Part Two of Instrumentation Job Interview Questions and Answers
These questions cover the process control, sensors, transmitters, safety systems, Communication and Networking and Instrumentation Calibration..
Process Control
1. What is the purpose of process control?
Answer: Process control is the automated or manual regulation of industrial processes to achieve and maintain desired output levels. It involves the use of sensors, controllers, and final control elements to manipulate process variables and ensure consistent product quality.
2. Can you explain the difference between open-loop and closed-loop control systems?
Answer: Open-loop control systems do not provide feedback regarding the controlled variable, whereas closed-loop control systems do. In a closed-loop system, the output is continuously monitored and compared with the desired set point, allowing the system to make adjustments as needed.
3. What is a PID controller, and how does it work?
Answer: A PID controller is a type of closed-loop controller that uses proportional, integral, and derivative terms to regulate the controlled variable. It calculates the error between the set point and the measured value, then applies a corrective action based on the proportional, integral, and derivative errors.
4. What is a cascade control system, and when is it used?
Answer: A cascade control system uses two or more controllers to regulate a single process variable.The outer loop sets the set point for the inner loop, which provides a fast and accurate response.
It is used when the primary controller cannot directly manipulate the process variable and requires a secondary controller to adjust a manipulated variable.
Sensors and Transmitters
1. What is the difference between a pressure gauge and a transducer?
Answer: A pressure gauge is a mechanical device used to measure pressure, while a transducer converts a physical quantity (such as pressure) into an electrical signal.
2. What is a smart transmitter, and how does it differ from a conventional transmitter?
Answer: A smart transmitter is a type of transmitter that can communicate digitally and provide more information than a conventional transmitter.
It's a digitally-enabled instrument that provides advanced functionality, such as self-diagnostics, data communication, and remote configuration. Smart transmitters can provide diagnostic data, self-calibration, and other advanced features.
Smart transmitters differ from conventional transmitters in terms of their ability to provide detailed information about the process and the health of the instrument.
3. Can you explain the difference between a thermocouple and a resistance temperature detector (RTD)?
Answer: A thermocouple is a temperature sensor made of two dissimilar metals that produce a voltage proportional to temperature. An RTD is a temperature sensor that changes resistance with temperature. RTDs are generally more accurate and stable than thermocouples but are also more expensive.
Safety Systems
1. What is a safety instrumented system (SIS), and what is its purpose?
Answer: A Safety Instrumented System SIS is a safety control system that monitors and regulates process variables to prevent or mitigate the consequences of hazardous situations. The purpose of an SIS is to ensure safety and prevent accidents by monitoring and controlling critical processes.
2. What is the difference between a safety shutdown system (SSDS) and an emergency shutdown system (ESDS)?
Answer: An SSDS is designed to prevent equipment or process damage, while an ESDS is designed to protect personnel and the environment.
3. Can you explain the concept of Safety Integrity Level (SIL) in safety systems?
Answer: Safety Integrity Level SIL is a measure of the reliability of an SIS. SIL is determined based on factors such as the probability of failure, the consequences of failure, and the risk reduction required. SIL is determined based on the risk associated with the process and the consequences of a failure.
4. What is the difference between SIS and BPCS (Basic Process Control System)?
Answer: SIS is a separate and independent safety control system designed to achieve safety functions, while BPCS manages and regulates the continuous process variables.
Communication and Networking
1. What is a fieldbus, and how does it work?
Answer: A fieldbus is a digital communication system used in process control. It allows devices such as sensors, actuators, and controllers to communicate with each other and with the control system.
2. What is a HART protocol?
HART (Highway Addressable Remote Transducer) is a communication protocol used in process control applications for transmitting digital data over an existing 4-20 mA current loop.
3. Can you explain the difference between HART and Modbus communication protocols?
Answer: HART (Highway Addressable Remote Transducer) is a digital communication protocol used to transmit and receive information between devices. Modbus is a communication protocol that allows for the exchange of data between devices over a serial link.
4. What is a Profibus, and how does it differ from a Modbus?
Answer: Profibus (Process Field Bus) is a digital communication system used in process control. It provides real-time communication between devices and can support both bus and tree topologies. Modbus is a communication protocol that allows for the exchange of data between devices over a serial link.
5. What is the difference between a fieldbus and a profibus?
Fieldbus and profibus are both communication protocols used in industrial automation systems. Fieldbus is a general term that refers to any digital communication protocol used in process control applications, while profibus is a specific type of fieldbus that uses a master-slave architecture.
Instrumentation Calibrations
1. What is the purpose of calibration, and how often should it be done?
Answer: Calibration is the process of adjusting and comparing an instrument's measurement accuracy against a reference standard to ensure accurate and reliable measurements.
It's the process of comparing a sensor's output to a known standard. It ensures accuracy and precision. Calibration should be done regularly, based on the manufacturer's recommendations or regulatory requirements.
2. What is a two-point calibration, and when is it appropriate?
Answer: A two-point calibration is a calibration method that involves calibrating a sensor at two points, typically the minimum and maximum range. It's appropriate for sensors that have a linear response and a limited range.
3. What is the difference between primary and secondary calibration?
Answer: Primary calibration involves comparing an instrument's output against a traceable standard, whereas secondary calibration involves the use of a calibrated reference standard.
4. What is a calibration certificate, and what information does it contain?
Answer: A calibration certificate is a document that provides evidence of a calibration's results and includes details such as the calibration date, measurement uncertainties, and the name of the calibration facility.
5. What is the difference between a fixed and a variable calibration range?
Answer: A fixed calibration range has a predetermined range of measurement, whereas a variable calibration range can be adjusted within a specified range.
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