22
Section 3
Model 580A 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
Changing the range of a Model 580A differential pressure switch typically
requires replacement of the DPU. Contact factory for assistance.
