300+ Instrumentation Engineering Interview Questions – Part 2

1. What is RTD?

RTDs (Resistance Temperature Detector) are temperature detectors whose resistance changes as the temperature rises. RTDs are used for continuous temperature measurement and have a positive temperature coefficient. When the temperature rises, the resistance of the RTD decreases.

Advantages of RTD:

1). More accurate

2). Linear output

3). High sensitivity

Disadvantages of RTD:

1). Cost is high

2). Need external power source

3). Base resistance is low

2. What is the RTD working principle?

The working principle of RTD is the relationship between the resistance and temperature of the metal, which means that the resistance of a metal varies with temperature. The amount of change in the resistance value of the material caused by a degree increase in temperature is measured, and the sensor is calibrated accordingly.

3. What is Pt100?

Pt100 denotes that platinum has a 100-ohm resistance at 0°C. Platinum resistance thermometers (PRT) have high precision over a wide temperature range (-200 to + 850°C).

4. What is the purpose of using a signal converter for Pt100?

The temperature sensor pt100 operates on resistance change; for example, if the ambient temperature is 0oC, the sensor’s equivalent resistance is 100 ohm.

When the thermostat pt100 is directly connected to the PLC, it must also be connected to the power supply in order to function; make a long wire.

Furthermore, 4-20mA is the internationally recognised standard.

5. Explain transducer?

A transducer is an electronic device that converts physical quantities such as flow, pressure, temperature, and level into proportional electric quantities.

6. Types of transducers?

1). Active.

2). Passive.

7. What is an active transducer?

It can generate its own output through mechanical displacement.

Piezoelectric pickup, for example.

8. What is a passive transducer?

The use of an auxiliary input that changes in response to mechanical displacement is required.

This type of device includes strain gauges, slide wire pots, capacitive pickups, and LVDTs.

9. What is Pulse transducer?

A transducer that converts measurand into the form of pulse is called pulse transducer

10.Why is platinum RTD preferred over copper and nickel?

Over a wide temperature range, the platinum resistance value is linear.

11.What is a Thermistor?

The term thermistor is a combination of the words “thermal” and “resistor.” It is a type of resistor whose resistance varies with temperature. They are made of metallic oxide that has been moulded into a bead, disc, or cylindrical shape before being covered in epoxy or glass.

12. Types of thermistors

a). Positive Temperature Coefficient (PTC) Thermistor

b). Negative Temperature Coefficient (NTC) Thermistor.

13. What is the primary distinction between RTD and Thermistor?

RTD has a positive temperature coefficient and Thermistor has a negative temperature coefficient:

-> For + ve temperature coefficient, as the temperature rises, so does the resistance of the sensing element.

-> For -ve temperature coefficient, as the temperature rises, the resistance of the sensing element falls.

14. What is the difference between an RTD and a thermocouple?

Connect the transmitter to a resistance Multimeter. If you read in ohms, it’s an RTD; otherwise, it’s a thermocouple.

The thermocouple always displays readings in millivolts.

15. What is the difference between a thermocouple’s cold and hot junctions?

The cold junction is the reference junction that is always kept at 0 ° C to prevent room temperature influence.

The hot junction is subjected to varying heat, which should be measured by temperature. Where the dissimilar metals should be joined.

16. What are the steps involved in valve selection?

a). Fluid nature

b). Function

c). Construction material

d). Disc type

e). Stem type

f). How to operate

g). Bonnet type

h). Body ends

i). Delivery time

j). Cost

k). Warranty

17. What is the function of a valve positioner?

The valve positioner’s goal is to improve the accuracy of the valve action. Positioners are used in the control loop to completely perform the function of the final control variable.

They position the valve by increasing or decreasing the air load pressure on the actuator.

18. Is it possible to use the valve without a positioner?

No, The purpose of the positioner is to control the stroke of the control valve in order to keep the valve in the desired position.

The positioner receives a signal from the controller and boosts the signal before sending it to the actuator to achieve the desired position. The positioner then cuts the boosted signal to the actuator and maintains the position.

19. What is a control valve?

The control valve is the last control element that allows the user to regulate or restrict the flow of fluid through a flow channel. It enables direct flow rate control and, as a result, regulation of process quantities such as pressure, temperature, and liquid volume.

