200+ instrumentation interview Q&A

1. What is Measurement?

This is a function that compares the unknown quantity to the standard, resulting in the knowledge of the unknown quantity’s quantity based on the standard used for comparison.

2. What is a 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 together and kept both the joints in two different temperatures, as a result, a voltage difference is produced proportional to the heat.

3. Types of Thermocouples

Type B

Type C

Type D

Type E

Type G

Type J

Type K

Type N

Type R

Type S

Type T

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4. Working Principle of Thermocouple?

The principle of thermocouple depends on the three effects;

See beck, Peltier, and Thompson.

See beck-effect

This type of reaction occurs when two metals with different properties are mixed. When heat is applied to any of the metal wires, electrons flow from the hot metal wire to the cold metal wire. As a result, direct current energizes the circuit.


Peltier’s effect is the total opposite of See beck’s. This effect states that by applying a potential difference between two dissimilar conductors, a temperature difference can be formed.


The voltage induces the total conductor’s length due to the temperature gradient as two disparate metals fix together and form two joints, according to this effect. This is a physical term that describes the rate and direction of temperature change at a specific location.

5. Advantages of thermocouple

• High-temperature range (-180 to 2320 °C)

• Inexpensive

• Large variety

• Large temperature range

• Rugged

• Self-powered

6. Disadvantages of thermocouple

• Non-linear

• Low voltage

• Require reference

• Less stable

7. What is RTD?

RTD stands for Resistance Temperature Detector. RTDs are sensors used to measure temperature

8. Types of RTD?

• Wire-Wound RTD

• Thin Film RTD

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9. Working principle of RTD?

The relationship between the resistance and temperature of the metal is the working principle of RTD, which means that the resistance of a metal varies with temperature. The amount of change in the material’s resistance value caused by a per degree increase in temperature is measured, and the sensor is calibrated accordingly.

10. Difference between 2 wire, 3 wire, and 4 wire transmitters.

A 2-wire transmitter transmits both power and signal over a single cable. Power and data signal is always directly proportional to the common ground in a 3-wire transmitter but in a 4-wire transmitter, there is a dedicated 2- wire each for both the power supply and data signals

11. Difference between Thermocouple & RTD?

12. Advantages of RTD

• More accurate

• Linear output

• High sensitivity

13. Disadvantages of RTD

• Expensive

• Require external current source

• Small

• Small base resistance

14. What is Thermistor?

The term thermistor is derived from “thermal” and “resistor”. It is a kind of resistor whose resistance is proportional to the temperature. They’re made of metallic oxide that’s been molded into a bead, disc, or cylindrical shape, then covered in epoxy or glass.

15. Types of thermistors

• Positive Temperature Coefficient (PTC) Thermistor

• Negative Temperature Coefficient (NTC) Thermistor.

16. What is a Positive Temperature Coefficient Thermistor?

In a PTC thermistor, the resistance is directly proportional to the temperature that means the value of resistance increases when the value of temperature increases. These types of sensors are used in fuses for the protection of circuits

17. What is a Negative Temperature Coefficient Thermistor?

In an NTC thermistor, the resistance is indirectly proportional to the temperature that means the value of resistance decreases when the value of temperature increases and its conductivity increases. When temperature increases the electrons are formed and increase the conductivity of an object.

NTCs are used in temperature measuring devices.

18. Advantages of thermistor

• High sensitivity to the temperature

• Low cost

• Small in size

• Easy to dismantle

19. Disadvantages of thermistor

• Limited range in measurement

• Non-linear characteristics for NTC

20. What is Process control?

It is the ability of a device to monitor and analyze the process to get the desired outcome. It helps to keep the quality and performance in the industry.

21. What is thermowells?

Thermowells are used to protect thermocouple elements installed in industrial processes. It is cylindrical in shape.

22. Why thermowells are used?

In most cases, it is not practical to place a temperature sensor on material directly. In these cases, we use thermowells to protect the sensor from damage, corrosion, erosion, high pressure, etc. It also protects the sensor element from physical damages too.

23. What materials are used in thermowells?

Stainless steel, Inconel, Monel, Alloy Steel, and Hastelloy

24. Different type of Level measuring instruments?

• Mechanical or Direct Method

• Inferential or Indirect Methods

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25. What is Zero suppression and Zero elevation?

