Top 100+ Instrument Technician Interview Questions and Answers

Published on July 3, 2025 | Category: answer
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This page provides a complete collection of interview questions and answers for Instrument Technicians,
ideal for job interviews, field engineers, maintenance staff, and freshers in instrumentation.

It covers key topics like sensors, transmitters, signal types, pressure and level switches, RTD, thermocouples, LVDT, PID control, and more.
Each question is written in simple, easy-to-understand language with accurate, real-world answers for practical use.

Whether you're preparing for an oil & gas role, power plant, process industry, or a maintenance job in any automation environment,
these questions will help boost your confidence and technical understanding.

1. What is an instrument switch?

An instrument switch is a device used in industrial automation to detect physical changes such as pressure, level, flow, or position and convert them into a discrete ON/OFF signal. It is commonly used for alarm, interlock, and control purposes in process plants.

2. What are the different types of instrument switches?

The major types of instrument switches include:

3. What is the difference between a switch and a transmitter?

A switch provides a digital ON/OFF signal based on a setpoint, whereas a transmitter provides a continuous analog signal (e.g., 4–20 mA) representing a real-time process value such as pressure, temperature, or level.

4. What is the basic working principle of a switch?

A switch works by detecting a process condition (pressure, level, flow, etc.) and mechanically or electronically opening or closing an electrical contact when the predefined threshold is crossed. This sends a digital signal to the control system.

5. What are NO and NC contacts?

NO (Normally Open) contact means the circuit is open in normal condition and closes when actuated. NC (Normally Closed) contact means the circuit is closed in normal condition and opens when actuated.

6. What is the use of switches in instrumentation?

Switches in instrumentation are used for:

7. How do instrument switches contribute to safety?

Instrument switches help in shutting down equipment during abnormal conditions. For example, a pressure switch may trigger shutdown during overpressure, and an emergency stop switch stops all motion immediately, reducing the risk of damage or injury.

8. What is a dry contact?

A dry contact (also called a volt-free contact) is a relay or switch contact that does not supply voltage on its own. It only opens or closes a circuit externally controlled by another voltage source.

9. What are SPST, SPDT, DPDT switches?

These define the pole and throw configuration of a switch:

10. What is contact bounce in switches?

Contact bounce refers to the rapid, unintended opening and closing of contacts when a switch changes state, due to mechanical vibration. It can cause noise or false signals and is often filtered using debounce logic or circuitry.

11. What is a limit switch?

A limit switch is an electromechanical device used to detect the presence or absence, position, or travel limit of an object. It operates by physical contact and changes its contact state when the object moves to a specific location.

12. What are the types of limit switches?

Common types include:

13. How does a mechanical limit switch work?

When a moving part comes in contact with the actuator of the switch, it mechanically moves internal contacts to either open or close the circuit, sending a digital signal to the PLC or control system.

14. What is a roller lever limit switch?

A roller lever limit switch uses a spring-loaded lever with a roller on the tip. When an object pushes the roller, it rotates the lever, actuating the internal contact to change its state.

15. Where are limit switches used?

Limit switches are used in:

16. What is a plunger-type limit switch?

A plunger-type limit switch is a push-button style device that is actuated when a part physically depresses its plunger. This movement opens or closes the contacts inside.

17. How to wire a limit switch to a PLC?

Wire the switch's NO/NC terminals to the digital input of the PLC. One terminal is connected to the common (0V or GND) and the other to the input channel. Use pull-up/down resistors if needed, and ensure power supply compatibility (e.g., 24V DC).

18. What is snap-action in limit switches?

Snap-action is a mechanism where the internal contacts switch state very quickly once a certain actuator travel point is reached, ensuring a clean and fast transition regardless of the speed of actuation.

19. How to test a limit switch?

Use a multimeter in continuity mode across the terminals. Actuate the switch and observe whether the contact state changes from open to closed or vice versa. Also, verify signal at the PLC input if installed.

20. How to maintain a limit switch?

Maintenance includes:

21. What is a pressure switch?

A pressure switch is a device that detects pressure changes in gases or liquids and activates a set of electrical contacts when a preset pressure is reached. It provides an ON/OFF signal to a control system.

