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100 Instrumentation Interview Questions and Answers

Instrumentation is a multi-faceted field that intertwines engineering and scientific principles to measure, control, and regulate various processes in industries like manufacturing, medicine, research, and more. Instrumentation forms the backbone of any automated process, dealing with the design, configuration, and operation of devices that can monitor or manipulate physical quantities. The instrumentation field encompasses both the theoretical understanding of phenomena and the practical ability to apply that understanding to real-world scenarios. As technologies continue to evolve, so too does the field of instrumentation, providing professionals with exciting and challenging opportunities to innovate and make significant contributions in various sectors.

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Instrumentation Interview Questions

Instrumentation Interview Questions and Answers

Our comprehensive guide of 100 Instrumentation Interview Questions and Answers to get deep insight into industrial instruments.

Can you explain what process control is?

Process control is a statistics and engineering discipline that deals with architectures, mechanisms, and algorithms for maintaining the output of a specific process within a desired range.

What is the purpose of a PID controller?

A PID controller is used to control a process by continuously adjusting the control inputs in order to minimize the difference between the desired setpoint and the measured process variable.

Can you describe the working principle of a thermocouple?

A thermocouple works on the principle of the Seebeck effect. It generates a voltage when its junction of two different metals is heated or cooled.

What is the difference between PLC and DCS?

PLC (Programmable Logic Controller) is ideal for discrete control tasks, while DCS (Distributed Control System) is designed for process control tasks. PLCs are used in applications that require robustness, simplicity, and a lower cost. DCSs are preferred for complex control processes where coordination between various processes is necessary.

Can you explain what Instrumentation Amplifiers (InAmps) are used for?

Instrumentation Amplifiers are used where great accuracy and stability of the circuit are required. They are used to amplify very low-level signals, reject noise, and also provide a high degree of isolation.

What is a 4-20mA signal and why is it widely used in industrial control systems?

A 4-20mA signal is a type of electrical current loop used in control systems. It is preferred because it can overcome the resistance of the line, is less susceptible to noise, and can provide a ‘live zero’ signal, where 4mA represents the lowest measurable value, and zero current indicates a fault.

What is a smart transmitter?

A smart transmitter is a type of process variable instrument that is able to perform functions beyond simple measurement. These include diagnostic functions, digital communication, and the ability to be configured for different parameters.

Can you explain the working of a flow meter?

Flow meters measure the rate or quantity of a moving fluid. The working principle depends on the type of flow meter. For example, a magnetic flow meter works by applying a magnetic field to the metering tube, which results in a potential difference proportional to the flow velocity perpendicular to the flux lines.

What is the difference between a solenoid valve and a motor-operated valve?

Solenoid valves are electrically controlled valves that typically handle smaller flow rates and are best for applications requiring fast opening and closing. Motor-operated valves are used for larger flow rates and are designed for applications where slower, controlled opening and closing are necessary.

What does HART communication protocol mean?

HART (Highway Addressable Remote Transducer) is a communication protocol widely used in industrial automation systems. It allows for two-way field communication and enables additional information beyond just the normal process variable to be communicated to/from a smart field instrument.

What is the role of a transducer in instrumentation?

A transducer is a device that converts one form of energy into another. In instrumentation, it’s typically used to convert a physical quantity like pressure or temperature into an electrical signal.

What is an RTD?

RTD stands for Resistance Temperature Detector. It’s a type of sensor used to measure temperature by correlating the resistance of the RTD element with temperature.

How does a level transmitter work?

A level transmitter measures the level of a substance in a container (for example, a tank) by measuring the pressure at the bottom of the tank due to the weight of the liquid.

What is the difference between a sensor and a transducer?

A sensor is a device that detects a physical quantity and generates a corresponding signal. A transducer, on the other hand, converts one form of energy into another. In some cases, a sensor can be a type of transducer, but not all transducers are sensors.

How does a pressure transmitter work?

A pressure transmitter works by measuring the force that fluid or gas exerts on a sensor element in the transmitter and converting that measurement into an electronic signal that can be read and interpreted.

Can you explain the principle of operation of an ultrasonic flow meter?

Ultrasonic flow meters work on the principle of measuring the speed of a fluid flow using ultrasound to calculate volume flow. They use a set of transducers that emit and receive ultrasonic waves. The difference in the transit times of these waves is used to calculate the flow rate.

