Pneumatic Control may be considered old but it is still alive and not yet dead.

Although controls in process industries has moved forward from analog 4 to 20 mA to complete digital controls, nevertheless pneumatics have remained ‘active’and finds its place in quite a few applications.

It is interesting to note that in the era of 4 to 20 mA, HART, FIELDBUS,

PROFIBUS etc. final control element/ components in the control loop are pneumatic devices. Electro Pneumatic converters, Valve positioners, Valve actuators, Damper drives, etc are the unique examples.

Pneumatic instruments therefore remained solidly entrenched and recognized as critical or rather important part of today’s ‘ HYBRID CONTROL SYSTEM’.

In the process industry where response time is much less of an issue and safety is a concern, pneumatic instruments find its usage. Pneumatic instruments by their very nature are passive device and are intrinsically safe. Pneumatic instruments are often abound in the hazardous process situation and installed cost in ‘Hazardous Area

Classification’ location is very much comparable to all electronic products.

Another interesting advantage of pneumatic instruments like transmitter, controller, chart recorders etc. are, in the event of power outage all these instruments continue to run and record process conditions which can be used to get safely on line manually in he event of such demand occurs.

Because pneumatic instruments are easy to understand and operate , offers reliable operations, long installed life, immune to electrical interference .

A direct nozzel system, which uses force balance principle is basis of many pneumatic instruments.

The high & low pressures are connected to opposite sides of a twin diaphragm capsule and the force on the capsule is transmitted thru’ a flexure to the lower end of the force bar. The diaphragm seal serves both as function for the force bar and as seal for the pressure chamber. The force is transmitted thru’ the flexure connector to the range bar which pivots on the range adjustment wheel.

Any movement of the range bar causes a minute change in the clearance between the flapper and nozzel.This produces a change in output pressure from the relay to the feedback bellows until the force in the feedback bellows balances the force on the diaphragm capsules. The output pressure which is established by this force balance is the transmitted signal.

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