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.
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.
The major types of instrument switches include:
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.
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.
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.
Switches in instrumentation are used for:
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.
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.
These define the pole and throw configuration of a switch:
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.
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.
Common types include:
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.
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.
Limit switches are used in:
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.
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).
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.
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.
Maintenance includes:
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.
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.
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.
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.
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.
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.
Types of pressure switches include:
Pressure switches are widely used in:
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.
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.
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.
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.
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.
Types include:
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.
Common applications:
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.
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.
Float switches provide a digital ON/OFF output. Depending on configuration, they use NO or NC contact logic, switching states based on liquid level.
Common issues include:
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.
Common types include:
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.
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.
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.
These refer to the transistor output type:
Connect:
Connect the sensor to a power supply and use the multimeter to check output voltage between output and ground. Place a target object to see if output changes (e.g., from 0V to 24V for PNP type).
A photoelectric switch uses light to detect objects. It consists of an emitter (light source) and a receiver. When the beam is interrupted or reflected by a target, the switch changes output state.
In this setup, the emitter and receiver are placed opposite each other. The object interrupts the beam between them to trigger the sensor output.
Both emitter and receiver are in the same unit, and a reflector is used opposite it. When an object blocks the reflected light, the sensor detects its presence.
The emitter and receiver are in the same unit. The sensor detects light reflected back from the object itself without a separate reflector.
It uses triangulation or dual receivers to detect only the object and ignore background reflections, improving accuracy and reducing false triggers.
Mount the sensor firmly, align emitter and receiver (if separate), avoid shiny or vibrating backgrounds, and adjust sensitivity or teach settings as per the manual.
It refers to accurately aligning the emitter beam with the receiver for consistent and reliable detection. Misalignment leads to missed objects or false signals.
Connect power (V+ and GND) and connect the output wire to a PLC digital input. Ensure correct PNP/NPN configuration and use resistors if required. Some sensors may have selection options for dark-on/light-on mode.
A level switch detects the level of solids or liquids in a container and provides an ON/OFF signal when a set level is reached. It is widely used in tanks, silos, and process vessels.
It uses vibrating metal forks that stop vibrating when in contact with a liquid or solid. This change is detected and converted into a switch signal.
It detects level by sensing changes in dielectric constant when material (liquid or solid) comes close to the probe. It is used for both conductive and non-conductive materials.
It uses radio frequency to measure admittance (inverse of impedance) and detects level by sensing changes in capacitance and resistance. It works well in sticky or build-up applications.
It uses a pressure-sensitive diaphragm that actuates a micro switch when material pressure pushes the diaphragm. It is commonly used in powder and grain silos.
To provide high and low-level alarms, pump control, overflow protection, and interlocks in processes like water treatment, chemical tanks, and storage vessels.
Connect the level switch to the pump’s control circuit or starter relay. Set the switch to activate the pump when the level drops (LOW level) and stop it when the level rises (HIGH level).
A float switch is a type of level switch using buoyancy to sense level, while level switch is a general term that includes float, capacitive, tuning fork, and other technologies.
Fail-safe switches are designed to go into a safe mode (e.g., open circuit or shutdown) during fault conditions like power loss or signal failure, preventing unsafe tank overflows or dry runs.
A flow switch monitors the flow of liquids or gases and provides a digital ON/OFF signal when the flow rate exceeds or drops below a set threshold. It is commonly used for pump protection and cooling circuits.
A paddle-type flow switch has a mechanical paddle inserted into the flow path. When the fluid flow pushes the paddle, it moves and actuates a switch to indicate flow presence.
A thermal flow switch uses temperature sensors to detect changes in heat transfer caused by fluid movement. No moving parts make it ideal for clean and sensitive applications.
A vortex flow switch uses the principle of vortex shedding, where fluid flow around a bluff body causes oscillations that are sensed and converted into an ON/OFF signal.
Flow switches are used for:
The minimum flow rate depends on switch size and type. It is specified by the manufacturer and must be above the setpoint to keep the switch activated.
Install the switch in a straight pipe section, avoiding bends or valves nearby. Ensure the paddle or sensor is correctly oriented with the flow direction arrow.