Can you explain what is meant by loop checking in instrumentation?

Loop checking in instrumentation is a process where the performance and connections of control loops are verified. It’s performed to ensure that the control loop is functioning correctly before the process is started.

Explain distributed control system?

A Distributed Control System (DCS) is a control system for a process plant where control elements are distributed throughout the system. This is in contrast to centralized systems that operate from one location.

What is the purpose of an orifice in a pipeline?

An orifice in a pipeline is used to change the fluid flow characteristics, typically to reduce pressure, restrict flow, or measure flow velocity.

How does a Vortex Flow Meter work?

A Vortex Flow Meter works on the principle of vortex shedding. As fluid passes a bluff body, vortices are shed alternatively from each side. The frequency of these vortices is proportional to the velocity of the fluid, and thus the flow rate can be calculated.

What is calibration in terms of instrumentation?

Calibration in terms of instrumentation is the process of checking and adjusting the output or response of a measurement instrument to ensure its accuracy and reliability. It’s typically done by comparing the instrument’s output with a known standard.

What is the difference between intrinsic safety and explosion-proof?

Intrinsic safety refers to equipment and wiring that is inherently safe. In other words, in an explosive atmosphere, it is incapable of causing an explosion. On the other hand, explosion-proof equipment is designed to prevent an internal spark or explosion from causing a much larger blast.

How does a pneumatic control valve work?

A pneumatic control valve works by converting an electrical signal into a pneumatic signal (air pressure). This air pressure is then used to actuate the valve and control the flow of a process fluid.

What are control valve actuators and what are the different types?

Control valve actuators are devices that move or control the valve’s closure member. They can be classified as pneumatic, hydraulic, electric, etc., based on the type of power used.

What is a control loop in instrumentation?

A control loop in instrumentation is a system where output is controlled by varying the input. It consists of a sensor, a controller, and a control element.

What is the difference between analog and digital signals?

Analog signals are continuous and change smoothly over time, while digital signals are discrete and change in steps. Analog signals can have an infinite number of values within a range, whereas digital signals have a limited number of distinct values.

How does a variable frequency drive (VFD) work?

A VFD works by varying the frequency of the electricity supplied to a motor, thereby controlling the speed and torque of the motor. It converts the incoming AC voltage to DC, and then back to a quasi-sinusoidal AC voltage, with the ability to vary the frequency and voltage.

How does a strain gauge work?

A strain gauge works on the principle that the electrical resistance of a wire changes proportionally with its strain within the elastic limit. When a load is applied, the resulting strain deforms the gauge and changes its electrical resistance.

What is SCADA and what is it used for?

SCADA stands for Supervisory Control And Data Acquisition. It’s a type of industrial control system used for gathering and analyzing real-time data, controlling industrial processes locally or at remote locations, and directly interfacing with devices such as sensors, valves, pumps, motors, and more.

How does a capacitive level sensor work?

A capacitive level sensor works by measuring the change in capacitance caused by the change in the level of a substance. As the level changes, the dielectric constant between the sensor and the substance changes, which in turn changes the capacitance.

What is a servo motor and where is it used?

A servo motor is a rotary actuator that allows for precise control of angular position, velocity, and acceleration. It’s used in applications such as robotics, CNC machinery, or automated manufacturing.

What is a fieldbus?

Fieldbus is a communication protocol used for real-time distributed control of process or manufacturing automation. It allows for digital communication between various field devices like sensors, actuators, and controllers.

What is SIS?

A Safety Instrumented System (SIS) is designed to prevent or reduce hazardous events by taking the process to a safe state when predetermined conditions are violated. It comprises sensors, logic solvers, and final elements.

What is Profibus communication?

Profibus is a standard for fieldbus communication in automation technology, initially promoted by Siemens. It’s used to exchange data between controllers and devices such as I/O devices, transducers, drives, etc.

What is the difference between a switch and a transmitter?

A switch operates by turning a device on or off when certain conditions are met. A transmitter, on the other hand, sends a continuous signal representing the measurement of a variable.

What is a mass flow meter and how does it work?

A mass flow meter is a device that measures the mass flow rate of a fluid flowing through a tube. It operates by measuring the force exerted by the fluid flow on a known mass.

What is the function of a manifold in instrumentation?