Zero elevation

The amount of the measured variable zero is above the lower range value for an elevated zero range. It can be expressed in percent of span or in units of measured variables.

Zero suppression

The amount of the measured variable zero is below the lower range value for a suppressed zero range. It can be expressed in percent of span or in units of the measured variable.

26. What is flow?

Flow is the volume per unit of time at a given temperature and pressure. Positive displacement or rate meters are commonly used to measure flow.

Units of flow are: kg / hr, litre / min, gallon / min, m3 / hr, Nm3 / hr.

27. What is temperature?

Temperature is the measure of hotness or coldness of an object expressed in terms of Fahrenheit or Celsius or Kelvin.

28. What is pressure?

Force acting per unit area.

Units: bar, Pascal, kg /cm2

29. What is the formula for calculating pressure?

P= F/A

P = Pressure

F = Force

A = Area

30. What is a differential pressure transmitter?

A differential pressure transmitter is a device that can create a pressure drop in a pipe consistently and accurately, then measure the pressure on both sides to determine the flow rate inside the pipe.

31. What are the types of pressure?

• Absolute

• Gauge

• Differential

32. What is absolute pressure?

The combined pressure of the atmosphere and the measured point is known as absolute pressure.

33. What is gauge pressure?

The difference between a measured point and the atmosphere

34. What are differential pressure switches?

Differential pressure switches are mechanical devices that detect the difference between two measured points.

35. Working principle of differential pressure switches

There are two ports in differential pressure switches. The differential pressure switch will analyze the pressure of the object when we placed it on the pressure source and it will either make or break the circuit after the pressure reaches a preset level. This will let the alarm ring and alert the user that the pressure has a variation or change.

36. Uses of differential pressure switches

Differential pressure switches are mainly used in the energy-saving method to regulate a device that is not working as it should. This can be used while in maintenance and other safety-required applications

37. Applications of differential pressure switches

• Air conditioners

• Determining the respiratory flow in medical devices

• Air proving

• Exhaust ducts

38. What is a float switch?

Float level switches are sensors that are used to detect the liquid level inside a tank

39. Types of float level switch

Horizontal float switch

Vertical float switch

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40. What is the difference between accuracy & precision?

Accuracy: It refers to how close an instrumentation reading comes to matching the true value of the quantity of the medium to be measured.

Precision: It is a measure of reproducibility

41. Explain Bernoulli’s theorem.

Bernoulli’s theorem states that the total energy of a liquid flowing from one point to another remains constant.

42. Applications of Bernoulli’s theorem


• Lift of an aircraft wing

• Blowing of roofs

• Bunsen burner

43. What are the primary elements used for flow measurement?

• Orifice Plate

• Venturi tube

• Pitot tube

• Flow Nozzle

• Weir

• Flumes

44. Electromagnetic flow meters are not suitable for which kind of fluids?

Non - conductive fluids.

45. Name different parts of a pressure gauge

• ‘C’ type bourdon tube.

• Connecting link

• Sector gear

• Pinion Gear

• Hair spring

• Pointer

• Dial

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

In a closed tank, the bottom of the tank is connected to the high-pressure side of the transmitter. The top of the tank is connected to the lower pressure side of the transmitter. Vessel pressure can be measured in this way.

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

The side with lower pressure is opened to the atmosphere in an open tank. so, the entire pressure is applied to the side with high pressure and measure the pressure through the high-pressure side.

48. Explain the working of the Rotameter.

Variable area meter is also a type of head meter. To hold the differential pressure constant there is a flow restrictor in variable area meters. The vertical tapered tube is present in the rotameter to make the fluid flow in the upward direction.

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

While a conductor moves through a magnetic field, there will be an electrical potential developed. The wire is used as a conductor in almost every piece of electrical machinery. This principle is also applicable for electrically moving conductive liquid.

The main device of a commercial magnetometer consists of an insulated straight cylindrical tube with a pair of electrodes that are approximately horizontal to the wall of the tube and located at the opposite end of the diameter tube. This device can be used only in electrically conducting liquid. The magnetic meter is mainly used in slurries and dirty liquids.

