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Hand
Pump
Pressure
Module
Pressure Input
•
The key to a good switch
test is repeatability.
Repeatability is best
achieved by applying a
slow change in pressure to
the switch as it approaches
its set or reset pressure.
•
When performing the test
find out where the switch
sets and make sure the
vernier/fine adjustment of
your test pump has enough
adjustment to vary the
pressure up to the set point.
In this way the pressure
can be changed slowly
capturing an accurate
switch set point pressure.
Repeat this procedure for
the reset point.
•
With practice you can get
the vernier of the pump
within range of the set and
reset point pressure and
get excellent repeatability
of your tests (within the
limitations of the switch
being tested).
TECH
TIPS
To get started testing the switch, connect as shown above. In this example
we will test dry contacts and continuity. To measure continuity for the test
select resistance measurement. Then toggle to the source screen mode
and select pressure to display the pressure generated by the hand pump
and measured by the pressure module. Advance the calibrator mode to the
split screen test mode.
The next step is to describe the switch and whether it is normally open
or closed at ambient pressure. The relaxed state of the switch is the
reset state. The set state is the condition of the switch it changes to with
applied pressure or vacuum. In this example the switch is normally open
and is expected to close when the pressure applied exceeds 10 psi. Next
the allowable pressure variance of the switch set state and deadband size
needs to be defined. In this example the ideal switch set value is
10 psi and is allowed +/- 1 psi of deviation. The allowable reset pressure
is described in the deadband tolerance. In this instance the reset state
must be more than 1 psi less than the found set pressure but not greater
than 3 psi less than the found set pressure.
Once the test tolerances are fully defined start the test. Increase the
pressure until the calibrator captures the set state pressure value. Then
decrease the pressure until the reset pressure is found. Repeat increasing
and decreasing the pressure across the switch looking for repeatability in
your set and reset pressure measurements. Once satisfied with the result
press done to get the pass/fail evaluation of the switch. If the switch fails
the test adjustment or replacement of the switch may be required. If the
switch is adjusted repeat the test to document the As-Left condition of the
switch before putting back into service. The test result is now documented
and ready for upload to calibration management software.
To perform the test:
With a modern documenting calibrator you can test for dry contacts opening and
closing on the switch or if you are using the Fluke 753 or 754 you can leave the
switch connected to the live voltage and the calibrator will measure the changing
AC voltage and interpret it as opening and closing of the switch.
One cautionary note: it is always safer to test a de-energized circuit, but this is not
always possible. Also, do not measure AC voltages above 300 V ac as that is the
maximum rating of the 75X family. 480 V ac 3-phase voltages must be de-energized
and disconnected from the switch if testing with the 75X family.
Additional resources
For more in depth information
about this application check out
these videos and application notes
from Fluke.
Pressure switch video
Pressure switch
application note
Pressure calibration
application note
STEP
1
STEP
2
STEP
3
33
Pressure Applications