Top 30 Instrumentation and control Interviews Questions & Answers

Summary

This video from Calibration Academy provides answers to the top 30 instrumentation interview questions, covering essential topics such as calibration, process variables, flow and pressure measurement, differential pressure transmitters, level measurement, thermocouples, RTDs, and smart transmitters. It explains practical procedures for installing, servicing, and troubleshooting instrumentation devices, along with the principles behind key instruments like magnetic flow meters and square root extractors. The content is tailored for both beginners and professionals preparing for technical interviews, offering clear explanations, units of measurement, and industry best practices, ensuring viewers gain a thorough understanding of fundamental instrumentation concepts.

Takeaways

• 😀 Calibration of instruments is essential to verify accuracy against known standards and correct any errors.
• 😀 The main process variables in instrumentation are flow, pressure, level, and temperature, each with specific units of measurement.
• 😀 Differential Pressure Transmitters (DPT) require proper valve operations for safe removal and reinstallation.
• 😀 Orifice plates and orifice tags help identify flow measurement points and provide critical information about diameter, material, and installation.
• 😀 The square root extractor is used to convert differential pressure signals into linear flow signals.
• 😀 Magnetic flow meters operate on Faraday’s law of electromagnetic induction to measure volumetric flow of conductive liquids.
• 😀 Smart transmitters can send both analog (4–20 mA) and digital signals, and are microprocessor-based for precise control.
• 😀 4–20 mA signaling is preferred over 0–20 mA because it allows detection of connection faults (0 mA) and clearly defines minimum process value (4 mA).
• 😀 Thermocouples and RTDs are common temperature sensors, with RTDs offering higher accuracy due to lead wire compensation in 3-wire configurations.
• 😀 Proper piping and installation techniques for liquids, gases, and steam are critical to ensure accurate differential pressure and level measurements.

Q & A

Why is calibration of instruments important?

Calibration is important because it allows you to check an instrument against a known standard and adjust its output if any errors are detected, ensuring accurate measurements.

What are the main process variables in instrumentation, and their units?

The main process variables are Flow (liters/sec, liters/min, m³/hr, kg/hr), Pressure (bar, Pascal, inches H2O, mH2O), Level (meters, millimeters, centimeters, percentage), and Temperature (°C, °F, Kelvin, Rankine).

How can you measure the milliampere output of a transmitter?

The milliampere output of a transmitter can be measured by connecting a multimeter in series with the transmitter and setting the multimeter to DC milliampere mode.

What are the primary elements used for flow measurement?

The primary elements for flow measurement include orifice plates, Venturi tubes, pitot tubes, anubars, flow nozzles, and weirs or flumes.

How do you remove and reinstall a differential pressure transmitter (DPT) from service?

To remove a DPT: close the low-pressure block valve, open the equalizing valve, then close the high-pressure block valve. To reinstall: start with all valves closed, open the equalizing valve, slowly open the high-pressure block valve, close the equalizing valve, and finally open the low-pressure block valve.

What is the purpose of a square root extractor in a differential pressure flow measurement?

A square root extractor converts the squared relationship of differential pressure to flow into a linear flow signal, allowing accurate flow measurement.

How does a magnetic flow meter work?

A magnetic flow meter operates on Faraday's law of electromagnetic induction. Conductive liquids passing through a magnetic field generate a voltage proportional to the flow, which is detected by electrodes.

What is the difference between two-wire, three-wire, and four-wire transmitters?

A two-wire transmitter uses a single cable for both power and signal. A three-wire transmitter separates power, signal, and common ground. A four-wire transmitter has two wires for power and two wires for signal, allowing independent power and measurement lines.

What are the differences between wet leg and dry leg methods in level measurement?

In a dry leg, the low-pressure side of the DPT is filled with gas or air. In a wet leg, it is filled with liquid. This affects calibration and measurement accuracy based on the application.

Why is a three-wire RTD preferred over a two-wire RTD in industry?

A three-wire RTD includes an extra lead to compensate for lead resistance, reducing measurement errors and making it more accurate than a two-wire RTD.