50+ Instrumentation Interview questions

1. What is the instrumentation Engineering?

Instrumentation engineering is the engineering specialization deals with principle and operation of measuring devices used in electrical, pneumatic, etc. design and configuration of automated systems.

Instrumentation engineers work with automated processes, such as chemical or manufacturing plants, for industries to enhance system efficiency, reliability, health, optimization and stability.

2. What are the primary control variables?

  • Temperature
  • Pressure
  • Flow

The level is excluded, which is not a primary control variable because the level is controlled by manipulating flow to tank.

3. What is the instrumentation measurement?

Instrumentation can be used to measure certain field parameters (physical values):

  • Pressure, either differential or static
  • Flow
  • Temperature measurement
  • Level Measurement
  • Density
  • Viscosity
  • Radiation
  • Current
  • Voltage
  • Inductance
  • Capacitance
  • Frequency
  • Resistivity
  • Conductivity
  • Chemical composition
  • Chemical properties
  • Various physical properties

4. What are the different flow meters?

Based on the working principle, there are 5 types of flow meters:

  • Differential pressure(DP) flow meter - example: Orifice plate, Venturi meter, Flow nozzle, Rotameter.

  • Velocity Flowmeter: Electromagnetic flowmeter, Ultrasonic flowmeter, Turbine flowmeter, Paddlewheel flow meter.

  • Positive displacement flowmeter: Nutating disk, Rotary vane, Gear, Diaphragm flowmeter.

  • Mass flowmeter: - Coriolis flowmeter, Thermal dispersion meter,

5. What are the different types of temperature measuring instruments?

6. What are different pressure measuring gauges?

. Liquid column elements:

  • Barometer

  • Manometer –u tube, enlarge leg well-inclined leg

Elastic element gauge:

Electrical transducers:

  • Resistance and inductance type

Force –balanced devices:

  • Deadweight gauge
  • Ring gauge
  • Bell gauge

7. What is Flow compensation?

In steam or gas flow measurement, the density of the steam or gas changes as pressure and temperature change. This change in density can affect the accuracy of the measured flow rate if it is uncompensated. Temperature, pressure compensated flow control is a mathematically adjusted flow control with one or more additional variables.

8. Why 4-20mA signal is prefered over a 0-10v?

If the instrument is faulty, it can be detected easily. If the output is 0 ma, it will be hard to determine whether it is a defective signal or a good signal.

20 ma is used as maximum because the human heart can withstand up to 30 ma of current only. For safety reason 20 ma chosen as the maximum value.

9. What is Ultrasonic flowmeter?

Ultrasonic flowmeters pass high-frequency sound wave to the fluid path to measure the fluid velocity. The ultrasonic method uses the change in the moving fluid’s propagation time. There are two main types of ultrasonic flowmeters: Doppler and form of transit time.

10. What are the options to measure the temperature of an object without contact?

  • Pyrometer: Pyrometry tests surface temperature without touching it. Each object above absolute zero(-273.15 ° C) emits radiation. This emission is radiation from the heat and depends on the temperature. The term infrared radiation is also used because most of this radiation’s wavelengths are in the infrared domain in the electromagnetic spectrum over a visible red light.

11. How DP transmitters are connected to closed tanks?

The bottom of the tank is attached to the transmitter’s high-pressure side in a closed tank. The top of the tank is fixed to the transmitter’s low-pressure leg. The pressure of the vessel can be measured in this way.

12. Do you know the working of a level troll?

The variation in buoyancy level resulting from a fluid level change varies the displacer’s net weight to increase or decrease the torque armload. This change is directly proportional to fluid level change and real gravity.

The subsequent torque tube movement shifts the rotor angular motion in RVDT resulting in a rotor shift proportional to the displacement of the rotor, which is transformed and amplified to a D.C. Up-to-date.

13. How does the DP flow transmitter system vary on gas and liquid pipeline?

To stop gas condensation in the signal line and HP & LP chamber, DP is mounted above the orifice plate for the gas line.

To stop gas trapping in the signal line and the HP & LP section, DP is mounted below the orifice plate for the liquid side.

14. Why is the Orifice tab provided?

Following reasons justify for providing orifice tab:

  1. Indication of the orifice plate in a line

  2. The orifice diameter is marked on it.

  3. The material of the orifice plate.

  4. The tag number of the orifice plate.

  5. To mark the inlet of an orifice.

15. How to recognize upstream in a pipeline a newly installed orifice plate?

The upstream can be marked with the Tag number markings of the orifice plate. Notes are always labelled on the orifice plate’s upstream.

16. What is the difference b/w volumetric and rate of flow?

Volumetric flow: the total amount of fluid that passes through a line of operation. Units: tube, meter cube.

Flow rate: the volume of fluid per time flowing through a process sheet.

17. Where we you use square root extractors?

Square root extractors are used across Orifice and other DP transmitters.

The relationship between the differential pressure (^P) and flow rate (Q) are not equal. DP is proportional to the square of the flow rate Q.

To convert the signal from the flow transmitter to one directly proportional to the flow rate, the square root of the signal from the flow transmitter must be obtained or extracted.

