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What do gauges show when vacuum boils off moisture?

JimM on Mon June 30, 2014 2:08 AM User is offline

Year: 1998
Make: Ford
Model: Explorer
Engine Size: 5.0 V8
Refrigerant Type: R134A
Ambient Temp: 80F
Country of Origin: United States


Thanks for the great forum!

I had a question about pulling a vacuum.

When I turn on the vacuum pump and open my low and high gauges, it pulls the system down to a steady -29Hg in about two minutes. If I close both manifold knobs to isolate the system after 2 minutes at a steady -29Hg, what should happen on the gauges?

If I get a slow rise from -29 back to 0 over about 10 minutes of being isolated, is this just the moisture boiling off and increasing the pressure in the system? Or does it definitely indicate a leak coming in from outside air?

This system has been open to the air for a few hours while replacing a condenser with a big hole in it. I'm trying to test for leaks before putting in the new accumulator/dryer and vacuuming for the 1hr recommended before adding oil and the refrigerant charge. I figured to pull a quick vacuum for a leak test between tightening bolts and replacing o-rings, but how long do I need to pull to get an accurate pass/fail about the seal of the system and not a false positive because of moisture boiling off inside the system?

Any help would be very appreciated, Thanks!

wptski on Mon June 30, 2014 8:11 AM User is offline

If it rises to zero in ten minutes, you have a leak. If it's moisture, it will rise but using analog gauges, you'd never see it. In order to see that increase, you'd need a micron gauge which normally reads starting around 29"hg. There are 24500 microns in an inch, if moisture is present it will rise to around 2500 and level off. The boiling of moisture starts below 500 microns. Using a analog gauge set "assumes" that you are below 500 microns because it's below 29"hg or looks like it on your gauge.

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