Interview questions - Basic instrumentation

Define physical quantity ?

Any quantity which can be measured is known as physical quantity.

Define Measurend ?

A physical quantity which is being measured.

Define sensors?

Sensors are devices which transform one physical quantity to another physical quantity.

Define Transducers ?

A device which provides a usable output in response to a specified measurend.

Define Active Instruments ?

An active instrument uses an auxiliary energy source to produce the desired output signal.

Define Passive instruments ?

A passive instrument can convert physical or chemical properties into another property without the aid of an auxiliary source.

Define Primary Instrument ?

A primary instrument is a scientific instrument,which by its physical characteristics is accurate and is not calibrated against anything else.

Define Secondary Instrument ?

Secondary instruments must be calibrated against a primary standard.

What is the difference between measured value and true value?

Measured Value - Any value or any reading calculated from measurement system or measuring instrument is known as measured value.

True value - Any value calculated from rated values is known as true value.


Interview Questions and Answers - Control Loop

What is a Primary element ?

Primary element is the measuring element that quantitatively converts the measured variable energy into a form suitable for measurement.

What is a transmitter ?

A transmitter is device which converts a non standard signal into a standard signal. 3-15 psi,4-20 ma etc.

What is meant by controlled variable?

A controlled variable is the value which is sensed to originate a feedback signal. Controlled variable is also known as process variable.

What is a controller?

A controller is a device which operates automatically to regulate a controlled variable.

What is Controller algorithm?

Controller algorithm is a mathematical representation of the control action to be performed.

What is set point?

Set point is an input variable which sets the desired value of the controlled variable.

Question - Draw a closed control loop?

what is error in a closed loop ?

The algebraic difference between the real value and ideal value of the measured signal.

What is manipulated variable?

A quantity or condition which is varied as a function of the algebraic error signal so as to cause a change to the value of the directly controlled variable.

What is feedback control?

Control action in which a measured variable is compared to its desired value to produce an actuating error signal which is acted upon in such a way as to reduce the magnitude of the error.

What Cascade control ?

Control in which the output of one controller is introduced as the set point for another controller.