Holding Vacuum When Evacuating, How Long??

[philipswanson]

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.

[Tim]

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

[philipswanson]

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

[tbirdtbird]

" 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

[Tim]

" 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.

[Cusser]

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.

[DetroitAC]

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

[tbirdtbird]

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

[philipswanson]

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

[tbirdtbird]

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