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22
Section 3
Model 581A Differential Pressure Switch
Example: Scale has range of 0-60 psid. Set point is 24 psid, with decreasing
pressure, (24/60 x 10 = 4).
1. Move the index pointer (item C) for the low switch from division 0 to
division 4 tick mark.
2. Tighten the switch lock (Item B) to a snug position, then turn an ad-
ditional 1/4 turn to place the set point within ±2% of full scale. Do not
overtighten.
Repeat steps 1 and 2 for the high switch, moving the index pointer from divi-
sion 10 to division 4.
Changing Set Point of an Out-of-Service Instrument
Use the following procedure to set a set point for an instrument that is out of
service—typically, an instrument that is disconnected from process lines or
mounted on a bench.
1. Drain and vent the housings.
2. Attach the calibration pressure source (air or N
2
) to the DPU HP housing.
3. Apply varying pressures in discrete steps and observe pointer readings
for accuracy. Use a pressure standard (Heise gage or equivalent) for
reference. A “bleed-pressure” method may cause errors.
4. Change the switch set point as described in
Changing Set Point of an In-
Service Instruments, page 21
.
5. Check the set point by changing measured pressure to actuate the switch.
For example: To verify a low-switch set point of 24 psid, apply approxi-
mately 30 psi. Reduce pressure to approximately 25 psid, hold for a few
seconds, then continue in 1/4 psid steps until the switch actuates. If the
set point is incorrect, continue instructions in
Changing Set Point of an
In-Service Instruments, page 21
.
6. To measure the switch deadband, reduce the pressure to zero, then in-
crease pressure until the switch resets.
7. To verify repeatability of a set point, repeat step 5 several times. For
improved accuracy, use smaller increments of pressure. Allow extra time
for gages with a slow response and for test systems that have long runs of
small-bore tubing.
High-alarm switches (right side of the scale) are adjusted in a similar manner.
Apply increasing pressure to establish the switch set point, and apply decreas-
ing pressure to measure the deadband.
Range Changes
The range of a Model 581A differential pressure switch can typically be
changed by replacing the range spring assembly inside the DPU. In some
cases, the replacement of the BUA may be required. See
Changing the DPU