Top Instrumentation Engineering Questions


Question. Define Pressure Switch?

Answer: As the name indicates, A switch that works on pressure is known as a pressure switch. A pressure switch is a switch that makes electrical contact when a certain set pressure has been reached at its input. The Pressure switch is used to provide on/off switching. The pressure switch is designed to make contact either on pressure rise or on pressure fall.

Pressure Switch has 3 contacts.


NO (Normally Open)

NC (Normally Closed)

A spring is used in pressure switch when pressure increases this spring comes in tension. A pressure is also set using adjusting screw when increasing or decreasing pressure comes to this pressure, contact changes from NC to NO or from NO to NC.

Question. How to check a Pressure Switch?

Answer: 2 wires are connected in a pressure switch One is connected in common and other is connected in NO or NC according to the application.

Step 1: first we check the continuity between common and normally closed (NC).

Step 2: Now after increasing the pressure when pressure is equal to set pressure we will check the continuity between common and normally open (NO).

Step 3: now we will decrease the pressure and when decreasing pressure is equal to the set pressure again we will check the continuity between common and normally closed (NC).

Question. What are the forms of analog input and analog output are used?

Answer: Analog inputs and analog outputs are used in the following forms

  1. 4-20 m Amp
  2. 0-20 m Amp
  3. 1-5 V DC
  4. 0-5 VDC

Question. Why 4-20 m Amp signal is used in place of 0-20 m Amp signal?

Answer: 4-20 m Amp signal is used in place of 0-20 m Amp signal, the reason can be well understood by the following table

4-20 mAmp output

4 0%

8 25%

12 50%

16 75%

20 100%

0-20 m Amp output

0 0%

5 25%

10 50%

15 75%

20 100%

It is clear from the table that at 0 % output, how can we recognize that the instrument we are using is giving output or not for example take transmitter as an instrument, in 4-20 m Amp signal system if output is 0 m Amp that means our instrument is not working but in 0-20 m Amp signal system we are unable to know whether our instrument is working or not.

Question. What different types of signals we use for PLC or DCS?

Answer: We use two different types of signals for PLC or DCS

Analog signal: An analog signal is a continuously varying signal for example signal from RTD, thermocouple, transmitter, motor rpm feedback and others.

Digital Signal: A digital signal is used for ON/OFF Switching for example signal ON/OFF Feedback, Local/ Remote feedback, Start/ Stop Command for a Motor.

Question . Define input and output signals in instrumentation?

Answer: Input Signal: An Input signal is a Feedback signal and comes from a field devices Example- signal from RTD, thermocouple, transmitters, ON/OFF Feedback, motor rpm feedback.

Output Signal: An output Signal is a Command Signal and it is given to field instruments or devices Example- Signal given to a Control Valve, Signal Given to Start/ Stop a motor.

Similarly, Analog and Digital Signals are of two types each.

Analog Input: Signal from RTD, thermocouple, transmitters, Flow-meters, motor rpm feedback.

Analog Output: Signal given to Control valves, the motor rpm command

Digital Input: Motor ON/OFF Feedback, Local/ Remote Feedback.

Digital Output: Motor Start/ Stop Command, Signal Given to operate Solenoid Valve (SOV).

Question. Suppose we are in front of a PLC or DCS panel, how we came to know whether it is digital signal panel or analog signal panel without checking with a multimeter?

Answer: We all know that in signals are of two types

digital signal

analog signal

in digital signals we use relays, but in analog signals, we do not use relays, so it is clear that the panel in which we use relays is a digital panel and the panel in which we do not use relays is an analog panel.

Question: Why we use relays in PLC or DCS Panels?

Answer: We use Relays in PLC or DCS Panels For digital Signals, As we all know that the digital signals are used for ON/ OFF Switching, So relays are used in these panels to show ON/ OFF Switching. For Example, if We give the command to start a motor, A relay will be light up to show that our signal is passing.

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