A manifold in instrumentation is used to connect one part of the system to another. It helps in isolating the pressure instruments from the process for maintenance and calibration without interrupting the process.

What is the purpose of a flame scanner in a boiler?

A flame scanner is used in a boiler to monitor the flame condition, typically using ultraviolet or infrared sensors. It ensures that a flame is present when fuel is being burned and triggers a shutdown if the flame goes out to prevent a dangerous situation.

Can you explain the principle of operation of a radar-level transmitter?

A radar-level transmitter works on the principle of time-of-flight. It sends a radar pulse toward the product surface, which then gets reflected back. The time taken by the pulse to return is proportional to the distance and hence, the level of the product.

What is the role of a junction box in instrumentation?

A junction box in instrumentation is used for connecting different instruments and for housing the terminations of multiple field cables. It provides a central location for wiring connections and helps in reducing clutter and confusion in wiring systems.

What is a PLC and where is it used?

A PLC, or Programmable Logic Controller, is a rugged computer used for industrial automation. It’s used to automate specific processes, machine functions, or entire production lines.

What is a bubbler-level measurement system?

A bubbler level measurement system measures the level of liquid by determining the pressure required to force a gas through a submerged tube at a constant flow rate. The pressure is proportional to the liquid level above the open end of the tube.

What is a Coriolis flow meter and how does it work?

A Coriolis flow meter measures the mass flow and density of a fluid based on the Coriolis effect. It uses a vibrating tube, through which the fluid flows. When the fluid moves through the vibrating tube, a Coriolis force acts on the tube, causing it to twist. This twisting is proportional to the mass flow rate of the fluid.

How do you select a control valve for a specific application?

Selecting a control valve for a specific application depends on factors such as the type of fluid, flow rate, pressure, temperature, the desired flow control characteristics, and the available actuation method.

What is a purge system and what is its purpose?

A purge system is used to clean or clear a process line, vessel, or system by introducing a purging medium, usually a gas or a liquid, to remove unwanted contaminants or residue. It’s used to maintain the cleanliness and integrity of the system.

What is a temperature transmitter?

A temperature transmitter is a device that converts the output of a temperature sensor, like a thermocouple or RTD, into a standardized analog or digital signal that can be read and interpreted by a control system.

How does an electro-pneumatic positioner work?

An electro-pneumatic positioner works by receiving an electrical input signal, converting it into a proportional pneumatic output signal, and then using that output to control the position of a control valve or actuator.

What is the difference between a potentiometer and a rheostat?

A potentiometer is a variable resistor with three terminals, used to measure voltage, while a rheostat is a variable resistor with two terminals, used to control current. Potentiometers are typically used for voltage division, whereas rheostats are used for current control.

What is a turbidity meter?

A turbidity meter is a device that measures the cloudiness or haziness of a liquid caused by suspended particles. It measures the intensity of light scattered by the particles present in the fluid.

How does a conductivity meter work?

A conductivity meter works by measuring the ability of a solution to conduct an electric current between two electrodes. The conductivity is directly related to the concentration of ions in the solution, which can be used to determine the concentration of a specific substance.

What is an I/P converter?

An I/P (Current-to-Pressure) converter is a device that converts a 4-20mA electrical signal into a proportional pneumatic output, typically 3-15 psi or 6- 30 psi. It’s used to interface electrical control devices with pneumatic actuators and valves.

What is the role of a pressure regulator?

A pressure regulator’s role is to control or maintain a pre-set downstream pressure. It reduces a source or inlet pressure to a lower outlet pressure and maintains this outlet pressure despite changes in the inlet pressure.

How does a proximity sensor work?

A proximity sensor works by emitting an electromagnetic or electrostatic field, or a beam of electromagnetic radiation (infrared, for instance), and looks for changes in the field or return signal. When an object approaches the sensor within its range, the change in the field or return signal is detected and processed.

What is an encoder in instrumentation?

An encoder is a sensor of mechanical motion that generates digital signals in response to motion. It can provide information about the position, velocity, and direction of a mechanical part or device.

What is the difference between active and passive sensors?

Active sensors require an external source of power to operate and produce an output signal, while passive sensors generate an output response from an external stimulus without the need for additional energy.

What is the principle of a pH meter?

A pH meter measures the hydrogen-ion activity in water-based solutions. It uses a pH-sensitive electrode (usually glass) to measure the voltage caused by the ion concentration, and this is used to determine the pH.

