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  1. #1
    Join Date
    May 2007
    Posts
    747

    Leak testing by vacuum.

    All the talk going on in the Nitrogen Pros Only thread put me in mind of this.

    Who uses vacuum as a leak test, I mean a good vacuum gauge, not your manifold? Does it really work well enough to not do an overnight nitrogen test?

    I'm real old school and think nitrogen is the best way to test. It seems to me that a vacuum test is still a pressure test but your test pressure is only 14.7 psi (at sea level).

    Vacuum test is the big thing where I work, and if somebody convinces me I should stop preaching nitrogen testing I'm sure my co-workers would appreciate it.

  2. #2
    Join Date
    Oct 2009
    Posts
    28
    How would you locate a leak with a vacuum test?
    Nitrogen and soap is the way to go.

  3. #3
    Join Date
    Apr 2010
    Location
    madison wi
    Posts
    139
    with a digi. micron gauge. yes I have done that, and I tend to think that can be a good way to go, but my thought is that pressureizeing the lines is also a good thing because up here we have had problems with small cracks / fishers in the pipe it's self. typically 5/8 but i have also heard 1/2 as well, my micron gauge is way more accurate than any manifold gauge I have. I may or may not notice +/-1/2 lb with the manifold gauge but will notice 500 up to 100 micron change with the micrometer. now finding the leak is a whole nother story.

  4. #4
    Join Date
    Mar 2007
    Location
    wedged in freezer shelf
    Posts
    7,210
    I thought you were supposed to check both ways
    not one or the other
    “If You Can Dodge A Wrench You Can Dodge A Ball”

  5. #5
    Join Date
    Apr 2010
    Location
    madison wi
    Posts
    139
    checking both ways is part of the job. you first pressureize to check for leaks, and then you have to pull a vaccume anyway so, when you are done pulling a good vaccume you should let it sit for a while and check back ... if you cant get below 300 microns you may have a issue somewhere vac oil, leak, ect. or at least that is how I was taught

  6. #6
    Join Date
    May 2007
    Posts
    747
    Let's say I'm working on a pretty good size system that takes a full bottle of N2 to make 150 PSI. What I think I'm hearing is you pressurize it just long enough to make sure nothing is spewing, since it can take quite some time for a bigger system to lose enough pressure to say for sure there's a leak, and then do the "real" test with vacuum.

    Is it impossible to get a system below 300 microns if there's a leak? Is that the magical number?

    I'm pretty ignorant about the range of possibilities, but I've noticed techs having difficulties with vacuum testing. More than a few times I have seen techs frustrated because the vacuum stopped dropping at some value like 500 microns. They would blank-off test the manifold and pump and get the manifold down to 50 microns so the possibility of leaking hoses, etc. is eliminated. They would just give up and gas up the system. And time would prove the systems were tight.

    My preference would be an overnight test at 150 PSI where it didn't drop at all. Then I don't have to worry about getting a perfect vacuum because pressure showed it tight.

  7. #7
    Join Date
    Apr 2010
    Location
    madison wi
    Posts
    139
    the vast majority of what i tend to do is supermarkets. we put up hangers, sling pipe, burn together and pressureize linesets. liq, and suct -we loop then and pressureize together to usually 150# for several days or even weeks. our line sets can be several hundred feet and, 200 ft of line set of 5/8 x 1 1/8 pipe is not uncommon at all. if we think we have a bad bunch of pipe then 400# to "prove the pipe" if a leak is suspected we seperate the suct and liq lines and pressureize ind. I was thinking about the micron thing, I may of been wrong on the # I think it is usually somewhere around 1000 (I have been off since oct, my boss didnt diversify with the companys that we do business for enough and our main one is not doing much lateley) good thing I own a snowplowing and lansdscapeing business on the side. so sorry, 1500-1000 was what i was thinking of. thats right the lower # of microns the lower the vaccume.

