Top 50 Instrumentation Engineering interview questions

1. What is Process control?

An engineering mechanism, process control uses operational variables like temperature, pressure, chemical content and algorithms, to continuously monitor industrial processes. It then uses all this information to adjust variables and finally reach product output specifications and objectives. Such process control systems always aim to achieve a process variable at the desired quantity, so that the system is said to be in controlled.

2. What is transmitter?

A transmitter is an electronic device that is generally mounted in the field in close proximity to a sensor. The sensor (also known as a transducer) measures a physical variable such as temperature or pressure and outputs a very low-level electronic signal. The basic function of the transmitter is to provide the correct electrical power to turn on (or excite) the sensor then to read the low-level sensor signal, amplify it to a higher level electrical signal and send that signal a long distance to a control or read-out device.

3. What is a “Smart” Transmitter?


A “Smart” transmitter is also called a microprocessor-based transmitter. It is the core of electronics in a device. Smart transmitters are used to communicate and also for the calibration process in remote areas. A sensor signal transmitter is a type of smart transmitter which sends a unified standard signal: 0/4-20mADC, 1-5VDC, 0-10VDc. The output control signal is standard in the form of 4-20mA. A good smart transmitter is genetally insensitive to noise.

4. What are the process variables and what are their units of measurement?

Flow, Pressure, Level, Temperature, Centigrade and Quality are process variables.

Their units of measurement are: 1. FLOW is measured in Kg I hr, Litter I min, Gallon I min. M3 I NM3 I HR. (GASES). 2. PRESSURE is the force acting per unit Area. The formula is P = F/A. Its units are Bar, Pascals, Kg, CM and Pounds. 3. LEVEL is the difference between 2 heights. Its units are Meters, mm, cm, and %. 4. TEMPERATURE is the degree of hotness or coldness of a body. Its units are Degree Centigrade, Degree Fahrenheit, Degree Kelvin, and Degree Rankin. 5. QUALITY basically deals with the analysis of PH, % of C02, % 02, conductivity and viscosity.

5. What Are The Primary Elements Used for Flow Measurement?

Orifice Plate, Venturi tube, Pitot tube, Annubars, Flow Nozzle, Weir & Flumes are used to measure flow.

6. What Are The Different Types Of Orifice Plates And State Their Uses?

types of orifice plates are Concentric, Segmental, and Eccentric.

7. Difference between 2 wire, 3 wire and 4 wire transmitter.

In a 2 wire transmitter, power and signal are transmitted through one common cable. In a 3 wire transmitter, data signal and power are always proportional to the common ground. In a 4 wire transmitter, 2 wires are used for power supply and two separate ones are used for signals.

8. Why 4-20 mA preferred over 0-20 mA signal?

In a 4-20mA signal, 4 would represent the minimal value and 20mA would represent the maximun value within a signal. With 0-20 mA, one cannot distinguish between minimum field value and connection breaks. In both the scenarios, the output will be 0mA.

In a 4-20 mA internal circuit, one can easily distinguish between connection breaks of minimal value. Normally when the value is minimum, a transmitter can render 4 mA while in case of connection breakage, it can render 0 mA.

9. What is Actuator ?

Within a closed-loop control system, the part of a final control element basically translates the control signal into an actionable element by the help of control device.

10. How to identify the Orifice in a Pipe Line?

An orifice tab is generally welded onto the orifice plate, that can extend the line. This is an indication of the orifice plate’s presence.

The purpose of an orifice tab is to indicate that the orifice plate stays in line and the orifice diameter is generally marked on top of it. Basically the material of an orifice plate has a tag no. on the orifice plate. It also has a mark which is the inlet of an orifice.

11. What is Bernoulli’s Theoremand where is it applicable?

Bernoulli’s theorem states the “Total energy of a liquid flowing from one point to another remains constant.” It is applicable for non compressible liquids.

12. How Do You Identify The H. P. Side Or Inlet Of An Orifice Plate In Line?

The marking is always done on the H. P. side of an orifice tab which gives an indication of the H. P. side.

13. How to Calibrate a D. P. Transmitter?

The following steps need to be followed while calibrating a D.P. Transmitter.

1.Adjust zero to X mtr.
2.Static pressure test: Give equal pressure on both sides of the transmitter. Zero should not shift. If it is shifting carry out static alignment.
3. Vacuum test: Apply equal vacuum to both the sides. The zero should not shift.

