Evacuation Procedure

Information on some common auto air conditioning topics.
Archived Forum

Moderators: bohica2xo, Tim, JohnHere

Post Reply
User avatar
JohnHere
Preferred Member
Posts: 1197
Read the full article
Joined: Sun May 13, 2018 10:20 am
Location: South Carolina Upstate - USA

Evacuation Procedure

Post by JohnHere »

"Your Mileage Might Vary," as the expression goes, but I normally try to evacuate a system for a minimum of one hour.

Special circumstances can and do present themselves, though. For example, if a vehicle with an A/C problem comes along and the owner needs it back in about two hours, that stipulation will certainly affect how much evacuation time it gets.

If I'm working on one of my own vehicles that I'm not in a hurry to finish, I'll evacuate for four or five hours and maybe even overnight depending on what time I start working on it. If I'm getting ready to hook-up the vacuum pump at, say, 11:00 p.m., I'll do so, turn in, and allow the pump to run overnight until 6:00 or 7:00 the next morning.

On the other hand, if I start evacuating at 4:00 p.m., I might keep the vacuum pump going until around 7:00 p.m., verify that the vacuum holds, proceed to charge it, and complete the job with enough time left to have a later-than-usual dinner, relax, and then call it a day.

If I know that a system is unusually "wet" for whatever reason or that it has been open for a long time, I'll go for an overnight evacuation, if possible.

I, too, use a micron gauge to supplement the low-side gauge on my Manifold Gauge Set. And I change the oil in my vacuum pump after every second evacuation.
Member – MACS (Mobile Air Climate Systems Association)

Thankful for the responses you have received? Please consider making a monetary donation to this Forum.
70monte
Posts: 42
Joined: Fri May 13, 2016 9:03 pm

Re: Evacuation Procedure

Post by 70monte »

JohnHere wrote: Sat Aug 13, 2022 6:10 pm "Your Mileage Might Vary," as the expression goes, but I normally try to evacuate a system for a minimum of one hour.

Special circumstances can and do present themselves, though. For example, if a vehicle with an A/C problem comes along and the owner needs it back in about two hours, that stipulation will certainly affect how much evacuation time it gets.

If I'm working on one of my own vehicles that I'm not in a hurry to finish, I'll evacuate for four or five hours and maybe even overnight depending on what time I start working on it. If I'm getting ready to hook-up the vacuum pump at, say, 11:00 p.m., I'll do so, turn in, and allow the pump to run overnight until 6:00 or 7:00 the next morning.

On the other hand, if I start evacuating at 4:00 p.m., I might keep the vacuum pump going until around 7:00 p.m., verify that the vacuum holds, proceed to charge it, and complete the job with enough time left to have a later-than-usual dinner, relax, and then call it a day.

If I know that a system is unusually "wet" for whatever reason or that it has been open for a long time, I'll go for an overnight evacuation, if possible.

I, too, use a micron gauge to supplement the low-side gauge on my Manifold Gauge Set. And I change the oil in my vacuum pump after every second evacuation.
Thanks for this information. I guess I will have to re-evaluate my evacuation procedure. I don't see myself doing it over night though. Even though I change my vacuum pump oil after every use per the owner's manual, it's 16 years old and I'm not sure I trust it to run all night without periodic checks to make sure it hasn't broke down or something.
User avatar
sherqa
Posts: 12
Joined: Thu Jul 21, 2022 1:39 am

Re: Evacuation Procedure

Post by sherqa »

Here in Middle East most of the local mechanics are using vacuum pump made from air-condition compressor. I am wondering will it pull sufficient vacuum ?
User avatar
JohnHere
Preferred Member
Posts: 1197
Joined: Sun May 13, 2018 10:20 am
Location: South Carolina Upstate - USA

Re: Evacuation Procedure

Post by JohnHere »

The only way I can think of to test a vacuum pump is to attach a Manifold Gauge Set directly to the pump, run it, and see what it pulls. If you're close to sea level, a good pump should pull down to about 29.94 Inches of Mercury. At higher elevations, the vacuum reading will be correspondingly less.
Member – MACS (Mobile Air Climate Systems Association)

Thankful for the responses you have received? Please consider making a monetary donation to this Forum.
tony1963
Posts: 68
Joined: Fri Nov 27, 2020 11:37 am

Re: Evacuation Procedure

Post by tony1963 »

If the system has been open for any period of time, it contains moisture just like the atmosphere. Pulling a vacuum for an hour is typically sufficient.

There are diminishing returns and the longer the time, the less air and moisture will come out. My experience shows that you can achieve a full vacuum of 29 in about 5 minutes. Of course, keeping it under vacuum longer will boil the moisture. But, a full hour versus two hours is not going to remove twice as much moisture, maybe only less than 1% more. So, it is all relative.

You could, in theory, leave it under vacuum for a year or into infinity.
swampy 6x6
Posts: 59
Joined: Wed Dec 02, 2020 11:14 pm

Re: Evacuation Procedure

Post by swampy 6x6 »

After reaching 500 micron at least 1 hr on the vac pump ,, using 4cfm or bigger 2 stage .
8 ---12 HRS if system been left open for any period of time
Bigger diameter the better for vac hoses
User avatar
JohnHere
Preferred Member
Posts: 1197
Joined: Sun May 13, 2018 10:20 am
Location: South Carolina Upstate - USA

Re: Evacuation Procedure

Post by JohnHere »

swampy 6x6: Note that this thread is more than one year old, so you probably won't receive a response.
Member – MACS (Mobile Air Climate Systems Association)

Thankful for the responses you have received? Please consider making a monetary donation to this Forum.
Post Reply