Automotive Air Conditioning Information Forum (Archives)

Provided by www.ACkits.com

We've updated our forums!
Click here to visit the new forum

Archive Home

Search Auto AC Forum Archives

A/C techs, when you are working on your own cars... Pages: 12

atikovi on Fri November 23, 2012 2:22 PM User is offline

How long do you vacuum out the system after replacing a part or doing an evac & recharge? I understand in a shop environment you may only do it for 15 or 30 minutes but when you have the time and are servicing your own car, how long would you let the vacuum pump run? Years ago I was told 2 hours but would that still apply to modern 134 systems?

GM Tech on Fri November 23, 2012 4:17 PM User is offline



-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

GM Tech on Fri November 23, 2012 4:18 PM User is offline

15 minutes

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Dougflas on Fri November 23, 2012 5:12 PM User is offline

As long as it takes to get to 500 microns. If it takes overnight, so be it.

Z2TT on Sun November 25, 2012 1:41 PM User is offline

Bare minimum 15 Minutes i'd say once more than 29inhg vacuum is reached. I always vacuum for 30 Minutes for peace of mind to make sure more moisture is removed.

NickD on Mon November 26, 2012 7:21 AM User is offline

Three hours for me, don't stand there and watch, go in the house and take a nap. But worthless unless you have means to at least put in an initial charge without removing hoses. That split second can kill that extra vacuum time. Just valve off the pump with the tank already teed in, just a deep vacuum clear up to that valved tank.

pippo on Mon November 26, 2012 7:53 PM User is offline

As an ametuer, I do 15 min, then stop, wait for 15 min to observe dial, then do 10-15 min again to suck out more boiled off gas/water vapor, if any. Then stop, watch needle again.


It doesnt take long to reach 29.9" Hg. Thats the easy part, to me.

-------------------------
beware of the arrival

Edited: Mon November 26, 2012 at 7:54 PM by pippo

Dougflas on Mon November 26, 2012 9:30 PM User is offline

I really think you're all missing the point. If you connected a micron guage to the system you'd see you're not doing much of a vacuum in the short time you have the pump connected. I had a 1988 Chev van, R12 system, over 100k miles with the original compressor and all other components. When I got the vehicle new, I pulled the r12 out, vacuumed for 12 hrs. (did not own a micron gauge then) and recharged.
My 1998 astrovan with 75K on it has a new compressor and drier. It took 6 hrs to get to 500 microns using a 2 stage pump, heavy duty vacuum rated hoses, and a vacuum tree on the pump. I was surprised it took that long but it did. And that was with a flushed system and new oil. The first 30 minutes showed 1200 microns if memory serves me correctly. granted, a repair shop will not normally spend that much time on a vehicle but this was my own vehicle.

Edited: Mon November 26, 2012 at 9:31 PM by Dougflas

Cussboy on Tue November 27, 2012 1:22 PM User is offline

I do at least an hour.

pippo on Tue November 27, 2012 7:12 PM User is offline

Quote
Originally posted by: Dougflas
I really think you're all missing the point. If you connected a micron guage to the system you'd see you're not doing much of a vacuum in the short time you have the pump connected. I had a 1988 Chev van, R12 system, over 100k miles with the original compressor and all other components. When I got the vehicle new, I pulled the r12 out, vacuumed for 12 hrs. (did not own a micron gauge then) and recharged.

My 1998 astrovan with 75K on it has a new compressor and drier. It took 6 hrs to get to 500 microns using a 2 stage pump, heavy duty vacuum rated hoses, and a vacuum tree on the pump. I was surprised it took that long but it did. And that was with a flushed system and new oil. The first 30 minutes showed 1200 microns if memory serves me correctly. granted, a repair shop will not normally spend that much time on a vehicle but this was my own vehicle.

Geez, didnt know that, Doug. Looks like I have to be more patient. Guess I got mis info in the past (from not sure where, though- so much info on web). Next time, Ill come here.

100,000 miles is a lot of miles, but we both know miles isnt the only thing in the equation- its also time. You could have 100,000m in 4 years time. Or you could take 10 years to get that. eh?

Also, whats the difference in 29.9" Hg and microns?

-------------------------
beware of the arrival

Edited: Tue November 27, 2012 at 7:13 PM by pippo

mk378 on Tue November 27, 2012 9:44 PM User is offline

Microns are a different unit of measurement which require a different type of gauge to detect, called of course a micron gauge.

Mastercool Micron Gauge

There is no such thing as a perfect vacuum, and there is still a lot of matter (air or water vapor) left when the needle on a dial gauge stops moving. Readings on a dial gauge are actually the difference between atmospheric pressure (which varies some with altitude and weather conditions) and the pressure in the system. On the other hand, a micron gauge is not affected by changes in atmospheric pressure. The micron scale starts at 0 (perfect vacuum, or at least a vacuum better than a micron gauge can measure) and goes up. A change of 1000 microns is 0.039 inches Hg, which you will never be able to detect on a manifold dial gauge.

