Cashco 764T Temperature Controller - Manual (Page 5)

IOM-764T 5
SEC TION III
sensed tem per a ture the supply air fl owing through
the sensor orifi ce in creas es, which elevates the
output signal. The pro por tion al band restriction
regulators the fl ow of ex haust through PORT A.
On reverse acting controllers the supply enters
PORT A and passes through the proportional
band re stric tion. With an increase in the sensed
tem per a ture the fl ow through the sensor or i fi ce
is increased, which reduces the output signal.
PORT B is the exhaust port.
Closing the proportional band adjusting screw
reduces the proportional band. Opening the screw
increases the proportional band.
The setpoint adjustment moves the sensor closer
to, or further away from, the sensor plate to change
the controlled temperature. Rotate clockwise
to increase temperature set point and counter-
clockwise to decrease temperature set point.
The 764T controller employs laminar fl ow to
de vel op the 3-15 psig (nominal 0.2 - 1.0 Barg)
output sig nal. Lam i nar fl ow was chosen because
it eliminates the need for range springs, levers,
pivots and other parts that pro duce friction and
lost motion.
A bimetallic thermal bulb is used to measure
tem per a ture. It is immersed in the fl uid whose
tem per a ture is to be controlled. A temperature
variation changes the length of the outer tube
more than it changes the length of the Invar rod.
This difference produces a minut movement of
the end of the Invar rod that contacts the sensor
plate. The sensor plate, in turn, throttles the fl ow
of instrument air through the sensor to develop
the 3-15 psig (0.2 - 1.0 Barg) output signal.
On direct acting controllers, see Figure 7, the
supply enters PORT B. With an increase in the
PRINCIPLE OF OPERATION III.
Figure 7: 764T Direct Action View Shows Port A & B
Rotated From Actual Position
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