basic air conditioner questions

I have a few basic air conditioner questions
1. When I check for R-134 leaks do I need to use a vacuum and pressure to check for leaks? Lets say I vacuum the system down to 30" for 4 hours and
find no leaks. Is this good or do I also need to apply , lets say 200 psi , to check for a positive leak. I know this sounds silly but will some seals show leaks when under a vacuum vs. pressure? Someone once told me you need to do both.
2. How do you flush a R-134 system.
I read a basic Automotive airconditoner Manual , but they didn't mention this.
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The leak check starts with a visual inspection. Any where dirt collects is a sing of a leak. Applying a vacuum can "temporally fix" a leaking o ring. You should leak test before you get to the point of evacuating the system (applying a vacuum ).

If it leaks it will show up no matter the pressure over atmosphere you do not need to see 200 psi. There are two ways to check for a leak. You can use a chemical dye ( I prefer ) or an electronic device ( called a sniffer, and expensive ).

By using the instructions that came with the flush kit.

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Hi Tom, Most so called leak detectors are just a lot of bunk...Only a real halogen leak dectector sold by places like McMaster Carr (check online sales) detect refrigerants in trace amounts. Color dyes in the system are a giant leak if you see them. A vacuum pump will not get the vacuum guage into the good range about 500 microns if there is a leak. Nor will it hold the vacuum for any legnth of time. Large leaks can be seen with just soap and water (palmolive dish soap) If a leak is ever found with the soap method it would be considered in lbs. per year. If you would leak 1 lb. of 134 a year would this be good? The system oil or anything oily in the air cond. system shows you have a leak. The system oil is the dryer... it hydroscopic oil it absorbs moisture, Only a high vacuum under 100 microns will boil off the moisture. Most car dealers don't have vacuum pumps good enough to get down there is why they flush it to remove moisture. McMaster also sells thre electronic vacuum guage in the micron range for air conditioning. Your just fooling yourself if you think you can get a vacuum of 30" by any other means than a electronic guage. Yea a mechanical gauge will show you 30" when it is really only 28". R 134 in a clean system will get down to 34 degrees in the dashboard vent.Without proper equipment you'll be lucky to get 52 degrees. That will be just a dank temperature. None of the humidity in the car will be extracted. That is accomplished by freezing it out on the coil, melting the coil and dripping below the car. Its a real tough thing without the right stuff. Be aware it will cost over $1000.00 to be able to get it right.....Change the oil first. Flushing with nitrogen gas removes moisture over a long period of time. 1/2 lb. flow for say 6 hours. Purge with freon, change the oil in the dryer all these help. Any shop not having a halogen leak detector, a vacuum pump inside of 50 microns and an electrinic vacuum guage- is just ripping off their customers.
Good Luck.

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HAH!
<snip>
You put it a little differently than I was going to, but you beat me to it... The humidity FREEZES out of the air? 52 F won't pull any humidity out? (Well... not if the dew point is less than 52) .... water won't boil off at 28" HG at higher temps? (Quite a bit above 70 F, though, I'm afraid.... it's about 77 F to boil water at 29" HG)
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Yes.
I did a search for "water boiling point vacuum" (no quotes in the search).
I eventually found a chart, but hell... I don't remember where either.

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Clem opined in

I found an abridged chart
Inches Vacuum (Hg) Temp. 29" 71 F. 25" 133 F. 20" 161 F. 15" 179 F. 10" 192 F. 5" 203 F. 0" 212 F.
Wanta read a DUM statement, this by an actual refrigeration tech?
"Yes, I have seen a mason jar with water in it but... what happens when oil and water are mixed? The water goes to the botom and the oil rises to the top. When you say have 2" layer of oil on top of a 1/4" layer of water, the water is not subjected to the vacuum pressure that is insulated by the 2" layer of oil. So without boiling the oil (which I am sure wouldn't boil at even 29.9"Hg) how would the water boil off? "
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Bob Vargo opined in

Tell you what... you call me on dum stuff I say, I'll call you on dum stuff YOU say.
And this is one of them:
"you'll be lucky to get 52 degrees. That will be just a dank temperature. None of the humidity in the car will be extracted. That is accomplished by freezing it out on the coil, melting the coil and dripping below the car."
While the evap HAS to be capable of freezing, the best operation is attained WITHOUT it freezing, which means sufficient airflow.
When vent temp is 55, high speed blower, there is a nice sensate cool and a good steady stream of condensate.
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Really? I've never heard this. (Not necessarily that I should have)
Why are they still so damned expensive, and how are they legally able to call it a dryer? Do they have any filtration?
The dryer purchase is the last thing I'm waiting for before taking on a compressor change on my '92 Cherokee. For $50 I'd HOPE there was more than just a can for holding extra refrigerant.
But the oil acts as the desiccant? Very interesting.
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Clem opined in

But you STILL need to replce it..(accumulator) because of the construction, it holds excess oil and on a conversion the old oil being trapped in there could block the oil pickup.
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