20. Components of a control valve?

a). Valve Body

b). Bonnet

c). Trim

d). Plug

e). Cage

f). Seat Ring

21. What is the function of a valve positioner?

a). Quick action

b). Valve hysteresis

c). Viscous liquids

d). Split range.

e). Line pressure changes on valve

f). Bench set not standard

g). Reverse valve operations

22. What is a solenoid valve?

A solenoid is an electrically operated valve. This is made up of a solenoid coil passing through a magnetic plunger. This plunger is connected to the pump and opens and closes the valve. There are two types of solenoid valves:

* Normally opened

* Normally closed

23. Name some primary elements of pressure measurement?

a). Bourdon tube

b). Diaphragm

c). Capsule

d). Bellow

e). Pressure spring

24. What are the different types of manometers?

a). Manometer with a U-tube

b). Manometer of the well type

c). Manometer of the inclined type

d). Manometer for ring balance

e). The micro manometer

25. Define I to P converter?

The instrumentation’s pneumatic signals are maintained at a maximum pressure of 3-15 psi. The air compressor system produces air at a much higher pressure, which is then reduced using restrictions and additional control of 3-15 psi. This restriction and control are provided by I/P converter.

26. What does an instrumentation technician/mechanic do?

Troubleshooting and repair of equipment and instruments is taught to instrumentation technologists, technicians, and mechanics. This trade is so intertwined with electricians, pipefitters, power engineers, and engineering firms that you will find yourself in a wide range of working conditions.

27. What is calibration?

Calibration is a set of documented operations that compare the measurement device to be calibrated to a traceable reference standard/device.

28. What are calibration certificates?

The calibration certificate is the official document that authorises instrument calibration and provides traceability to the National Institute of Standards and Technology.

Most businesses issue the calibration certificate as a historical record. Some, on the other hand, enter data from the certificate into a local database to keep track of the status of their test equipment inventory.

29. What’s the difference between IS and non-IS cables?

Non-IS cables are used in non-hazardous areas where protection is not required.

Intrinsically safe cables, or IS cables, are those that are safe in faulty situations, are protected from external electrical or magnetic field damage, and are used in intrinsic circuits.

30. What differs intrinsic earthing from conventional (usual) earthing?

The IS ground connection must be kept separate from the plant earth and any other electrical grounds. When used in IS cables, the screens should only be grounded at one point, typically at the same point as the interface devices, and should be isolated from one another.

31. What is a ground, and what function does it serve?

Article 100 of the NEC defines a ground as “connected to ground or to a conductive body that extends the ground connection.” There are two distinct topics when discussing grounding.

Earth grounding is the intentional connection of a circuit conductor, usually the neutral, to an earth electrode.

Grounding equipment ensures that all operating equipment within a structure is properly grounded.

32. What is a good ground resistance value?

There is no universal ground resistance threshold recognised by all agencies. The NFPA and IEEE, on the other hand, recommend a ground resistance of 5.0 ohms or less.

The NEC requires that the system impedance to ground be less than the 25 ohms specified in NEC 250.56. It should be 5.0 ohms or less in facilities with sensitive equipment.

The telecommunications industry has frequently used 5.0 ohms or less for grounding and bonding. The goal of ground resistance is to achieve the lowest ground resistance value that makes economic and physical sense.

33. What is cryogenic?

Cryogenic means to work in lower temperature range (-50 and below than that).

34. What is a hazardous area?

The hazardous areas is a probability analysis and risk assessment of a manufacturing area or process that processes a potentially flammable atmosphere, with the sole focus on minimising or eliminating electrical energy as a potential ignition source.

35. What are the various types of calibration?

a). Calibration of transducers

b). Calibration of the data system

c). End-to-end physical calibration

36. How often should an instrument be calibrated?

-> Calibrate instruments according to the manufacturer’s instructions.

-> Prior to and following the critical measuring project

-> Once a month, quarterly, or semi-annually

37. Why should calibration be performed?

Calibration describes the precision and efficiency of a machine component’s measurement. When machinery uses specific techniques or tests specific parameters such as heat and humidity, it begins to deviate from its precision over time. And calibration ensures the safety of the process machine. Calibration can help to reduce costs associated with manufacturing errors.

38. Explain in detail how to calibrate a linear instrument.

The Zero-span method is used for linear instrument calibration.


1). Apply the instrument’s low range value (LRV) and record the reading.

2). If the reading is off, rotate the Zero adjustment until the instrument is accurately registered at this point.

3). Now, use the upper range (URV) value on the instrument and record the result.

4). If the readings do not function properly, rotate the span adjustment until the instrument displays the correct URV reading.