What is the purpose of grounding in instrumentation?

Grounding in instrumentation is used to protect the instruments and personnel from electrical faults. It also helps to maintain signal integrity by preventing electrical noise and interference from affecting the measurement signals.

How does a photoelectric sensor work?

A photoelectric sensor works by emitting a light beam (visible or infrared) from its light-emitting element. This beam is then received by a light-receiving element. When an object passes through the beam, the sensor output changes, indicating the presence of the object.

Can you explain Bernoulli’s principle in terms of flow measurement?

Bernoulli’s principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure. This principle is used in flow measurement, where a restriction in the flow path causes an increase in fluid velocity and a decrease in pressure. This pressure difference can then be used to calculate the flow rate.

What is a vibration sensor and how does it work?

A vibration sensor, also known as a vibration transducer, converts vibrations into an electrical equivalent output. Common types of vibration sensors include piezoelectric and accelerometer sensors. They work by producing an electrical signal proportional to the vibration.

What is the function of a pneumatic system in instrumentation?

A pneumatic system in instrumentation uses compressed air as a medium for transmitting control signals. These systems are often used in industrial control applications due to their reliability, simplicity, and safety.

What is a load cell and how does it work?

A load cell is a type of sensor that is used to measure force or load. It converts a force into an electrical signal. This is accomplished through a mechanical arrangement that causes strain in the device when a force is applied, changing its resistance.

How does a piezoelectric sensor work?

A piezoelectric sensor works based on the piezoelectric effect. When force, pressure, or acceleration is applied to the sensor, it produces an electrical charge that is proportional to the applied force.

What is a limit switch and where is it used?

A limit switch is a type of sensor that detects the presence or absence of an object when the object reaches a specific physical limit or boundary. They’re often used in industrial controls to control the motion of machines and systems.

What is the principle of a magnetic flow meter?

A magnetic flow meter, also known as a mag meter, measures flow by using Faraday’s Law of electromagnetic induction. It applies a magnetic field to a conductive fluid flow, and the voltage induced across the fluid flow is proportional to its velocity.

What is the function of a relay in an electrical circuit?

A relay is an electrically operated switch. It uses a small amount of power to control a larger amount of power by opening or closing contacts in another circuit. When a voltage is applied to the coil, it generates a magnetic field that moves the contacts to turn the relay on or off.

What is the difference between RTD and thermocouple?

Both RTDs (Resistance Temperature Detectors) and thermocouples measure temperature, but they do so in different ways. RTDs work by correlating the resistance of the RTD element with temperature, and they are known for their accuracy. Thermocouples measure temperature based on the Seebeck effect, where a voltage is produced by the temperature difference between two different metals.

What is the role of a solenoid valve in a control system?

A solenoid valve in a control system is an electromechanical device used for controlling liquid or gas flow. The solenoid acts as an actuator on the valve body to open, close, or partially obstruct the flow.

How does a differential pressure transmitter work?

A differential pressure transmitter works by sensing the difference in pressure between two points in a system. This is done by having two pressure inlets. The transmitter then outputs a signal proportional to this pressure difference.

What is the HART protocol?

HART (Highway Addressable Remote Transducer) protocol is a communication protocol widely used in industrial automation systems. It’s known for its ability to communicate over legacy 4-20mA analog wiring, carrying both the 4-20mA signal and digital communication.

What is a transducer and how does it work?

A transducer is a device that converts one form of energy into another. For instance, a pressure transducer converts pressure into an electrical signal. The transducer works by changing its physical properties in response to the input energy, generating a corresponding output signal.

How do you calibrate a pressure transmitter?

To calibrate a pressure transmitter, you need to compare its output against a known standard at different input levels. The input levels are typically at the low end, mid-range, and high end of the scale. Adjustments are made until the transmitter’s output matches the expected values at these points.

What is a data logger and what is it used for?

A data logger is an electronic device that records data over time or in relation to location. It’s equipped with a sensor to receive information and a computer chip to store it. Data loggers are used in various industries for different purposes, like monitoring temperature, humidity, pressure, and more.

How does a pneumatic actuator work?

A pneumatic actuator converts energy formed by vacuum or compressed air into either linear or rotary motion. This is achieved by the compressed air forcing the piston to move within the cylinder, which in turn moves the valve stem and actuates the valve.

