Field Instrumentation Interview Questions and Answers 2021


Instrument Engineers can use this question bank to prepare for Field Instrumentation Interview Questions and Answers in Oil and Gas Plants.

Field Instrumentation Questions

1. What are the different types of orifice plates? State their uses.

Different orifice plates are: 1. Concentric 2. Segmental 3. Eccentric


These plates are suited for liquid, gas, and steam applications. A concentric orifice refers to the presence of concentric holes in these plates.


This plate has a hole in the shape of a circle segment. This plate is used to quantify colloidal and sherry flow.


The eccentric holes are on this plate. This plate is intended to monitor viscous and sherry flow.

2. How do you identify an orifice in the pipeline?

The orifice plate has an orifice tab welded on it that extends out of the line, indicating the orifice plate.

3. Why is the orifice tab provided?

The reasons for having an orifice tab are as follows:

1. The presence of an orifice plate in a line.

2. It is marked with the diameter of the orifice.

3. The orifice plate’s material.

4. The orifice plate’s tag number.

5. To indicate the orifice’s inlet.

4. Explain Bernoulli’s theorem. State its application.

The total energy of a liquid flowing from one point to another remains constant, according to Bernoulli’s theorem. It can be used with non-compressible liquids. Bernoulli’s equation alters depending on the type of liquid flow.

The speed of a fluid, its dynamic pressure, and its kinetic energy are all proportional. It can be utilized in a variety of real-world scenarios, such as measuring pressure on an aircraft wing or calibrating an airspeed indicator.

It can also be used to reduce pressure in the carburetor’s venturi tubes.

5. How can a D.P. transmitter be calibrated?

The following steps can be used to calibrate a D.P. transmitter:

1. Set the transmitter’s zero.

2. Run a static pressure test by applying the same amount of pressure to both sides of the transmitter. There should be no shift in zero on either side. If the zero moves, perform static alignment.

3. Suction test: Apply the same amount of vacuum to both sides. The value of zero should not change.

4. Calibration procedure: Supply the transmitter with 20 psi air and vent the L.P. side to the atmosphere. Connect the instrument’s output to a conventional test gauge. Make a zero adjustment. Adjust the span by applying the required pressure to the high-pressure side. If required, adjust the zero gain.

6. How is flow measured in the square root?

Flow is proportional to the square root of pressure. As a result, F=K is the square root of the applied pressure. Because the flow is proportional to the square root of the differential pressure. The flow is not directly indicated by the pressure pen.

The square root of the pen can thus be used to calculate flow. Assume the pen only reads half of the chart. Using the pen measure in the chart, flow can be determined.

7. Name different parts of a pressure gauge. Explain the use of hairspring in the pressure gauge.

Pressure gauge includes the following components:

a. ‘C’ type bourdon tube.
b. Connecting link
c. Sector gear
d. Pinion Gear
e. Hairspring
f. Pointer
g. Dial

8. How D.P. transmitter can be applied to the close tank?

The bottom of the tank is connected to the high-pressure side of the transmitter in a closed tank.

The lower pressure side of the transmitter is attached to the top of the tank. The pressure inside the vessel can be measured this way.

9. How D.P. transmitter can be applied to open tank?

The lower pressure side of an open tank is vented to the atmosphere. The high pressure side receives all of the pressure.

The high pressure side of the vessel is used to measure the pressure.

10. Explain the working of an electronic level troll?

The change in buoyancy caused by a change in liquid level changes the net weight of the displacer, increasing or decreasing the torque armload.

This variation is proportional to changes in the liquid’s level and specific gravity. In an RVDT, the ensuing torque tube movement changes the rotor’s angular motion, resulting in a rotor change proportional to the rotor displacement.

11. Explain the working of an enraf level gauge.

The servo-powered null balance approach is used in the Enraf level gauge. A continuous level sensing element is a displacer. A capacitive balance system controls a two-phase servo motor that coils and unwinds the measuring wire until the tension in the weight springs is equal to the weight of the displaced component immersed in the liquid.

12. What is the constant voltage unit?

A rectifier, CR, and filter capacitor make up the constant voltage circuit. There are two steps of zener regulation after that. The lamp line voltage Zener regulation is offered using abridge configuration.

13. Explain the burnout feature.

If the input circuit is open, Burnout gives the warning function of driving an indicator at the end of the scale. There is a burnout resistor that creates a voltage drop between the measuring circuit and the amplifier.

When there is an open circuit in the input, the polarity of the signal dictates the direction of the servo drive.

R-value 10 M for upscale burnout

R-value 22 M for downscale burnout.

14. Why thermowells are used? What materials are used in thermowells?

Exposing a temperature sensor directly to a substance is not desirable or practical in many situations. As a result, wells are utilized to defend against corrosion, abrasion, abortion, and high-pressure processes.

A thermowell is also beneficial for preventing physical harm to a sensor during handling and routine operations.

Thermowells are made of stainless steel and Inconel.

15. How is automatic reference junction compensation carried out in temperature recorders?

Variable nickel resistors are utilized in automatic reference junction adjustment. Its resistance fluctuates as the temperature changes. This reference junction compensator is positioned so that it will be at the reference junction’s temperature.