39. How a positioner work?

A positioner is a device that is installed in a valve to ensure that it is in the correct position of opening according to the control signal. An I/P converter only transmits the opening/closing request to the valve and cannot confirm its position.

The positioner detects valve openings via a position feedback link connected to the valve stem, which serves as its input signal. The output of an I/P converter serves as its setpoint input. The difference between these two is the error signal, which is used to position the valve to the correct position in order to reduce error to zero.

As a result, a positioner is nothing more than a pneumatic feedback controller. Controlled external supply air to the positioner provides power to the positioner in order for it to position a valve. A positioner is also used in a valve when the operating signal range of the valve differs from the output range of the I/P converter.

40. Explain how the Rotameter works?

1). The Rotameter is made up of a vertical tapered tube through which the metered fluid flows upward.

2). As the flow changes, the ‘float’ rises or falls, causing the area of the passages to change so that the differential across it balances the gravitational force on the ‘float.’

3). The difference in pressure is kept constant.

4). The rate of flow is measured by the position of the ‘float.’

41. What is the magnetic meter’s operating principle?

When a conductor moves across a magnetic field, an electric potential is created. The conductor in most electrical machinery is a wire. The same principle applies to a moving, electrically conductive liquid. The primary device of commercial magnetic metres is a straight cylindrical electrically insulated tube with a pair of electrodes located at opposite ends of a tube diameter and nearly flush with the tube walls. This device can only work with electrically conducting liquids. The magnetic metre is well-suited to measuring slurries and dirty fluids.

42. Explain the Turbine meter’s mechanism.

Turbine metres are made up of a straight flow tube within which a turbine or fan can freely rotate about its axis, which is fixed along the tube’s centre line. A magnetic pick up system detects the rotation of the rotor via the tube walls. Because the rotor speed is directly proportional to the flow rate, the turbine metre is a flow rate device. The output is typically in the form of magnetic pick-up electric pulses with a frequency proportional to the flow rate.

43. What is the function of a single-seated valve?

When an absolute shut off is required, a single seated valve is used on smaller sizes. The pressure drop across the valve in the closed or nearly closed position limits the use of single seated valves.

44. What is the function of a double-seated valve?

The upward and downward forces on the plug due to fluid pressure reduction are nearly equalised in double seated valves. It is typically used on larger valves and high-pressure systems. Actuator forces are reduced.

45. Name different types of bourdon tubes.

a). C type

b). Spiral

c). Helix

46. What is an intrinsically safe system?

Intrinsic safety is a method of designing electrical equipment for safe use in environments made hazardous by the presence of flammable gases or vapours in the air. Any spark or thermal effect produced either normally or under specified fault conditions is incapable of causing ignition of a specified gas or vapour in air mixture at the most ignited concentration in an intrinsically safe circuit.

47. What is a zener diode? What exactly is a voltage regulator?

A p-n diode’s breakdown region can be made very sharp, and almost vertical diodes with an almost vertical breakdown region are known as zener diodes. In the breakdown region, a zener diode is equivalent to a battery. Because of this, the current flowing through the zener diode can vary while the voltage remains constant. Because of this constant voltage, the zener diode has become an important device in voltage regulation.

The output of a voltage regulator remains constant despite changes in the input voltage caused by the zener effect.

48. What is the principle of force balance? List some of its benefits.

A force balance controller generates an output signal by opposing torque. The input force is applied to the input bellows, causing the beam to move. This causes nozzle back pressure to crackle. The balancing bellows detects nozzle back pressure and brings the beam into balance. For full scale output, the baffle movement is very small, around 0.002.


a). There are fewer moving parts.

b). Baffle movement is minimal.

c). Frictional losses are reduced.

49. What is the motion balance principle?

A controller that generates an output signal through the motion of its components. The baffle is being pushed closer to the nozzle. Back pressure at the nozzle will rise. This increase in back pressure acting on the balancing bellows causes the bellows to expand. As a result, the nozzle moves upward. The nozzle will move until its motion is nearly equal to the motion of the input baffle.

50. What is a SCADA system?

Supervisory Control and Data Acquisition (SCADA) is a system that uses a coded signal to control a remote device via a communication channel (usually using a communication channel at a remote station).

300+ Instrumentation Engineering Interview Questions Part 1

300+ Instrumentation Engineering Interview Questions – Part 3

300+ Instrumentation Engineering Interview Questions – Part 4

300+ Instrumentation Engineering Interview Questions – Part 5

300+ Instrumentation Engineering Interview Questions – Part 6

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