22. What is the working principle of a pressure switch?

When pressure is applied, a diaphragm or piston mechanism deflects. At a specific pressure, it actuates a microswitch inside to open or close electrical contacts. The contacts revert once the pressure falls below the reset value.

23. What is setpoint and reset point in pressure switches?

The setpoint is the pressure value at which the switch activates (changes state). The reset point is the value at which it deactivates and returns to its original state. The difference between the two is called the dead band.

24. What is the difference between pressure switch and transmitter?

A pressure switch gives a digital output (ON/OFF) when a specific pressure is reached, while a pressure transmitter gives a continuous analog output (e.g., 4–20 mA) proportional to the measured pressure.

25. What is dead band in a pressure switch?

Dead band (also called hysteresis) is the pressure difference between switch activation (setpoint) and deactivation (reset point). It prevents the switch from rapidly toggling when pressure fluctuates near the threshold.

26. What is an SPDT pressure switch?

An SPDT (Single Pole Double Throw) pressure switch has three terminals – Common (C), Normally Open (NO), and Normally Closed (NC). It allows control of two circuits based on pressure status.

27. What are the types of pressure switches?

Types of pressure switches include:

28. Where are pressure switches used?

Pressure switches are widely used in:

29. How to calibrate a pressure switch?

Use a calibrated pressure source and gradually increase pressure. Note the pressure at which the switch activates. Adjust the setpoint screw or knob to align it with the desired pressure. Repeat for reset point.

30. How to wire a pressure switch to DCS or PLC?

Connect the NO or NC contact of the pressure switch to a digital input of the PLC/DCS. Ensure the switch is rated for the system voltage (e.g., 24VDC). Use shielded cables for noisy environments.

31. What is a float switch?

A float switch is a device that detects the level of liquid in a tank or vessel. It uses a buoyant float that moves with the liquid level to open or close an electrical contact.

32. What is the principle of a float switch?

As liquid level rises or falls, the float moves accordingly. The float contains a magnet or tilting mechanism that triggers a switch when it reaches a set position, sending an ON/OFF signal.

33. What is a magnetic float switch?

A magnetic float switch uses a float with a magnet inside. When the float moves, the magnet actuates a reed switch enclosed in a stem tube, changing the electrical signal state.

34. What are the types of float level switches?

Types include:

35. How to install a float switch?

Install vertically or horizontally based on type. Ensure proper sealing and avoid obstacles inside the tank. Adjust float position to match desired setpoint. Secure wiring through a junction box.

36. Where are float switches used?

Common applications:

37. How to connect float switch to pump starter?

Use the float switch in series with the control circuit of the pump starter. When the level reaches the ON or OFF setpoint, it will break or complete the circuit to start or stop the pump.

38. What is a top-mounted float switch?

It is installed on the top of a tank, with the float hanging vertically down a stem. The float rises with liquid, triggering internal reed switches to change signal status.

39. What is the output of float switch?

Float switches provide a digital ON/OFF output. Depending on configuration, they use NO or NC contact logic, switching states based on liquid level.

40. What are common float switch faults?

Common issues include:

41. What is a proximity switch?

A proximity switch is a non-contact sensor that detects the presence of nearby objects using electromagnetic fields, light, or capacitance. It provides a discrete (ON/OFF) signal to the control system when an object enters its sensing range.

42. What are the types of proximity switches?

Common types include:

43. What is an inductive proximity switch?

It is a sensor that detects metallic objects using a magnetic field generated by an internal coil. When a metal object enters the sensing range, it changes the coil’s field and triggers the output signal.

44. What is a capacitive proximity switch?

This sensor detects both metallic and non-metallic objects (like plastic, water, wood) by measuring changes in capacitance when the target enters the sensing area.

45. What is the sensing distance in proximity sensors?

The sensing distance is the maximum range at which the sensor can detect an object. It varies by sensor type and size, commonly between 2 mm and 30 mm. Larger and shielded sensors may have longer ranges.

46. What is NPN and PNP in proximity switches?

These refer to the transistor output type:

Selection depends on the PLC input type.

47. How to wire a proximity switch to PLC?

Connect: