IOM-764P / PD
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SECTION III
III. PRINCIPLE OF OPERATION
1. The 764 controllers employ laminar flow to
pro duce the 3-15 psig (nominal 0.2–1.0 Barg)
output signal. Lam i nar flow eliminates the need
for range springs, levers, pivots and other
parts that pro duce friction and lost motion.
2. The sensing diaphragm in the 764P and 764PD
has a high spring rate and any change in the
sensed pressure produces a minute diaphragm
movement which strokes the sensor plate.
The sensor plate, in turn, throttles the flow of
in stru ment air through the sensor to develop
the 3-15 psig (0.2-1.0 Barg) output signal.
3. On direct acting 764P controllers (see Figure 7)
the supply air enters PORT A and passes through
the pro por tion al band restriction. With an increase
in the con trolled pres sure the flow through the
sen sor is reduced which in creas es the output
signal. The proportional band restriction regulates
the flow rate of supply air into PORT A. PORT B
is the ex haust port. A 764PD op er ates the same.
4. On reverse acting 764P controllers (see Figure
8) the supply enters PORT B (not PORT A). With
an increase in the controlled pressure, the supply
air flow ing through the sensor decreases, which
re duc es the output signal. The proportional band
restriction regulates the flow of exhaust through
PORT A. A 764PD operates in a similar manner.
5. Closing (CW rotation) the proportional band
ad just ing screw re duc es the proportional band.
Open ing the screw in creas es the pro por tion al band.
6. The setpoint adjustment positions the sensor so
the diaphragm must deflect its maximum for its
highest con trolled pres sure setting and hardly
de flects for the lowest con trolled pressure setting.
SUPPLY AIR
OUTPUT SIGNAL
EXHAUST (VENT)
LOWER SENSED PRESSURE
HIGHER SENSED PRESSURE
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