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    Acceptable Vacuum in mm Hg


    Hi - in case any of you were wondering, I have now passed my CITB. (Huzzah!) However, I was checking through my Safe Handling of Refrigerants Reference Manual today and found a reference to the degree of acceptable vacuum you should aim to achieve. It states " as a guideline at least 10" Hg, 250 mm Hg vacuum should be achieved".

    I thought that the general guidance was that a vacuum of 300 to 500 microns should be achieved. Am I being thick, or isnt 300 microns equivalent to 0.3 mm Hg / Torr ? If so, the CITB guide is recommending a vacuum of 250,000 microns... I am totally unfamiliar with the whole concept of microns, mm Hg etc, so if I am totally missing the point, please tell me!!



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    Re: Acceptable Vacuum in mm Hg

    Asuming no gages will show you the right vacuum, 29" Hg will do.

    Personaly, my old Gould vacuum pump changes the sound when reaches the maximum vacuum,

    Congratulations!

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    Re: Acceptable Vacuum in mm Hg

    Quote Originally Posted by chemi-cool View Post
    Asuming no gages will show you the right vacuum, 29" Hg will do.

    Personaly, my old Gould vacuum pump changes the sound when reaches the maximum vacuum,

    Congratulations!
    OK, that sounds good...

    But how do you know that the vacuum you achieved is standing and there is no moisture boiling off in the system?

    Digital micron gauge every time

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    Re: Acceptable Vacuum in mm Hg

    Well, I look at the gages and remember where the hand was.

    Nothing in any gages gives you any info about moisture in the system or, I have missed something during the years.....

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    Re: Acceptable Vacuum in mm Hg

    My vacuum pump pulls down to 30", is that good enough?
    To start off with, you need to recognize that no standard refrigeration gauge set is accurate enough at low vacuum levels to give you much useful information at all. For example, I can assure you that your pump does not pull down to 30". I know your gauge may say 30" but a perfect vacuum (unattainable on earth) is 29.92". That may sound close enough until you realize that you can't remove all the moisture until the vacuum level gets down to 29.72" (5,000 microns) or lower. If we assume that your gauge set is off by only .5 psi (equal to 1" of mercury) then when it reads 30" it is actually at 29.00" or nearly 5x higher pressure than the vacuum level required to dehydrate your system.
    Transvestites are men who like to eat, drink, and be Mary.

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    Re: Acceptable Vacuum in mm Hg

    Quote Originally Posted by chemi-cool View Post
    Well, I look at the gages and remember where the hand was.

    Nothing in any gages gives you any info about moisture in the system or, I have missed something during the years.....

    No, you are right there, nothing on your standard gauges will tell you if there is moisture in the system.

    That's why it is so important to use a proper micron gauge, see the post above by Phil...

    On a micron gauge, you will see the pressure reducing to say 1000 microns if you then shut the vacc pump off you will see the pressure slowly increasing. This is not a sign of a leak, it is moisture boiling off.

    Only when you achieved a deep vacuum that is standing (i.e. the vacc pump shut off and the micron gauge not rising for 1/2 hour) will you know that there is no moisture present in your system.

    This is vital on modern units working on synthetic oil where any moisture will cause the oil to become acidic.

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    Re: Acceptable Vacuum in mm Hg

    Thanks for all your responses, but what I really want to know is whether the reference manual is incorrect when it says "at least 10" Hg, 250 mm Hg vacuum should be achieved". 250 mm Hg is 250,000 microns, isnt it?

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    Re: Acceptable Vacuum in mm Hg

    Quote Originally Posted by Newgirl View Post
    Thanks for all your responses, but what I really want to know is whether the reference manual is incorrect when it says "at least 10" Hg, 250 mm Hg vacuum should be achieved". 250 mm Hg is 250,000 microns, isnt it?
    Yes, you are right. 1 mm Hg is the same as 1.000 micron.

    Your reference manual is way out

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    Re: Acceptable Vacuum in mm Hg

    In the company where I work, we say that vacuum should be between 3 and 7 mbar (2-5 mm hg) to be acceptable.

    and during the drop test it should not rise above 7 mbar
    (7 mm hg) in a 12 hour period.



    We use gauges from tempress to measure the vacuum, never did trust the electronic gauges and havent used one since I was in school

    The vacuum meter is my most priced tool and is kept in a special box with a foamcutout holding it in place. and we have a vacuum gauge mounted in the workshop that we can check our pumps and calibrate our gauges against.

    http://www.tempress.dk/getfile.php?objectid=467508
    Last edited by Tycho; 28-06-2008 at 09:20 PM.
    -Cheers-

    Tycho

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    Re: Acceptable Vacuum in mm Hg

    ooops, double post
    -Cheers-

    Tycho

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    Re: Acceptable Vacuum in mm Hg

    THIS is a good informative page that will answer your question.

