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  #1  
Old 03-30-2006, 08:53 AM
R Leo's Avatar
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A/C Vacuum Question

I've lot plently of hermetic systems under my belt but it's been years since I've done a car a/c. Consequently, I need some guidance on evacuating my a/c system.

I've turned the corner on Sybill, the SDL, by installing the new evaporator and starting to reassemble the HVAC 'object' and installing it in the car. Remember, I've replaced the condenser, filter-dryer, compresssor and, evaporator on this car.

The refrigeration connections are now all made and I decided it would be a good idea to pull a vacuum on the system to make sure there weren't any leaks. I ran the pump (a Robinair vane pump) for about 15 minutes and shut it down, closed the valves on the refrigerant gauges and let it sit over night.

Immediately after shut down, the low side gauge indicated approximately 29" of vacuum. After 10 minutes, there was little change in the vac reading (came up maybe 2-3").

After sitting overnight, the vacuum was still there but had decreased to about 18".

I know that mechanically connected refrigerant systems can't hold a deep vacuum but what can they be expected to maintain? Based on the info above, is my system 'tight'? Also, I'm suspicious of the '134a 'adapters' I bought. These gizmos permit you to connect a set of standard R12/R22 hoses to 134a quick connect...they aren't the greatest things I've ever seen so I suspect that they can leak too.
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Last edited by R Leo; 03-30-2006 at 08:59 AM.
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  #2  
Old 03-30-2006, 09:19 AM
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Quote:
Originally Posted by R Leo
Based on the info above, is my system 'tight'?
No.
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  #3  
Old 03-30-2006, 10:02 AM
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I wouldn't be so fast to condemn it after 15 minutes on the pump. There could still be moisture in the system.
Evacuate for another hour and let it sit overnight again.

When did you replace the filter dryer? If it's not brand new then you need to replace it.

The system should hold a deep vacuum why would you think it wouldn't? The fittings almost always leak. They should have caps which should be on and tight at all times when the gauges arre not connected. This is for leaks and to keep them clean.

FYI your procedure is wrong. First you shut the valves then you turn the pump off. Or in other words, the pump goes on first and off last.

DAnny
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Last edited by dannym; 03-30-2006 at 10:07 AM.
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  #4  
Old 03-30-2006, 10:09 AM
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That's too much of a drop. A tight system will show almost no movement overnight - perhaps a needle's width on the low side guage.

However. Has the system been run since the condensor/compressor/dryer were replaced? The 'used' oil can hold a surprising amount of refrigerant, even while open to the atmosphere. Your drop in vacuum may be due to refrigerant migrating out of the oil under vacuum.

Basically, we can't tell if your system is tight or not. Vacuum it for several hours - perhaps 4 - then let it sit overnight. If you get a lot less vacuum loss, that's a good sign.

- JimY
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  #5  
Old 03-30-2006, 10:55 AM
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15 minutes may not be enough to boil off all the moisture. Also, you're not going to charge it with 134a after all that work, are you? Besides the better performance, R12 will develop lower system pressures, which will let your compressor run quieter and longer.
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  #6  
Old 03-30-2006, 11:00 AM
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Quote:
Originally Posted by dannym
When did you replace the filter dryer? If it's not brand new then you need to replace it.
Replaced the drier with the condenser, compressor et al, ad nausaeum. When I discovered that the evap was leaking, I shot some refrigerant into the system and left it in there (it would maintain about 30psi...above that it leaked) until I pulled it apart and reassembled it yesterday.

Quote:
Originally Posted by dannym
The system should hold a deep vacuum why would you think it wouldn't?
I know for a fact, that a mechanically connected, belt-driven system will not hold a deep vacuum. And that they generally leak around the compressor drive shaft because it is designed to hold pressure, not a vacuum.

Quote:
Originally Posted by dannym
FYI your procedure is wrong. First you shut the valves then you turn the pump off. Or in other words, the pump goes on first and off last.
I know that. I wrote that wrong; In practice, I closed the valves and then shut the pump off.
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  #7  
Old 03-30-2006, 11:16 AM
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Quote:
Originally Posted by jcyuhn
That's too much of a drop. A tight system will show almost no movement overnight - perhaps a needle's width on the low side guage.

However. Has the system been run since the condensor/compressor/dryer were replaced? The 'used' oil can hold a surprising amount of refrigerant, even while open to the atmosphere.

Basically, we can't tell if your system is tight or not. Vacuum it for several hours - perhaps 4 - then let it sit overnight. If you get a lot less vacuum loss, that's a good sign.

- JimY
Thanks!

The system was never 'run' because the evap was leaking. However, it would hold pressure (10-20psi) so I shot gas into it while I waited for the evaporator to arrive.

I'll run the pump for a few hours when I get home and then let it sit overnight to see what happens.
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  #8  
Old 03-30-2006, 12:47 PM
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Is there still water in the system after 15 minutes on the pump? Of course.

Could water in the system by itself completely account for the observed rise in pressure? I don't see how it could, but I'm ready for someone who knows more about it to educate me!

Think about it this way. If one had liquid water present in part of a system, then evacuated the system, and then isolated said system from the vacuum pump by a valve (still with some liquid water present in the sytem), what would the pressure of the system be when equilibrium has been reached between water (liquid) and water (vapor)? It's a function of temperature, so consider it for a typical ambient temperature for Randy.

