A/C System Evacuation - A Question for the Professionals

[jgl1]

The compound gauges of a refrigerant manifold are good for gross measurements but are not accurate enough to determine exact vacuum levels. The last inch of vacuum (29 to 29.9" Hg) represents the difference between 23,000 and 250 microns, which the gauge face is simply incapable of reporting.

Without an accurate means of measurement, determining the correct evacuation time becomes guesswork; you waste time by running the pump long after evacuation is complete or, if not long enough, you risk not removing all the moisture.

Does anyone routinely use a thermistor or electronic vacuum gauge in conjunction with their manifold during system evacuations? If so, is there a preferred gauge manufacturer?

[mbdoc]

I would doubt that very few people would go to that extreme!
Watched MB pull vacuum at factory on new cars for 45 seconds & then charge them. OF course with all new parts not as much chance for moisture to collect. WE always run vacuum for 1/2 hour & install new drier.
30 years ago when I started this business the time for a *good* vacuum was 24 hours.

Brother of the Benz
You are really taking the pressure you can't read to an extreme.
And yes the minus pressure scale on my manifold set is of no real value while evacuating, as time is the controlling element.
With time at 20-29"hg, water will boil and this vapor will be removed by the pump.
What is a realistic time, ?
I pump a unit down that has been opened to the atmosphere at least 30 minutes.
If the system has lost it's gas, just long enough to introduce enough gas for leak testing.
The prime use of the vac side of the gauge is for rate of rise. How long will the system maintain this inacurate pressure and will the needle creep to atmospheric.
So you see that portion of the gauge really has 2 functions.
Try this thinking and I believe you will be more at ease.
Happy Trails Beep Beep from The Spiderman in Houston!!!

[Benzmac]

And remember, leaks are found with the system under pressure. You will have to find it with a leak tester or with soapy water or with the trace of oil it leavs behind.

If this sytem is under a good 29" vacuum, holding for 20min. or so, you will know if it leaks or not. If it leaks the vacuum will be lost to 26-24"

[Arthur Dalton]

I have several vac. pumps , but the one I like the best is an old "Crosley" [ remember them?] refrigerator compressor.
I have a plastic tube on the discharge [high] side that I put in a clear glass of oil.
When the bubbles stop for 5 min. , I have a good vac.
[29"]
When I have the time and want to be through, I pull the vac, cap the system, and recheck the next morning.
No leaks should hold the vac.
Any system that does not pull down in 1/2 hour has a leak
or a bad vac. pump. The pumps capacity is easily checked
by capping the suction hose end and pulling a vac againt the gage.

[stevebfl]

I have and have used a micron style vacuum guage made by Robinair. It turns out that they are very sensitive to oil or refrigerant contamination.

You are absolutely right about the necessity. Twenty nine inches is not enough and none of the gauges made are accurate enough to tell whether you are doing anything or not. I would guess that around 90% of all evacuations never boil water. You can suck all year long at 29inches and you won't boil water unless it is awful hot out, probably well above 100deg F. I haven't looked at a table in a few years.

The electronic tester was really only good to gauge what type of service level we needed to maintain on our vacuum pumps to be able to achieve the necessary vacuum. They never worked accurately for more than a season.

Brother of The Benz
Steve, you really got the gray matter whirling.
As I remember pressures and temperatures, water boils at 33*F at 1 micron Torr. Help me out if these figures are in error. I think you get the drift of it!
For our auto applications R-12 boils at 32*F at 30psig evaporator pressure.
With these figures we can see what occurs at the expansion valve should there be water, ever so small an amount, it will freeze and restrict the flow of refrigarant. This can be seen on the low pressure gauge, blank-off from the compressor.
After having your system emptied by a certified Freon re-cycler, evacuate the system as long as you have time. You can never overdo it.
Burr my R134A with the addition of a "Hot Shot" charge does fine.
Happy Trails Beep Beep from The Spiderman in Houston!!!

[stevebfl]

I will see if my micron guage still has the chart attached. getting water to boil at 85deg ambient will take a lot less than one micron but it will still be a lot more than 29inches.

[Edited by stevebfl on 05-12-2001 at 11:52 AM]

[engatwork]

Finally got the '85 300D blowing cold air on 134a. The lowest it will blow is 52 dF as indicated by a thermometer stuck in one of the center vents (I can tell when I accelerate too fast - it slides out ).

Anyway, upon completion of the replacement of one of the a/c hoses (thanks Fastlane/Partshop.com) I took it to the guy that evacuates and charges it - good person and excellent cars "a/c and electronics" independent - anyway, he allowed it to evacuate at a minimum 3 hours and replaced the schrader valve on the high pressure side, all for $50 and I was pleased to get 52 dF "cold" air.