    (Because I'm not sure...)

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    Re: Acceptable Vacuum in mm Hg

    Quote Originally Posted by Tycho View Post
    In the company where I work, we say that vacuum should be between 3 and 7 mbar (2-5 mm hg) to be acceptable.
    Interesting. In comercial refrigeration (HFC/HCFC) it is normal to aim for not more than 1000 micron (1 mm Hg) as the final pressure.

    Are you working mostly on amonia systems Tyco?

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    Re: Acceptable Vacuum in mm Hg

    An accurate reading cannot be made with a standard refrigeration gauge.
    Remember 29” of mercury as read on a compound gauge equals 23,368 microns of vacuum. If there is a minute quantity of moisture in the system, you wouldn’t see it with standard gauges. You need a vacuum gauge which will indicate moister being present because the gauge would rise.
    Therefore, if you’re trying to achieve 300 micron and moisture remains in the system, the vacuum could rise to say 1000 micron but you would not see this rise on a standard pressure gauge. That’s why you need to use a suitable vacuum gauge, digital ones are becoming very poular.

    Chemi-cool is in the middle east and, probably finds the ambient there is much drier than the UK where we are at risk of getting moisture into the system during installation and service repairs. I think to rely solely on standard gauges, unless you’re absolutely sure of the system and conditions prevailing, could prove to be risky. If you’re going to do the job to a professional standard then do it right first time out.

    Better safe than sorry?.

    These sites are very useful.

    http://www.mitsubishielectric.ca/hvac/PDF/brochure_City_Multi_R410A_Installation.pdf

    http://www.engineeringtoolbox.com/vacuum-converter-d_460.html

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    Re: Acceptable Vacuum in mm Hg

    Quote Originally Posted by chemi-cool View Post
    Well, I look at the gages and remember where the hand was.

    Nothing in any gages gives you any info about moisture in the system or, I have missed something during the years.....



    Yes it does if you use and accurate vacuum gauge.

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    Re: Acceptable Vacuum in mm Hg

    Quote Originally Posted by SteinarN View Post
    Interesting. In comercial refrigeration (HFC/HCFC) it is normal to aim for not more than 1000 micron (1 mm Hg) as the final pressure.

    Are you working mostly on amonia systems Tyco?

    yup, mostly large industrial plants. the volume on those plants compared to commercial plants are worlds apart, so 3 mbar on a plant that size is pretty good.

    you also have to take the surrounding temperature into account, but if you achieve 3 mbar and it stays there during a 12 hour droptest, it's dry enough
    -Cheers-

    Tycho

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    Re: Acceptable Vacuum in mm Hg

    Quote Originally Posted by chemi-cool View Post
    Well, I look at the gages and remember where the hand was.

    Nothing in any gages gives you any info about moisture in the system or, I have missed something during the years.....

    I beg to differ... I can also listen to my vacuumpump and know when it's achieved vacuum.

    at that time I connect my vacuum gauge so I can see that it is at the desired vacuum, I then leave the gauge connected, write down the time and the reading and close the valve the vacuum gauge is connected to.

    after that, I do a reading every hour and note the time and pressure.

    if I get a graph looking like this:


    There is water/moisture in the system

    if I get a graph looking like this:


    There is a leak somewhere
    -Cheers-

    Tycho

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    Re: Acceptable Vacuum in mm Hg

    Quote Originally Posted by Newgirl View Post
    Hi - in case any of you were wondering, I have now passed my CITB. (Huzzah!) However, I was checking through my Safe Handling of Refrigerants Reference Manual today and found a reference to the degree of acceptable vacuum you should aim to achieve. It states " as a guideline at least 10" Hg, 250 mm Hg vacuum should be achieved".

    I thought that the general guidance was that a vacuum of 300 to 500 microns should be achieved. Am I being thick, or isn't 300 microns equivalent to 0.3 mm Hg / Torr ? If so, the CITB guide is recommending a vacuum of 250,000 microns... I am totally unfamiliar with the whole concept of microns, mm Hg etc, so if I am totally missing the point, please tell me!!
    Good Post Newgirl!