14. How to adjust 20 psi air supply to a transmitter?

  • Vent the L.P. side to the atmosphere.
  • Connect the output of an Instrument to a standard test gauge. Adjust the zero.
  • Apply required pressure to high pressure side of the transmitter and adjust the span.
  • Adjust zero again if necessary.

15. What is the Seal Liquid used for filling impulse lines on crude and viscous liquids?

Glycol is the seal liquid.

16. What do you mean by self regulation?

Self regulation means the output moves from one steady state to another steady state for a sustained change in input. This means for change in some input variable, the output variable will rise until it reaches a steady state so that inflow = outflow. The process adopts a specific value of controlled variable for nominal load with no control operations though this method.

17. Name different test inputs.

Step, Ramp, Impulse, Sinusoidal, Pulse inputs are various step inputs.

18. Define interacting systems with an example.

In an interacting system, the load changes within the first process and affects the second process and vise versa. Once both are connected in a series, it is called an interacting system. Eg. Two level tanks are connected in series.

19. Define thermocouple?

It is a junction of two materials used to convert heat into electrical energy. When a pair of wires made up of two completely different metals, are joined at one end, a temperature difference can produce a voltage proportional to the heat.

20. What are the advantages of a thermocouple?

It has a rugged construction, and the temperature range lies between -270 degree Celsius to -2700 degree Celsius. This couple is cheap and highly reproducible.

21. How to check a gas monitor without test gas?

Some gas monitors are provided with a self-test facility to check its alarm and shutdown functions (levels).

22. What are the two characteristics of a first order process modeling?

The smaller the value of the time constant, steeper would be the initial response of the system. A first order lag process is self regulating the ultimate value of the response = Kp or steady state gain of the process for a unit step change in the input.

23. Define the function of a controller.

A controller is an instrument used to control a process variable for measurement. It’s job is to monitor the error signal continuously and give a corrective output to the final control element.

24. Define Process control.

It is an engineering mechanism which describes how much the manipulated variable should change in order to remain controlled. It continuously monitors an industrial process‘ operational variables.

25. What is mathematical modeling.

It is a set of equations that characterize the process. It is also the art of translating problems from an application area into tractable mathematical formulations with a theoretical and numerical analysis, which can provide an insight, answers, and guidance useful for the originating application.

26. Mention two drawbacks of derivative action.

Derivative control itself is a crude prediction of the error in future, based on the current slope of the error and the time of error is measured with the Td. Some of its drawbacks are: (i) The output of controller is zero at constant error condition.
(ii) It will amplify the noise present in the error signal.

27. What are the steps involved in designing the best controller?

In order to design a fool proof controller, one has to define the appropriate performance criteria (ISE, IAE, and ITATE). Then they need to compute the value of these performance criteria using P, PI, or PID controller with the best settings for all adjusted parameters (Kp, Ti, and Td). Now one can select the controller which gives the best value for the performance criteria mentioned above.

28. Explain the operating principle of a gas monitor?

Gas monitor measures the imbalance in the current loop caused by its active and in-active filaments in the presence of a combustible gas.

29. What is the important factor set on a gas monitor loop?

Depending on the manufacturer, either it is the loop current or the voltage at the sensor head which must be set precisely for better accuracy and sensitivity.

30. What is the constant voltage unit?

The constant voltage circuit consists of a rectifier, CR and a filter capacitor. It is followed by two stages of zener regulation. Abridge configuration is provided to lamp line voltage zener regulation. Regulation 1 and regulation 2 provides relatively provide constant current. Resistors form a bridge that may remoment line voltage effects.

31. Explain the burnout feature.

Burnout provides the warnsug feature of driving indicator at the end of scale, if the input circuit is open. A burnout resistor is provided which develops a voltage drop between the measuring circuit and amplifier. The polarity of the signal determines the direction of the servo drive upon an open circuit in the input.
Upscale burnout: R value 10 M
Downscale burnout: R value 22 M.

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

In closed tank, bottom of the tank is connected to the high pressure side of the transmitter. Top of tank is connected to the lower pressure side of the transmitter. In this way vessel pressure can be measured.

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

In open tank the lower pressure side is vented to the atmosphere. All pressure is applied to the high pressure side. This vessel pressure is measured through high pressure side.

