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  • ppm to ib/hr

    Hi,
    I'm measuring total VOC with a PID from a pipe containing air. The flow rate is 100 cfm. I would like to convert the ppm to ib/hr. what other parameters that I need for the calc, and better what would be the calculation. Thanks in advance!

  • #2
    Originally posted by Floki1Frith2 View Post
    Hi,
    I'm measuring total VOC with a PID from a pipe containing air. The flow rate is 100 cfm. I would like to convert the ppm to ib/hr. what other parameters that I need for the calc, and better what would be the calculation. Thanks in advance!
    You need to know what kind of ppm, mass ratio, volume ratio, molar ratio. Being a gas, probably molar ratio. Based on temperature and barometric pressure, you need to convert your exhaust flow of100 cfm to moles/min. Multiply by ppm )expressed as a decimal, 1 ppm is 0.000 001) to get moles/min of VOC. Now the big problem -- to convert to mass, you need an average molar mass of the VOCs, but by definition, it is a mixture. You need at least an approximate analysis of the composition.

    The details of the calculation may be spelled out in law. If you have doubts about how to procedure, you may need to consult an engineer faamiliar with the calculations necessary for compliance.

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    • #3
      Thanks John,
      That is most helpful!
      The PID is calibrated using isobutylene, maybe use that for the average molar mass?

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      • #4
        Originally posted by Floki1Frith2 View Post
        Thanks John,
        That is most helpful!
        The PID is calibrated using isobutylene, maybe use that for the average molar mass?
        That would certainly be correct for isobutylene.

        I am not a professional environmental engineer and can't offer a professional opinion. Where I have been involved, my experience is that the procedure is usually spelled out in law and varies slightly from place to place. If this involves complianc, I would first look to the ful text of the law.

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