    Basically no-one is wrong with their desired Target points.
    I would like to point out the obvious to many of the members.
    But as you say you are NEW!
    If I have opened a system to work on it and I have followed best practise.
    When I vacuumed it out, I would be happy with 10 TORR or thereabouts or 5,000 Microns.
    Provided there is no rise in pressure. ( indicates Moisture present which is being forced to boil)
    Then I would re-instate the system.

    If I am concerned about the presence of moisture then yes I would pull down to maybe 5 Torr or lower.
    Repeating this several times if necessary having each time broken the vacuum with Nitrogen.
    This is sometimes referred to as a triple evac.

    I have rarely ever had to pull lower than maybe 2 Torr and am not to keen on pulling much lower.
    Anyways as Tyco points out on larger systems trying to pull down to very low levels is in-practical and rarely achievable.

    The most important thing is don't loose sight of what is trying to be achieved.
    And that is to force any moisture (Water) to boil
    off and thereby be removed.

    Don't forget the pressure required to do so is directly related to the standing ( ambient) temperature of the plant.
    The COLDER it is the lower the vacuum needs to be to force the water to boil.

    So despite our many different desired Vacuum levels.
    As long as the pressure reached is low enough to force the water to boil.
    And there is no rise in pressure over a reasonable period.
    Then there is no moisture present......
    Job done.
    Grizzly
    supplied in Vikings post....

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    Re: Acceptable Vacuum in mm Hg

    pt chart for water gives required vacuum according to ambient

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    Re: Acceptable Vacuum in mm Hg

    Quote Originally Posted by Contactor View Post
    pt chart for water gives required vacuum according to ambient
    Well said.
    I was going to mention "Steam Tables as a reference.
    But decided that I had woffled on long enough !
    Cheers Grizzly

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    Re: Acceptable Vacuum in mm Hg

    Thank you guys, point was taken, I will shop for a high quality vacuum Gage tomorrow.

    A few things I would like to emphasize,
    As I see it' there are two kinds of vacuum, a new system and working system.

    In the first case, I leave the system in vacuum for at least 48 hours, in high ambient and low humidity [35%].
    In the second case, after recovering the system, there is always some refrigerant in it and after doing what needs to be done, just pull the vacuum once and fill the system up.

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    Re: Acceptable Vacuum in mm Hg

    Hello

    I few things I have learned.

    Size the pump to suit the system. A plug in 1 phase vacumn pump is no good for a large ammonia system, you need a large three phase single stage industrial pump to remove the bulk of the moisture.

    Triple evacuation is alway essential.

    Heat is needed to boil of the moisture in the system.

    2500 microns is fine for field service. 1000 microns for a new system.

    A good pressure test is essential before fitting a vac pump.

    Drop test, a rise and steady off is moisture, a continual rise is a leak.

    Fit the vacumn gauge well away from the pump.

    Change the oil in the pump when any visible change in the oil is seen.

    Hope this helps.

    Kind Regards Andy
    If you can't fix it leave it that no one else will

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    Re: Acceptable Vacuum in mm Hg

    I just replaced my failed first floor system in my home a few weeks ago. I placed my torr gauge at the lowest spot in the system. Thas is also where I located the pump. I have a tiny vacuum pump, so it took a while, perhaps four hours before I was satisfied with a double evacuation process. Both times, the torr gauge remained steady once things were valved off. With the connections right and tight, I had no trouble other than time waiting to establish slightly lower than 500 micron. Valved off, it wandered to 600 micron after 45 minutes or longer. I was content with the measurements and results.

    I will also comment that part way through this process, during the second evacuation, when I reached 1200 microns or so, the pump stopped pulling the system down. We watched for quite a while with no progress and then changed the pump oil. The system promptly sought a better and lower vacuum.

    You can't see any difference between 500 and 2000 torr or a leak on your manifold gauge. That means there is no proof the system is dry unless you use a torr gauge that measures vacuum properly. If you go by vacuum pump sound, it, might work. It might not. The customer pays the price because the consequence is the equipment won't last as long as it might.

    I've fixed stuff myself based on time and vacuum pump sounds. I won't go on that again after my observations and experiments. I have also run my vacuum pump into an empty R22 canister and watched with only a tiny leak in hoses and fittings, the torr gague is never satisfied when it is valved off. I cannot support comissioning a system based on a pointer gauge or based on vacuump pump sound as adequate for one's customers.

    I can say that if the system or evacuation system is leaking anywhere, you won't valve it off and see this result of almost no rise in 3/4 hour. I've tried this on an empty canister or R22 and found the quality of flares, fitting, hoses and connections is essential. I went to all copper lines to get things right and dry.

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