Instrumentation Interview question and answers

1. What is Intrinsic safety?

By a method by which limiting the amount of electrical energy in a circuit is too low to avoid an explosion in hazardous areas.

2. What is IS and non IS cables?

IS (Intrinsic Safety) cables are with some specific considerations with voltage, current, inductance, and capacitance.

The color is blue with a tough sheath to avoid mechanical damage. It connects to intrinsically safe devices in the field. Electromagnetic interference is avoided.

Non IS cables are with no special considerations and to be layed in non-hazardous areas. Identified easily with its black sheath.

Generally IS cables and non IS cables are segregated while laying in cable trays.

3. What is a Transducer? How many types are they?

A transducer is a device that converts physical quantities into proportional electric quantity such as voltage, current.

Transducers are two types

Active.

Passive.

Active transducer: produces its own electrical output as a function of mechanical displacement  Ex: Piezoelectric pickup.

Passive transducer: requires an auxiliary input which it modifies as a function of mechanical displacement.

Ex: Strain gauges, Slide wire pots, capacitive pickups, LVDT.

4. What are Load cell and strain gauge?

The load cell is one application of strain gauge.

A strain gauge is a transducer that converts force to change in electrical resistance.

Strain gauges can be used to find force, thrust, load, pressure, torque, displacement, and flow.

The gauge factor is a change in resistance due to a change in length. The gauge factor varies with temperature and strain.  

Sensing element material – metallic or semiconductor.

Metallic strain gages are constantan, nichrome, dynaloy, platinum.

Semiconductor strain gauges have more sensitivity than metal strain gauges. These gauges are attached to the paper, ceramic material, or bonded to a load.

5. How many orifice pipe tap connections are there? Also for venturi, flow nozzle, pitot tube.

Flange taps, corner taps, veena contract taps, radius taps, pipe taps

Flange taps: 1” upstream and 1” downstream.

Flange taps are recommended for pipe sizes less than 2”. because veena contract may be less than 1” from orifice plates.

Corner taps: corner taps are located directly at the face of the orifice, they are useful for pipe sizes less than 2”. where veena contracta may occur inside the dimensions of the standard flange taps.

Veena contracta taps: are made use for highest differential pressures are available.

Upstream = 1pipe diameter.

Downstream- at veena contracta where pressure is minimum. However minimum pressure varies with the d/D ratio, thereby introducing an error if the plate bore is changed.

Radius taps- upstream: 2 1/2 pipe diameters.

Downstream – 8 pipe diameters.

Venturi Tube: Pipe taps 1/2 pipe diameters upstream.

                      Middle of the throat.

Flow nozzle: 1 diameter upstream

          1/2 diameter downstream.

Pitot tube: it is used to measure fluid velocity.

Impact opening: directly in the line flow.

Static opening: 90 degrees from impact opening.

A pitot tube can be used to measure flow rates of water in rivers and air velocity in airplanes. It can be used to measure airflow in ducts or pipes.

6. What is the difference between thermocouple and pyrometer?

A thermocouple works on the principle of the Seebeck effect, which states that when a junction of dissimilar metals is heated emf will be generated.

Whereas the pyrometer is a remote sensing thermometer is used to measure high temperatures such as kilns, furnaces, etc., It is a non-contact device.  

7. What is a thermistor?

A thermistor is defined as a type of resistance whose electrical resistance varies with the change of temperature. A thermistor – a combination of two words, THERMAL, and RESISTOR.

8. What are the two types of thermistors

Negative Temperature thermistors (NTC).

Positive Temperature Thermistors (PTC).

9. What are the uses of thermistors?

Used in household applications such as Fridge, Micro ovens, water heaters, Electrical iron box.

10. What are the temperature and resistance relationship?

The temperature and resistance relationship is non-linear. That means the temperature and resistance relationship is inversely proportional.

11. What are the different modes of thermistors?

Bead type, rod type, disc type.

12. What are the advantages of thermistors?

Thermistors are small in size

More accurate, fast response than RTD

Cheaper cost-wise.

13. What is the general thermistor measuring range?

General thermistors used to measure temperature -55 to +150 deg C.

14.Why Platinum RTD is preferred over Cu and NI.

Platinum resistance values are linear over a wide range of temperatures.

15.Law of intermediate metals

The use of the third metal does not affect the temperature measurement, as long as both wires are maintained at the same temperature. 

16.Law of intermediate temperatures.

Sum of voltages generated by a thermocouple with a junction at temperature T1 and T2 another thermocouple with a junction at temperature T2 and T3 is equal to the emf generated equal to T1 and T3.