Jim
'85 300D
'95 E320
'97 CRV

Brothers of The Benz
Steve, my memory isn't as sharp as it used to be and your chart will confirm your figures. Your correct, I left the decimal point off of the 1 micron Torr, it is 0.10 micron Torr.
What You and I are saying is you must maintain a condition of low pressure to dry the system. Whether it is 29"hg or 0.10 micron.
Jim, 52*F discharge air Temp is quite good depending on the ambient temperature. If it is 85*F ambient you have a drop of 33*F or 38% cooling. That's good. And this isn't considerring the humidity.
In discussing the various methods we all have performed or seen done shows how forgiving the automotive airconditioner is. The better we apply our knowledge the better the A/C performs.
The automotive A/C is a far cry from a sub-zero type where using a different refrigirant requires the driest of system.
This brings to mind the first units I worked on. It was on Chrysler Automobiles and they used R-22 in a Servel trunk unit. The discharge temp at the nozzles on the rear package shelf was 45-50*F.
There is an additive that can be added to the R134A to improve on the coolig. It is "Performance Booster" Additive. It improves the cooling by 8%. It's a 3 oz can and cost me $10.45.
Possibly by the fumbling of an over the hill mechanic and the help of Steve your A/C will perform up to your expectations.
Happy Trails Beep Beep from The Spiderman in Houston!!!

[jgl1]

dlswnfrd - I must disagree with your statements that "time is the controlling element" and "..evacuate the system as long as you have time. You can never over do it." with regards to A/C system evacuation.

You cannot equate evacuation duration with vacuum pump efficiency: a pump with a factory micron rating of 1000 microns will not scavenge a system any lower than that, regardless of how long it is allowed to run. Nor can you compensate for a pump operating at reduced efficiency by allowing it to run overnight. With the inaccuracy inherent in compound gauges, a problem could exist with a vacuum pump and one would never know it. A vacuum of 10,000 microns is indistinguishable from a vacuum of 1,000 microns on a compound gauge, yet the former is an unacceptable level of evacuation for a refrigerant system.

For optimal system evacuation, a vacuum pump's performance should be checked prior to starting an evacuation; most currently manufactured pumps (non-laboratory) have micron ratings of 20 to 50. If the pump is unable to reach the factory rating, then pump maintenance is in order.

Additionally, one should have knowledge of the pump's capacity and minimum pump down times. Robinair's 6 CFM vacuum pump, with a micron rating of 20 (Model 15600), requires 60 minutes to reach a vacuum of 600 microns under laboratory conditions. This two stage 6 CFM pump is no slouch, listed as applicable for A/C systems up to 50 tons, including tractor/trailers, buses and rooftop A/C systems. Smaller pumps, such as 1.2 CFM or 4 CFM, must be expected to take longer to reach the same level.

Given this, a minimum evacuation time of 30 minutes for an opened system is possibly sufficient but more likely not. But how would one know using a compound gauge alone? I suspect Steve is correct in stating that a majority of all evacuations are insufficient.

Clean oil is imperative for peak vacuum pump performance. Moisture and other contaminants quickly deteriorate oil purity, thinning the oil and reducing the pump's ability to achieve deep vacuum. Contaminated oil reduces a vacuum pump's ability to remove moisture yet how many do-it-yourselfers routinely change their vacuum pump oil, let alone even know that it is part of vacuum pump maintenance? All vacuum pump manufacturers call for frequent oil changes; Robinair, e.g., calls for oil renewal following:

a) evacuation of a system that was overly moisture-laden.

b) evacuation of a system with compressor burnout.

c) oil contamination (oil looks cloudy or milky)

d) failure of the vacuum pump to pull to factory specifications when blanked off to an electronic thermistor vacuum gauge.

e) every 10 hours of operation.


Modern automotive A/C systems are built tighter and charges are more critical. These systems have greater sensitivity to contaminants, including moisture, which makes proper evacuation more critical than ever before.

It is well known that the presence of moisture in a refrigeration systems can cause freeze-up. However, this is not the only problem caused by moisture. Refrigerant oil rapidly absorbs and retains water (which is why you should only use refrigerant oil from sealed containers). Water, in the presence of refrigerants containing chlorine, will form hydrochloric acid, which results in corrosion. The water-formed acid also combines with the refrigerant, resulting in a closely linked mixture of fine globules. This "sludging" greatly reduces the lubricating properties of refrigerant oil, which can be the death knell for certain compressors, such as the Nippondenso 10Pxx series.

Brother of The Benz
jgl1, I can not disagree with everything you replied.
Too True, probably 99.99999% of automotve reffers have never had the pressures applied as you are referencing.
Stating time is the factor means as long as you have time; evacuate to a time convient to your work load in the shop. Ofcourse if the work load is light give it time as you feel comfortable with, opposite if the shop is busy.
You probably will not be able to tell one from the other.
My little Welch pump has a blank off of 1500(just a guess as I don't have a gauge for this pressure and the name tag and manual are gone) microns with the non-condensable vent valve closed. Less with it open which is where I operate it initially.
The low pressure scale on the low gauge of the manifold set is just a for reference.
When A/C was offered in the GM automobile, Kent-Moore Tools(provider of toolig for GM) furnished the dealerships with a vac/pump that was a Frigidaire domestic compressor from that division of GM. These pumps were used for many many years until manufactures as Welch and Stokes came on the Toolig provider list.
We are blessed in that the automotive A/C is so over designed to provide us with cooling with less than perfect servicing technique.
I will still pump down as long as I have the time to wait for my little pump to stop burping and then a little more.
I am satisfied with what I do but I wish I could get you to service my A/C for you would do better than my fumbling.
Happy Trails Beep Beep from The Spiderman in Houston!!!