Instrument Engineer Interview Questions

What is the use of single seated valve?

The single seated valve is used on smaller sizes where an absolute shut off is required.

The use of single seated valve is limited by pressure drop across the valve in the closed or almost closed position.

What is the use of double seated valve?

In double seated valves the upward and downward forces on the plug due to reduction of fluid pressure are nearly equalized.

It is generally used on bigger size valves and high pressure systems. Actuator forces required are less.

What is the use of valve positioner?

Valve positioner can be used for following reasons:

a. Quick action
b. Valve hysterisis
c. Viscous liquids
d. Split range.
e. Line pressure changes on valve
f. Bench set not standard
g. Reverse valve operations

What are primary elements of measuring pressure? Which type of pressure can be measured by these elements?

Primary elements of measuring pressure are:

a. Bourdon Tube
b. Diaphragm
c. Capsule
d. Bellows
e. Pressure springs

These elements are known as elastic deformation pressure elements.

Name different types of bourdon tubes.

Types of bourdon tubes:

  1. C type
  2. Spiral
  3. Helix

What are different types of control valves?

The commonly used control valves can be defined as follows:

a. Depending on Action:

Depending on action there are two types of control valves 1. Air to close 2. Air to close

b. Depending on body:

Depending on body there are 4 types of control valves

  1. Globe valves single or double seated
  2. Angle valves
  3. Butterfly valves
  4. Three way valves

What is furnace draft control?

Balanced draft boilers are generally used negative furnace pressure. When both forced draft and induced draft are used together, at some point in the system the pressure will be same as that of atmosphere.

Therefore the furnace pressure must be negative to prevent hot gas leakage. Excessive vacuum in the furnace however produces heat losses through air infiltration. The most desirable condition is that the one have a very slight negative pressure of the top of furnace.

What is intrinsically safe system?

Intrinsic safety is a technique for designing electrical equipment for safe use in locations made hazardous by the presence of flammable gas or vapours in the air.

Intrinsically safe circuit is one in which any spark or thermal effect produce either normally or under specified fault conditions is incapable of causing ignition of a specified gas or vapour in air mixture at the most ignited concentration.

What is zener diode? What is voltage regulator?

The breakdown region of a p-n diode can be made very sharp and almost vertical diodes with almost vertical breakdown region are known a s zener diodes.

A zener diode operating in the breakdown region is equivalent to a battery. Because of this current through zener diode can change but the voltage remains constant. It is this constant voltage that has made the zener diode an important device in voltage regulation.

Voltage regulator: The output remains constant despite changes in the input voltage due to zener effect.

What is force balance principle? State some of its’ advantages.

Force balance principle: A controller which generates an output signal by opposing torque. The input force is applied on the input bellows which moves the beam. This crackles nozzle back pressure.

The nozzle back pressure is sensed by the balancing bellows which brings the beam to balance. The baffle movement is very less about 0.002 for full scale output.


a. Moving parts are fewer.
b. Baffle movement is negligible
c. Frictional losses are less

What is motion balance principle?

A controller which generates an output signal by motion of its parts. The increase in the baffle is to move towards the nozzle. The nozzle back pressure will increase.

This increase in the back pressure acting on the balancing bellows, will expand the bellows. The nozzle is moved upward due to this. The nozzle will move until motion almost equals the input baffle motion.

How will you test a transistor with a multimeter?

  1. Emitter +ve of meter and base -ve output =Low resistance
  2. Emitter -ve of meter and base +ve output =High resistance
  3. Collector +ve and base -ve output =Low
  4. Collector -ve and base +ve output =Low

Emitter: Collector = High resistance
PNP: Opposite Results

Explain ratio control system.

A ratio control system is characterized by the fact that variations in the secondary variable don’t reflect back on the primary variable. A ratio control system is the system where secondary flow is hold in some proportion to a primary uncontrollable flow.

If we assume that the output of a primary transmitter is A. and the output of the secondary transmitter is B, and that the multiplication factor of the ratio relay is K, then for equilibrium conditions which means set valve is equal to measured valve, we find the following relation:

KA-B=0 or B/A = K where ‘K’ is the ratio setting off the relay.

What is solenoid valve? Where it is used?

A solenoid is electrically operated valve. It consists of solenoid coil in which magnetic plunger moves. This plunger is connected to the plug and tends to open or close the valve.

There are two types of solenoid valves:

  1. Normally Open
  2. Normally closed

Use: It is used for safety purpose in different electric work.