18. Why thermowells are used?

Thermowells are equipment that protects temperature measuring devices from exposing the device to the measuring substance.

Wells are used to protecting against corrosion damage erosion and processes of high pressure. A thermowell can also be used to protect a sensor during handling and routine operations from physical damage.

19. What is RTD?

RTDs ( Resistance Temperature Detector) are temperature detectors that change their resistance as temperature rises. RTDs are used for continuous measurement of temperature, indicating a positive coefficient of temperature. The resistance of RTD decreases when the temperature increases.

20. What is Pt100?

Pt100 means at 0°C temperature platinum gives 100-ohm resistance. Platinum resistance thermometer (PRT) offer excellent precision over a wide range of temperatures (-200 to + 850°C).

21. Why use a signal converter for Pt100?

The temperature sensor pt100 operates on the resistance change; i.e. if the ambient temperature is0oC, the equivalent resistance of the sensor is 100ohm.

When the thermostat pt100 is connected directly to the PLC, it must be connected to the power supply for operation; make long wire.

And also 4-20mA is the internationally accepted standard.

22. What is the main difference between RTD and Thermistor?

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

For + ve temperature coefficient, when the temperature increases the resistance of the element used for sensing increases.

For - ve temperature coefficient, when the temperature increases the resistance of the element used for sensing decreases.

23. How to identify between RTD & Thermocouple?

Connect the transmitter to a multimeter in resistance mode. If you read in ohms, otherwise it’s RTD, it’s a thermocouple.

The thermocouple always shows reading in mV.

24. What are Cold junction and Hot junction in a thermocouple?

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

Hot junction is exposed to the varying heat that should be measured by the temperature. Where to join the dissimilar metals.

25. What is the control valve?

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

26. What are the main parts of a Control valve?/ Mark the parts of the control valve in the picture below:


  • Actuator Part

  • Diaphragm

  • Actuator spring

  • Actuator stem

  • Valve positioner

  • Valve body

  • Bonnet

  • Trim

  • Valve seat

  • Valve stem

  • Packing

  • Packing Box

  • Seat ring

  • Yoke

  • Plug

27. What are different types of control valve you know?

The major types of control valves are classified into two types linear motion and rotary motion. All other types are mentioned in the picture.

28. What is the use of valve positioner?

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

By increasing or decreasing the air load pressure on the actuator, they place the valve in the right position.

29. Can we use the valve without positioner?

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

The positioner receives a signal from the controller and sends the boosted signal to the actuator to reach the desired position as the positioner cuts the boosted signal to the actuator and keeps the position.

30. What are the uses of valve positioner?

  • Quick action
  • Valve hysteresis
  • Viscous liquids
  • Split range.
  • Line pressure changes on valve
  • Bench set not standard
  • Reverse valve operations

31. What is the solenoid valve?

A solenoid is a valve that is electrically operated. This consists of a solenoid coil passing in a magnetic plunger. This plunger is attached to the pump, opening or closing the valve. Two styles of solenoid valves are present:

  • Normally opened
  • Normally closed

32. Name some primary elements of pressure measurement:

  • Bourdon tube
  • Diaphragm
  • Capsule
  • Bellow
  • Pressure spring

33. What are the types of manometers?

  • U – tube manometer
  • Well type manometer
  • Inclined type manometer
  • Ring balance manometer
  • Micro manometer

34. What is I to P converter?

The pneumatic signals of the instrumentation are sustained at a maximum of 3-15psi. The air compressor system delivers air at a much higher pressure which is then decreased employing restrictions and further control of 3-15psi. The I/P converter is used to provide this restriction and control.

35. What are the different types of Bourdon tubes?

  • C type
  • Spiral type
  • Helix

36. State the principle of LVDT

The LVDT’s secondary winding differential output voltage is proportional to the displacement felt by the mass induced by the Core acceleration; the differential output voltage becomes an acceleration metric.

37. State the principle of Mc leod gauge

Compression of a low-pressure gas sample to a point that is high enough to be read using a simple manometer.

38. What is the force balance principle?

Principle of force balance: a controller that produces a torque-opposed output signal. The input force is applied to the beam moving reference bellows. This breaks the backpressure of the nozzle.

The backpressure of the nozzle is felt by the rotating bellows that support the projector. The baffle movement for full-scale output is much less than 0.002.

39. What is motion balance principle?

A controller that moves its parts to generate an output signal. Moving towards the nozzle is the increase in the baffle. The backpressure of the nozzle is going to increase.

The bellows will extend this change in the backpressure acting on the balancing bellows. As a consequence, the nozzle is pushed upwards. The nozzle moves until movement is almost equal to the movement of the input baffle.

40. What is the difference between differential pressure and Delta pressure?

Differential pressure means the difference between high pressure & low pressure. Delta Pressure stands for the difference between two measured pressure values. This can be measured either at different times/dates or at different positions in a system.

41. How DP transmitter can be applied to open tank?

The lower pressure side is released into the atmosphere in the open tank. All the pressure is applied to the hand of the high pressure. The pressure of this vessel is determined by the side of the high pressure.

