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24
Section 3
Model 581A Differential Pressure Switch
Table 3.2—Troubleshooting
Problem
Possible
Source
Probable Cause
Corrective Action
Switch
Drifts
(set point
not repeat-
able)
Process
Changes
Transients or surges cause
switches to actuate prema-
turely
Add time delay gages or add time
circuit
Set point and/or deadband are
too wide in pressure values
Specify DP range as low as
practical; set point repeatability
and deadband are percentage of
full range
Electrical overloads affect the
spring properties of the leaf
actuator in the switch
Examine circuits for voltage,
amperes
DC inductive loads cause
arcing and burning of contacts
Consider arc-suppression devices
or relays
Vent gas or drain liquids from
signal lines
Calibration
Techniques
Failure to check set point after
locking
Verify set point repeatability after
locking switch plate
Rapid pressure change or vent-
ing system
During calibration, make pressure
changes in slow, discrete steps
Pressure application in reverse
Test low-alarm with decreasing
pressure and high-alarm with
increasing pressure
Reference gage inaccuracy
Suitable pressure standard such
as manometer, dead-weight
tester, or Heise-type gage may be
required
Damage to switch contacts
Adjust plunger screws carefully
to avoid damage to internal parts
of switch