Instrumentation Engineering Interview Preparation Notes

Basic Instrumentation

Q: What is instrumentation?

A: The science of measuring and controlling process variables in industries.

Q: What is a transmitter?

A: Device converting a physical signal into a standard output like 4–20 mA.

Q: Difference between sensor and transducer?

A: Sensor detects change; transducer converts one form of energy to another form.

Q: Why use 4–20 mA signal?

A: It avoids signal loss and allows live zero detection.

Q: What is a control loop?

A: A system that measures, compares, and corrects a process variable automatically.

Process Measurement

Q: What is an RTD?

A: Resistance Temperature Detector; measures temperature via change in resistance.

Q: What is a thermocouple?

A: Temperature sensor made from two dissimilar metals generating a voltage by temperature difference.

Q: How does a DP transmitter measure level?

A: By sensing hydrostatic pressure difference between two points.

Q: What is an orifice plate?

A: A restriction used for flow measurement by creating a differential pressure.

Q: What is a vortex flowmeter?

A: Measures flow using vortices shed by a bluff body in the fluid stream.

Control & Automation

Q: What is PLC?

A: Programmable Logic Controller used for automation and logic control, it is industrial computer the can process sensor and instrument signals.

Q: What is DCS?

A: Distributed Control System for process automation with multiple control loops.

Q: Explain PID control.

A: Proportional-Integral-Derivative control for stable process operation.

Q: What is SCADA?

A: Supervisory Control and Data Acquisition for remote monitoring and control.

Q: Difference between PLC and DCS?

A: PLC is logic-focused; DCS is process-focused with distributed controllers.

Signal & Communication

Q: What is HART protocol?

A: Digital communication superimposed on 4–20 mA analog signal.

Q: What is Foundation Fieldbus?

A: Digital two-way communication protocol for smart field devices.

Q: What is Modbus?

A: A serial communication protocol used in automation systems.

Q: Difference between analog and digital signal?

A: Analog is continuous; digital is discrete (binary).

Q: What is intrinsic safety?

A: Design ensuring no spark or heat sufficient to ignite hazardous atmospheres.

Calibration & Maintenance

Q: What is calibration?

A: Comparing instrument output with a known standard and adjusting if needed.

Q: What is zero and span?

A: Zero is lower limit; span is the difference between upper and lower range.

Q: What is LRV and URV?

A: Lower Range Value and Upper Range Value of instrument measurement.

Q: How to check an RTD?

A: Measure resistance with a multimeter and compare with standard tables.

Q: What is loop checking?

A: Testing the complete instrument loop from field device to control system.

Control Valves & Actuators

Q: What is a control valve?

A: Valve that regulates fluid flow as per control signal.

Q: What is a positioner?

A: Device ensuring valve travel matches the control signal.

Q: What is Cv?

A: Flow coefficient indicating valve capacity for a given pressure drop.

Q: What is split range control?

A: One control signal operates two valves in different ranges to cover full flow.

Q: Air-to-open vs air-to-close?

A: Describes valve fail direction: air-to-open opens with air, air-to-close closes with air.

Safety & Standards

Q: What is SIL?

A: Safety Integrity Level defining reliability and performance of safety functions.

Q: What is IP rating?

A: Ingress Protection rating indicating dust and water resistance of enclosures.

Q: What is ATEX?

A: EU certification for equipment intended for explosive atmospheres.

Q: What is grounding?

A: Connecting system to earth to ensure safety and reduce noise.

Q: What is shielding?

A: Using conductive layer around cables to block electromagnetic interference.

Technician-Level Practical Questions

Q: What is cable glanding?

A: Securing and sealing cable entry into equipment to maintain IP rating and strain relief.

Q: What is a junction box?

A: Enclosure for cable terminations and wire connections in the field.

Q: What is a marshalling cabinet?

A: Panel collecting field signals and routing them to DCS/PLC inputs.

Q: What is a continuity test?

A: A test to verify a conductor is continuous and not open-circuited.

Q: How to identify 4–20 mA loop fault?

A: Check loop current with a multimeter/loop calibrator and inspect wiring, power, and HART errors.