50. What is direct and indirect level measurement?

Direct means that the fluid level being measured is in direct contact with the sensor.


• Sight glass type

• Float type

• Magnetic level gauge

Indirect The level of a liquid is calculated by a variable that changes according to the level in the indirect method of level measurement.


• Pressure gauge type

• Differential pressure type

• Ultrasonic type

• Radar type

51. What is point and continuous level measurement?

A point-level sensor detects liquid levels at specific points in a tank. This is mostly used when a high or low level needs to be controlled.

A continuous level sensor is used to detect liquid levels continuously at every point in a chamber or tank. This is mainly used in processes where it is always very important to know the level and for applications where precision increased is necessary.

52. What is a flowmeter?

Flow meters are a device used to measure the rate of fluid flow rate. It can be used to detect the linear, non-linear, mass, or volumetric flow rate of both gas and liquid.

53. Types of flow meters

• Orifice Flow Meters

• Glass Tube Flow Meter

• Electromagnetic Flow Meter

• Coriolis Flow Meter

54. What are the different types of Orifice plates?

• Concentric

• Segmental

• Eccentric

55. State use of different orifice plates

• Concentric: This plate is ideal for liquid, gas, and steam service. As the name implies these plates have concentric holes

• Segmental: This plate has holes in the form of circular segments. This tray is used to measure the flow of colloids and sherry.

• Eccentric: This plate has unusual holes, and this plate is used to measure viscose and sherry flow.

56. Why is the orifice tab provided?

• A line indicating the orifice plate

• It has a mark on it that indicates the diameter of the orifice.

• The orifice plate’s material.

• The orifice plate’s tag number.

• To indicate the orifice’s inlet.

57. What is the disadvantage of orifice meters?

A permanent pressure loss.

58. Why orifice meters are preferred in industries?

Due to the low cost

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59. What is P&ID?

P&ID stands for Process and Instrumentation Diagram. It shows the process equipment’s interconnection that is used to control a process

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60. What are the documents and drawings used in Instrumentation?

• Process Flow Diagram

• Piping & Instrumentation Diagram (P&ID)

• Loop Diagram

• Functional Diagram

• Hoop-up drawing

• Instrumentation index

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61. How is flow measured in the square root?

The flow rate is directly related to the square root of the pressure. So, F = K is the square root of the applied pressure. Flow rate changes for the square root of the pressure drop.

62. What is impulse tubing?

Normally impulse tube is a small diameter tube or pipe made of stainless steel material used to transmit the pressure signal from the process tapping to the transmitter.

63. What are the different types of cabinets involved in control system design?

• System Cabinet

• Marshalling cabinet

• Intermediate cabinet

• Network cabinet

• Power Distribution cabinet

• Remote IO cabinet

• Remote RTU cabinets

• Fibre optic patch panel wall mount console cabinet

64. What is a Safety barrier?

A safety barrier is required to ensure if any failure in the safe/non-hazardous zone (control room) will not be able to produce enough energy to ignite the gaseous atmosphere in the hazardous zone. This Safety barrier becomes a crucial element in an Intrinsically safe circuit design.

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65. What is a Junction box?

A junction box is an electrical box that is used as an intermediate junction between field devices and marshaling panels. The basic elements of a junction box are terminal blocks.

66. What are the different types of signal connections?

• RF (Radio Frequency) Modulated Television

• Composite Video

• S-Video

• Component Video

• RGB and its variants: RGSB, RGBS, RGBHV:


67. What is conventional type signal connection?

In a conventional type connection, the signal from the field transmitter will be sent in the form of electromagnetic waves through copper wires to the field signal processor (like Field Terminal assembly) for further signal processing and connection to the control system. Normally the transmitted signal will be in the range of 4-20 mA.

68. What is the Foundation field bus connection?

Foundation Fieldbus (FF) is one of the new network communication protocols, which is used to connect field devices with the control system. This system uses a digital, multi-drop, serial two-way communication to connect field devices. This digital protocol enables two-way communication between field devices and the control system. Apart from the field signal we can calibrate, configure and pull various diagnostics information from the FF-based smart field transmitters and final control elements.

69. What is Trunk cable?'

The main H1 or HSE link single pair cable used to connect the control system with the Foundation Fieldbus junction box in the field is termed as Trunk Cable.

