Instrumentation Questions

300+ Instrumentation Engineering Interview Questions – Part 4

1. What are leveltrol’s limitations?

A leveltrol’s limitations are that it cannot be used for lengths longer than 72 inches.

2. How will you check the zero of a level D.P. transmitter while it is in operation?

a. Close the isolation valves on both sides.

b. Open the vent valves on the L.P. and H.P. legs.

c. Check and, if necessary, adjust the zero.

3. Classification of temperature measurement?

Contact Type: Thermocouple, RTD

Non Contact Type: Pyrometer

4. What is the burnout feature? 

If the input circuit opens, burnout provides a warning feature by driving the indicator to the end of the scale. A burnout resistor is included to create a voltage drop between the measuring circuit and the amplifier. When there is an open circuit in the input, the polarity of the signal determines the direction of the servo drive.

5. What is the purpose of a converter in a temperature recorder?

The converter is intended to convert a direct current input voltage into an alternating current input voltage proportional in amplitude to the input.

6. What kind of sensing element would you use to detect extremely low temperatures?

R. T. D. (Resistance Temperature Detector) is the sensing element used to measure extremely low temperatures.

7. What are thermocouples for skin temperature?

Skin thermocouples are those that are directly connected to the process and do not require a thermowell.

Used to measure the skin temperature of heaters, furnaces, flue gas, and other appliances.

8. Define Wheatstone bridge Principle? 

The Wheatstone bridge operates on the principle of null deflection, which means that the resistance ratios are equal and no current flows through the circuit. Under normal circumstances, the bridge is unbalanced because current flows through the galvanometer.

9. Define potentiometer?

Potentiometer is a device used to measure an electromotive force by balancing it against the potential difference produced by passing a known current through a known variable resistance.

10. Define difference gap control?

Differential gap control is similar to on off control, but there is a band or gap around the control point.

11. On off control used in?

When precise control is not required.

Processes with enough capacity to keep the final operator up to date with the measurement cycle.

It is most commonly found in refrigeration and air conditioning systems.

12. Use of difference gap control?

In industry, differential gap control is frequently used in non-critical level control applications where the only goal is to keep a tank from flooding or drying. When a measured variable exceeds the upper gap, the control valve will fully open or fully close. Similarly, if it exceeds the lower gap, it will fully open or close.

13. What is dry leg calibration and where is it used?

Dry leg calibration is only used for level measurement in a closed tank containing liquids at room temperature. For level measurement in a closed tank with hot and extremely cold liquids, the wet leg calibration method is required.

14. HART stands for

Highway Addressable Remote Transducer

15. What is HART protocol?

HART is a bidirectional communication protocol that allows intelligent field instruments and host systems to exchange data. A host can be any software application, ranging from a technician’s handheld device or laptop to a plant’s process control, asset management, safety, or other system that employs any control platform. Communication takes place between two HART-enabled devices, which are typically a smart field device and a control or monitoring system. Standard termination practises and instrumentation grade wiring ensure dependable communication.

16. How HART works?

The term “HART” stands for Highway Addressable Remote Transducer. The HART Protocol employs the Bell 202 Frequency Shift Keying (FSK) standard to superimpose low-level digital communication signals on top of the 4-20mA.

17. What are the modes in HART?

Master-Slave Mode

Burst Mode

18. What is reset-wind up?

When a reset action is used in a controller where the measurement is away from the set point for an extended period of time, the reset may drive the output to its maximum, resulting in reset wind up. When the process is restarted, the output will not be reduced until the measurement crosses the so point, resulting in large overshoots. This issue can be avoided by including an anti-reset wind up circuit, which eliminates output saturation.

19. Define anti-reset wind up?

If the limit acts in the control amplifier’s integral circuit’s feedback section, the controller output will immediately begin to drive in the opposite direction as soon as the process signal crosses the set point. This method is known as antireset wind up.

20. Define de-saturators?

When long transient responses, such as in a batch process, are expected and a sustained deviation exists, the controller integral action continuously drives the output to a minimum or maximum value. This is known as “integral saturation of the control unit.”

21. Explain how a turbine metre works.

Turbine metres are made up of a straight flow tube within which a turbine or fan can freely rotate about its axis, which is fixed along the tube’s centre line. Straightening vanes upstream of the turbine reduces the possibility of fluid flow rotational components. A magnetic pick-up system senses the rotation of the rotor through the tube wall in most units. Because rotor speed is directly proportional to flow rate, the turbine metre is a flow rate device. The output is typically electrical pulses from the magnetic pick-up with a frequency proportional to flow rate. Turbine metres are mostly used to measure clean, non-corrosive hydrocarbons.

22. Where does the integral orifice used?

Small flow rates are measured using an integral orifice. It is directly attached to the secondary device.

The integral orifice diameter ranges from 0.020 inch to 0.250 inch. The integral orifice is commonly used in laboratories and pitot plants.

integral orifice

23. Where can you find a quadrant orifice?

Quadrant orifices are preferred when the fluid is viscous and the operating Reynolds number is low.

24. What exactly is the Reynolds number?

The correspondence of fluid forces in two systems is implied by dynamic similarity. There are numerous classes of forces that influence the behaviour of fluids in general. Inertial viscous, gravitational, compressibility, pressure, and elastic forces are a few examples. Fluid properties are used to develop dimensionless ratios. Dimension and velocity, which are essentially force ratios.

The Reynolds number is the most important of these.

