This topic came up because I was testing out the new MR45 digital recovery machine and that machine goes off by itself when it hits a 20″ Hg vacuum. This is a cool feature but it is good to know when that level of vacuum is overkill and when it’s not enough according to EPA requirements.
Why would you need to recover into a vacuum you might ask? Well, so long as you are above a PERFECT VACUUM (and you always do) there are still molecules of refrigerant in a system even at 0 pisg (14.7 PSIA at sea level). In low pressure systems like centrifugal chillers the entire system charge can often be in a vacuum when the system is off, this means that recovery on these systems means you START below 0 PSIG and go down from there.
First off let’s pretty much assume that none of you are using recovery machines OLDER than 1993 so really only look at the right side of the chart above.
If you are working on an air conditioning system with UNDER 200 lbs you are safe taking your recovery to 0 or atmospheric pressure. If the system you are working on has OVER 200 lbs of refrigerant or if you are working on a medium pressure or low pressure system you will need to pull the system into a vacuum.
The EPA does make an exception if the system has a know leak and pulling into a vacuum will result in contamination of the recovered refrigerant. Here is an excert from the EPA final rule summary from 1995 (still in force)
Also let me clarify that 25mm hg absolute is another way of saying 25 torr or 25,000 microns, it’s just a finer scale and it goes from 760 torr (760,000 microns) down as the vacuum gets deeper whereas inches of mercury (“hg) goes up as the vacuum gets deeper.