70. What is Spur cable?

The H1 or HSE link cable used to connect the field instruments such as transmitters, switches, valves, solenoid valve, etc., with Foundation Fieldbus junction box is termed as a Spur Cable.

71. What is Profibus connection?

Profibus, also known as Process Field Bus, is a field bus communication standard used in the automation field. A network designed for industrial computers that can handle a lot of noise. Different network topologies, such as the star, tree, and line, or a mix, are feasible with the two-wire Profibus cable.

72. What are the different types of communication protocols used in a control system environment?

• ControlNet

• DeviceNet

• DirectNet

• Ethernet Global Data (EGD)

• EtherNet/IP

• Ethernet Powerlink

• FOUNDATION Fieldbus – H1 & HSE

• HART Protocol

• Modbus Plus

• Profibus


• AS-i – Actuator-sensor interface

• BSAP – Bristol Standard Asynchronous Protocol

• CC-Link Industrial Networks

• CIP (Common Industrial Protocol)

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73. What is the difference between a Transducer and a Transmitter?

A transducer is a device that converts any form of field signal into an equivalent electrical signal.

A transmitter is a device that converts reading from a sensor or a transducer into a standard electrical signal and transmits that signal to a controller (DCS/PLC Controller).

74. Why is a 4-20mA analog signal preferable to a 0-20mA analog signal?

The following are the main reasons for using 4-20mA instead of 0-20mA:

• The value of 4mA is considered engineering zero. Due to this, 0.0mA is referred to as an invalid input or dead zero. As a result, 0mA to the analog input indicates that the circuit is open and no current is flowing.

• Wire breaks are easy to find with 4-20mA.

75. Why 20mA is used in 4-20mA signal connection?

As per Ohms law V= IR

Where V = Voltage, I = Current and R = Resistance

Above shown is the illustration of the conventional 4-20mA signal connection diagram. Due to line voltage drop issues, the voltage signal from the transducer is converted to a current signal inside the transmitter and transmitted through pair of copper wires to the controller input card, where a resistance of 250 ohms is connected across the wire to convert the signal back to a voltage signal of 1 – 5V DC level so that controller input card can read the value.

The signal conversion table is shown below


This is the reason why 20mA is used on the higher side of the current signal.

76. Why voltage signals are not used?

The voltage signal is more non-linear than that of the current signal, so the current signal is preferred over the voltage signal.

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77. Why earthing is required in the control system?

During operation, all equipment in the control system, whether low-power or high-power, produces some electrical leakage and noise. As a result, when we connect these devices to the earth, they will stay grounded.

• Preventing electric shocks keep people safe.

• It protects electrical appliances and devices from harm.

• It reduces the risk of fire produced by current leakage, which could otherwise result in a fire.

78. What is Safety earth?

Safety earth is a connection of an installation’s exposed conductive portions to the installation’s primary earthing connection. The safety earth’s function is to ensure a safe low resistance path for any leakage current in the body of an installation, equipment, or device to flow to the earth so that personnel can work in the installation, equipment, or device safely.

79. What is Instrument/Signal earth?

The instrument earth/signal earth’s function is to provide a low impedance path to the noise currents introduced by unwanted RFI/EMI interference, which may result in the induction of faulty unwanted noise signals into the 4-20mA analog signals. Shields of multi-pair analog instrument signal cables are connected to this earth in the marshaling cabinets.

80. Why shield cables are used in multi-core cables?

It is utilized to minimize and prevent the effects of EMI and other risks prevalent in busy, industrial environments.

81. What should be the resistance value of safety earth?

Less than 5 Ohms

82. What should be the resistance value of Instrument/signal earth?

Less than 1 Ohm

83. What is Megger?

The Megger is the instrument used for measuring the resistance of the insulation.

84. Why test the insulation of a cable?

When cable insulation is pulled through raceways, it can be nicked or scraped from individual conductors. Moisture, high heat, and certain chemicals can all cause damage to insulation. An insulation resistance test must be performed after installation to ensure that the insulation has not been damaged during the installation procedure.

85. What is HART?

HART (Highway Addressable Remote Transducer) Protocol is an open standard used globally to send and receive digital information using analog wiring between smart devices and control systems.