  8. #8
    Join Date
    May 2010
    Location
    Trenton, Mo
    Posts
    47
    Pressure test with a 20% mixture of R22 is by far the best way to leak check, but nothing is fool prof, and we all miss leaks once and a while no matter how careful we are. The vacuum leak check does however have its place. If you have a system that you suspect has standing moisture in it, the vacuum test can tell you for sure. Pull the system down to at least 500 microns and let it stand for 24hrs if the microns rise to 1000 to 1200 then there is moisture in the system. If it stays below 800, no leaks or moisture, finish the evacuation process. If it goes above 1200 microns you still have a leak. These values are approximate and there are many variables to factor in. For instance not ever micron gauge reads the same. I hope this helps, and answers your question, please let me know. Contact me anytime.

  9. #9
    Join Date
    Jan 2010
    Posts
    6,838
    I would be hesitant to do a standing vac test. Why would you want to find out there is a leak and you pulled in moisture all night ?

  10. #10
    Join Date
    Apr 2010
    Location
    San Diego, CA
    Posts
    228
    There's a lot of numbers on this subject that people throw around arbitrarily. I've heard people say things like "if you pull down to 500 microns, blank off and wait 5 minutes, and it only goes up to 1200 then its moisture, and if it goes above 1400 then its a leak". The reality is that in either case it could be moisture and it could be a leak and it could also be that you just did the blank off test too early. The size of the system, the size of your vac. pump, quality of your hoses, etc all come into play here.

    If you want to be sure the best way to do it is to pressure test your system with nitrogen and use the vacuum blank off test as both a leak test and a test to make sure you had it under vacuum long enough to bring all the nitrogen/air/moisture out.

    A thermistor vacuum guage reads high if there's moisture in the system. As in, artificially high. The vacuum level could be 100microns but the moisture will make your vacuum guage read higher than that. You should be pulling a vacuum for as long as it takes to bring the system to less than 500 microns and have it hold less than 500 microns in a 15 minute blank off test. If it passes that test then you know it's evacuated to the best of your ability, isn't so full of moisture that your new compressor dies a horrible death in 6 months, and isn't leaking. How long should it be under vacuum? The answer is as long as it takes to pass a proper blank off test. This is where 3/8th vacuum hoses, full flow manifolds, and the biggest vacuum pump you can carry come in handy :-)

  11. #11
    Join Date
    Apr 2010
    Location
    madison wi
    Posts
    139
    There's a lot of numbers on this subject that people throw around arbitrarily. I've heard people say things like "if you pull down to 500 microns, blank off and wait 5 minutes, and it only goes up to 1200 then its moisture, and if it goes above 1400 then its a leak". The reality is that in either case it could be moisture and it could be a leak and it could also be that you just did the blank off test too early. The size of the system, the size of your vac. pump, quality of your hoses, etc all come into play here.

    If you want to be sure the best way to do it is to pressure test your system with nitrogen and use the vacuum blank off test as both a leak test and a test to make sure you had it under vacuum long enough to bring all the nitrogen/air/moisture out.

    A thermistor vacuum guage reads high if there's moisture in the system. As in, artificially high. The vacuum level could be 100microns but the moisture will make your vacuum guage read higher than that. You should be pulling a vacuum for as long as it takes to bring the system to less than 500 microns and have it hold less than 500 microns in a 15 minute blank off test. If it passes that test then you know it's evacuated to the best of your ability, isn't so full of moisture that your new compressor dies a horrible death in 6 months, and isn't leaking. How long should it be under vacuum? The answer is as long as it takes to pass a proper blank off test. This is where 3/8th vacuum hoses, full flow manifolds, and the biggest vacuum pump you can carry come in handy :-)

    I would agreee with this, niceley stated

  12. #12
    Join Date
    Jan 2010
    Posts
    6,838
    CM Quote (and the biggest vacuum pump you can carry come in handy :-)


    CM,
    Bigger is not necessarily better when it comes to pumps.

  13. #13
    Join Date
    Dec 2008
    Location
    Nor•Cal
    Posts
    221
    Nitrogen to 150 psi to test for leaks (with a trace of r22 if using a detector). I only pull a vacuum after getting past the pressure test. Vacuum is only for removing moisture/non-condensables. Ideally, a rise in microns should only indicate moisture boiling off inside the system since the pressure test already checked for leaks... although sometimes it can indicate gases bubbling out of the walls of charging hoses if your micron guage is sensitive enough.

    14.7 psi doesn't seem like much of a leak test to me.

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