Automotive Air Conditioning Information Forum

When evacuating the system in a motorhome, how long should it be able to hold vacuum? Mine dropped a little overnight. Does that mean I have a leak? It held steady for a few hours when I started.

I never understood why it was suggested to watch vacuum overnight. I would charge it and do a good leak test with proper equipment.

OK, can you tell me the best method for doing the leak test? Thanks.

" Mine dropped a little overnight"
How are you determining this? Vacuum gauge?
If you are using the typical red/yellow/blue charging hoses, they are not totally impervious to vacuum, at all, and overnight (not necessary) your vacuum may decay a bit.
Generally we vacuum for 30-60 min.
Then you can puff in a trace charge maybe bring your gauges (both sides) to about 20 psi, then use an electronic sniffer, which is what we prefer here

tbirdtbird wrote: ↑Tue Jul 06, 2021 1:48 pm " Mine dropped a little overnight"

Generally, we vacuum for 30-60 min.
Then you can puff in a trace charge maybe bring your gauges (both sides) to about 20 psi, then use an electronic sniffer, which is what we prefer here

Electronic was our preferred method.

Disclaimer: although I have a fair amount of AC experience and knowledge, plus my own AC equipment, I AM NOT an AC professional !!!

I generally evacuate a system like 45 minutes (since I'm not a shop, and good time to break for lunch). I generally do a 15 to 30 minutes test to see if the system holds vacuum, then I begin to add refrigerant.

I do this for a living in a test lab. If we change a lot of components, or using something new, dry nitrogen at 10 bars (about 150 psig), we see pinholes instantly by dropping pressure readings. This is also how I do it at home, but I only have analog gouges, so I'm waiting 1/2 day or letting it go overnight. This is obviously not practical for an A/C shop working on a customer car. I'd suggest testing with pressure, a small charge of R134a, low enough pressure to make sure it's only vapor. I believe this is also allowed by EPA 609 and 608 regulations, but admittedly I haven't read them recently. 50psig and check it the next day when the ambient Air is the same temperature

It is allowed.
For example on residential/commercial you would usually puff in a trace charge of refrigerant, then fully pressurize with dry nitrogen and sniff away.
I don't think too many are using dry nitrogen in MVAC but you can still puff in a trace charge

Doesn't the dry nitrogen mix dilute the effectiveness of the sniffer?

no. it allows you to pressurize the system to 100psi. The number of refrigerant molecules present has not changed