Instrumentation Specialist Interview Preparation Guide
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Instrumentation Specialist based Frequently Asked Questions by expert members with experience as Instrumentation Specialist. These questions and answers will help you strengthen your technical skills, prepare for the new job test and quickly revise the concepts

109 Instrumentation Specialist Questions and Answers:

1 :: Do you know how is flow measured in square root?

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.
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2 :: Tell me how is automatic reference junction compensation carried out in temperature recorders?

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.
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3 :: Tell me what is the use of valve positioner?

Valve positioner can be used for following reasons:
a. Quick action
b. Valve hysterisis
c. Viscous liquids
d. Split range.
e. Line pressure changes on valve
f. Bench set not standard
g. Reverse valve operations
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4 :: Explain what is the purpose of measuring dissolved oxygen?

Dissolved oxygen is measured in boiler feed water for addition of dosing compounds to remove oxygen to avoid corrosion in steam boilers. In wastewater plants it is measured to estimate quality of effluent. (oxygen demand)
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5 :: Explain me where should we use “ Open to alarm “ and “Close to alarm “?

A) Where probability of an open circuit fault is higher we use “open to alarm” and where probability of a closed circuit fault is higher we use “close to alarm” strategy. “Open to alarm” strategy is usually preferred.
B) Selection should be such that whenever component/ instrument/ system fault occurs, system should give alarm.
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6 :: Explain what does DN stand for in valve size?

DN stands for Diameter Nominal. It is a flange size of the valve.
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7 :: Do you know advantages of Displacer type Level switch to float type switch?

Displacer type switches involve very little movement whereas float type switches involve large movement of float and hence are more likely to get stuck-up. However float switches can be used over a much longer range than a displacer switch.
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8 :: Explain what is a thermal relief valve?

A thermal relief valve is typically used to relieve over pressure due to thermal expansion of liquid in pipelines.
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9 :: Tell me what is motion balance principle?

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.
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10 :: Explain basic difference between DCS & PLC?

1) DCS handles more nos of I/Os rather than PLC.
2) PLC is faster system than DCS.
3) DCS can handles handsome quantity of I/Os so that can be used for total plant automation. Where as PLC has own limitations so it generally used for small but for important(Safety point of view) units, like boiler automation, Make-up compressor automations Etc.
4) In the above mentioned case the these PLC’s can be get connected with the DCS with the help of soft link. Generally this is used to make alert to both the operator.
5) As I heard the PLC used to handle the DI/DO signals so it can take fast actions. Some of the time it is used to handle few nos of AI/AO. 6) DCS & PLC’s speed depends on the scan rate of I/Os.
6) For both the system Marshaling panels, Consoles and other faculties of Ethernet Etc can be used according to the need.
7) According to the Cause and effects diagrams the System programmer assigns the control action block into the system, we can call them as memory assigning.
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