A/C vacuum pump specs

I used to have an old A/C vacuum pump, but I sold it because it was leaking oil and not making much vacuum anymore. Now I need to invest in one as I have a couple A/C projects coming up.

What specs should I look for? I saw some specs listed on some pumps at HF and realized that I know nothing about them. They're often listed as X microns of vacuum. Microns are a unit of distance. WTF. What specs are good and bad? I know that CFM makes quicker work, but I don't mind leaving the pump on for a half hour, but how much vacuum do I need?

curtis73 wrote: They're often listed as X microns of vacuum. Microns are a unit of distance. WTF.

Micrometer of Hg.

In reply to curtis73:

Water boils at 29.2 inches of Hg at typical room temp of 72F, so you need a pump that can do that long enough to boil all the moisture out of the system. In my experience, many lower priced vacuum pumps aren't capable of attaining and/or holding the needed vacuum long enough to boil all the moisture out, so buying one is a waist of money. Stick with Robinair, Yellow Jacket, or some other brand name.
Atmospheric pressure is equal to about 760k microns. When applying a vacuum, the microns go down, so the lower the micron rating the better. The exact amount depends on temperature, but at room temp you'll need to be somewhere around 10k Microns or lower. You also need to pay attention to the CFM rating, and a minimum of 5 CFM is best for automotive A/C applications.

I don't know the specs but I bought the HF model that hooks up to my Air compressor. I have been very impressed with it for the very low cost and small size. I have only used it on my BMW which only has a small leak (charge lasts all summer).

Chris

vacuum pumpI see it will only pull 28.3 inches so it may not be good enough, but it worked for me to get mine up and running.

Chris

bravenrace wrote: In reply to curtis73: Water boils at 29.2 inches of Hg at typical room temp of 72F, so you need a pump that can do that long enough to boil all the moisture out of the system. In my experience, many lower priced vacuum pumps aren't capable of attaining and/or holding the needed vacuum long enough to boil all the moisture out, so buying one is a waist of money. Stick with Robinair, Yellow Jacket, or some other brand name. Atmospheric pressure is equal to about 760k microns. When applying a vacuum, the microns go down, so the lower the micron rating the better. The exact amount depends on temperature, but at room temp you'll need to be somewhere around 10k Microns or lower. You also need to pay attention to the CFM rating, and a minimum of 5 CFM is best for automotive A/C applications.

Altitude will make a difference too ... Not sure whether the op is at sealevel or not

Ok. So I can convert from mMHg to inches Hg. Got it. As long as it pulls more than 29.2 I'm good. Thanks!

Slippery wrote: Altitude will make a difference too ... Not sure whether the op is at sealevel or not

I'm right about 1000' above sea level, but I often travel to places which are much lower altitudes, and rarely much higher than 2000'

Most decent vacuum pumps will pull to 29.9"Hg, which will cover almost any altitude differences. Altitude really doesn't make any difference anyway, as the pressure measured is inside an isolated system.

curtis73 wrote: I used to have an old A/C vacuum pump,

ewwwwww...

hey, can you send it back when you're done with it?

bravenrace wrote: Most decent vacuum pumps will pull to 29.9"Hg, which will cover almost any altitude differences. Altitude really doesn't make any difference anyway, as the pressure measured is inside an isolated system.

Ok... so I have "convert" software, but it doesn't include micrometerHg. So, I turned to the intarwebz and decided to convert 30"Hg to micrometersHg to know how to shop for a pump.

Um...... 30"Hg is equal to about 763,000 microMHg?

Most pumps I've seen are rated in the 15-25 micron range. What am I doing wrong?

In reply to curtis73:

I've never used microns to rate a pump, only "Hg, but I'll look when I get back to work tomorrow and let you know.

curtis73 wrote:

bravenrace wrote: Most decent vacuum pumps will pull to 29.9"Hg, which will cover almost any altitude differences. Altitude really doesn't make any difference anyway, as the pressure measured is inside an isolated system.

Ok... so I have "convert" software, but it doesn't include micrometerHg. So, I turned to the intarwebz and decided to convert 30"Hg to micrometersHg to know how to shop for a pump. Um...... 30"Hg is equal to about 763,000 microMHg? Most pumps I've seen are rated in the 15-25 micron range. What am I doing wrong?

Looks like microns are backwards -- 0 is an absolute vacuum, 760,000 is atmospheric pressure.

http://www.engineeringtoolbox.com/vacuum-converter-d_460.html

Ah... ok.

Looks like I would need a pump capable of 0.75 microns in order to get 29.89"Hg. 1.0 microns would be 29.88. But if I buy a more common 15 micron pump I'm only getting about 29.2"Hg. I suppose that would be OK as long as I vacuum on a day that the temperature of the components is higher than STP? Or is 29.2" enough for all intents and purposes?