86. What is Sourcing and sinking?

Sinking and sourcing are two important concepts to understand when connecting a PLC to the outside world. These two concepts only have an impact on DC modules.

The sinking circuit connects the load to the ground (-DC ).

The sourcing circuit provides a +24-volt source to the load (+DC).

87. Define controller.

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

88. What are the types of controllers?

• Proportional controllers.

• Integral controllers.

• Derivative controllers

We can make the combination of the above controllers and make the following controllers

• Proportional and integral controllers (PI Controller)

• Proportional and derivative controllers (PD Controller)

• Proportional integral derivative control (PID Controller)

89. What is a proportional controller?

Every controller is designed to meet some specific use. The proportional controllers are designed for the following reasons.

• We can use proportional controllers only if the deviation is not large.

• Can use only if the deviation is sudden

90. Advantages of proportional controllers

• It makes the system more stable by reducing steady-state error

• We can make an overdamped system much faster

91. Disadvantages of proportional controllers

• We will have some offsets due to the use of this controller

• It increases the maximum overshoot of the system

92. What is an integral controller?

As the name implies, in integral controllers the actuating signal (also called the output) is directly proportional to the integral of an error signal.

93. Advantages of integral controllers

Following a disturbance, integral controllers can return the controlled variable to its exact set point

94. Disadvantages of integral controllers

Due to the slow response to the produced error, it makes the unstable system.

95. What is a derivative controller?

We can’t use the derivative controllers alone due to the disadvantages. Check next Q&A for knowing the disadvantages

96. Disadvantages of a derivative controller?

• It never reduces steady-state error.

• It generates saturation effects and amplifies the system’s noise signals.

97. Why we cannot use a derivative controller alone?

• It never reduces steady-state error.

• It generates saturation effects and amplifies the system’s noise signals.

98. Advantages of Derivative Controller

It helps to improve the system’s transient response

99. What are the span and zero of a transmitter?

Span: The difference between the lowest point and the highest point in a configured measurement range

Zero: It is the lowest pressure at which the transmitter was calibrated

100. What is a range of a transmitter?

The range of a transmitter is defined as setting the scale for the 4mA and 20mA points. “Calibrated range” or “calibration range” are two other names given to the scale. This defines the reading at which transmitter output is at 4mA known as the Lower Range Value (LRV) or as “zero” meaning 0% and the reading at which the input is 20mA known as Upper Range Value (URV), also known as “full scale”, meaning 100%.

101. What is a cable schedule?

A cable schedule is a document that lists all of the instrument cables available. Each instrument or connection requires a cable and a gland as shown in this paper.

The cable schedule contains the following information:

• Number of Cables

• Type / Specification of Cable

• Size of the Cable

• Length of the Cable

• Termination descriptions at the source and destination

• For each incoming cable, determine the kind and size of the cable gland.

102. What is JB Schedule?

JB Schedule is a file that contains a JB list. Each instrument or connection requires a different cable and gland as shown in this document/file.

The JB schedule contains the following information:

• JB Type / Specification

• JB Number

• JB Location/Area

• Termination point

103. What is Instrument Index?

It is a list of documents that contains all the instruments in a plant or industry

104. What are the different modes of a PID?

• Manual

• Auto

• Cascade

105. Various types of PIDs?

• Proportional Control

• Standard Type PID Controller

• Real-Time PID Controllers

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106. What is Feedback control?

A feedback control system is one in which the output is regulated by a feedback signal generated by the system’s measurement. This feedback signal is compared to a reference signal to produce an error signal, which is then filtered by a controller to produce the control input for the system.

107. What is Feed-Forward Control?

Feedforward control is designed to anticipate the impact of a measured disturbance on the process variable and deploys control action to counteract the impending disruption in a timely manner. This will significantly improve disturbance rejection performance, but only specific to the particular disturbance variable which is being measured.

108. What Ratio control?

Ratio control is used when we need to mix two fluids in a specific ratio

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109. What is three-element Control?

A three-element controller is used in the level control system that usually measures the flow rate of steam from the boiler, water level, and flow rate of water to the boiler to control the water flow into the boiler.

