Preventing contamination of pressure standards is very important. Contamination can lead to costly repair or even replacement of pressure standards.
By far, the most frequent sources of contamination are test devices (also called test gauge, gauge, UUT or DUT) that have liquid, particulate matter or other media in them.
Supply gas (especially compressed air from a house air system that has not been dried and filtered) can also contaminate a controller. However, most users of precision pressure controllers use proper care in providing supply gas.
Vacuum pumps that are used to set sub-atmospheric pressures can also be a source of contamination. If the controller, test device, and all interconnect hardware (i.e. test system) are under vacuum, then vented or brought to a positive pressure, care should be taken to ensure that the gas that fills the test system is from a clean source. Also note that if a test system is under vacuum, and the vacuum pump is turned off, then the vacuum remaining in the test system might pull oil from the vacuum pump. One way to prevent this is to use vacuum pumps with valves on the inlet port that vent when the pump's power is turned off (most vacuum pumps from Fluke Calibration have these). Also note that if a test system is under vacuum, and a vacuum pump with a self-venting valve is turned off, then the test system will be filled with gas through that valve and through the vacuum line from the vacuum pump to the controller. Any oil filled vacuum pump might have some oil residue in its vacuum line. Traps and filters on the vacuum pump can minimize this but need to be maintained.
The most frequently contaminated pressure standards, that are the most difficult to decontaminate, are gas operated pressure controllers (like Fluke Calibration 6270A, PPC4, 7250, 7252).
What is contamination? Contamination is anything that you don't want to get in your pressure standard. With a gas operated pressure controller contaminants can be water, oils, fuels, alcohol, other liquids and/or solids (particulates, dirt). Note that if a pressure gauge is normally used to monitor a water line it will be full of water. This isn't a contaminant when the gauge is in use, but it is a contaminant if you're going to calibrate the gauge with a gas operated pressure controller.
How to prevent contamination of a gas pressure controller?
* Clean the gauges before calibrating them - There are a limited number of gauge cleaning systems available. At it's most basic form, a gauge cleaner could be draining (as best possible) a gauge. An additional step could be filling the gauge with a cleaning liquid, then draining. Vacuum and/or positive pressure might assist with this. If doing this make sure that the gauge can withstand the vacuum without being damaged. See this article for more details and information on Gauge Cleaning.
* Include contamination prevention as part of the gas pressure controller or the calibration system
1. The 6270A pressure calibrator has the Contamination Prevention System (CPS) available as an option. The gauge is mounted on the CPS and is pressurized by the 6270A pressure controller. Pressure is decreased through purge and vent valves on the CPS, preventing contaminants from entering or passing through the 6270A pressure controller. The internal filters are accessible for cleaning or replacement.
2. The 8270A, PPC4, PPC3, PPC2-AF and PPC2+ pressure calibrators have models of Self Purging Liquid Trap (SPLT) available.
* For the PPC controllers, the SPLT is an inline filter with a vent valve. The vent valve is automatically opened by the pressure controller to vent the pressure when it's below 100 psi (700 kPa or 7 bar). It is not continuously active for decreasing pressure like the CPS. The internal volume of the SPLT is about 20 cc and is not effective for liquid contaminant volumes greater than about 10 cc. The SPLT is rated to 2000 psi (14 MPa or 140 bar). See the SPLT, PPC3 and PPC4 manuals for more information on operation and use of SPLT. The filter housing itself is by Parker and could be used with a 7250, 7252 or 7750. It is Parker model 95S6 and is rated to 5000 psi. See the attached data sheet from Parker.
Product page is here, SPLT Self Purging Liquid Trap
* For the 8270A controller see the SPLT-40M instructions here SPLT-40M Instructions (flukecal.com)
3. The 7250, 7252 and 7750i pressure calibrators have the 7000-102 Liquid Trap available as option. The Liquid Trap has a sump to collect liquids and a manually operated drain valve. Internal volume is 150 mL (9 in2) and is rated to 3000 psi (20 MPa or 200 bar).Product page is here, 7000-102 Liquid Trap
4. The PIDT600 dirt traps are smaller than other devices and have a small sump to trap contaminants.
PIDT600-2 (1/8 NPT, 300 psi/20 bar)
PIDT600-8 (1/4 BSP, 3000 psi/210 bar). Similar to above but aluminum housing (not see through)
PIDT600-9 (1/4 NPT, 3000 psi/210 bar)
The CPS, SPLT and Liquid Trap have limited use as gauge cleaners. In general, a gauge is pressurized to about 100 psi (700 kPa or 7 bar) and the device is used to purge and/or vent. See the product manuals for 6270A, SPLT and PPCx for more instructions.
Both gauge cleaning and contamination prevention can be effective at preventing contamination of gas operated pressure controllers.
For questions contact Fluke Calibration Pressure Technical Support at Pressure Technical Support or by calling +1.877.355.3225. For quotes on any of the above products contact Sales at Fluke Calibration Sales