Of course, it's a fine idea to give the system more time on the pump and repeat the rate-of-rise experiment. Nothing trumps reality. We expect a report, Randy!
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  #9  
Old 03-30-2006, 09:14 PM
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R Leo,

This site (the one linked) is considered the premier MVAC forum by many. The thread in the link discusses your question. The pro tech (screen name "Nacho") is very knowledgable. So are Warren W., iceman, Tom G., GMTech and some others. Many pros on this site.

http://acsource.net/acforum/viewtopic.php?t=1420&highlight=vacuum+overnight
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Last edited by jbaj007; 03-30-2006 at 09:22 PM.
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  #10  
Old 03-30-2006, 09:35 PM
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Quote:
Originally Posted by eskimo
Could water in the system by itself completely account for the observed rise in pressure? I don't see how it could, but I'm ready for someone who knows more about it to educate me!
If I understand the question, I don't believe you will see a significant increase in pressure at ambient temperatures. Lets assume the temperature is 70F, at that temperature water turns to vapor (boils) at 0.36 psia (that's a vacuum of about 14.3 psig, or 29 in of Hg). Therefore, if you pump the system down to 29 inches some of the water will turn to vapor until the partial pressure of the water vapor reaches the equilibrium pressure. At that point the water should stop turning to vapor until the system is pumped down again. IMHO, the only way the pressure would continue to increase would be to increase the temperature. Is that what you were asking?
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  #11  
Old 03-30-2006, 09:51 PM
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Ok here goes. Yes the system WILL hold a deep vacuum, which is about 250-500 microns. We use a deep vacuum on all A/C systems. A vacuum is just really low pressure. You cannot really do a complete pressure test with the system in a vacuum. Why? Becaue the difference in perssure is only 14.7:1. You A/C system runs at much higher pressure differentials when under operation. To test the system properly, you must pressurize it with Nitrogen and a trace amount of R-22 to 300psi. Then if it does not leak, you can say its leak free. Almost no body does this on automotive A/C systems.
On residential and commercial systems this is much more common.

Now your gauge set up is designed to read pressure accurately, and much less accurately, vacuum. That is why we use the micron gage to get an accurate vacuum reading. But you probably dont have one. After pulling a deep vacuum or what you think is a deep vacuum according to your A/C gages, then you let the system rest for about 1-2 hours. If the vacuum decreases some (pressure rises), then you have moisture that has turned to vapor and is raising the pressure in the system. (Dalton's Law of Confined gases, the total system pressure is the sum of the pressure of the individual gases in the mixture) Vapor was present and hence raised the pressure. You then start a deep vacuum again. A good rule of thumb for a auto system and not using a micron gage is about 45-60 minutes of running time with the vacuum pump. You can do this cycle a few times and then should remove most all moisture and no increase in pressure after sitting.

If the gage reading increase so much that it moves out of a vacuum then the atmosphere pressure is pushing into the sytem and that means you have a leak.

So by your figures it looks like you need to do a few more cycles with the vacuum pump.

You did change the drier I hope, before you did all this?
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  #12  
Old 03-30-2006, 10:02 PM
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Quote:
Originally Posted by Craig
If I understand the question, I don't believe you will see a significant increase in pressure at ambient temperatures. Lets assume the temperature is 70F, at that temperature water turns to vapor (boils) at 0.36 psia (that's a vacuum of about 14.3 psig, or 29 in of Hg). Therefore, if you pump the system down to 29 inches some of the water will turn to vapor until the partial pressure of the water vapor reaches the equilibrium pressure. At that point the water should stop turning to vapor until the system is pumped down again. IMHO, the only way the pressure would continue to increase would be to increase the temperature. Is that what you were asking?
There were a couple of posts suggesting that 15 minutes wasn't long enough to get rid of all the water in the system. In my post, I was trying to politely suggest that although that is almost certainly true, I didn't think that water could, by itself, account for the pressure increase that R Leo observed.

At 70 degrees Fahrenheit, the vapor pressure of water is about 0.74 inches of mercury. Randy saw the pressure rise by 11 inches of mercury.

His reading after letting it sit overnight - 18 inches of mercury - is nearly 40% of atmospheric pressure!
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  #13  
Old 03-30-2006, 10:05 PM
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This is a copy of some info from a pump manufacturer's site. FWIW

Pressure Rise Test

We previously mentioned that the only difference
between deep vacuum and the methods of the past is
that we can measure what we have done. This is called
the Pressure Rise Test.
When the sensor reads between 300 and 400
microns,
DV-29 Method. Blank-off the high and low side ball
valves attached to the system.
General Hookup Method. This includes using
copper tubing or metal hose to the high and low side.
Close blank-off valve on the pump. This will isolate the
gauge from the pump.
Wait for at least 5 to a maximum of 20 minutes to
allow system pressure to equalize. The reading you see
at the end of this test will be very close to what you
actually have in the system. A rapid rise during this test
to atmospheric pressure indicates a leak, while a slower
rise to around 1500 microns indicates moisture is
present.


There are many evacuation level recommendations
including the statement “evacuate the system to below
200 microns.” This should not be considered. Note we
say “system” because it is possible to evacuate piping or
some component other than the compressor to below
this level. Refrigeration oil has a vapor pressure and by
going below 200 microns, you will degas particles of the
refrigeration oil. By changing the makeup of the oil, it will
no longer be a true lubricating oil.
Printed in U.S.A.
2004 J/B Industries Inc.
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Last edited by jbaj007; 03-30-2006 at 10:59 PM.
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  #14  
Old 03-31-2006, 07:14 AM
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It's definintely leaking. I pumped it down again (ran pump for about 4 hours) and let it sit overnight. This morning it is up to just a few inches of vacuum remaining.

I'll pressurize the system with some 134a and start sniffing for leaks.

Thanks all!
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Old 03-31-2006, 09:09 AM
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Did you use Nylog on your o-rings?

http://www.ackits.com/merchant2/merchant.mvc?Screen=PROD&Store_Code=AMA&Product_Code=RT200R

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