42. What is a P&ID?

Piping and Instrumentation Diagrams (P&ID) are schemes of pipelines, equipment, instrumentation, control systems, from a process system found in Oil Refinery, Chemical Plant, Paper Mill, Cement Plant, etc.

43. Why to use P&ID?

To describe all plant and equipment. This indicates the flows in a plant and the sensors or other devices in question. It gives names and parameters to each sensor and other equipment.

It is a pictorial image of the processing facilities:

  • Key piping and instrument details
  • Control and shutdown schemes
  • Safety and regulatory requirements
  • Basic start up and operational information

44. What are the items that will listed in a P&ID?

  • Instrumentation and designations
  • Mechanical equipment with names and numbers
  • All valves and their identifications
  • Process piping, sizes and identification
  • Miscellanea – vents, drains, special fittings, sampling lines, reducers, increasers and swaggers
  • Permanent start-up and flush lines
  • Flow directions
  • Interconnections references
  • Control inputs and outputs, interlocks
  • Interfaces for class changes
  • Computer control system
  • Identification of components and subsystems

45. Identify the valve P&ID symbols for valves:

46. Identify the P&ID symbol for functional diagram:

47. How to read a P&ID diagram?

Consider the below figure.

The symbol of the instruction consists of a tag number and letter of identification. Numbers in instrumentation diagrams on the P&ID symbols represent the instrument tags number.

Individual instruments are shown by circular symbols or circle on P&ID drawings. Elements of shared control / display are circles surrounded by a cube. A hexagon shows machine functions and a programmable logic controller function is shown as a diamond inside a square.

The addition of a single horizontal bar through any of the four graphical elements indicates that the function is in the category of the main location. An auxiliary location is indicated by a double line and no line places the device or instrument in the field. A dotted line is used to display devices located behind a panel in some other inaccessible location.

48. What an instrumentation technician/mechanic do?

Instrumentation technologists, technicians and mechanics are trained in troubleshooting and fixing equipment and instruments. This trade is so interconnected with electricians, pipefitters, power engineers, and engineering firms that in extremely diverse working conditions you will find yourself.

49. What is calibration?

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

50. What is calibration certificates?

The calibration certificate is the official document authorized for the calibration of instruments and provides traceability to the National Institute of Standards and Technology.

The calibration certificate is issued by most businesses as a historical record. However, some enter data on the certificate in a local database to track the status of their test equipment inventory.

51. What is the difference between a Standard (Traceable) and Accredited Calibration Certificate?

Standard (Traceable) Certificate: Instrumentation is calibrated to specifications with traceable standards. A full data report is optional with the calibration certificate.

Accredited Certificate: meets the ISO / IEC 17025 requirements and the accrediting body of the laboratory. The unit under test is calibrated within the authorized scope of accreditation of the supplier in accordance with ISO / IEC 17025.

52. What are the different types of calibration?

  • Transducer calibration

  • Data system calibration

  • Physical end to end calibration

53. How frequently an instrument should be calibrated?

  • Calibrate instruments on the basis of manufacturer recommendation.
  • Before and after critical measuring project
  • Monthly, quarterly, or semiannually

54. Why calibration should be carried out?

Calibration describes the precision and efficiency of the measurement obtained using a machine component. Over time, machinery begins to drift from its precision when it uses specific techniques or tests particular parameters such as heat and humidity. And:

  • Calibration keeps process safe
  • Machine Calibration can help to reduce costs from manufacturing errors

55. Briefly describe how to calibrate a linear instrument?

Linear instrument calibration is so-called as the Zero-span method. Procedure:

  • Apply the low range value (LRV) to the instrument and note the reading.

  • If there is an irregularity in the reading, rotate the Zero adjustment until the instrument is accurately registered at this point.

  • Now apply the upper range (URV) value to the instrument and note the reading.

  • Rotate the span adjustment until the instrument displays the correct reading of the URV if the readings do not function properly.

56. What is intrinsic safety system?

A system that involves equipment and interconnecting wiring in which any spark or thermal effect in any part of the system intended for use in hazardous areas is unable to cause ignition.

57. What are Intrinsic Safety Barriers?

The safety barriers for the zener diode are voltage regulators. There are two different types of barriers, one way and two-way barriers.

There are two types of safety barriers:

  • Active barriers
  • Passive barriers

58. What is the difference between IS and non IS cables?

Non IS cables are used in non-hazardous areas where protection needs no special consideration.

Cables that are safe in faulty situations, protected from external electrical or magnetic field damage and used in intrinsic circuits are referred to as intrinsically safe cables or IS cables.

59. What differs Intrinsic earthing from usual earthing?

The IS ground connection must be separated from plant earth and other electrical ground. When used in IS cables, the screens should only be grounded at one point, usually at the same point where the interface devices are grounded, and should be isolated from each other.

60. What is a hazardous area location?

The hazardous areas is a probability analysis and a risk assessment of a manufacturing area or process that processes a potentially flammable atmosphere that focuses solely on minimizing or eliminating electrical energy as a source of potential ignition.

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