User guide

IOM-764P / PD

SECTION III

III. PRINCIPLE OF OPERATION

1. The 764 controllers employ laminar ﬂ ow 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 ﬂ ow

of in stru ment air through the sensor to de-

velop 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 ﬂ ow through the sen-

sor is reduced which in creas es the output signal.

The proportional band restriction regulates the

ﬂ ow 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 ﬂ ow ing through the sensor decreases, which

re duc es the output signal. The proportional band

restriction regulates the ﬂ ow 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 deﬂ ect its maximum for its

highest con trolled pres sure setting and hardly

de ﬂ ects for the lowest con trolled pressure setting.

SUPPLY AIR

OUTPUT SIGNAL

EXHAUST (VENT)

LOWER SENSED PRESSURE

HIGHER SENSED PRESSURE