Basic Instrumentation - Interview Questions and Answers

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Interview Questions and Answers - Basic Instrumentation

What is closed loop ?

A Closed control loop exists where a process variable is measured,compared to a setpoint and action is taken to correct any deviation from setpoint.

What is open loop?

An open loop is more of a manually controlled control loop exists where the process variable is not compared and action is taken not in response to feedback on the condition of the process variable but is instead taken without regard to process variable conditions.

Whar are the common process variables used in industrial application ?

Pressure,Flow,Level,Temperature
Density,Ph,Mass,Conductivity.

What is setpoint?

Setpoint is a value for a process variable that is desired to be maintained.

What is error (Control loop) ?

Error is the difference between the measured variable and the setpoint and can be either positive or negative.

What is controller in a Control loop?

A controller is a device that receives data from a measurement instrument,compares that data to a programmed setpoint and if necessary signals a control element to take a corrective action.

What is a PLC (Programmable Logic Controller ) ?

PLCs are usually computers connected to a set of input/output devices. The computers are programmed to respond to input by sending output to maintain all processes at setpoint.

What is a Distributed Control Systems ?

DCSs are controllers that,in addition to performing control functions,provide readings of the status of the process,maintain databases and man-machine interface.

What is an Indicator?

An indicator is a human readable device that displays information about the process.

What is a recorder ?

A recorder is a device that records the output of measurement device.

What is an actuator ?

An actuator is the part of a final control devices that causes a physical change in the final control device when signaled to do so.

Eg: Valve,Motor etc…

What is meant by Manipulated variable ?

The fuel or energy that is physically altered by the final control element.

What is a Transducer?

A transducer is device that translates a mechanical signal into an electrical signal.

What is a Transmitter?

A transmitter is a device that converts a reading from a sensor or transducer into a standard signal and transmit that signal to a monitor or controller.

What is a Pneumatic Signal?

Pneumatic signals are signals produced by changing the air pressure in a signal pipe in proportion to the measured change in a process variable.

3-15 psig

What is an Electric (Analog) Signal?

Electric analog signals produced by changing the electrical property in proportion to the measured change in a process variable.

4-20 ma,1-5 volt,