Instrumentation and Control Engineering Interview Question and Answers

 Instrumentation and Control Engineering Interview Question and Answers

1. What Is Instrumentation Control?
Answer: In addition to measuring field parameters, instrumentation is also responsible for providing the ability to modify some field parameters.
Some examples include:- Device Field Parameter(s) Valve Flow, Pressure Relay Voltage, Current Solenoid Physical Location, Level Circuit breaker Voltage, Current.
2. What Is Instrumentation Engineering?
Answer: Instrumentation engineering is the engineering specialization focused on the principle and operation of measuring instruments which are used in design and configuration of automated systems in electrical, pneumatic domains etc. They typically work for industries with automated processes, such as chemical or manufacturing plants, with the goal of improving system productivity, reliability, safety, optimization and stability.

3. What Is Instrumentation?
Answer: The technology which is used to measured and control the process system of plant is called instrumentation.
4. What Is Instrumentation Measurement?
Answer: Instrumentation can be used to measure certain field parameters (physical values)
These measured values include
  • pressure, either differential or static
  • density
  • viscosity
  • radiation
  • current
  • voltage
  • inductance
  • capacitance
  • frequency
  • resistivity
  • conductivity
  • chemical composition
  • chemical properties
  • various physical properties
5. What Is Reynolds Number?
Answer: We know with help of reynolds number what type of flow in fluid.
6. Why Do We Use 4-20ma Signal Instead Of 0-10 Ma?
Answer: To elevate zero so that we can come to know whether it is dead zero or from signal.
7. What Is The Difference Between Dry Leg And Wet Leg?
Answer: Dry leg means in lab. And wet leg means in feild
8. How Can We Calibrate A Positioner?
Answer: A positioner is a device put into a valve to ensure that it is at a correct position of opening as per the control signal. An I/P converter only sends the opening/closing request to valve but can not confirm its position. Positioner senses the valve opening through a position feedback link connected to valve stem which is its input signal. I/P converter output is its setpoint input. The difference between these two is the error signal based on which the positioner positions the valve to correct position to reduce error to zero. Hence positioner is nothing but a pneumatic feedback controller. Controlled external supply air to positioner provides power to positioner to position.a valve. Also positioner is used in a valve when valve operating signal range is different from I/P converter output range.
9. Explain What Is The Working Principle Of The Magnetic Meter?
Answer: 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 electrode 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.
10. Tell Me The Mechanism Behind The Turbine Meter?
Answer: 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.
11. How To Choose Differential Range?
Answer: 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.
12. What Is The Working Of Rota Meter?
Answer: 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.
13. Explain What Are De-saturators?
Answer: 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.
14. Do You Know How Is Automatic Reference Junction Compensation Carried Out In Temperature Recorders?
Answer: 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.
15. How To Test A Transistor With A Multimeter?
  • Emitter +ve of meter and base -ve output =Low resistance
  • Emitter -ve of meter and base +ve output =High resistance
  • Collector +ve and base -ve output =Low
  • Collector -ve and base +ve output =Low
Emitter: Collector = High resistance
PNP: Opposite Results
16. What Is Motion Balance Principle?
Answer: 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.
17. What Is Force Balance Principle? State Some Of Its' Advantages?
Answer: 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.
  • Moving parts are fewer.
  • Baffle movement is negligible
  • Frictional losses are less
18. What Is Zener Diode?
Answer: 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.
19. What Is Voltage Regulator?
Answer: Voltage regulator: The output remains constant despite changes in the input voltage due to zener effect.
20. What Is Intrinsically Safe System?
Answer: 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.
21. What Is Furnace Draft Control?
Answer: 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.
22. What Is Ratio Control System?
Answer: 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.
23. What Are Different Types Of Control Valves?
Answer: The commonly used control valves can be defined as follows
a. Depending on Action:- Depending on action there are two types of control valves
  • Air to close
  • Air to close
b. Depending on body:- Depending on body there are 4 types of control valves
  • Globe valves single or double seated
  • Angle valves
  • Butterfly valves
  • Three way valves
24. What Is Solenoid Valve? Where It Is Used?
Answer: 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
  • Normally Open
  • Normally closed
Use:- It is used for safety purpose in different electric work
25. What Is The Use Of Single Seated Valve?
Answer: 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.
26. What Is The Use Of Double Seated Valve?
Answer: 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.
27. What Is The Function Of 3-way Manifold Valve?
Answer: excessive pressure to your differential pressure transmitter, you could damage your instrument. This is known as over-ranging the transmitter. A three-way manifold valve is a device that prevents the instrument from being over ranged. It also allows the isolation of the transmitter from the process loop (an option which could be used generaly for maintenance or re-calibration or fitting new equipment).
28. Can We Use A Control Valve Without Positioner?
Answer: Control valve can not be without positioner. The purpose of the positioner is to control the control valve stroke so as to keep the valve in desired position. The positioner receives signal from the controller, and send the boosted signal to the actuator to reach the desired position as and when the valve reaches the desired position the positioner cuts the boosted signal to the actuator and keeps the position.

