In a pneumatic control system, compressed air is supplied to the controlling element. As the value of the measured variable changes, the pneumatic output of the controlling element changes with it. A flapper-nozzle mechanism provides the means of controlling pneumatic output.
Presently most pneumatic instruments are designed to give an air output of 3 PSI or 0.2 Kg/ cm2 at zero scale reading and 15 PSI or 1.0 Kg/ cm2 at maximum scale. Prior to about 1947, the span of pneumatic insts. Were different for various manufacturers. Such as 2-14,3-15,3-18,5-20 PSI etc. were not uncommon. This made interchange difficult . Since 1949, most makers have standardized on the 3-15 PSI or 0.2-1.0 kg/cm2 span. This brings us back to lower pressure on the nozzle system. The nozzle system operates under pressure much below the 15 PSI maximum of the relay output. The pressure change on the nozzle system for the 3-15 PSI output change in the relay system may not be over ¾ PSI.
This brings out the fact that the relay must be an amplifier of the nozzle pressure. Change for a given pointer change is called the amplification factor or gain of the relay. Thus if a nozzle pressure of two pounds causes an output pressure change of 10 pounds in the instt. output , the gain of relay is 5. The increased output from the relay was connected to two exactly equal spring and bellows assemblies. The receiving unit is adjusted so that the recorder reads zero on the chart at 3 PSI or 0.2 Kg/cm2 and maximum chart reading of 15 PSI or 1.0 Kg/cm2. When the flapper is moved against the nozzle ,air can not escape and maximum air pressure passes to the amplifier. When the flapper is moved away from the nozzle, air can escape, thus reducing the amount of air pressure to the amplifier. As the the flapper moves from one extreme position to another , it serves to control the amplifier , which produces an air pressure proportional to the measured variable and of sufficient signal strength of transmission over the required distance.
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