110. What is loop check?

The loop check is the last stage of commissioning a project. We could say that loop checking is a process to check individual analog and digital loops connection is proper as per the engineering documents and also to check that each loop is working or not properly.

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111. What is Manifold?

Instrumentation manifolds are a kind of process connection system that provides various types of pressure management applications such as isolation, ventilation, and equalization. Manifolds are usually used in other measuring instruments like pressure transmitters, gauges, and other pressure sensing devices.

112. What is the equalization valve in the DP transmitter?

It is used to make ensure that the same pressure will be applied to both sides of the transmitter i.e., zero differential pressure.

113. What is an Analyser?

An analyzer is a tool that is used to evaluate process data of various process variables.

114. Different types of analyzers?

• Combustion Analyser

• Gas Analyser

• Liquid Analysis

• pH Analyser

115. Brief about vibration measurement?

The vibration measurement system is a device that measures vibration levels. Optional features include frequency measurement, a frequency-weighting network, and a display such as a meter, printer, or recorder.

116. What is a density meter?

A density meter is a device that is designed to calculate the density of the liquid. Density meters are mainly used to measure the density of slurries, sludges, and other liquids that flow through pipelines.

Density is usually represented as either rho or D.

117. Types of density meters

• Red Meter

• Coriolis

• Nuclear

• Microwave

• Ultrasonic

118. Major control system manufacturers

• ABB.

• Emerson Electric.

• Honeywell International.

• Siemens.

• Yokogawa Electric.

• General Electric.

• Metso

• Mitsubishi Heavy Industries.

119. Various units of Pressure measurement?

The SI unit for pressure is pascals (Pa). Other units of pressure include torr, barr, atm, at, ba, psi, and manometric units like mm Hg and fsw.

120. Various units of Temperature measurement?

• Celsius

• Fahrenheit (°F)

• Celsius (°C)

• Kelvin (K)

121. Various units of flow measurement?

• Cubic feet per second (CFS)

• Cubic meters per second (cms)

• Gallons per minute (GPM)

122. What is a weighing scale?

The weight scale is a device used to measure the weight or mass of an object. These are also known as mass scales, mass balances, and weight balances.

123. What is the working principle of the weighing scale?

The weighing scale’s working principle is based upon the Electro-Magnetic Force Restoration(EMFR). The fundamental idea is similar to that of a simple beam balance. As a result, the coil is attached to the other side and seeks to travel away from the object’s magnetic field.

124. What are various PLC programming languages?

• Function Block Diagram (FBD)

• Instruction List (IL)

• Ladder Diagram (LD)

• Sequential Function Chart (SFC)

• Structured Text (ST)

125. What is IP rating?

The IP rating or IP code classifies the degree of protection given by an enclosure.

The international standard EN 60529 defines IP ratings.

126. What is ExProof?

Explosion-Proof is the acronym for exproof. Just like the name implies, it is used to prevent the sparks in the industry from igniting vapors, gases, dust, etc.

127. What are different international standards available for measuring instruments?

• ISO 463:2006

• ISO 463:2006/COR 1:2007

• ISO 463:2006/COR 2:2009

• ISO 1502:1996

• ISO 3274:1996

• ISO 3274:1996/COR 1:1998

128. What is UPS?

An uninterruptible power supply (UPS) is a device that allows a system to keep running for at least a short time when the primary power source is lost. Power surges are also protected by UPS equipment.

129. What is an inverter?

It is electrical equipment used to convert D.C power into A.C power from the battery.

Battery saves electricity as D.C, so we need an inverter to convert it into A.C because most of the equipment is designed to run in A.C power.

130. What is a rectifier?

A rectifier is an electrical device that converts alternating current (AC) to direct current (DC).

131. What is MCC?

A motor control center (MCC) is a system for centrally controlling some or all-electric motors. It is made up of many covered parts connected by a common power bus, each with a combination starter that includes a motor starter, fuses or circuit breakers, and a power disconnect.

132. What does MMWC mean?

MMWC is a Millimetres water column. It is a less commonly used unit of pressure

133. What is a relay?

Relays are electrically operated switches that are designed to perform closing and open the circuits electronically as well as electromechanically.

134. What is a solenoid valve?

A solenoid is an electrically operated valve. It consists of a solenoid coil in which a magnetic plunger moves. This plunger is connected to the plug and tends to open or close the valve.

