤ Use spring-loaded sensors, flexible conduits, and avoid resonance.

30. Why do we use insulated vs. grounded thermocouples?
    ➤ Insulated: For noise reduction, signal stability.
    ➤ Grounded: Faster response but may introduce ground loops.

---

🛠️ Calibration & Troubleshooting (31–40)

31. How do you calibrate a thermocouple?
    ➤ Compare with a reference source using a dry block or bath; record actual vs. indicated values.

32. What is a dry block calibrator?
    ➤ A solid-state portable device with temperature-controlled block for sensor calibration.

33. What is the reference junction compensation in thermocouples?
    ➤ Cold junction temperature compensation using internal/external methods to correct voltage output.

34. How do you check an open circuit in a thermocouple?
    ➤ Use a multimeter for continuity or check signal drop; broken circuit gives max negative offset.

35. Why is lead wire resistance important in RTDs?
    ➤ It adds to sensor resistance; 3- or 4-wire configurations compensate for it.

36. How do you perform a loop check for temperature transmitters?
    ➤ Inject known signal, verify mA output, check sensor-to-PLC loop for correctness.

37. What is a temperature transmitter and how does it work?
    ➤ Converts sensor signal (ohms or mV) to standard 4–20 mA or digital signal (HART/FF).

38. What are common faults in temperature measurement systems?
    ➤ Open/broken wires, EMI, sensor drift, ground loops, cold junction error.

39. How can you detect a broken thermocouple or RTD?
    ➤ Unstable reading, max or min value displayed, continuity test.

40. How do you calibrate a smart temperature transmitter?
    ➤ Connect to HART communicator or software, apply known input, adjust span/zero.

---

📏 Standards, Protocols & Advanced (41–50)

41. What standards govern temperature measurement?
    ➤ IEC 60751 (RTD), ASTM E230 (Thermocouples), IEC 60584.

42. What is a 3-wire and 4-wire RTD? Why are they used?
    ➤ 3-wire: Compensates lead resistance. 4-wire: More accurate, used in labs.

43. How do you compensate for lead wire resistance in RTD circuits?
    ➤ Use 3- or 4-wire methods; the transmitter subtracts lead resistance.

44. What is HART protocol and how is it used in temperature transmitters?
    ➤ Hybrid analog + digital protocol for configuring and reading diagnostics over 4–20 mA.

45. What are the advantages of using FOUNDATION Fieldbus or Profibus PA?
    ➤ Multi-variable data, lower wiring cost, diagnostics, reduced loop components.

46. What is linearization in temperature instruments?
    ➤ Converting nonlinear sensor response (e.g., thermocouples) into linear output for accuracy.

47. What are span and zero in temperature transmitters?
    ➤ Zero = Lower range value (LRV), Span = URV – LRV.

48. How does ambient temperature affect measurement accuracy?
    ➤ It can alter transmitter electronics and junctions; need for compensation.

49. What is sensor drift and how do you manage it?
    ➤ Gradual deviation over time; manage by periodic calibration and diagnostics.

50. How do you select a temperature sensor for cryogenic applications?
    ➤ Use sensors rated for low temperatures (e.g., Type T thermocouple, special RTDs), ensure insulation and protection.