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  • converting ppb to ug/m3

    im attempting to convert ppb to ug/m3

    i want to take pressure and temperature into account when converting

    i have seen the archive for this forum
    [inset this domain]/archive/index.php?t-413.html

    ppm(stp) = ppm(@ Tm) x Tstp / Tm

    where Tm is the temperature measured and T stp is the standard temp & pressure
    but how do i alter the equation to take into account that the ppm was measured at a pressure that is not the standard

  • #2
    Re: converting ppb to ug/m3

    Originally posted by Unregistered
    im attempting to convert ppb to ug/m3

    i want to take pressure and temperature into account when converting

    i have seen the archive for this forum
    [inset this domain]/archive/index.php?t-413.html



    but how do i alter the equation to take into account that the ppm was measured at a pressure that is not the standard
    Ppm or ppb is an ambiguous term. Is it a ratio based on molar fraction, weight, or volume?

    If you are talking a gas pollutant in air, there is no such adjustment. Draw a control volume. Both the air and the pollutant inside change volume with temperature and total pressure, but their ratio remains unchanged (one of the reasons for using it).

    Now when you convert to mass/volume, things do change with temperature and pressure. The mass of pollutant inside the control volume remains unchanged, by the volume of the mixture changes (essentially following ideal gas law).

    It isn't clear to me exactly what you are trying to do or how to offer help.

    Comment


    • #3
      Re: converting ppb to ug/m3

      we are measuring NO2 in air (ppb) as well as the pressure and temperature

      im not sure on the method of te measurement of the (NO2) we are using an ML9841B Nitrogen Oxides Analyzer ive looked in the manual but cann't find nything telling me if it is Mw or volume

      in order to report our findings to defra we have to convert to ug/m3

      using the defra conversion method you just multiply by 1.91 and defra state that this assumes that the temperature is 25C and the pressure is 1013mbar i can't find any information on how this conversion factor is obtained i only know it is used across Europe (which i feel is inappropriate due to the massive range of temperatures)

      i am wanting to compare a correct conversion method that takes the temp and pressure at the time the sample was taken with the defra method in order ascertain if the air quality is better/worse than we currently think it is

      Comment


      • #4
        Re: converting ppb to ug/m3

        i decided to register as this forum is rather useful

        i found this equation

        ug/m3 = (ppb x Mw x Tsd x Pm)/(22.4136 x Tm x Psd)
        ppb = concentration
        Mw = molecular weight
        Tsd = standard temperature
        Pm = measured presure
        Tm = measured temperature
        Psd = standard pressure

        from ht__tp://www__.gasdetection.com/__TECH/__convert.html
        (just remove the underscores as i cann't yet post urls)
        it is in the ppm to mg/m3 excel spreadsheet

        i can't work out is Tst and Pst are the T & P at which the molar weight are measured or if it is 0C and 1 bar

        Comment


        • #5
          Re: converting ppb to ug/m3

          Since it is a gas in air, it is almost certainly based on molar fractions. The US (EPA and OSHA) requires this for gas pollutants, weight/volume is ONLY permitted for dusts and particulates. When stated as mole fraction, the value will not change with temperature and pressure.


          Molar weight is a property of the NO2 and independent of temperature and pressure. The MW term converts the mole fraction of NO2 to a mass. The 22.4136 term is molar volume at "normal," it is converting the moles of air to a volume at 0 C and 101.325 kPa, the conditions with "sd" subscript. The other terms correct that volume for measured conditions. Some powers of ten are being "concealed" as they internally cancel. Ppb is mol of pollutant divided by kmol of mixture, and the pollutant is converted to g, and the mixture to m.

          The equation looks correct, except 22.4136 seems slightly odd. The NIST CODATA value is nearer 22.41400 L/mol, for an ideal gas. However, if they are correcting for Z, compressibility factor, a value of 0.99998 would explain the difference. Air is so close to 1 at "normal" that I have never looked for a Z value. It is probably much closer than the accuracy of measurement and I wouldn't worry about it.

          Comment


          • #6
            Re: converting ppb to ug/m3

            thanks

            does the unit in which i input the pressures matter as our kit measures in mbar (so i was going to use 1000 mbar ie 101.3Kpa as the Psd)

            as i assume the units of the pressure will cancel anyway

            or do i need to convert it to Kpa
            Last edited by garethrees; 09-08-2008, 06:28 AM.

            Comment


            • #7
              Re: converting ppb to ug/m3

              Originally posted by garethrees
              thanks

              does the unit in which i input the pressures matter as our kit measures in mbar (so i was going to use 1000 mbar ie 101.3Kpa as the Psd)

              as i assume the units of the pressure will cancel anyway

              or do i need to convert it to Kpa
              The pressure units don't matter as long as they are absolute, not gauge, and normal and measured both use the SAME units. (Your spreadsheet used mm Hg).

              Oops. 1000 mbar != 101.3 kPa, you can use 1013 mbar, but 22.4136 is NOT the correct molar volume for 1 bar, that is nearer 22.7 L/mol. 1 bar = 100 kPa, exactly. "Standard" pressure is agreed by convention 1013.25 mbar, 101.325 kPa, 760 mm Hg. etc. even if it rarely occurs.

              The temperatures also need to be absolute, kelvin recommended. However, any other absolute scale, such as Rankine would work, if both are in same unit.

              Comment


              • #8
                Re: converting ppb to ug/m3

                thanks for that ive altered the standard pressure to 1013.25mbar

                im using Kelvin for the temperature

                thanks for the help

                Comment


                • #9
                  Re: converting ppb to ug/m3

                  1ppb So2 to ug/m3

                  Comment


                  • #10
                    Re: converting ppb to ug/m3

                    Originally posted by Unregistered View Post
                    1ppb So2 to ug/m3
                    See formula in post #4 and discussion in 5-8. You just need to determine the molecular weight of SO2 (approx 64)

                    Comment

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