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PSI .. PSIG.. PSIA help please

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  • PSI .. PSIG.. PSIA help please

    Ok I know that PSI = lb/ft^2 and the G stands for gauge and a stands for absolute. My question is do you convert between the three.

    For example I am given a number 50 PISA ... how do I convert that? I don't understand and can't find anything on the net. When solving problems I want to be in PSI, right? So therefor when given PSIG x 144= PSI correct? please help i have the test tomorrow and I don't want a simple conversion messing my mark up.

  • #2
    Re: PSI .. PSIG.. PSIA help please

    PSIG is the pressure read from a guage which reads the difference between the pressure in the pipe and the pressure of the atmosphere. PSIA is the total pressure including the pressure of the atmosphere.

    PSIG + 1 atmosphere = PSIA
    PSIA - 1 atmosphere = PSIG

    1 atmosphere is approximately 14.7 PSI, so use that value, or the acutual atmosphere measurement for your location if you have it.

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    • #3
      Re: PSI .. PSIG.. PSIA help please

      thank you very much!

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      • #4
        Re: PSI .. PSIG.. PSIA help please

        psia is the absolute pressure

        psig is the gauge pressure. It is lower than the absolute pressue by the atmospheric pressure in the room where the gauge is being used.

        atomospheric pressure is generally assumed to be 14.7 psi

        psi is not specific, but generally assumed to be the same as psig

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        • #5
          Re: PSI .. PSIG.. PSIA help please

          if psi and psig are same then how atmospheric pressure be 14.7 psi. it should be 0 psi.
          I think it is 14.7 psi 0 psig. so, generaly psia and psi are same.

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          • #6
            how to convert PSIG to Kg/sq.cm

            please help me how to convert the PSIG to Kg/sq.cm

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            • #7
              Re: PSI .. PSIG.. PSIA help please

              1 psi = 1/14.5 bar = 1e5/14.5 Pa (or N/m2)

              Therefore 1 psi = 1e5/14.5 x 1e-4 = 10/14.5 N/cm2 = 0.6897 N/cm2

              1N = 1 kg.m/s2 = 1/9.8067 kgf

              I'm assuming you mean kgf, otherwise the units of your conversion would be kg/cm/s2...

              Assuming you mean kgf/cm2:

              1 psi = 0.6897/9.8067 = 0.0703 kgf/cm2

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              • #8
                Re: PSI .. PSIG.. PSIA help please

                Originally posted by Unregistered
                if psi and psig are same then how atmospheric pressure be 14.7 psi. it should be 0 psi.
                I think it is 14.7 psi 0 psig. so, generaly psia and psi are same.
                Atmospheric pressure at sea level is 14.7 psia = 0 psig.

                psig is pressure relative to a reference point, typically ambient pressure (usually safe to assume 14.7 psia, unless your calculation is for a pressure relative to that in a liquid column, e.g. at seabed conditions).

                psi is a unit of pressure difference, hence the common assumption that if a pressure is given in psi it is equivalent to a gauge pressure. Unless it is obvious I would ask for it be clarified as the term 'psi' tends to be a bit misused - usually when the originator does not know if they mean gauge or absolute!

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                • #9
                  Re: PSI .. PSIG.. PSIA help please

                  Originally posted by Grace
                  1 psi = 1/14.5 bar = 1e5/14.5 Pa (or N/m2)

                  Therefore 1 psi = 1e5/14.5 x 1e-4 = 10/14.5 N/cm2 = 0.6897 N/cm2

                  1N = 1 kg.m/s2 = 1/9.8067 kgf

                  I'm assuming you mean kgf, otherwise the units of your conversion would be kg/cm/s2...

                  Assuming you mean kgf/cm2:

                  1 psi = 0.6897/9.8067 = 0.0703 kgf/cm2
                  so if i have air at 200psi its 214.7psig?

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                  • #10
                    Re: PSI .. PSIG.. PSIA help please

                    Psia "absolute" =14.7 At Sea Level
                    Psig "guage" = 0 At Sea Level If Your Reading Is Sensative To Altitude For Plumbing Or Avaionics Or Even Engeniring This Would Be A Critical Issue 0 Psi Or Psig {same} =14.7 Psia

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                    • #11
                      Re: PSI .. PSIG.. PSIA help please

                      so if i have air at 200psi its 214.7psig?
                      If you have 200psi and it is not specified to be gage or absolute, it is assumed that you have a gage reading 200psi, so it is equal to 200psig.

                      The absolute pressure would be 214.7psia.

                      Take a tire for example. The 200psi is the pressure of air in the tire greater than the atmospheric pressure outside the tire. Since the atmospheric pressure is 14.7 psi, the total pressure (absolute) in the tire is 214.7psia.

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                      • #12
                        Re: PSI .. PSIG.. PSIA help please

                        what is the conversion from PSI to PSIA?
                        is it the same?

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                        • #13
                          Re: PSI .. PSIG.. PSIA help please

                          There really is no "conversion"
                          psia = pounds / sq. ft in absolute terms.
                          psig = pounds / sq. ft as shown on a gauge.

                          I guess the "conversion" is psia = psig+14.7 psi at sea level ( or = psig + [whatever ambient pressure you're at])

                          If you're at sea level and you have a sealed container with a gauge reading 20 psi, the absolute pressure in that container is (20+14.7 =) 34.7 psia. If you stick that container in a plane and fly up to where the atmospheric pressure is only 5 psi (and you're wearing a pressure suit so you don't pass out), that same guage will read 32.7 psi.

                          Say the tank has a big ol' label on it saying "This tank will explode at 25 psi." Will it explode in your plane?
                          Probably not, because in general, 25 psi would mean a gauge reading of 25 psi at sea level, which corresponds to 39.7 psia. No matter what the guage reads, the pressure in the tank has not changed, it's still 34.7 psia.

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                          • #14
                            Re: PSI .. PSIG.. PSIA help please

                            Originally posted by Unregistered guy
                            Say the tank has a big ol' label on it saying "This tank will explode at 25 psi." Will it explode in your plane?
                            Probably not, because in general, 25 psi would mean a gauge reading of 25 psi at sea level, which corresponds to 39.7 psia. No matter what the guage reads, the pressure in the tank has not changed, it's still 34.7 psia.
                            I agree with your first two paragraphs, but I disagree with this. The stress in the walls of the tank relate to the differential (or gauge) pressure. In the example, the rated pressure will be exceeded if atmospheric pressure drops. This is why divers breathing compressed air have to exhale as they come to the surface (and come slowly).

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                            • #15
                              Re: PSI .. PSIG.. PSIA help please

                              So up above you'll see the correct difference between psig and psia several times (psig + 14.7 psi = psia). But I notice that there is a lot of variation on what people are calling psi. Let me try to clear it up a little for people who may still find this thread when searching for help on pressure.

                              psi is not specific to gauge or absolute pressure. Which it is greatly depends on where you find it (if I had my way, it would always be specified as gauge or absolute).

                              With school, in equations you're using the "psi" you should use is almost always absolute (psia). In general, with equations you want to use absolutes (absolute temperature like Kelvin, absolute pressure like psia, etc).

                              Outside of theory and equations (such as labels on tank or objects), "psi" usually refers to gauge pressure.

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