29. Why 4-20 Ma Signal Is Preferred Over A 0-10v Signal?
Answer: Generally in a transistor some amount of voltage is required for turning it on. This voltage is the cut in voltage. Up to this voltage the exists a nonlinearity in its characteristics. Beyond this cut in voltage the char. of transistor is linear. This nonlinear region lays between 0-4ma, beyond 4ma it linear. This is why 4-20 ma range is used.
30. What Is Remote Mount With Mta Option? (in Case Of Dual Sensor Vortex Flowmeter, Rosemount Has Written In Its Catalog Drawings)?
Answer: Remote mount with MTA means MTA connector for mounting remote connection. MTA is the Connector used for connection. Generally we called it MTA Connector. There are many types of MTA connector such as MTA 100connectors, Mta 156 connectors etc.
31. How To Write The Program For Single Push Button By Using Ladder Diagram?
  • Create a Push button
  • Connect a Counter series to PB
  • Create a compare button
  • Initialize 1 at one end
  • Counter output in another
  • Connect the Coil which has to be energized
  • Initialize 2 at one end
  • Counter output in another
  • Connect the Coil which has to be reenergized.
32. What Is Plc?
Answer: PLC means Programmable Logic Controller. It is a class of industrially hardened devices that provides hardware interface for input sensors and output control element. The field I/p include element like limit switches, sensors, push button and the final control elements like actuator,  solenoid/control valves, drives, hooters etc. PLC Senses the input through I/P modules, Processes the logic through CPU and memory and gives output through output module.

