Instrumentation Specialist Interview Questions And Answers

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.

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.

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

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)

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.

DN stands for Diameter Nominal. It is a flange size of the valve.

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.

A thermal relief valve is typically used to relieve over pressure due to thermal expansion of liquid in pipelines.

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.

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.

Inductive proximity switches are better suited for detection of conducting metal objects and are easily tested for proper operation. Capacitive switches are typically used for detecting non-conductive materials.

For more than 400 or 450 degree Centigrade thermocouple is used, below this temperature RTD is used?

In the process plant Control room built considering the non-hazardous area. So in case if fire/Explosion takes place in the plant than that has to be restricted from entering into the control room. So MCT(Multiple cable transient) blocks are used. They are designed to sustain the fire for a fixed time duration. That block hold the cables which are entering into the CCR.

Some tachometers provide analog output with almost instantaneous response time. They are highly suitable for speed control in some applications.

Fuse blowing sometimes. Power circuits are most likely to fail.

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.

Contacts having not potential. E.g. Relay contacts/ field switches contacts. They are used in logic circuits. A potential free contact is usually wired into an electrical circuit. However it must be ensured that the contact ratings are suitable for the service in which it is used.

For providing leak proof seal.

A) An offset is difference between a desired value and an actual value. The phenomenon of a dependent variable assuming a different set of values when the corresponding independent variable is increasing than when it is decreasing is known as hysteresis.
B) Offset is fixed bias from desired value. Hysterisis is difference between set and reset.
“ Pressure switch with setting of 0.5 kg/cm2 (head pressure included) was found in calibration report . What is the significance of head pressure?
Head pressure refers to the pressure exerted by fluid inside the impulse pipe connecting the pressure switch to the vessel. This pressure acts in addition to the actual process pressure in the vessel. Pressure acting on switch = process pressure + head pressure.

Types of bourdon tubes:
1. C type
2. Spiral
3. Helix

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.

D.P. transmitter can be calibrated using following steps:
1. Adjust zero of Xmtrs.
2. 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.
3. Perform vacuum test: Apply equal vacuum to both the sides. Zero should not shift.
4. 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.

Pretty much the same thing since both convert one type of energy into a proportional different type of energy. Like the pressure cell inside a pressure transmitter which we call a transducer or sensor and a I/P converter which we call a converter. Other examples are a analog to digital converters or the sensors in a car's engine which are also called transducers since they are stand alone and their transmitters or monitors are in a different place.

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.

The term galvanic isolation refers to electric isolation of two circuits using a transformer or an optoisolator. However a transformer provides a better isolation.