Instrumentation Specialist

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.

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.

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.

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.

Types of bourdon tubes:

1. C type

2. Spiral

3. Helix