Robertshaw 84374 Series Quick Start Manual page 4

OPERATION (See Figure 2)
NOTE: For descriptive purposes the operation of
the direct acting (automatic reset) sensor shown in
Figure 2 will be explained. The operation of the
reverse acting model is the same although reversed.
It vents on a decrease in high process pressure.
As shown, pressure entering the CONTROL port of the
sensor will flow around the stem and sleeve, through the
bellows head, and out the VENT port. The bellows head is
at the limit of its downward travel that is restricted by the
reset/ differential adjustment screw. By limiting the
downward travel of the bellows head, this adjustment screw
defines how low the process pressure must drop before
causing the sensor to reset and stop venting. (See
Adjustments - Differential).
A decrease in the Process Pressure will cause the stem to
travel downward as the force exerted downward on the
stem by the range spring overrides the force exerted upward
by the Process Pressure. The sleeve will travel downward
with the stem until it seats on the O-ring in the bellows
head.
When the sleeve seats on the O-ring, it will block the
control pressure flowing around the stem and stop the
venting of the sensor. The control pressure will then build
around the outside of the bellows and cause the bellows to
compress. As the bellows compresses, the bellows head will
move upward carrying the sleeve with it until the tubular
portion of the bellows head contacts the upper diaphragm
seat.
Figure 2
84374-A1 Shown
(Inlet port rotated 90° from true position)
When the bellows head seats, upward travel will be
stopped and the sensor reset. The sensor will remain in this
position unless the Process Pressure increases enough to
overcome the force exerted by the range spring.
As the Process Pressure increases, the stem will be forced
upward against the range spring until travel is sufficient to
lift the sleeve off the O-ring in the bellows head. When the
sleeve lifts off the O-ring, the pressure inside the bellows
will equalize with the pressure on the outside of the
bellows.
When the pressure on the inside is equal to the pressure
on the outside, the bellows will attempt to recover to its
free length carrying the bellows head downward until
stopped by the reset/ adjustment screw. The sensor will
again be in the venting configuration as shown.
On Manual Reset models, the automatic reset/differential
adjustment screw will be replaced with a spring loaded
plunger as shown in the inset. The reset plunger will allow
the bellows to extend downward to a point the sleeve
cannot reach regardless of how much the Process Pressure
drops. For this reason, the plunger must be depressed
(while pressure is supplied to the CONTROL port) to raise
the bellows head enough to allow the sleeve to contact the
bellows head O-ring. With this accomplished, the sensor
will reset as previously described.
NOTE: Depressing the plunger without a CONTROL
PRESSURE present will NOT reset the sensor.
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