PeachParts Mercedes-Benz Forum - View Single Post

Diesel Giant · #11 03-30-2006, 09:51 PM

Ok here goes. Yes the system WILL hold a deep vacuum, which is about 250-500 microns. We use a deep vacuum on all A/C systems. A vacuum is just really low pressure. You cannot really do a complete pressure test with the system in a vacuum. Why? Becaue the difference in perssure is only 14.7:1. You A/C system runs at much higher pressure differentials when under operation. To test the system properly, you must pressurize it with Nitrogen and a trace amount of R-22 to 300psi. Then if it does not leak, you can say its leak free. Almost no body does this on automotive A/C systems.
On residential and commercial systems this is much more common.

Now your gauge set up is designed to read pressure accurately, and much less accurately, vacuum. That is why we use the micron gage to get an accurate vacuum reading. But you probably dont have one. After pulling a deep vacuum or what you think is a deep vacuum according to your A/C gages, then you let the system rest for about 1-2 hours. If the vacuum decreases some (pressure rises), then you have moisture that has turned to vapor and is raising the pressure in the system. (Dalton's Law of Confined gases, the total system pressure is the sum of the pressure of the individual gases in the mixture) Vapor was present and hence raised the pressure. You then start a deep vacuum again. A good rule of thumb for a auto system and not using a micron gage is about 45-60 minutes of running time with the vacuum pump. You can do this cycle a few times and then should remove most all moisture and no increase in pressure after sitting.

If the gage reading increase so much that it moves out of a vacuum then the atmosphere pressure is pushing into the sytem and that means you have a leak.

So by your figures it looks like you need to do a few more cycles with the vacuum pump.

You did change the drier I hope, before you did all this?