# 300+ Instrumentation Engineering Interview Questions – Part 3 ## 1. What exactly is a SCADA network?

Supervisory control and data acquisition is an abbreviation for a computer system that collects and analyses real-time data. SCADA systems are used to monitor and control industrial machinery.

## 2. Define Supervisory Control?

Supervised control is a catch-all term for the control of a large number of individual controllers or control loops, whether human or automatic control systems, but almost all real-world systems are a mix of the two.

## 3. Describe the ratio control system.

A ratio control system is distinguished by the fact that changes in the secondary variable have no effect on the primary variable. A ratio control system is one in which secondary flow is held in some proportion to an uncontrollable primary flow.

If we assume that the output of a primary transmitter is A and the output of a secondary transmitter is B, and that the multiplication factor of the ratio relay is K, then we find the following relationship for equilibrium conditions where set valve equals measured valve:

KA-B=0 or B/A = K, where ‘K’ is the ratio that activates the relay.

## 4. Explain signal isolator

A signal isolator is an electrical device that is used to remove earth loop errors caused by noise and signal interference problems. These isolators employ a variety of electronic methods to interrupt the two grounds connections while transmitting an exact or low-loss signal.

## 5. Define straight through valve?

Valve in which the closing operation of the valve is accomplished by turning the closing element 90 degrees.

## 6. Define trim?

Trim is made up of stems, seat surfaces, back seat bushings, and other small internal parts that normally come into contact with the surface fluid.

API 600

## 8. Define sensor trim?

Sensor trim is a two-point sensor calibration that applies two end-point pressures and linearizes all output between them. It enables the user to choose between low trim and high trim end point values to provide digital calibration over the required measurement range.

## 9. Define PLC Level and its type?

Based on their output, PLCs are grouped into three types: relay output, transistor output, and triac output. The relay output is ideal for both AC and DC output devices. PLCs with transistor outputs use switching operations and are found inside microprocessors.

## 10.What exactly is MCC?

A motor control centre (MCC) is a system that controls some or all electric motors from a central location. It is composed of numerous covered parts linked by a common power bus, each with a combination starter consisting of a motor starter, fuses or circuit breakers, and a power disconnect.

## 11. What does MMWC mean?

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

## 12. Why is the MMWC unit used?

MMWC stands for millimeter of water column. Hydrostatic pressure is generated by a liquid column head of water, and this principle is commonly used in U tube water column manometers, where the difference in water level between each side is measured in millimetres or inches.

## 13. What is the meaning of slope in PH transmitter?

The slope of a pH sensor is the linear relationship between a raw voltage reading and a pH value. The slope determines how much the raw voltage reading must change to see a one-pH change. At 25 °C, the theoretical slope value, or ideal condition, is 59.16 mV.

## 14. What is difference between differential Pressure & Delta Pressure?

Differential pressure is the difference between high and low pressure.

Delta pressure denotes a pressure drop.

## 15. What are the key elements of a Control Loop?

In its most basic form, a closed control loop consists of a sensor, transmitter, controller, signal converters, and final control elements.

In reality, many other instruments will be present in a practical loop to support the operation of the aforementioned instruments.

## 16. How do you use a level transmitter in a closed tank?

The maximum allowable distance above the lower tap limits the transmitter location in closed systems. This is the same as the 1 atmosphere equivalent seen previously in pressurised systems. The transmitter in sub-atmospheric systems (vacuum systems) should be mounted at or below the lower tap. This ensures that the transmitter sees a positive pressure on both the measurement and reference sides at all times.

The distance between the taps becomes the reference offset from zero in two seal systems. Regardless of where the transmitter is mounted, the calculations are the same.

## 17. What are the automatic controller modes?

The modes of control are as follows. PID Control, PI Control, ON/OFF Control, Cascade Control, Override Control, Feedback Control, and Feed Forward Control are all examples of automatic controls.

These controls can be set to MANUAL or AUTO Mode. When set to MANUAL, the mode of control is disabled; when set to AUTO, the mode of control is enabled.

## 18. Difference between signals and waves?

Signal means information. A wave is something that transports information from one location to another.

## 19. Why gold is added in P-N junction?

To reduce the recombination time.

## 20. Latch vs flip flop?

The flip flop is triggered at the edge, whereas the latch is triggered at the level. In a latch, the gate serves as a control signal. Clock is used as a control input in FF.

## 21. Advantages of synchronous circuit?

Synchronous circuits share a common clock and use it to operate. Asynchronous circuits, on the other hand, use different clock frequencies and operate independently.

## 22. Why transistor BJT is called current controlled device?

Because the transistor’s input current controls the output current, for example, in the CE configuration, the base current Ib controls the output current Ic.

## 23. What is the difference between emulator and simulator?

A simulator is simply software that acts like hardware. Inside, you can see all of the electronic components, and you can connect them in various ways to simulate o/p as well as normal hardware. Before performing tests on real hardware, we can test on emulator, which is an exact copy of the real hardware on which we wish to work.

