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  1. #1
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    Does a vacuum pump actually create negative pressure?



    If you use a vacuum pump to evacuate the refrigerant system inorder remove moisture and contaminates, does it actually create a vacuum or negative pressure within the system? My manifold gauge definitely reads negative pressure but I am wondering if the vacuum pump is just making the pressure in the system lower than atmospheric pressure. Atmospheric pressure is roughly 14.7 psia at sea level, so my theory is that the vacuum pump just lowers the pressure within the system below that which is atmospheric pressure outside of the system, which in fact is not a negative pressure, but in a sense yes it is a vacuum but only because the pressure in the system is lower than that outside of the system. It just seems to me if you actually went below 0 psia then you would be creating a black hole. Can anybody explain this to me?



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    Re: Does a vacuum pump actually create negative pressure?

    Congratulations you’ve just discovered what Evangelista Torricelli did in 1643!

    Atmospheric pressure exists and a vacuum pump makes it disappear in a quantum micro black hole.

    When the pump has pushed everything into the black hole, fortunately enough for us it evaporates instantly and all you are left is with zero absolute pressure.

    If you start measuring from this point up, then you won’t have to deal with those nasty negative numbers any more.

    The only problem is that nobody agrees what atmospheric pressure is, some say 14,696 psi others 1 atm, yet others 1 kgf/cm2. But at least everybody agrees that we can measure it from zero absolute up! Never down because you cannot take anything back from the black hole.

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    Re: Does a vacuum pump actually create negative pressure?

    Quote Originally Posted by Hutch. View Post
    If you use a vacuum pump to evacuate the refrigerant system inorder remove moisture and contaminates, does it actually create a vacuum or negative pressure within the system? My manifold gauge definitely reads negative pressure but I am wondering if the vacuum pump is just making the pressure in the system lower than atmospheric pressure. Atmospheric pressure is roughly 14.7 psia at sea level, so my theory is that the vacuum pump just lowers the pressure within the system below that which is atmospheric pressure outside of the system, which in fact is not a negative pressure, but in a sense yes it is a vacuum but only because the pressure in the system is lower than that outside of the system. It just seems to me if you actually went below 0 psia then you would be creating a black hole. Can anybody explain this to me?
    You need to understand what a vacuum is, in order to be able to create one.

    There is no such thing as negative pressure (or negative temperature come to that).
    We are all under pressure, because we are all on this planet and the planet has an atmospher.

    The weight of the atmospher will depend on were you are on the surface of the world,
    but you are correct in that most people understand it to be about 14.7 psi (1 bar) at sea level.
    It could also be written in differant ways dependant on the scale you use ( millibar, pascals,
    milimeters of murcury, inches of murcury or microns ).

    We are part way up a scale and if you create a vacuum or a "negative" pressure you only
    lower the pressure below what we are at.

    Nobody (not even at SERN) nobody can create a pressure below nothing.

    If you ever get low enough to nothing there is, abosolutely nothing, no pressure, no heat,
    no sound, no movement and some scientists argue that even time does not exist.

    So when you create a vacuum with you vac pump and measure it with your gauges, all
    you are doing is removing a little bit of pressure and getting closer to nothing.

    The better the pump and the more accurate the gauge will allow you to get lower,
    but nobody on this earth has ever got down to nothing.

    Nothing is called Absolute Zero by the way and the absolute scale is pascals and it starts
    at Zero...............

    All the best

    taz

    .
    Last edited by taz24; 19-04-2012 at 09:29 AM.

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    Re: Does a vacuum pump actually create negative pressure?

    The vacuum pump removes moisture, contaminents (which you could call air).
    Only want refrigerant in system, nothing else to make a pure correct cycle.
    If you live in outer space, you could probably leave vacuum pump at home.

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    Re: Does a vacuum pump actually create negative pressure?

    .

    To corectly measure something you need to know where you are starting from and where you
    are going to end up. Those 2 points then can be measured. What you measure and the scale you
    measure in does not mater, all you are doing is measure the distance (in any scale) between
    2 (or more) points.

    Where people stuggle with pressure and temperature, is the start point.

    If you do not know where you are starting from you can't possibly measure how far you have
    traveled.


    Hundreds of years ago a couple of guys found (or they thought they found) the start point.
    Pascal and Torricelli were doing this 300 or 400 years ago. That is why we measure pressures
    in torrs's and pascal's.

    All the best

    taz

    .

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    Re: Does a vacuum pump actually create negative pressure?

    Troll!!!!!
    Tool's ? check ! Condom's ? check !
    If you can't fix it , f*ck it !!!

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    Re: Does a vacuum pump actually create negative pressure?

    Quote Originally Posted by Hutch. View Post
    If you use a vacuum pump to evacuate the refrigerant system inorder remove moisture and contaminates, does it actually create a vacuum or negative pressure within the system? My manifold gauge definitely reads negative pressure but I am wondering if the vacuum pump is just making the pressure in the system lower than atmospheric pressure. Atmospheric pressure is roughly 14.7 psia at sea level, so my theory is that the vacuum pump just lowers the pressure within the system below that which is atmospheric pressure outside of the system, which in fact is not a negative pressure, but in a sense yes it is a vacuum but only because the pressure in the system is lower than that outside of the system. It just seems to me if you actually went below 0 psia then you would be creating a black hole. Can anybody explain this to me?
    Even though this might look like a Troll, there are some valid points, and quite interesting so.