17.Seebeck effect

When a junction of two dissimilar metals is heated emf will be generated.

18.Peltier effect

Peltier effect states that if current flows across the junction of two dissimilar metals, which are maintained at the same temperature the heat are either released or absorbed depending upon the direction of the current.

19.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.

20.Why Orifice plate is preferred over the Venturi tube.?

Orifice :

Orifice plates are small in size and easy to install or remove.

Measure a wide range of flow rates.

They are most suitable for gases and liquids.

They are inexpensive

Price does not increase dramatically.

Venturi:

They are large in size, can not be used where space is limited.

Maintenance is not easy

Require large laying length.

The initial cost is very expensive and also installation and maintenance.

21. What is the transmitter turndown ratio?

It is the ability to adjust the sensing range of the transmitter.

If the transmitter operates 25 psi full scale to 150 psi full scale, it’s turndown is 6:1

22. What is a Vortex flowmeter

A vortex flow meter is a flow measuring device, used to measure the flow of liquids and gases. Its sensor detects the pressure pulses and converts these into electrical signals.

23. What are the different sensors used in Vortex flowmeters?

1.Thermal sensor.

2.Mechanical Sensor.

3.Capacitance Sensor.

4.Piezoelectric.

5.Strain gauge.

6.Ultrasonic.

The most popular is the piezoelectric sensor.

24. What are the advantages of Rotameter over others?

It gives a linear relationship between float position and flow. The scale is graduated linearly.

No power is required.

The very low initial setup cost.

Simple and robust

It can handle varieties of corrosive liquids and gases.

25. What is an ultrasonic flow meter?

It is a flow meter used to measure the velocity of a fluid with ultrasound to calculate volumetric flow.

Sending sound waves at a frequency beyond human hearing typically 05, 1, or 4 MHz. Ultrasonic flowmeter sends signals with.

Insertion transducer – making contact with liquid.

External transducer – making contact with the pipe wall.

26. Is there any problem with using a magnetic flow meter for the flow measurement of petrol?

One can not use a magnetic flow meter to measure the flow of petrol. It does not work. The liquid to be measured should have electrical conductivity. Magnetic flow meter generally not work for hydrocarbons, distilled water.

27. What is the working principle of a Magnetic flow meter?

A magnetic flow meter works on the principle of faradays law of electromagnetic induction.

It states that the voltage induced across any conducting material that is passing through the right angle in a magnetic field is directly proportional to the velocity of the conductor.

E= kVBD

E = emf generated.

V = Velocity of the conductor.

B= Magnetic field strength.

D= Length of the conductor. I.e distance between two electrodes.

The induced voltage is measured by two electrodes placed within the flow tube.

28. What are the advantages of Rotameter over others?

It gives a linear relationship between float position and flow. The scale is graduated linearly.

No power is required.

The very low initial setup cost.

Simple and robust

It can handle varieties of corrosive liquids and gases.

29. What are the disadvantages of variable area flowmeters?

1. When opaque fluid is used, the float may not be visible.

2. It must be installed in a vertical position only.

3. Some types of glass tubes are subject to breakage.

30. What is the Turbine flow meter?

The turbine flow meter is a volumetric flow measuring instrument.

When the liquid passes through the meter body, it causes the freely suspended turbine rotor blades to rotate. The velocity of the turbine blades is directly proportional to the velocity of fluid passing through the flow meter.

A magnetic pickup mounted on the flow meter body sense each rotation of blade passing, causing the sensor to generate a pulse.

The frequency of the electrical pulses is directly proportional to the volume of liquid or gas.

31. Why is the rotameter only used vertically?

A variable area flowmeter requires gravity to balance the float position against the flow of liquid through it.

32. What is the difference between static characteristics precision and accuracy?.

Accuracy is defined as the closeness of the measured variable to a standard value. Suppose if your box is 5 kgs weight but upon measurement, it is 7 kgs. Then your measurement is not accurate.

Precision is defined as the closeness of two or more measured variables to each other.  

For example: Suppose if you weigh a box four times. Upon measuring your weight the readings are 50 kgs, 50.2kgs, 50.15 kgs, 50.3 kgs. The measurements are precise.

33. What is the drift?

Drift in the change of calibration of the instrument.

The calibration of the instrument gradually shifts over a long period of time is called drift.

34. What is sensitivity?

The smallest change in the measured variable for which the instrument responds.

35. What does Pt100 mean?

Pt100 means 100 Ohms at 0 Deg C.

Pt range is -220 to 1050 Deg C.