135. What is a final control element?

A mechanical device that physically adjusts a process in reaction to a change in the control system setpoint is referred to as a final control element. Valves, dampers, fluid couplings, gates, and burner tilts, to name a few are final control elements that are pertinent to actuators. Final control elements are essential parts of a control loop.

136. What are the different types of final control elements?

• Control valve

• ON/OFF Valve

• Variable frequency drive

• Solenoid valve

• Damper

137. What are the types of valves?

• ON/OFF Valve

• Control valve

• Isolation Valves

• Regulation Valves

• Safety Relief Valves

• Non-Return Valves

• Special Purpose Valves

138. What are the different types of control valves?

• Globe Valve

• Gate Valve

• Diaphragm Valve

• Pinch Valve

• Ball Valve

• Butterfly Valve

• Plug Valve

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139. What is control valve stroking?

This method is used to show that the actuator can open and close the valve with enough force. The technique must show that there will be no mechanical damage or permanent deformation of valve components and that all accessories will work properly.

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140. What is Valve Positioner?

Positioners are devices designed to help actuator position in a control valve with the help of control signal

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141. Components of a control valve?

• Valve Body

• Bonnet

• Trim

• Plug

• Cage

• Seat Ring

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142. What is the use of a single-seated valve?

Single-seat valves are used in smaller sizes that require absolute shutoff. The use of single-seat valves is limited by the pressure drop across the valve in the closed or near closed position.

143. What do you mean by self-regulation?

For getting a sustained change in input, the output moves from one steady state to another steady state is called self-regulation. The output variable changes regarding the value change in input until the output value reaches a steady state. The process of using a specific value of the controlled variable for the rated load without any control operation through this method.

144. Name different test inputs.

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

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

Glycol is the seal liquid.

146. What Is the SCADA System?

Supervisory Control and Data Acquisition (SCADA) is a system that controls a remote device by operating with a coded signal over a communication channel (usually using a communication channel at a remote station).

147. What is SCADA Network?

Acronym for supervisory control and data acquisition, a computer system for gathering and analyzing real-time data. SCADA systems are used to analyze and control industrial equipment

148. What is meant by Supervisory Control?

Supervised control is a generic term for the control of many individual controllers or control loops, whether human or automatic control systems, but almost all real-world systems are a combination of the two.

149. State some of the force balance advantages.

• Moving parts are fewer.

• Baffle movement is negligible

• Frictional losses are less

150. Why damping torque is required in an indicating type of instrument?

To suppress the oscillations of the pointer at steady-state position 151. What is a ring counter?

Counter based on the register.

152. What is an embedded system?

A Microcontroller-based system designed for a specific task.

153. What is the other name of Multiplexer and De-Multiplexer?

Multiplexer: Data selector

De-Multiplexer: Data Distributer

154. What is an Opto-coupler

It is a semiconductor device that allows an electrical signal to be transmitted between two isolated circuits

155. Opto-coupler is also known as


156. What are the advantages of Opto-coupler?

• One-way transmission of a signal

• Complete electrical isolation between input and output

• Suited for isolating a low voltage circuit from a high voltage circuit.

157. Which device is suited for isolating low voltage circuits from high voltage circuits?

Opto-coupler (Opto-isolator)

158. Why null type instruments are more accurate?

The null-type instruments consume almost negligible power at the end of the measurement.

159. Why silicon is used in the fabrication of Zener diode?

Very low reverse saturation current (nA).

160. The Active transducer also known as?

Self-generating type of transducers.

161. What device do you require for the purpose of interfacing of microcontroller with a DC motor?


162. What is the range of LVDT?

1.25mm to250m

163. LVDT stands for

Linear Variable Differential Transformer.

164. What is impedance?

The opposition to the flow of current in an AC circuit

165. What is reactance?

Capacitance or inductance creates resistance to the flow of alternating current.

166. What happens when the impedance of an electrical load is equal to the internal impedance of the power source?

The source can deliver maximum power to the load

167. What is an actuator?

A device that converts various forms of energy into rotary or linear mechanical energy to generate automatic motion. It is a device that translates an electrical control signal into physical action.