Standard atmospheric pressure is 768,000 microns. So a "good" reading in the garage of 500 microns means that only about 1/1500 of the original amount of air in the lines remains. That amount is small enough to not interfere with cooling performance.

Edited: Tue November 27, 2012 at 9:49 PM by mk378

NickD on Wed November 28, 2012 6:01 AM User is offline

No extended amount of vacuuming will remove the moisture out of that hygroscopic PAG. Only way to get rid of it, is to flush it out and pour in new stuff.

Z2TT on Wed November 28, 2012 9:54 AM User is offline

Isn't that the Job of the new Receiver/Drier, that should filter the moisture out of the oil?

If replacing PAG, replace it with PAO oil which is non hygroscopic.

Also choosing a warmer day to vacuum will probably aid better in removing moisture.

Cussboy on Wed November 28, 2012 1:32 PM User is offline

Vacuum does two things:
removes air from the system
reduces the pressure so water will boil at the ambient temperature

Vacuming can also help give an idication of the leak-tightness of the system (for example, if one cannot achieve good vacuum)

Agree: will not "pull" the moisture out of the hydrophilic oil.

iceman2555 on Wed November 28, 2012 4:27 PM User is offlineView users profile

It is not necessarily the amount of time for a proper evacuation but the ambient temperature also. A short vacuum maybe acceptable if the ambient temp is sufficient however, if the ambient temp is low then time becomes less important....almost to the point it makes not difference if it is 15 minutes....an hour or more......to boil any liquid....heat must be present. The boiling point of water/moisture at 29.52 hg is 53 degrees......and this takes into consideration that the system is totally sealed....the vacuum pump is 100% operational. Haven't serviced the pump/changed pump oil and it maybe that moisture will never be removed from the system. The use of a micron gauge is always a plus.
A suggestion, insure the system is intact and sealed.....the pump should be properly serviced......operate the vacuum pump for 5-10 minutes....this should bring the system to a deep vacuum. Shut down the gauges....observer for possible loss of vacuum...should be no more than 2 in/hg in 5 minutes. Also keep in mind if the system does loose vacuum....it could be from the manifold/hose/connectors....always check these with a good leak detector. Re energize the vacuum pump....open gauges....start the vehicle....this will add extra heat to the components of the system and aid in moisture removal. Add in the micron gauge and monitor both......a good 29/29.XX (maintainable) should be sufficient.....esp with the extra heat of the engine. This also takes into effect that the vacuum pump is a electrical driven pump.......dual stages are great......high cfm is good....however, I have an old single stage, 1/2 hp pump purchased in 1986 that still works great. The most expensive/highest tech tool is not always necessary.

-------------------------
The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

iceman2555 on Wed November 28, 2012 4:37 PM User is offlineView users profile



Very old pump....still works...!!!!

Check this photo....notice the degree of vacuum.....ambient temp......micron gauge.......this was after 1.5 hours of operation. The third photo indicates the amount of moisture remaining in the test bottle after this evacuation. App. 3/4 of a teaspoon is remaining. 2 teaspoons were added to the beaker prior to the test. Simply adding heat to the beaker utilizing a heat gun was sufficient to remove all residual liquid. At this point the micron gauge also began to fall drastically. The increase upon the beaker insured the total removal of the liquid.



-------------------------
The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson



Edited: Wed November 28, 2012 at 4:39 PM by iceman2555

ice-n-tropics on Thu November 29, 2012 12:52 PM User is offline

Hey ice#1,
What's that last pic.?

My 2 cents is there are different answers to vac time based on prior conditions:
1) A prior fully charged system can frost the Accumulator or R/D for up to an hour at lower ambients unless heated w/ a hair drier. This delays vaporization of recently introduced moisture until refrigerant is boiled out of the low points or lub.
2) A freshly installed dry system is OK @ 15 min. vac
Residual flush can also delay good vacuum.
Single end capped PAG (SEC) has higher hygroscopicity than double end capped PAG (DEC)
The job of fresh desiccant is to remove moisture from the oil and refrigerant. It also holds acid.
[email protected]

-------------------------
Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy.
AMAZON.com: How To Air Condition Your Hot Rod

TRB on Thu November 29, 2012 4:09 PM User is offlineView users profile

I am still a fan of a micron gauge for proper results. With that said, normal procedure if I were doing a repair. 30 minutes - turn off close valves for 15 minutes - open valves another 30 minutes.

-------------------------

When considering your next auto A/C purchase, please consider the site that supports you: ACkits.com
Contact: ACKits.com


Edited: Thu November 29, 2012 at 8:57 PM by TRB

pippo on Thu November 29, 2012 7:26 PM User is offline

Thanks, for the explanation. Lots to learn. Good point on temp being a factor.