## 24. Why is the Op-Input amp’s impedance high?

The first stage, i.e. the input stage, is a dual i/p balanced o/p opamp with a very high i/p resistance. If we use a FET instead of a BJT for the first stage diffamp, the i/p resistance will be very high in M ohms.

## 25. What is a Thermistor with a Positive Temperature Coefficient?

In a PTC thermistor, resistance is directly proportional to temperature, which means that resistance increases as temperature increases. These kinds of sensors are used in fuses to protect circuits.

## 26. What is a Thermistor with a Negative Temperature Coefficient?

The resistance of an NTC thermistor is indirectly proportional to the temperature, which means that as the temperature rises, so does the resistance, and vice versa. When the temperature rises, electrons form, increasing an object’s conductivity.

Temperature measuring devices employ NTCs.

## 27. What is direct level measurement?

The term “direct” refers to the fluid level being measured being in direct contact with the sensor.

## 28. Examples of direct level measurement

a). Sight glass

b). Float

c). Magnetic level indicator

## 29. What is indirect level measurement?

In the indirect method of level measurement, the level of a liquid is calculated by a variable that changes according to the level.

## 30. Examples of indirect level measurement

a). Pressure gauge

b). Differential pressure

c). Ultrasonic technology

## 31. What is point level measurement?

A point-level sensor detects liquid levels in a tank at specific points. This is typically used when a high or low level must be maintained.

## 32. What is continuous level measurement?

A continuous level sensor detects liquid levels at every point in a chamber or tank on a continuous basis. This is primarily used in processes where it is always critical to know the level and in applications where increased precision is required.

## 33. Define P&ID?

P&ID is an abbreviation for Process and Instrumentation Diagram. It depicts the interconnection of process equipment used to control a process.

## 34. How is flow measured in the square root?

The flow rate is proportional to the square root of the pressure. As a result, F = K represents the square root of the applied pressure. The flow rate is affected by the square root of the pressure drop.

## 35. Define impulse tubing?

An impulse tube is a small diameter stainless steel tube or pipe used to transmit pressure signals from the process tapping to the transmitter.

## 36. Types of cabinets involved in control system design?

a).System Cabinet

b). Marshalling cabinet

c). Intermediate cabinet

d). Network cabinet

e). Power Distribution cabinet

f). Remote IO cabinet

g). Remote RTU cabinets

h). Fibre optic patch panel

## 37. Define Safety barrier?

A safety barrier is required to ensure that any failure in the safe/non-hazardous zone (control room) does not produce enough energy to ignite the hazardous zone’s gaseous atmosphere. This safety barrier becomes a critical component in the design of an intrinsically safe circuit.

## 38. What is a junction box?

A junction box is an electrical box that serves as a link between field devices and marshalling panels. Terminal blocks are the fundamental components of a junction box.

## 39. Define trunk cable?’

Trunk Cable refers to the main H1 or HSE link single pair cable that connects the control system to the Foundation Fieldbus junction box in the field.

## 40. Define Spur cable?

Spur Cable refers to the H1 or HSE link cable used to connect field instruments such as transmitters, switches, valves, solenoid valves, and so on to the Foundation Fieldbus junction box.

## 41. Why are voltage signals not used?

Because the voltage signal is more nonlinear than the current signal, it is preferred over the voltage signal.

## 42. What is the purpose of an inclined manometer?

It is used to increase the instrument’s scale. Because the manometer is angled with respect to the vertical.

## 43. What is the basic operation of a pressure gauge?

Hooks’ law is used to apply pressure.

“Measuring Stress in an Elastic Medium” Principle

## 44. What happens if the displacer collapses while in line?

The output will be at its peak.

## 45. What happens if the displacer develops a hole while in line?

The output will be as low as possible.

## 46. What are the applications of suppression and elevation?

Suppression and elevation are used in level applications when (1) transmitters are not mounted on a level and (2) the leg is wet. Condensable vapours, in other words.

## 47. What is leveltrol?

The leveltrol is a device that measures the level of liquids in a closed vessel.

## 48. Explain how a leveltrol works?

It operates on the Archimedes principle, which states that “the weight loss of a body immersed in a liquid is equal to the amount of liquid displaced by the body.”

## 49. Explain how an electronic leveltrol works.

The change in buoyancy caused by a change in liquid level causes the net weight of the displacer to change, increasing or decreasing the load on the torque arm. This variation is proportional to the change in liquid level and specific gravity. The resulting torque tube movement changes the angular motion of the rotor in the RVDT (Rotary Variable Differential Transformer), resulting in a voltage change proportional to the rotor displacement that is converted and amplified to a direct current.

## 50 How will you reverse a leveltrol action?

The reversing arm serves as the torque tube’s motion take off arm. It has a slot on each side of the centre so that the link can be connected for either reverse or direct action.