SD66
3
CAUTION
Synthetic compressor lubricants in the air stream at the instrument may cause it to fail.
The requirements for a quality instrument air supply can be found in the Instrument Society of America's "Quality
Standard for Instrument Air" (ISA-S7.3). Basically, this standard calls for the following:
Particle Size — Maximum particle size in the air stream at the instrument should be no larger than 3 microns.
Dew Point — The dew point, at line pressure, should be at least 10°C (18°F) below the minimum temperature to
which any part of the instrument air system is exposed at any season of the year. Under no circumstances should the
dew point, at line pressure, exceed 2°C (35.6°F).
Oil Content — Maximum total oil or hydrocarbon content, exclusive of non-condensable, should not exceed 1 ppm
under normal operating conditions.
PRINCIPLE OF OPERATION
The input-to-output ratio is determined by the effective areas of the input diaphragm and output diaphragm
assembly. The input diaphragm of an Amplifying Relay has a larger effective area than the output diaphragm
assembly, which requires the output to be proportionally higher for the relay to be in balance (see Figure 2,
Amplifying View). Conversely, the input diaphragm of a Reducing Relay has a smaller effective area than the
output diaphragm assembly, which requires the output to be proportionally lower for the relay to be in balance (see
Figure 2, Reducing View).
Operation
An increase in input pressure forces the input diaphragm and output diaphragm assembly downward, closes the
exhaust port, and moves the plunger to open the supply port; see Figure 2. Supply pressure is admitted to the output
and to the output diaphragm assembly. The output will increase until the upward force of the output pressure acting
on the output diaphragm assembly equals the downward force of the input pressure acting on the input diaphragm.
All actions are reversed for a decrease in input pressure.
Figure 2 Schematic
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