36. Why Orifice Tab is provided?

The tab is provided to check

Tag.No,

Inlet mark of an orifice

For an indication of the orifice plate in line.

Orifice dia

The material of orifice plate for eg: SS or some other.

37. Why Thermowell is used?

Thermowell is used to avoid direct exposure to the process gases or liquids.

To protect the sensor from corrosion and erosion.

38. What is Hazardous area classification as per the IEC standard?

Zone 0: Explosive / Hazardous atmosphere is present always.

Zone 1: Explosive/Hazardous atmosphere is likely to occur in normal conditions.   

Zone 2: Explosive/Hazardous atmosphere is not likely to present. If present, it is only for short periods.

39. Why is the mA signal is preferred for signal transmission?

As no current drop in long transmission and reducing the noise.

40. Why does a transmitter output signal start from 3-15 psi or 4-20 mA?

Linearity and can check up whether it is a live zero or a dead zero.

41. What is a control valve?

The control valve is a valve used to control fluid passage by the variation of the area of the flow passage as per the signal from an electronic or pneumatic controller.

42. What are the different types of actuators?

The different types of actuators are

Diaphragm operated.

Piston operated.

43. What is # in the flange rating of a control valve?

# stands for pressure “class” of a valve.

As per ASME B16.34, “ class” followed by a number is the designation for a particular pressure and temperature rating.

In other words, pressure and temperature rating are specified in classes.

#150 means “class 150 “ flange with certain pressure and temperature ratings.

44. What are the different valve characteristics? and describe it in brief.

Linear, equal percentage, and quick opening.

Linear: Flow capacity increase with valve travel.

Equal Percentage: Equal increments of valve travel produce equal percentage changes in Cv.

Quick opening: Very large changes in flow for very small changes in lift.

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

Orifice plate, Venturi tube, Flow Nozzle, Pitot tube, Annubar.

46. What are the different orifice plates?

Concentric, Eccentric, Segmental and Quadrant edge.

Concentric: liquids, steam, and gas.

Eccentric: viscous and slurry flow.

Segmental: slurry and colloidal flow.

Quadrant edge: viscous fluids.

47. Where does integral orifice is used?

The integral orifice is used to measure small flow rates.

48. What is the seal liquid for filling impulse lines on crude/viscous liquids.?

It is Glycol.

49. What is meant by cavitation in the control valve?

When a liquid enters a valve and static pressure at vena contracta drops to less than fluid vapour pressure and recovering to above fluid vapour pressure. This pressure recovery causes the implosion or collapse of the vapour bubbles at the vena contracta.

50. What is the ambient temperature?

The average temperature of the surrounding air which comes in contact with equipment and instruments under test.

51. What is flashing in the control valve?

When liquid flows through the control valve, the formation of vapours takes place. If the pressure remains below the vapour pressure of a liquid and does not recover to the previous state is called “ flashing”

52. When do you use a valve positioner?

The addition of a positioner can correct many variations including changes in packing friction due to dirt, corrosion, or lack of lubrication, variations in dynamic forces of the process in operation.

The valve positioner also allows split ranging the controller signal between more than one valve.

53. What is the need for an I/P converter in the control valve?

In some process loops, the controller output is electronic and the final control element is pneumatically operated. To connect these two we need a device that should linearly convert electrical to air pressure ( 4-20mA to 3-15 psi). Such a device is called the I/p converter.

54. Why is the rotameter used vertically?

Rotameter is a variable area flow meter, requires gravity to balance the float position against the flow of liquid through it.

55. What is the turbine flow meter?

A turbine flow meter is a volumetric measuring device. When liquid passes through the meter body, it causes the freely suspended turbine blades to rotate. The velocity of turbine blades is directly proportional to the velocity of fluid passing through the flowmeter.

A magnetic pickup is mounted on the flow meter’s body which senses each rotor blade passing causing the sensor to generate frequency output. The frequency is directly proportional to the volume of liquid or gas.

56. What is Bernoulli’s theorem and where it is applicable?

Bernoulli’s theorem states that the overall energy of the fluid as it enters into the system is equal to the overall energy of the fluid as it leaves.  

It applies to incompressible liquids.

57. How do you carry out piping for a differential pressure transmitter on liquid, gas, and steam services.?

On the liquid lines, the transmitter is mounted below the orifice plate because liquids have a property of self-draining.On the liquid lines, the transmitter is mounted below the orifice plate because liquids have a property of self-draining.  

 On gas service, the transmitter is mounted above the output because gases have a property of self-venting secondly condensate formation.

On steam service, the transmitter should be mounted below the orifice plate which condensates pots.  Condensate pot should be at the same level.

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