# Temperature Instrument Interview questions and Answers

### Temperature is one of the most important and crucial variables to be measured in industries like fertilizer , oil and gas . In this article, a set of interview questions which is frequently asked during interview.

1: Define temperature ?

Answer: Temperature is a measure of the heat produced by the continuous motion of molecular particles in a body. It describes the relative hotness or coldness of a body by which it may give away or accept energy from other bodies.

2: Define heat ?

Answer: Heat is a form of energy. When heat is applied to a body, the amplitude of vibration of its molecules increases. This increases the temperature of the body. The increase in temperature is always proportional to the heat energy of the system.

3: What are the different temperature scales?

Answer: The temperature scales are, Fahrenheit (°F), Celsius or Centigrade (°C), Rankine (°R) and Kelvin (K).

4: Give the conversion formula between the Fahrenheit scale and Celsius scale.

Answer: To convert °F to °C, °C = (°F – 32) * 5/9

To covert °C to °F, °F = (°C *9/5) + 32

5: Convert 119°F into Kelvin scale. ?

Answer: Convert the 119°F into °C

°C = (119 – 32) * 5/9 = 48.333°C

Convert 48.333°C into Kelvin

K = °C + 273.15 = 48.333 + 273.15 = 321.483 K

6: Define absolute zero.

Answer: Absolute zero is the temperature at which there is no molecular motion in a system. At absolute zero, the energy of the molecule is zero.

7: Define Thermal Conductivity.

Answer: Thermal Conductivity is the flow or transfer of heat from a high-temperature region to a low-temperature region.

8: What are the modes of heat transfer?

Answer: The basic methods of heat transfer are,

1. Conduction – Heat flow through a material
2. Convection – Heat transfer due to the motion of higher temperature particles in a material
3. Radiation – Emission of energy by electromagnetic waves that travel at the speed of light through materials that do not conduct electricity.

9: Categorize the different methods of temperature measurement. ?

Answer: The different methods of temperature measurement are,

1. Expansion of material to give a visual indication or dimensional change (Example: Thermometer, Bimetallic thermometer)
2. Electrical Resistance change (Example: RTD, Bolometer)
3. Semiconductor characteristic change (Example: Thermistor, IC temperature sensors)
4. The voltage generated by dissimilar methods (Example: Thermocouple)

10: State a few applications of the bimetallic strip.

Answer: 1. Bimetallic strips are used in bimetallic thermometers for local temperature measurement in process industries.

1. The spiral bimetallic strip is used in air conditioning thermostats.
2. The bimetallic strip is used for simple ON/OFF switching applications.

11: What is Pt 100?

Answer: Pt 100 is a Platinum Resistance Temperature Detector (PRTD) which has a resistance of 100Ω at 0°C with a temperature coefficient of 0.00385Ω/°C.

12: Why Platinum is preferred as an element in RTD

Answer: Platinum has a constant value of resistance temperature coefficient of 0.004Ω/°C for a temperature range between 0 to 100°C. Platinum can withstand high temperatures; maintain excellent stability and accuracy and it is less susceptible to contamination. So it is an obvious choice for measurement purpose.

13: List a few applications of RTD in industries.

Answer: 1. RTDs are used in power plants to monitor fuel and coolant temperatures entering and leaving heat exchangers.

1. RTDs are used in precise air temperature control during the manufacturing of microprocessors.
2. RTDs are used in pharmaceutical and food industries for the verified accurate temperature measurement and control.

14: What is a thermistor?

Answer: Thermistor is a thermally sensitive resistor which produces a change in resistance with the change in temperature. Thermistors are made up of semiconductor materials. There are two types of thermistors,

1. Negative temperature coefficient (NTC) – Resistance decreases with increase in temperature.
2. Positive temperature coefficient (PTC) – Resistance increases with increase in temperature.

The disadvantages are nonlinearity and self-heating problem.

16: State a few applications of a thermistor. ?

Answer: 1. Thermistors are preferably used in a small temperature range where they are linear and very sensitive. Example: To measure the temperature of a reference junction of a thermocouple.

1. Thermistors are used in temperature monitoring systems. Example: safety switches in a coffee maker.
2. Thermistors are used in circuit protection. A PTC thermistor may operate as a resettable fuse in electric circuits.

17: Explain the working principle of the thermocouple.

Answer: The operation of a thermocouple is based on the Seeback Effect. The Seeback Effect states that when the heat is applied to a junction (hot junction) of dissimilar metals, an emf is generated at the other junction (cold junction). The emf generated is proportional to the difference in temperature between the two junctions.

18: List the different types of thermocouples.

• Type E – Chromel / Constantan
• Type J – Iron / Constantan
• Type K – Chromel / Alumel
• Type R – Platinum / Platinum Rhodium 13%
• Type T – Copper / Constantan

The other types are,

• Type B – Platinum Rhodium 30% / Platinum Rhodium 6%
• Type N – Nicrosil / Nisil
• Type S – Platinum / Platinum Rhodium 10%

Answer: The advantages of a thermocouple are self-powered, low cost, wide temperature range, and rugged.

The disadvantages of a thermocouple are low sensitivity, nonlinear, and the need for reference.

20: Arrange RTDs, Thermistor, and Thermocouples in the order of increasing sensitivity. ?

Answer: Thermocouple < RTDs < Thermistor

21: Arrange RTDs, Thermistor, and Thermocouples in the order of increasing linearity.

Answer: Thermistor < Thermocouple < RTDs

22: What is a Thermopile?

Answer: Thermopile is a number of thermocouples connected in series, to increase the accuracy and sensitivity by increasing the output voltage when measuring small temperature differences. Their prime application is the detection of thermal radiation.

Answer: The advantages are low cost, good sensitivity, easy interface to control devices.

The disadvantages are a slow response, self-heating and the need for a power source.

24: What is the working principle of radiation pyrometer?

Answer: Radiation Pyrometers receive thermal radiation, from a distance, emitted by the body whose temperature is to be measured. The radiation is converted into an electrical signal from which the temperature is calculated.

25: Where are radiation pyrometers employed?

Answer: Pyrometers are used to measure higher temperatures above the range of thermocouples. It is also used to measure the temperature of rapidly moving objects and objects with which physical contact is impossible.

26.. Why does RTD measurement use 3 wires for a field signal connection?

3 wire system is used in temperature measurement by an RTD to compensate the line resistance. Three wire  system provides a Wheatstone Bridge in the measuring instrument.

27. What instruments are used to measure temperature ?