    First, I just want to say THE METRIC SYSTEM RULES!!! ALL ELSE DROOLS

    that out of the way, here goes

    The pressure in a fluid is defined as

    "the normal force per unit area exerted on a imaginary or real plane surface in a fluid or a gas"

    The equation for pressure can expressed as:

    p = F / A (1)

    where

    p = pressure [lb/in2 (psi) or lb/ft2 (psf), N/m2 or kg/ms2 (Pa)]

    F = force [1), N]

    A = area [in2 or ft2, m2]

    1) In the English Engineering System special care must be taken for the force unit. The basic unit for mass is the pound mass (lbm) and the unit for the force is the pound (lb) or pound force (lbf).

    relationships between absolute pressure and gauge pressure

    Absolute Pressure

    The absolute pressure - pabs - is measured relative to the absolute zero pressure - the pressure that would occur at absolute vacuum. All calculation involving the gas laws requires pressure (and temperature) to be in absolute units.

    Gauge Pressure

    A gauge is often used to measure the pressure difference between a system and the surrounding atmosphere. This pressure is often called the gauge pressure and can be expressed as

    pg = ps - patm (2)

    where

    pg = gauge pressure

    ps = system pressure

    patm = atmospheric pressure

    Atmospheric Pressure

    Atmospheric pressure is pressure in the surrounding air at - or "close" to - the surface of the earth. The atmospheric pressure vary with temperature and altitude above sea level.

    Altitude and Air Density

    Standard Atmospheric Pressure

    Standard Atmospheric Pressure (atm) is used as a reference for gas densities and volumes. The Standard Atmospheric Pressure is defined at sea-level at 273oK (0oC) and is 1.01325 bar or 101325 Pa (absolute). The temperature of 293oK (20oC) is also used.

    In imperial units the Standard Atmospheric Pressure is 14.696 psi.

    1 atm = 1.01325 bar = 101.3 kPa = 14.696 psi (lbf/in2)= 760 mmHg =10.33 mH2O = 760 torr = 29.92 inHg = 1013 mbar = 1.0332 kgf/cm2 = 33.90 ftH2O

    Pressure Units

    Since 1 Pa is a small pressure unit, the unit hectoPascal (hPa) is widely used, especially in meteorology. The unit kiloPascal (kPa) is commonly used design of technical applications like HVAC systems, piping systems and similar.

    1 hectoPascal = 100 Pascal = 1 millibar
    1 kiloPascal = 1000 Pascal

    Some Pressure Levels

    10 Pa - the pressure below 1 mm of water
    1 kPa - approximately the pressure exerted by a 10 g of mass on a 1 cm2 area
    10 kPa - the pressure below 1 m of water, or the drop in air pressure when moving from sea level to 1000 m elevation
    10 MPa - nozzle pressure in a "high pressure" washer
    10 GPa - pressure enough to form diamonds

    Some Alternative Units of Pressure

    1 bar - 100,000 Pa
    1 millibar - 100 Pa
    1 atmosphere - 101,325 Pa
    1 mm Hg - 133 Pa
    1 inch Hg - 3,386 Pa

    A torr (torr) is named after Torricelli and is the pressure produced by a column of mercury 1 mm high - equals to 1 / 760th of an atmosphere.

    1 atm = 760 torr = 14.696 psi

    Pounds per square inch (psi) was common in U.K. but has now been replaced in almost every country except in the U.S. by the SI units. Since atmospheric pressure is 14.696 psi - a column of air on a area of one square inch area from the Earth's surface to the space - weights 14.696 pounds.

    The bar (bar) is common in the industry. One bar is 100,000 Pa, and for most practical purposes can be approximated to one atmosphere even if

    1 Bar = 0.9869 atm

    There are 1,000 millibar (mbar) in one bar, a unit common in meteorology.

    1 millibar = 0.001 bar = 0.750 torr = 100 Pa
    blatantly copied from here http://www.engineeringtoolbox.com/pressure-d_587.html

    Tho I imagine that if I posted just the link, none of you would have looked at it


    Very interesting that I came over this a few weeks ago, and now I find this question here
    -Cheers-

    Tycho

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    Re: Does a vacuum pump actually create negative pressure?

    Quote Originally Posted by Hutch. View Post
    If you use a vacuum pump to evacuate the refrigerant system inorder remove moisture and contaminates, does it actually create a vacuum or negative pressure within the system? My manifold gauge definitely reads negative pressure but I am wondering if the vacuum pump is just making the pressure in the system lower than atmospheric pressure. Atmospheric pressure is roughly 14.7 psia at sea level, so my theory is that the vacuum pump just lowers the pressure within the system below that which is atmospheric pressure outside of the system, which in fact is not a negative pressure, but in a sense yes it is a vacuum but only because the pressure in the system is lower than that outside of the system. It just seems to me if you actually went below 0 psia then you would be creating a black hole. Can anybody explain this to me?
    A vacuum is a name for a low pressure area. A potential difference of pressure having a lower pressure than a higher pressure that when connected causes energy to flow in a form of a current that will flow until the potential difference is equalised or balanced.

    The divers who have to live for 3 months in a pressure chamber to climatise to working within an atmospheric pressure beyond atmospheric pressure at sea level, ie 14.7 psig, 1 barg, 100kpa, 750 torr. Are dwelling in an atmosphere where everything outside the chamber is a lower pressure area or a vacuum equivalent to the one we would pull down within a system. If they were to step outside that chamber before it had depressurised over a few weeks they would explode and atomise.

    It would be the same as pulling an instant vacuum on a system or container with a known quantity of moisture causing it to vapourise and evaporate, depending on the ambiant temperature.

    That is how I would define a vacuum in simplified terms.
    50 hertz but 100s worse

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    Re: Does a vacuum pump actually create negative pressure?

    Pressure is a function of the number of molecules present. When you create a vacuum, you are reducing the number of molecules present. A good vacuum pump should be able to pull below 25 microns (or millitorr). However, most techs I know are satisfied to get a system below 500 microns. The only way to tell that is with an electronic vacuum gauge. It's impossible to do with a low-side compound gauge.

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