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    nadeem majeed's Avatar
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    Question measuring vacuum



    when we vacuum the a/c unit the vacuum gauge goes to -1bar. can we get more vacuum on the same atmospheric pressure



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    Re: measuring vacuum

    Quote Originally Posted by nadeem majeed View Post
    when we vacuum the a/c unit the vacuum gauge goes to -1bar. can we get more vacuum on the same atmospheric pressure
    Anyone who uses their compound guage to read and measure a vacuum is a cowboy and shouldn't have their licence.

    A vaccustat is the only way to know what vacuum you have in a system and your codes of practice will tell you the minimum requirements for all systems.

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    Re: measuring vacuum

    Hi nadeem

    It sounds like you are quoting -1 bar from your manifold gauge set?
    This is not suffiecient for accurate measurement of vacuum.

    The scale is vast and mbar is expressed differently in low vacuum industry/applications.
    1000mbar or atmospheric will appear as 1 x 10 3 (this should be a small 3 next to the 10).

    With a rotary vane vacuum pump the lowest point really necassary for dehydration in the refrigeration industry is anything <3 mbar.
    This type of pump will go to 1 x 10-3 or 0.001 mbar.

    Lower vacuum is possible and the best recorded in test conditions is 1 x 10-14 or 0.0000000000000mbar!

    Hope this helps....

    Sturt.

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    Re: measuring vacuum

    I use microns as the unit for measuring a vucuum.

    My Vac pump pulls a vacuum of 27 microns.

    On a new install I like to pull a vac of 50 microns. If I shut the system off and leave for the lenght it takes me to have a ciggarette and the vac reads 1 micron above what I've pulled. I start to worry about a leak.

    On existing "wet" plant I pull at least 1000 microns, however I usually tripple vac and pull to 500 or less.

    -DOGMA_

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    Re: measuring vacuum

    Dogma, 500 microns is far above vapor pressure of water at room temperature.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: measuring vacuum

    Extract from Australia/NZ Codes of practice.


    PART 1
    6 Evacuation
    This section refers to evacuation in the field only – not evacuation during the manufacturing
    process.
    As the systems covered by this code are supplied pre-charged with refrigerant there is
    no need to evacuate the system upon installation. If evacuation is required at a later stage,
    however, the following procedure must be followed:
    6.1 Instructions must be followed if the system manufacturer has supplied instructions
    for evacuation, except where the instructions specify a practice that will lead to
    emission of refrigerant.
    6.2 The system must be evacuated to less than 117 Pa absolute (900 microns of mercury)
    if the system manufacturer has not supplied instructions with the system for
    evacuation.
    6.3 After the system has been evacuated the vacuum pump should be isolated from
    the system. As a guide, with constant ambient conditions, the vacuum should not
    rise more than 13 Pa (100 microns of mercury) in one hour. A greater rate of rise may
    indicate a leak or the presence of moisture (see also 7.1.9).
    6.4 Absolute vacuums must be measured using accurate measuring equipment selected for the specific application.


    PART 2
    6 Evacuation
    This section refers to evacuation in the field only – not evacuation during the manufacturing process.
    6.1 The manufacturer’s instructions for evacuation must be followed if the system is
    factory-matched (ie: the manufacturer has supplied a matched evaporator and
    condenser) and the manufacturer has supplied instructions with the system, except
    where the instructions specify a practice that will lead to emission of refrigerant.
    Provided the instructions do not specify a practice that will lead to emission of
    refrigerant, if the manufacturer’s instructions are followed then the installation is
    exempt from items 6.1.2 to 6.1.5.
    The relevant parts of this section must be complied with if there are any parts of the
    evacuation procedure not covered by the manufacturer’s instructions.
    Installation of all other systems, or systems where manufacturer’s instructions are not
    supplied, must comply with section 6 of this code in its entirety.
    6.2 Evacuation should be carried out with dedicated evacuation hoses (large diameter
    / as short as practical) and gauges and not service manifolds / gauges.
    6.3 The system must be evacuated to remove moisture and non-condensables after
    determining that there are no refrigerant leaks when the system is pressurised,
    6.4 Evacuation must be either the deep evacuation method, or triple evacuation using dry
    nitrogen only as the moisture absorber, following the procedures described below.
    Deep vacuum method: Pull a deep vacuum to a pressure of less than 65 Pa absolute
    (500 microns of mercury). After isolating the vacuum pump, allow the system to stand for
    60 minutes to ensure the vacuum is maintained at or below 78 Pa absolute (600 microns of mercury); OR
    Triple evacuation method: Use a vacuum pump to pull a vacuum to a pressure
    of at least 65 Pa absolute (500 microns of mercury). Break the vacuum with dry
    nitrogen and allow the system to stand. Re-evacuate the system and repeat the
    procedure twice more, breaking the vacuum each time with dry nitrogen.


    That is the guide I use Peter_1 but I always prefer to go beyond the recomendation.

    Like I said though, in a new/dry system I go to 50u and 500 for existing/wet plant.


    I do usually try to reach 50u on all systems if time persists.

    I was working with my boss on industrial plant a couple months back and he used a vac stat for the first time. He reached 3000 u and said that was enough. I kept my mouth shut.

    Last edited by dogma; 08-11-2008 at 10:34 AM. Reason: more info included

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    Re: measuring vacuum

    Quote Originally Posted by Peter_1 View Post
    Dogma, 500 microns is far above vapor pressure of water at room temperature.

    Nope! Vapor pressure of water at 20°C is 0,0234 Bar which is 17,55144338 Torrs or 17.551,44338 microns.
    That mean that 500 microns is way below vapor pressure of water at room temperature.

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    Re: measuring vacuum

    Hi nike123 and Peter 1.

    What are the recomendations and codes in your countries for evacuation etc?.... and do you have a ph diagram for water/steam you can post?



    -Dogma_

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    Re: measuring vacuum

    Quote Originally Posted by dogma View Post
    Hi nike123 and Peter 1.

    What are the recomendations and codes in your countries for evacuation etc?.... and do you have a ph diagram for water/steam you can post?



    -Dogma_
    This is from CPS:
    2000 microns lowest average industrial requirements for A/C systems
    1000 microns medium average industrial requirements for A/C systems
    600 microns highest average industrial requirements for A/C systems
    400 microns lowest average industrial requirements for refrigeration systems
    200 microns medium average industrial requirements for refrigeration systems
    100 microns highest average industrial requirements for refrigeration systems
    25 microns deep vacuum for special requirements and for testing of vacuum pump efficiency


    This could be helpful!

    This is water vapor pressure temperature table!

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    Re: measuring vacuum

    Sorry Dogma, misinterpreted IP units. You're 100% right.
    I also use the triple vacuum method but use now OFN instead of refrigerant like in the old days.
    What we have to use legally now is the EN378 which states lower than 290 Pa.
    I personally say in school lower than 100 Pa, preferable in the region of 50 Pa to 70 Pa.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: measuring vacuum

    Quote Originally Posted by Peter_1 View Post
    "...but use now OFN instead of refrigerant like in the old days."
    OFN? please explain

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    Re: measuring vacuum

    Quote Originally Posted by dogma View Post
    OFN? please explain
    Oxygen Free Nitrogen (O2 < 0,5 ppm)
    Last edited by nike123; 09-11-2008 at 12:20 PM.

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    Re: measuring vacuum

    Ahh... I see..


    nadeem,


    buy 1 of these mate.


    http://www.toolstop.co.uk/cps-vg200-...um-gauge-p1327
    Last edited by dogma; 10-11-2008 at 07:06 AM. Reason: bit more in

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