What is a sensor’s sensitivity?

A sensor’s sensitivity is the ratio of the output signal or response to the input quantity or stimulus. It indicates how much the sensor’s output changes when the measured property changes. High sensitivity is desirable as it allows the sensor to detect small changes in the measured property.

What is an ultrasonic flow meter and how does it work?

An ultrasonic flow meter measures the velocity of a fluid with ultrasound to calculate the flow rate. It sends an ultrasonic wave through the fluid, and the change in the wave’s speed as it travels upstream and downstream is used to calculate the flow rate.

What is the function of a level transmitter?

A level transmitter is used to measure the level of a substance in a container. It can be used to measure the level of liquids, powders, or other granular materials. The level measurement can be either continuous or point values.

What is a variable frequency drive (VFD)?

A variable frequency drive (VFD) is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor. VFDs are used in applications ranging from small appliances to large compressors.

How does an inductive proximity sensor work?

An inductive proximity sensor works by generating an electromagnetic field in a coil and detecting changes in this field caused by a metallic object. When a metal object comes close to the coil, it changes the field’s strength and induces a current in the coil, which the sensor detects.

What is a thermal mass flow meter and how does it work?

A thermal mass flow meter measures the flow of gas directly by detecting the cooling effect of the gas as it passes over a heated surface. As more gas flows over the surface, more heat is removed. This change in temperature is used to determine the mass flow rate.

How does a gas chromatograph work?

A gas chromatograph works by injecting a sample gas mixture into a flow-stream, then passing it through a column. Different components in the sample separate based on their chemical properties and interaction with the column’s material. As the separated components exit the column, they’re detected and quantified.

What is the function of a diaphragm in a pressure gauge?

The diaphragm in a pressure gauge serves as a barrier between the process fluid and the pressure measuring mechanism. When pressure is applied, the diaphragm flexes and transfers this movement to the measuring element.

What is a PID controller?

A PID controller is a control loop feedback mechanism widely used in industrial control systems. PID stands for Proportional, Integral, Derivative. These three control actions allow the controller to adjust the control output to drive the process variable towards the setpoint.

What is the purpose of a heat exchanger?

A heat exchanger is a system used to transfer heat between two or more fluids, without them mixing together. Heat exchangers are used in many applications, including heating and cooling in buildings, power generation, and in many industrial processes.

What are the common types of process control valves?

Common types of process control valves include gate valves, globe valves, ball valves, butterfly valves, and plug valves. Each has its unique characteristics and applications.

What is the difference between a contact and non-contact temperature sensor?

A contact temperature sensor requires physical contact with the object to measure its temperature. Examples include thermocouples and RTDs. A non-contact temperature sensor measures temperature by capturing the amount of thermal radiation emitted by the object, such as an infrared thermometer.

What is an optical sensor and how does it work?

An optical sensor is a device that converts light rays into electronic signals. It measures the physical quantity of light and then translates it into a form that is readable by an integrated measuring device. It works based on either the reflection or interruption of light.

What is the main advantage of a pneumatic control system?

The main advantage of a pneumatic control system is its simplicity and reliability. They are particularly effective in harsh environments where electronic systems might fail. They are also safer to use in hazardous environments where a spark could trigger an explosion.

What is the purpose of a data acquisition system?

The purpose of a data acquisition system is to collect information from different locations and convert the collected signals into digital values for processing and analysis. It serves as an interface between the real world of signals or physical parameters and the digital world of processing and data analysis.

What is a pneumatic signal?

A pneumatic signal is a type of control or measurement signal used in a pneumatic system. It typically consists of a pressure air signal, where changes in pressure represent different values of the process variable. A common standard for pneumatic signals is 3-15 psi.

What is Modbus protocol?

Modbus is a serial communication protocol developed for use with its programmable logic controllers (PLCs). It allows for communication between many devices connected to the same network, for example, a system that measures temperature and humidity and communicates the results to a computer.

How does a vortex flow meter work?

A vortex flow meter measures the flow rate of fluids by detecting the vortices created by the fluid passing a bluff body in the flow stream. The frequency of these vortices is proportional to the flow rate, and the meter translates this into a usable output signal.

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March 17, 2024

Very great questions with brief explanations! I want to thank to everyone who made these list!

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