34. Explain the working of an electronic level troll.

The variation in level of buoyancy resulting from a change in liquid level varies the net weight of the displacer increasing or decreasing the load on the torque arm. This change is directly proportional to change in level and specific gravity of the liquid. The resulting torque tube movement varies the angular motion of the rotor in RVDT providing a rotor change proportional to the rotor displacement, which is converted and amplified to a D.C. current.

35. Explain the working of an enraf level gauge.

Enraf level gauge is based on the ser powered null balance technique. A displacer serves as continuous level sensing element. A two phase ser motor controlled by a capacitive balance system winds unwinds the the measuring wire until the tension in the weight springs is in balance with the weight of the displaced part immersed in the liquid. The sensing system in balance measures the two capacitance formed by the moving central sensing rod provided by the two capacitor plates and the si plates.

36. Why thermo wells are used? What materials are used in thermo wells?

In numerous applications, it is neither desirable nor practical to expose a temperature sensor directly to a material. Wells are therefore used to protect against damage
corresion, arosion, aborsion and high pressure processes. A thermo well is also useful in protecting a sensor from physical damage during handling and normal operations.
Materials used in thermo wells: Stainless steel, Inconel, Monel, Alloy Steel, Hastelloy

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

Different orifice plates are: 1. Concentric 2. Segmental 3. Eccentric
– Concentric: These plates are used for ideal liquid as well as gases and steam service. Concentric holes are present in these plates, thats why it is known as concentric orifice.
– Segmental: This plate has hole in the form of segment of the circle. This plate is used for colloidal and sherry flow measurement.
– Eccentric: This plate has the eccentric holes. This plate is used in viscous and sherry flow measurement.

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

An orifice tab is welded on the orifice plate which extends out of the line giving an indication of the orifice plate.

 39. How is flow measured in square root?

Flow varies directly as the square root of pressure. Thus, F=K of square root of applied pressure. Since this flow varies as the square root of differential pressure. The pressure pen does not directly indicate flow. Thus flow can be determined by taking the square root of the pen. Assume the pen reads 50% of the chart. So, flow can be calculated using the pen measure in the chart.

40.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.

41. 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.

42. 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.

43. 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

44. 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.

45. 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

46. 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.

47. 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

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

In automatic reference junction compensation, variable nickel resistor is used. As the temperature changes, so does its resistance. This reference junction compensator is located, so that it will be at the temperature of the reference junction. The reference junction is at the poset where the dissimilar wire of the thermocouple is rejoined. This joint is invariably at the terminal strip of the instrument.

49. What are de-saturators?

When, in some processes, e.g. batch processes, long transient responses are expected during which a sustained deviation is present the controller integral action continuously drives the output to a minimum or maximum value. This phenomenon is called ‘integral saturation of the control unit’. When this condition is met, then this unit is de-saturated.

50. Explain the working of Rota meter.

Variable area meters are special form of head meters. Where in the area of flow restrictor is varied. So as to hold the differential pressure constant. The rota meter consists of a vertical tapered tube through which the metered fluid flows in upward direction. As the flow varies the ‘float’ rises or falls to vary the area of the passages that the differential across it balances the gravitational force on the ‘float’. The differential pressure is maintained constant. The position of the ‘float’ is the measure of the rate of flow.

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

An electric potential is developed when a conductor is moved across the magnetic field. In most electrical machinery the conductor is a wire. The principle is equally applicable to a moving, electrically conductive liquid. The primary device of commercial magnetic meters consist of straight cylindrical electrically insulated tube with a pair of electrodes nearly flush with the tube walls and located at opposite end of a tube diameter. This device is limited to electrically conducting liquids. The magnetic meter is particularly suited to measurement of slurries and dirty fluids.

52. Explain the mechanism behind the turbine meter.

Turbine meters consist of straight flow tube within which a turbine or fan is free to rotate about it s axis which is fixed along g the centre line of the tube. Mostly, a magnetic pick up system senses the rotation of the rotor through the tube walls. The turbine meter is a flow rate device
, since the rotor speed is directly proportional to the flow rate. The output is usually in the form of electric pulses from the magnetic pick up with a frequency proportional to the flow rate.

53. How would you choose differential range?

The most common range for differential range for liquid measurement is 0-100. This range is high enough to minimize the errors caused by unequal heads in the seal chambers. It is also dependent on the differences in the temperature of the load lines. The 100 range permits an increased in capacity up to 400. While decrease down up to 20 by merely changing the range tubes or range adjustments.

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