The reference junction is located at the poset, where the thermocouple’s dissimilar wires are reunited. This joint is always found at the i’s terminal strip.

16. What are de-saturators?

When extended transient reactions are predicted in some processes, such as batch processes, and a sustained deviation is present, the controller integral action constantly drives the output to a minimum or maximum value.

The term for this situation is “integral saturation of the control unit.” This unit is de-saturated when this condition is met.

17. Explain the working of Rotameter.

Variable area meters are a type of head meter that is unique. The location of the flow restrictor varies. In order to maintain a steady differential pressure. The rotameter is made comprised of a vertical tapered tube that allows the measured fluid to flow upward.

The ‘float’ rises or descends in response to the flow, varying the area of the channels so that the disparity across it balances the gravitational force on the ‘float.’ The differential pressure is kept constant at all times. The rate of flow is measured by the position of the ‘float.’

18. What is the working principle of the magnetic meter?

When a conductor is transported across a magnetic field, an electric potential is created. The conductor in most electrical machinery is a wire. A moving, electrically conductive liquid can also use this technique.

Commercial magnetic meters’ primary equipment is a straight cylindrical electrically insulated tube with a pair of electrodes approximately flush with the tube walls and a locator.

Only electrically conductive liquids can be used with this gadget. Slurries and filthy fluids are particularly well-suited to the magnetic meter.

19. Explain the mechanism behind the turbine meter.

Turbine meters are made out of a straight flow tube in which a turbine or fan can freely rotate around its axis, which is fixed to the tube’s center line. A magnetic pick-up device detects the rotor’s rotation primarily through the tube walls.

Because the rotor speed is directly related to the flow rate, the turbine meter is a flow rate device.

20. How would you choose differential range?

The 0-100 range is the most typical difference range for liquid measurement. This range is wide enough to reduce errors caused by seal chambers with uneven heads.

It is also influenced by the temperature differential between the load-lines. The capacity of the 100 range can be expanded to 400. By just changing the range tubes or adjusting the range, you can save up to 20%.

21. What is the use of a single seated valve?

The pressure drop across the valve in the closed or nearly closed state limits the use of single seating valves.

22. What is the use of a double seated valve?

The upward and downward stresses on the plug owing to reduced fluid pressure are essentially equalized in double seated valves.

It’s commonly seen on larger valves and high-pressure systems. The amount of force required by the actuator is reduced.

23. 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

24. What are the 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

25. Name different types of bourdon tubes.

Types of bourdon tubes:

1. C type
2. Spiral
3. Helix

26.What are the different types of control valves?

The commonly used control valves can be defined as follows:

a. Depending on Action:

Depending on the action there are two types of control valves

1. Air to close 2. Air to close

b. Depending on the body:

Depending on the 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

27. What is furnace draft control?

Negative furnace pressure is commonly employed with balanced draught boilers. When both forced draught and induced draught are utilized together, the pressure in the system will equal that of the atmosphere at some point.

To prevent hot gas leakage, the furnace pressure must be negative. However, excessive vacuum in the furnace results in heat loss due to air penetration.

28. What is the intrinsically safe system?

Intrinsic safety is a method of designing electrical equipment that is safe to operate in areas where there are flammable gases or vapors in the air.

Any spark or thermal action produced normally or under specified fault conditions is incapable of inducing ignition of a specified gas or vapor in an air mixture at the most ignited condition in an intrinsically safe circuit.

29. What is Zener diode? What is a voltage regulator?

A p-n diode’s breakdown zone can be made very acute, and diodes with almost vertical breakdown regions are known as s Zener diodes.

In the breakdown zone, a Zener diode is equivalent to a battery. As a result, the current flowing through the Zener diode can fluctuate, but the voltage remains constant. The Zener diode has become an important device in electronics because of its constant voltage.

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

A controller that generates an output signal by opposing torque is known as a force balancer. The beam is moved by the input force, which is applied to the input bellows.

This causes nozzle back pressure to crackle. The balancing bellows detect the nozzle back pressure and bring the beam into equilibrium. For full scale output, the baffle movement is quite little, around 0.002.

31. What is motion balance principle?

A controller generates an output signal by the movement of its components. The baffle is being raised in order to move closer to the nozzle. Backpressure at the nozzle will rise.

The balancing bellows will expand as a result of the increased backpressure exerted on them. As a result, the nozzle is shifted higher. The nozzle will move until its motion is about equivalent to that of the input baffle.

32. 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

33. Explain the ratio control system.

The fact that changes in the second variable do not affect the primary variable distinguishes a ratio control system. A ratio control system is one in which secondary flow is maintained in a proportion to an uncontrollable primary flow.

If we suppose that the output of the main transmitter is A and the output of a secondary transmitter is B, and that the multiplication factor of the ratio relay is K, we may derive the following relationship for equilibrium conditions in which the set valve equals the measured valve:

KA-B=0 or B/A = K, where K is the ratio that sets the relay in motion.

34. What is a solenoid valve? Where it is used?

A solenoid is a valve that is controlled by electricity. It is made up of a solenoid coil that moves a magnetic plunger. This plunger is attached to the plug and has the ability to open and close the valve.

Solenoid valves come in two varieties:

1. Open Normally

2. Normally shut

It’s used in a variety of electric jobs for safety reasons.

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