I'll apologize up front for coming across a bit harsh but this question smacks of reasons refrigerants are regulated and why the EPA seems it necessary to require anyone servicing AC or refrigeration systems to have some certification.

The whole concept of mechanical refrigeration relies on the "latent heat of vaporization". In other words it is the change of state from liquid to gas and back that makes this possible. In order for this work the refrigerant has to bas pure as possible. Non condensable stuff like "air" or liquids that do not vaporize like refrigeration oils all reduce the efficiency of the system. Also, any air or moisture in the system will cause internal corrosion and enough water vapor can cause ice crystals that can clog parts such as the expansion valve which can cause a blockage dead heading the compressor and causing immediate damage. Part of charging an AC system is to pull a deep vacuum so that only pure refrigerant is in the system. Of course a new filter-dryer helps as the desiccant can deal with some moisture.

As for leak testing, since atmospheric pressure is only 1 bar, (14.1 psi) a vacuum is really a poor leak test. It will only find the big ones and AC gauges are not that responsive in the vacuum range. A properly equipped AC shop will pressurize the system with nitrogen or a dry air source say to 300 psi and make sure everything holds. Then after this test is successful, the vacuum is pulled. For a system that has been open this should be allowed to go at least 4 hours or more. This will boil all the contaminants out including what is absorbed in the refrigerant oil. Stepping back if using oil from an open container, put the required amount in before the vacuum.

Now the system can be charged, by weight, do not rely on pressure to determine fill, especially with R-134a. Relying on pressure will result in over fill most every time. For most Vanagons you need~1,300 grams. For R-134a you use ~85% of the amount required for R-12.

Now here is where things can get ugly. As or after filling you find that you do still have a leak or you find that another part needs repair or replacement. You have no means to recover the stuff in the system. So here you let leak out or release it yourself to get rid of the pressure. The intentional release or charging a known leaking system are violations at the federal level. While individuals get away with this any regulated shop may not. You need the recovery equipment to service AC systems.

Dennis

-----Original Message----- From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of John Rodgers Sent: Monday, July 04, 2011 5:36 PM To: vanagon@GERRY.VANAGON.COM Subject: A/C - what it the purpose of the vacuum check

What does this vacuum check do for the system in terms of servicing the A/C system.

When I put my A/C system back together next week, the pressure in the system will be ambient. Is the only purpose in pulling a vacuum check to enable the refrigerant to enter the system more easily? Or a check for leaks? And do both sides of the system et checked?

Thanks,

John

-- John Rodgers Clayartist and Moldmaker 88'GL VW Bus Driver Chelsea, AL Http://www.moldhaus.com