Instrumentation Specialist

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.

Refer to instruction manual for the positioner / control valve. The cams are often marked with limited amount of information, which may help an experienced person.

ATEX/FM/CSA certifications generally refer to certification for suitability of instruments for use in hazardous area when installed in accordance with recommended guidelines. Any certification, which is locally acceptable as per statutory requirements, may be used. We typically accept American/European/Indian certifications/approvals in India.

Transmitter : Tube routing, maintenance area, man approach, Valve : Hand wheel operations, Maintenance area, Etc. Loop power indicators: Man approach Illumination from Electrical if instrument is not going to provide.

Pressure gauge includes following components:

a. ‘C’ type bourdon tube.

b. Connecting link

c. Sector gear

d. Pinion Gear

e. Hair spring

f. Pointer

g. Dial

Use of hair spring: Hair spring is responsible for controlling torque. It is also used to eliminate any play into linkages.

Primary elements of measuring pressure are:

a. Bourdon Tube

b. Diaphragm

c. Capsule

d. Bellows

e. Pressure springs

These elements are known as elastic deformation pressure elements.

Seal pots or condensate pots are used to provide buffer volume of liquid for impulse lines where wet leg measurement is used. Small quantities of draining from these lines produces very small changes in head pressure and therefore reduces errors compared to the case when no seal pots are used. Also in many seal pots it is possible to fill liquid into the impulse line when the line is completely drained.

Not mandatory. This is controller specification. Direct OR reverse.

This can be typically achieved by two ways:

By connecting o/p of one I/P converter to two positioners adjusted suitably for split range operation of control valves. Taking two AO from DCS. Split range to be defined in DCS. Both I/P converters and positioners to be calibrated with input as 4to20 ma dc and 3to15 psi respectively.

Usually there is a blocking diode to protect the transmitter against supply reversal and almost zero current signal should be transmitted.

The design engineers / equipment manufacturers follow/publish certain guidelines w.r.t. different types of cables and the voltages/currents and types of signals carried by them.

A minimum loop resistance is required so that modulated current signal produces a modulated voltage signal, which may be detected by the receiving equipment.