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Top 30 Instrumentation Interview Questions

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process control and instrumentation interview questions

Are you a graduate looking to pursue a career in an emerging field such as electrical engineering, robotics, or electrical designing? Then you need to learn the process control and instrumentation questions bit prior to an interview. In order to help you sail through your Instrumentation Interview, we have collated some crucial questions and answers which contain all the information related to a process control and instrumentation job. Most industries like steel, chemical, power production and oil and petrol comprise of a dedicated process control and instrumentation department. This department hires specialist engineers.

Read Data Science Interview Questions for 2020 here.

One you apply to a process control and instrumentation job, you will receive an interview call. After clearing the same, you will get a chance to design, research and develop new devices which can be used to monitor and control various industrial operations. Here are some Instrumentation Interview Questions to help you crack a job interview with ease.

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.

instrumentation and process control interview

Instrumentation and process control interview questions for freshers

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.

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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.

11. What is the purpose of an Orifice Tab?
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.

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12. What is Bernoulli’s Theorem and 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.

13. 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.

14. 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.
Calibration Procedure:

15. How to adjust 20 psi air supply to a transmitter?
1.Vent the L.P. side to the atmosphere.
2.Connect the output of an Instrument to a standard test gauge. Adjust the zero.
3.Apply required pressure to high pressure side of the transmitter and adjust the span.
4.Adjust zero again if necessary.

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

Glycol is the seal liquid.

17. 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.

18. Name different test inputs.
Step, Ramp, Impulse, Sinusoidal, Pulse inputs are various step inputs.

19. 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.

20. 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.

21. 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.

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22. Explain temperature measurement using thermocouple?
A thermocouple is an electrical device consisting of two dissimilar electrical conductors forming electrical junctions at differing temperatures. It can produce temperature-dependent voltage due to a thermoelectric effect, and this voltage can be interpreted to measure temperature. The standard configuration for thermocouple usage is shown in the figure.

Image result for standard configuration for thermocouple usage

PC: Wikiwand

This is a K-type thermocouple (chromelalumel) and has a standard thermocouple measurement configuration. The measured voltage \scriptstyle V can be used to calculate temperature \scriptstyle T_\mathrm{sense}, provided that temperature \scriptstyle T_\mathrm{ref} is known.

\boldsymbol \nabla V = -S(T) \boldsymbol \nabla T,

The standard measurement configuration shown in the figure shows four temperature regions and thus four voltage contributions:

  1. Change from \scriptstyle T_\mathrm{meter} to \scriptstyle T_\mathrm{ref}, in the lower copper wire.
  2. Change from \scriptstyle T_\mathrm{ref} to \scriptstyle T_\mathrm{sense}, in the alumel wire.
  3. Change from \scriptstyle T_\mathrm{sense} to \scriptstyle T_\mathrm{ref}, in the chromel wire.
  4. Change from \scriptstyle T_\mathrm{ref} to \scriptstyle T_\mathrm{meter}, in the upper copper wire.

The measured voltage turns out to be

V = \int_{T_\mathrm{ref))^{T_\mathrm{sense)) \left( S_{+}(T) - S_{-}(T) \right) \, dT,

where \scriptstyle S_{+} and \scriptstyle S_{-} are the Seebeck coefficients of the conductors attached to the positive and negative terminals of the voltmeter, respectively (chromel and alumel in the figure).

Credits: Wikiwand

23. 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.

24. 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.

25. 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.

26. 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.

27. 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.

28. 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.

29. What is the relationship between proportional band and proportional gain?
The reciprocal of proportional gain can be expressed as a percentage and this is called proportional band. The formula is Kp=100/PB or 

where PB is the proportional band.

30. Why is the electronic controller preferred over pneumatic controller?


PC: United Trades

Pneumatic vs. Electronic Controls: Basically electronic signals operate over great distances without any time lag. These electronic signals can be made compatible with digital controllers. Electronic devices can be designed to be almost maintenance free as intrinsic safety techniques eliminate electrical hazards. Also, they are less expensive to install and more energy efficient. Due to these positive attributes and favorable properties, electronic controllers are preferred over pneumatic controllers.

Hope the above instrumentation interview questions help you prepare for an instrumentation engineering interview or a process control and instrumentation interview. Crack it like a pro! All the best.

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