33. Applications Of Plc?
Answer: PLC can be used in almost all industrial application solutions right from small machine to large manufacturing plants. Even it caters applications of redundant systems at critical process plants.
34. Explain The Role Of Plc In Automation ?
Answer: PLC plays most important role in automation. All the monitoring as well as the control actions are taken by PLCs. PLC Senses the input through I/P modules, Processes the logic through CPU and memory and gives output through output module.
35. Explain The Role Of Cpu ?
Answer: This component act as a brain of the system. CPU consist of Arithmetic Logic Unit, Program memory, Process image memory, Internal timers and counters, flags It receives information from I/P device, makes decisions depending upon the information and logic written and sends information through the O/P devices. 
The CPU’s are distinguished with following features
  • Memory capacity
  • Instruction set supported
  • communication option
  • time required to execute the control program.
36. Explain The Role Of Power Supply In Plc System ?
Answer: Power supply provides system power requirement to processor, I/O and communication modules. Typically the power supply has input voltage 120 V – 230 V AC or 24 V DC and back plane output current 2 A to 5 A at 5 V DC
37. Explain The Role Of Rack Or Chassis In Plc System ?
  • A hardware assembly, which houses the processor, communication and I/O modules.
  • It does following functions.
  • Power distribution
  • Containment of I/O modules
  • Communication path between I/O module and CPU
  • The chassis are available in different slots in various PLC systems. Additional chassis can be connected using chassis interconnecting cable.
38. What Is Role Of I/o Modules?
Answer: Electronic plug in units used for interfacing the i/p and o/p device in the machine or process to be controlled. I/P module receives data from i/p devices (Pushbutton, Switches, Transmitters) and send it to processor. The O/P module receives data from processor and send it to output device  (Relay, Valves).
Digital/Discrete :- Sends and Receives On/Off signal
Analog :- Sends and receives variable input or output signals
39. Explain The Role Of Eeprom Memory Module?
Answer: This module is inserted into processor system for maintaining a copy of project (PLC program). This is helpful in case of memory corruption or Extended power loss.
40. Difference Between Fixed And Modular Plcs?
  • In non modular PLCs the processor will have inbuilt power supply and I/Os in one unit.
  • The modular PLC, will have separate slots for components like Power supply, I/O modules.
  • You can select the I/Os or power supply as per the need.
41. What Are The Types Of I/os?
Answer: Local:-  These are the I/Os placed in the PLC main rack containing CPU. These I/Os are connected to CPU through backplane.
Distributed/Remote:-  These are the I/O placed at remote location from the main rack containing the CPU. These I/O’s are to be connected on communication bus like control net, device net or FIP I/O.
42. What Is Meaning Of Resolution In I/o Cards In Plcs ?
Answer: It is the minimum change in i/p parameter which can sensed by the i/p card. As far as Digital I/O is concerned it takes only one bit for operation. In case of analog input the resolution determines how much bits are used for input or output. For example a 12 bit resolution card means the input will come as 0 to 4095 count (2^12). For 16 bit data the counts will be from 0 – 65536(2^16). More the resolution the data will be more accurate
43. What Is An Analog Input Module ?
Answer: An I/O module that contains circuits that convert analog input signals to digital values that can be manipulated by the processor. The signals for pressure, flow, level, temperature transmitters are connected to this module. Typically the input signal is 4-20 mA, 0-10 V
44. What Is Analog Output Module ?
Answer: An I/O module that contains circuits that output an analog dc signal proportional to a digital value transferred to the module from the processor. By implication, these analog outputs are usually direct (i.e., a data table value directly controls the analog signal value).
45. What Is Meaning Of Universal Analog Input Card?
Answer: Normally there are different cards for different signals. But in universal input card the same channels can be configured for RTD, Thermocouple, Current or voltage input.
46. Give Examples Of I/p And Output Connected To Plcs?
  • Digital I/P (Pushbutton, Switches)
  • Analog I/P (Temp, Pressure, Flow, Level)
  • Digital O/P (Solenoids valves, Contactors)
  • Analog O/P (Control Valves, Speed)
47. Explain Source And Sink Concept ?
Answer: Sinking Source type modules gives out the current to the field digital devices while sink type modules draw current when the device is in high state
Sinking:- When active the output allows the current to flow to a common ground.
Sourcing:- When active, current flows from a supply, through the output device and to ground.
48. What Is Forcing Of I/o ?
  • Forcing the I/Os means making the desired status of I/O in PLCs irrespective of its status coming from the field.
  • In certain cases when there is problem in receiving field input /output, we can force the i/Os so that the logic takes desired state.
49. What Is Meaning Of Scan Time In Plc?
Answer: Scan time is the Time required to read the I/P, Process the logic and update the output in one cycle.
50. What Is Typical Scan Time In Plcs ? What Affects Scan Time ?
Answer: Typically it is less than 10 ms. It depends on the complexity of logic, PID algorithm etc.
51. How To Program Plcs ?
Answer: Every PLC manufacturer have their own software for programming the PLC. For example Siemens uses Simantic S7 Manager, Allen Bradley uses RS Logix and Modicon uses PLC pro programming software. The programming language used is
  • Ladder Logic (LD),
  • Statement List (STL),
  • Functional Block Diagram (FBD),
  • Sequential Function Chart (SFC),
  • Instruction List (IL) etc
52. What Is Ladder Diagram ?
Answer: This is a programming language, which expresses a program as a series of “coils” and “contacts”, simulating the operation of electromechanical relays. The resultant program is the equivalent of an equation, which is executed continuously in a combinatorial manner. The advantage of this language is the familiarity many electricians have with the simple operation of relays. Disadvantages include the complexity of large, cross-connected programs, and the difficulty of expressing such non-binary functions as motion control and analog I/O
53. Do You Know How Do You Identify An Orifice In The Pipeline?
Answer: An orifice tab is welded on the orifice plate which extends out of the line giving an indication of the orifice plate. 
54. Explain What Are Different Types Of Orifice Plates? State Their Uses?
Answer: Different orifice plates are
  • Concentric
  • Segmental
  • 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.
55. Name Different Types Of Bourdon Tubes?
Answer: Types of bourdon tubes
  • C type
  • Spiral
  • Helix
56. What Are Primary Elements Of Measuring Pressure? Which Type Of  Pressure Can Be Measured By These Elements?
Answer: Primary elements of measuring pressure are
  • Bourdon Tube
  • Diaphragm
  • Capsule
  • Bellows
  • Pressure springs
These elements are known as elastic deformation pressure elements.
57. What Is The Use Of Valve Positioner?
Answer: Valve positioner can be used for following reasons
  • Quick action
  • Valve hysterisis
  • Viscous liquids
  • Split range.
  • Line pressure changes on valve
  • Bench set not standard
  • Reverse valve operations
58. Name Different Parts Of A Pressure Gauge Explain The Use Of Hair Spring In The Pressure Gauge?
Answer: Pressure gauge includes following components
  • ‘C' type bourdon tube.
  • Connecting link
  • Sector gear
  • Pinion Gear
  • Hair spring
  • Pointer
  • Dial
Use of hair spring: Hair spring is responsible for controlling torque. It is also used to eliminate any play into linkages.
59. Tell Me How Is Flow Measured In Square Root?
Answer: 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.
60. Explain How Can A D.p. Transmitter Be Calibrated?
Answer: D.P. transmitter can be calibrated using following steps
  • Adjust zero of Xmtrs.
  • Perform static pressure test:- Give equal pressure on both sides of transmitter. Zero should not shift either side. If the zero shifts then carry out static alignment.
  • Perform vacuum test:- Apply equal vacuum to both the sides. Zero should not shift.
  • Calibration procedure:- Give 20 psi air supply to the transmitter and vent L.P. side to atmosphere. Connect output of the instrument to the standard test gauge. Adjust zero. Apply required pressure to the high pressure side and adjust the span. Adjust zero gain if necessary.