Also, good point on the dryer taking care of whatever imperfection our systems may inherently have )dont dryers suck up up to 10 mls of water?). Its like we cal in lab experiments- "experimental error". Cant get rid of all of it, no matter what.

-------------------------
beware of the arrival

NickD on Fri November 30, 2012 6:38 AM User is offline

Automotive components like a starter drive engagement solenoid or a fuel pump use to come in a screwable can. Was cheap and simple to replace the contacts or the diaphragm with a screwdriver, and make these components like new again. Today these components come in a sealed throwaway can.

To the best of my recollection, dryers such as a receiver or an accumulator always came in a throwaway sealed can. When the only component on the interior that would go sour is the desiccant bag. For R-134a the desiccant is XH-9 Zeolite and once exposed to moisture, is non-reversible, where is my trashcan?

With the introduction of R-134a, both the EPA and SAE had to up the moisture limits when we were forced to use R-134a, and the old calcium oxide or silica gel desiccant had to be improved with X-9 Zeolite's. Kind of like trying to remove the flour to put it back in the bag after baking a cake you didn't care for. Again, where is that trashcan.

You can't see the desiccant to determine its condition without destroying the can, but risk getting acid buildup in your restored AC system without replacing that can. This is a 44 magnum, the most powerful handgun, did I shoot 5 or 6 shots, are you ready to risk it? Kind of thing.

When Mitch and Detroit_AC were on this board, two really great guys, discussed another phenomenon I frankly don't comprehend. A kind of reverse osmoses in R-134a systems where moisture can seep into a high pressure system destroying the desiccant. Implying that the receiver or accumulator should be replace every five years or so.

Ha, in my neck of the woods with road salt, most vehicles that old are throwaway items due to severe unrepairable unibody rust. So really not a concern for me.

Z2TT on Mon December 03, 2012 10:50 AM User is offline

I've found it extremely hard to think how Atmospheric Air pressure from outside of the system can work it's way through the thick layers of a/c hose, and get inside a system where pressure is always higher,
this really has just been a theory and I don't think it's ever been proven.

I'm also wondering, are Guage Scales accurate enough to tell rough vacuum? E.g if my Guage goes down to 30in/hg can i be sure that it's definately more than 29?
Or can they get very inaccurate quickly? Not sure if I'm ready to invest in a Micron gauge yet.

I'm not all that clued up on Micron guages, but is it simply just a easier way of measuring vacuum...... or does the micron gauge "Know" that moisture is still in the system?

bohica2xo on Mon December 03, 2012 12:50 PM User is offline

Quote
Originally posted by: Z2TT
I've found it extremely hard to think how Atmospheric Air pressure from outside of the system can work it's way through the thick layers of a/c hose, and get inside a system where pressure is always higher,

this really has just been a theory and I don't think it's ever been proven.


And you would be wrong to think that.

Moisture ingress to a closed & pressurized system is documented, and is expressed in grams of water per square centimeter per year. There is even an SAE spec associated with it. That is why we tell people with 50+ feet of hose under a motorhome to replace the dryer more often.

Here is a Goodyear hose page showing the published specs including moisture ingress:

Goodyear Galaxy refrigeration hose


A micron gauge precisely measures vacuum. The 29" mark on the typical manifold set is subject to the accuracy of the gauge itself, which is +/- 2% of full scale. Sort of like using a yardstick to measure piston rings. There are 25,400 microns in an inch. a 1500 micron change would not be visible at all on a manifold gauge set.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

mk378 on Mon December 03, 2012 3:17 PM User is offline

Look up osmosis. The only thing that matters is the partial pressure of water inside the hose (low) vs. the concentration of water outside (high). The presence of other substances even at much higher pressures does not affect the tendency of each individual substance to migrate from where it is more prevalent to where it is less.

Edited: Mon December 03, 2012 at 3:17 PM by mk378

NickD on Tue December 04, 2012 3:36 PM User is offline

A micron gauge is an absolute above zero pressure gauge. As opposed to a manifold gauge that is relative where maximum ambient pressure varies with both weather and altitude. Generally lose about one inch in maximum vacuum for each 1000 feet above sea level, weather can change anothe +/- an inch.

Average barometric pressure is defined as 29.95"/Hg at sea level, at say 10,000 above sea level, all you can get is 20.6"/Hg. Whereas if you can hit 0 with a precision micron gauge, this would be a perfect vacuum regardless of weather or altitude. As Bohica pointed out, resolution is also a factor.

mollygomez on Fri November 01, 2013 1:08 AM User is offline

Half an hour I suppose will be good enough top do so.

Back to Automotive Air Conditioning Forum

We've updated our forums!
Click here to visit the new forum

Archive Home

Copyright © 2016 Arizona Mobile Air Inc.