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OPERATION - See Figure 2
In the direct-acting models, process pressure
pushes the stem against the range spring. The bellows
head moves toward the range spring and compresses
the bellows due to the inlet port (control) pressure.
When the process pressure reaches the range spring
setpoint, the stem pushes the sleeve off its O-ring
seat. Inlet pressure flows past the O-ring and into the
inside cavity of the bellows. This equalizes the
outside-to-inside pressure of the bellows and allows
the spring-action of the bellows to "snap" the bellows
head toward the reset and differential adjustment
screw. The inlet port pressure is thus vented past the
diaphragm seat and out the vent port.
point, the range spring forces the stem and sleeve
toward the bellows head. In the automatic reset
models, the bellows head is stationary at this time
against the reset and differential screw. This
stationary position allows the sleeve to seat itself on
the O-ring after sufficient process pressure is lost. In
the manual reset models, the reset button must be
pressed to force the bellows head toward the sleeve
to allow seating as the sleeve itself cannot move a
distance sufficient to contact the O-ring. This sleeve
and O-ring contact causes the loss of inlet port
pressure inside the bellows. The bellows head moves
toward the range spring and resets the sensor.
process pressure.
PROCESS PORT
4
With the lowering of process pressure below set-
Reverse-acting models vent on a decrease in
Figure 2
84370-A 1 shown
(Inlet port rotated 90° from true position)
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