Temperature is one of the important and crucial variables to be measured in industries. In this article, a set of interview questions and answers on temperature measurement is presented.
Question 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.
Question 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.
Question 3: What are the different temperature scales?
Answer: The temperature scales are, Fahrenheit (°F), Celsius or Centigrade (°C), Rankine (°R) and Kelvin (K).
Question 4: Give the conversion formulae 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
Question 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
Question 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.
Question 7: Define Thermal Conductivity.
Answer: Thermal Conductivity is the flow or transfer of heat from a high-temperature region to a low-temperature region.
Question 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.
Question 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)
5. Radiated Energy (Example: Pyrometer)
Question 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.
2. The spiral bimetallic strip is used in air conditioning thermostats.
3. The bimetallic strip is used for simple ON/OFF switching applications.
Question 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.
Question 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.
Question 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.
2. RTDs are used in precise air temperature control during the manufacturing of microprocessors.
3. RTDs are used in pharmaceutical and food industries for the verified accurate temperature measurement and control.
Question 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.
Question 15: Give the advantages and disadvantages of a thermistor.
Answer: The advantages are low cost, high sensitivity, and fast response.
The disadvantages are nonlinearity and self-heating problem.
Question 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.
2. Thermistors are used in temperature monitoring systems. Example: safety switches in a coffee maker.
3. Thermistors are used in circuit protection. A PTC thermistor may operate as a resettable fuse in electric circuits.
Question 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.
Question 18: List the different types of thermocouples.
Answer: The common types are,
· 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%
Question 19: State the advantages and disadvantages of the thermocouple.
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.
Question 20: Arrange RTDs, Thermistor, and Thermocouples in the order of increasing sensitivity.
Answer: Thermocouple < RTDs < Thermistor
Question 21: Arrange RTDs, Thermistor, and Thermocouples in the order of increasing linearity.
Answer: Thermistor < Thermocouple < RTDs
Question 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.
Question 23: State the advantages and disadvantages of IC temperature sensors.
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.
Question 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.
Question 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.
When we are applying heat near the LM35, why motor rotating? And when we are removed the heat ,why motor stop rotating?
ReplyDeletePlease explain the observation and relating with the operation of LM35
ReplyDelete