168. Which transducer converts heat energy into electrical energy?


169. Which phenomenon depends on Avalanche breakdown?


170. Which phenomenon depends on Zener diode?

High-intensity electric field

171. Which material is used to make the core of a high-frequency transformer?


172. The Frequency is measured by _________ bridge?

Wien’s bridge

173. Hays bridge is used to measure___________ and Schering bridge is used to measure____________

Inductance, Capacitance

174. When a sine wave is given as input to Schmitt trigger its generates:

Square wave

175. What is the primary function of multiplexing?

To allow two or more signals to make use of a single communications channel

176. What is the effect of doping intrinsic semiconductors?

To move the Fermi level away from the center of the forbidden band

177. What is the difference between circuit and network?

The network is an electrical elements interconnection that may or may not have a closed path.

A circuit is also an electrical interconnection, but it must have at least one independent source and one closed path.

178. What is the Hall effect voltage in intrinsic silicon?


179. How many flip-flops do we need to construct Mod -12 counter?


180. What is a Transducer?

A transducer is a device that converts physical quantities like flow, pressure, temperature and level into proportional electric quantities.

181. What are the types of transducers?

• Active.

• Passive.

182. What is an active transducer?

It can produce its own output by the mechanical displacement

Ex: Piezoelectric pickup.

183. What is a passive transducer?

Requires the use of an auxiliary input that changes in response to mechanical displacement.

Strain gauges, slide wire pots, capacitive pickups, and LVDTs are all examples of this type of device.

184. What is a pyrometer?

A pyrometer is a type of thermometer. We can calculate the distant object’s temperature

185. What is a thermistor?

It is a type of resistance whose electrical resistance and change of temperature are proportional that means the value of electrical resistance changes with the change of temperature.

The thermistor is a combination of thermal and resistor.

186. What are the two types of thermistors?

• Negative Temperature thermistors (NTC).

• Positive Temperature Thermistors (PTC).

187. Applications of thermistor

Used in home appliances like fridge, microwave Owen, iron box, etc.

188. What are the advantages of thermistors?

• Smaller in size

• More accurate

• Fast response than RTD

• Low cost

189. Why Platinum RTD is preferred over Cu and NI?

The platinum resistance value is linear over wide temperature ranges.

190. What is the Thomson effect?

The evolution or absorption of heat when current passed through a conductor in which there is a difference in temperature along its length.

191. What is a Vortex flow meter?

A vortex flow meter is a flow meter that is used to measure liquid and gas flow. Its sensor detects and converts pressure pulses into electrical signals.

192. What is the drift?

Changes that occurred in the instrument calibration are called drift.

Drift is the term used to describe how an instrument’s calibration shifts over time.

193. What is the definition of sensitivity?

The instrument responds to the smallest change in the measured variable.

194. Stroboscope is used for the measurement of____?

Angular velocity

195. Which materials do not have a covalent bond?


196. What is the constant voltage circuit?

This is the power circuit used to drive the output at a constant level.

197. What are De-saturators?

When long transient responses are expected in some processes, such as batch processes, and a sustained deviation is present, the controller integral action continuously drives the output to a minimum or maximum value. The term for this phenomenon is “integral saturation of the control unit.” This unit is de-saturated when this condition is met.

198. What is the force balance principle?

The principle of force balance: A controller produces an output signal by a torque in the opposite direction. An inlet force is applied to the inlet bellows which moves the beam. This nozzle back pressure is cracked. Nozzle back pressure detected by the balance bellows helps to balance the beam. Baffle movement is much smaller around 0.002 for full scale output.

199. 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 insight, answers, and guidance useful for the originating application

200. What is the motion balance principle?

A controller generates an output signal by the 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 backpressure 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.

201. Purpose of Pirani gauge

Low-pressure measurement

202. Psi stands for

Pounds per square inch

203. Low pressure expressed in


204. What is reset-wind up?

When a reset action is applied to controllers where the measurement has been far from the setpoint for an extended period of time, the rest can bring the output to its maximum, resulting in the break being terminated. When the process is restarted, the output will remain at its maximum until the measurement crosses the point, resulting in large overshoots. This issue can be avoided by incorporating an anti-reset winding circuit, which eliminates the issue of output saturation.