Reynolds number

25. What types of bonnets would you use for high and very low temperatures?

High temperature: Bonnets are equipped with radiation fins to protect the glad packing from damage.

Extended bonnets are used in extremely cold temperatures to keep gland packing from freezing.

26. How are you going to work on a control valve while it is in service?

While the control valve is in place or in service, it must be bypassed, and the line must then be depressurized and drained.

27. What does a valve positioner do?

The valve positioner is employed to control valve with quick action.

28. Butterfly valves used?

Butterfly valves are only used in systems that allow for a small pressure drop across the valve.

When the disc rotates 90 degrees, the butterfly is fully open. One disadvantage of this valve is that even minor angular displacement causes a significant change in flow.

29. What are three-way valves used for?

Three-way control valves are only used in special systems where a controlled ratio of flow dividing or mixing is required.

30. What are the various types of plugs?

Contoured plug

V. port plug

31. Define V-port Plug?

V-port plug: A ported plug is commonly found on double-seated valves. This is due to the fact that ported plugs have a more consistent offbalance area.

32. Define Contoured plugs

Contoured plugs are typically used on single-seated valves with small trim sizes.

33. What is a cage valve?

A cage valve employs a piston with a piston ring seal connected to a single seated valve “plug.” The hydrostatic forces acting on the top of the piston or below the valve plug tend to cancel out in this situation. A cage holds the seat ring in place. Cage valves are commonly used to reduce noise.

34. Define Camifex valve?

Camifex valves are a type of valve that sits between a globe valve and a butterfly valve. For full opening, the plug rotates 60′.

35. Advantages of Camifex valve?

The required actuator forces are extremely low.

Extended bonnet allows for use on any service, including high and low temperatures.

Flow variations

Low weight

36. What is the purpose of the link attached to the valve positioner?

The link acts as a feedback loop to the value. This link detects ant valve movement. The valve position may change due to changes in line pressure on H.P. service; the link detects this change and the positioner produces an output that returns the valve to its original position.

37. What are booster relays used for?

Booster relays are essentially self-contained pressure regulators for air lines.

38. Classification of booster relays?

They are divided into three categories:

a. Volume Boosters: These are devices that increase the available volume of an air signal.

b. Ratio Relays: Used to multiply or divide an input signal’s pressure.

c. Reversing Relays: These generate a decreasing output signal in response to an increasing input signal.

39. Angle valves used for?

Angle valves are used in extreme conditions where very high pressure drops are required. Where erosion would damage a conventional type of valve.

40. What are the various valve characteristics?

The various valve characteristics are as follows:

a. Linear

b. Percentage of the total

c. Rapid opening.

41. Difference between linear, equal percentage, and quick opening?

Linear: The relationship between valve opening and flow rate is linear.

Equal percentage: For equal increments of valve opening, the flow rate range will be equal. The flow will be small at small openings.

Quick opening: The flow rate increments are greater at small openings. The flow rate becomes constant as the opening increases.

42. Where is an Air to close control valves used?

a. Reflux lines.

b. Cooling water lines.

c. Safety Relief services.

43. Where is an Air to open control valves used?

a. Feed lines.

b. Steam Service.

44. What exactly is a constant voltage unit?

A rectifier, CR, and a filter capacitor comprise the constant voltage circuit. Then there are two stages of zener regulation. A bridge configuration is provided to control the line voltage zener. Regulations 1 and 2 both provide relatively constant current. Resistors form a bridge, which can modify line voltage effects.

45. Define cascade control valve?

Cascade refers to the connection of two controllers in series. The first is the primary or master controller, and the second is the secondary or slave controller. The secondary controller’s output drives the final control element.

46. What exactly is Furnace Draft Control?

Negative furnace pressure is commonly used in balanced draught boilers. When both forced draught and induced draught are used together, the pressure in the system will reach atmospheric pressure at some point. To prevent hot gas leakage, the furnace pressure must be negative.

However, excessive vacuum in the furnace causes heat loss through air infiltration. The most desirable condition is to have a very slight (about 0. 1 ” H20) negative pressure at the furnace’s top.

47. What exactly is anti-surge control?

The ratio of compressor pressure rise to inlet flow rate is used to set the flow in the by-bass loop in this method of surge control. When the suction pressure drops and the discharge pressure rises, the compressor begins to surge. The PDT detects this and sends a signal to the FRC, which opens the by-pass valve.

48. Define Surge?

When the discharge head of a turbo compressor cannot be sustained at the available suction flow, surge occurs.

Surge occurs at specific head and flow combinations defined by the compressor manufacturer’s performance curves.

49. Define the term “spectroscopy.”

The measurement and interpretation of radiation emitted, scattered, or absorbed by various atoms, molecules, and other chemical species is known as spectroscopy.

50. Define the term chromatography.

Chromatography is defined as the physical and chemical separation of a mixture’s various components into pure fractions or bands of each component.

300+ Instrumentation Engineering Interview Questions – Part 1

300+ Instrumentation Engineering Interview Questions – Part 2

300+ Instrumentation Engineering Interview Questions – Part 3

300+ Instrumentation Engineering Interview Questions – Part 5

300+ Instrumentation Engineering Interview Questions – Part 6

Rabert T

As an electrical engineer with 5 years of experience, I focus on transformer and circuit breaker reliability in 110/33-11kV and 33/11kV substations. I am a professional electrical engineer with experience in transformer service and maintenance. I understand electrical principles and have expertise troubleshooting, repairing, and maintaining transformers, circuit breakers, and testing them. Tweet me @Rabert_infohe

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