Pressure Module
TECH TIPS
Hand Pump
• 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).
Pressure Input
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-energised 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 Fluke 75x family. 480 V AC 3-phase voltages must be de-energised and disconnected from the switch if testing with the 75x family.
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.
STEP 1
STEP 2
Additional resources For more in depth information about this application check out these videos and application notes from Fluke.
STEP 3
Pressure switch video
Pressure switch application note Pressure calibration application note
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Process Calibration Tools Pre sure Applicati n
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