AC Evacuation/Vacuum Question

[76eldo]

I was working with Joe Caristo last night on his 78 Biarritz.

He installed a new compressor, new O rings, and all other connections look tight.

I recently bought a vacuum pump and he has a set of gauges.  After more than an hour, the vacuum pump would only pull the vacuum down to a minus 25, not the minus 30 that you normally see.

I looked on youtube at some AC videos and some said it should pull all the way to -30 and some just said needs to be below zero for over an hour.  Assuming the vacuum holds after the pump is shut off, does the -25 versus -30 make a big difference?

Thanks,

Brian

[Dave Shepherd]

If it holds  25" of vacuum for half an hour, you are good to go.

[Scot Minesinger]

I usually leave the vacuum pump on over night just to kill it.  -25 inches is OK though.  However, vacuum is to remove moisture, the longer it is at -25,(hence overnight) the more moisture will be removed.  Vacuum is not a leak test, although it will show a bad leak.

[Dan LeBlanc]

I would pressurize the system with nitrogen first before charging.  What may not leak under vacuum may leak under pressure.

[TJ Hopland]

It could just be your gauge that isn't very sensitive or accurate in the vac range.   I have a medium to upper end set that on the vac side of the scale does not have much range and have noticed that sometimes it does vary by what appears to be 5 or more with no apparent change in what its hooked to.  I recently used a friends gauge set and noticed on his the vac range takes up like 1/4 of the gauge face and is calibrated as fine as the pressure side.  Not sure what brand it was but it looked to be pretty old and high quality.    Beware of many of the inexpensive gauge sets that are available today.  I have tangled with a few of them and it was not a good experience.  Its also what lead me to spend $350 instead of $80.    Just changing to a better gauge set improved my success rate on AC systems dramatically.   Its also nice that the knobs have not broken off yet like happened on the $80 set the first time I used it. 

["Cadillac Kid" Greg Surfas 15364]

Brian,
IF you are working on an R-134a system with PAG oil it is possible that being as hygroscopic as it is, the oil is still releasing or trying to release moisture.  I would run the pump overnight and see what you have.
Greg Surfas

[TJ Hopland]

I didn't know PAG was hygroscopic.   I guess that explains why you often find a kinda slimy foamy mess. 

Where does mineral and Ester fall in the hygroscopic scale?

["Cadillac Kid" Greg Surfas 15364]

Mineral is the least.
Greg Surfas

[TJ Hopland]

Interesting.  I can see that 'feature' of PAG leading to a lot of problems.   Moisture is bad enough for an AC system to start with but if the oil is 'holding' on to it its going to be even harder to remove from the system and any that stays in there I assume is going to change the characteristics of the oil which is not going to be good for the system long term. 

So it seems we should treat PAG oil like brake fluid?  Don't keep open containers around?  Or isn't it that bad?

[76eldo]

It's an R-12 job.
Maybe it needs to be on longer.

We only left it on about 90 min the first time.

Brian.

[mikanystrom]

What altitude are you at? 

Once I ran a vacuum pump overnight and it was still "burping" the next morning (stuff cooking off in the oil under vacuum).  I was sucking out water and flush agent slowly.  There were no leaks.

[76eldo]

Approximately 213 feet above sea level.

I spoke to a mechanic friend this morning.  He said on a not-so-humid day a minus 25 reading is OK. 

Thanks,

Brian

[Scot Minesinger]

The longer the vacuum pump is running the more moisture is removed.  Almost all the a/c work I do results in an overnight vacuuming.