Holding Vacuum When Evacuating, How Long??

Friendly format provided to inquire about automotive a/c systems.
Archived Forum

Moderators: bohica2xo, Tim, JohnHere

philipswanson
Posts: 73
Read the full article
Joined: Fri Jan 04, 2019 9:32 am

Holding Vacuum When Evacuating, How Long??

Post by 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.
User avatar
Tim
Site Admin
Posts: 1104
Joined: Tue Apr 12, 2016 2:19 pm
Location: Scottsdale, Arizona
Contact:

Re: Holding Vacuum When Evacuating, How Long??

Post by 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.
------------------------------
Please support ACKITS.com for your Auto A/C Parts and Tool needs.

Help Support the Forum
philipswanson
Posts: 73
Joined: Fri Jan 04, 2019 9:32 am

Re: Holding Vacuum When Evacuating, How Long??

Post by philipswanson »

OK, can you tell me the best method for doing the leak test? Thanks.
tbirdtbird
Preferred Member
Posts: 1241
Joined: Sat May 02, 2020 1:48 pm
Location: Texas

Re: Holding Vacuum When Evacuating, How Long??

Post by 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
When considering your next auto A/C purchase, please consider the site that supports you: www.ACKits.com
User avatar
Tim
Site Admin
Posts: 1104
Joined: Tue Apr 12, 2016 2:19 pm
Location: Scottsdale, Arizona
Contact:

Re: Holding Vacuum When Evacuating, How Long??

Post by Tim »

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.
------------------------------
Please support ACKITS.com for your Auto A/C Parts and Tool needs.

Help Support the Forum
User avatar
Cusser
Preferred Member
Posts: 846
Joined: Sun Apr 24, 2016 6:29 am

Re: Holding Vacuum When Evacuating, How Long??

Post by 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
Preferred Member
Posts: 267
Joined: Mon Oct 17, 2016 10:33 am
Location: SE Michigan

Re: Holding Vacuum When Evacuating, How Long??

Post by 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
Preferred Member
Posts: 1241
Joined: Sat May 02, 2020 1:48 pm
Location: Texas

Re: Holding Vacuum When Evacuating, How Long??

Post by 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
When considering your next auto A/C purchase, please consider the site that supports you: www.ACKits.com
philipswanson
Posts: 73
Joined: Fri Jan 04, 2019 9:32 am

Re: Holding Vacuum When Evacuating, How Long??

Post by philipswanson »

Doesn't the dry nitrogen mix dilute the effectiveness of the sniffer?
tbirdtbird
Preferred Member
Posts: 1241
Joined: Sat May 02, 2020 1:48 pm
Location: Texas

Re: Holding Vacuum When Evacuating, How Long??

Post by tbirdtbird »

no. it allows you to pressurize the system to 100psi. The number of refrigerant molecules present has not changed
When considering your next auto A/C purchase, please consider the site that supports you: www.ACKits.com
Post Reply