Test by simulating flow and observing output signal or LED indicator. A multimeter can check continuity across switch contacts. Flow simulators or bypass valves may also be used.
Flow switches provide a digital ON/OFF contact, usually SPDT, which indicates whether flow is present or not based on the setpoint.
Check for:
An E-stop switch is a safety device used to immediately halt machinery or processes during an emergency. It is usually a large red push-button that latches when pressed.
Types include:
A latching E-stop remains in the triggered state until manually reset. A non-latching E-stop returns to normal when released. Latching is preferred for critical safety.
E-stops should be placed:
Press the E-stop and ensure the machine or system halts. Check PLC input or contact status. Reset the switch and confirm normal operation resumes.
A large, round, red push-button typically used for emergency stop applications. It is easy to press in panic situations and often latches mechanically.
An OFF switch stops the machine in normal operation, while an E-stop is used in emergency situations for immediate disconnection and safety interlocking.
E-stop switches are designed to meet safety categories like SIL (Safety Integrity Level) or ISO 13849 PL (Performance Level). They often integrate with safety relays or PLCs.
Yes, if the E-stop switch is certified as Ex-proof (explosion-proof) for hazardous locations, following ATEX/IECEx standards.
Connect the NC contact of the E-stop switch to the safety relay input. The relay monitors the loop for continuity, and when broken (E-stop pressed), it trips output safety contacts.
A push button switch is a manual switch used to control a circuit by pressing its actuator. It can be momentary or maintained type, depending on design.
A control unit with two buttons (typically green for START and red for STOP) used in motor or pump control circuits for local operation.
Use NO push-button for START and NC for STOP. Connect them in a control circuit with contactor coil, often with a holding contact to keep the motor running after releasing START.
A switch that requires a key to operate. It is used for authorized access or control in restricted areas like electrical panels, control rooms, or mode selection.
A rotary switch that allows selection of multiple modes such as Auto/Manual, Forward/Reverse, or Speed settings. Commonly used in control panels.
A mechanical switch with a lever that toggles between ON and OFF positions. Used for lighting, machine power, and instrumentation.
A floor-mounted switch operated by foot. It allows hands-free control of machines or tools. Common in textile, metalworking, and medical devices.
A membrane switch is a flat, flexible switch consisting of multiple layers including printed conductive traces. It operates when pressure is applied to close the circuit between layers. It is commonly used in HMIs, medical devices, and keypads.
Typical layers include:
Membrane switches are widely used in:
A rope pull switch is an emergency switch that can be activated by pulling a cable/rope along a conveyor or long machine. Pulling the rope actuates the switch and stops equipment.
Used in conveyor systems, assembly lines, textile plants, and bulk handling equipment to allow emergency stop from any point along the machine.
After activation, most rope pull switches require manual reset using a push button, key, or knob to restore operation and resume the process.
The IP (Ingress Protection) rating defines the protection level of a switch against dust and water. For example, IP67 means dust-tight and protected against immersion in water.
A switch certified for use in explosive or flammable environments (e.g., Zone 1, Zone 2) following standards such as ATEX or IECEx. It prevents ignition sources from triggering explosions.
Debounce logic eliminates multiple unwanted signals caused by mechanical bouncing of switch contacts. It can be implemented in software (PLC program) or hardware.
Selector switches are used to choose modes such as:
It uses a cam mechanism to operate contacts in a fixed sequence. Commonly used in rotary selector switches, it offers multiple positions and outputs based on shaft rotation.
A panel mount switch is designed to be installed into a control panel or enclosure. It usually has locking threads, nuts, or clips for secure mounting and connection from the rear side.
A switch with internal illumination (LED) that lights up during operation or to show status. Used in low-light areas, control panels, and vehicles.
Use:
A self-reset (spring-return) switch automatically returns to its original position after activation. Common in push buttons and momentary switches.
Switches are used to form interlocks to prevent unsafe operations. For example, a limit switch may prevent a door from opening unless a machine is stopped.
Switch bounce is the rapid toggling of contacts during actuation. It can be avoided using: