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Finding flow rates for different gases

  1. Apr 8, 2013 #1
    (Please excuse me if this is not the right forum for this question)

    I have been setting up some old regulators for use with welding gases and need a simple way of converting flow from one gas to another since I often find myself trying to measure flow of one gas with a flowmeter calibrated for another. I assumed conversion tables must exist for this purpose and did not think it would be a problem. However once I got doing it I found that there are vast differences between the results to be had by using the conversion factors to be found on different websites on the net, weirdly so.

    For example, a Dartmouth Engineering site says this:
    Flowmeters are calibrated for a specific gas. To calculate the flow rate of another gas, multiply the reading by the ratio of the conversion factor of the desired gas to the conversion factor for the calibration gas.
    Air 1.00
    Oxygen .996
    Argon 1.443
    Helium 1.454

    Matheson Gas, on their Basic Flowmeter Principles page
    gives these flow rate conversion factors

    Air 1.00
    CO2 .92
    Oxygen 1.05
    Argon 1.18
    Helium .37

    The Omega Instruments site gives this;
    Air 1.00
    CO2 .74
    Oxygen 1.05
    Argon 1.45
    Helium 1.45

    And another site whose name I lost
    Air 1.00
    CO2 .85
    Argon 1.52
    Helium 3.97

    Another welding site, Scottgross, gives the flows differently, as SCFH through a .032 (the standard size used on welding regulators for flow measurement) orifice at 30 psi, but we can convert those to conversion factors:
    Air 37 (conversion factor)
    CO2 30 .81
    Oxygen 35 .94
    Argon 32 .86
    Helium 100 2.70

    Most of these pages caution that they factors should be considered accurate only within 10%, which is good enough for my purposes, so minor differences are understandable. But look at argon, which drifts from .86 to 1.53 And what are we to make of the helium numbers? It's hard to avoid thinking that someone just did their math backwards.
    I am tempted to go with the scottgross factors, as they appear to be derived from actual measurements using the same size orifice as I will be using, but it is odd that those numbers seem be be the inverse of everyone else's.

    I know there are formulas that would let me figure this out for myself, but my math skills are prehistoric and I can't believe there isn't an easier way.
  2. jcsd
  3. Apr 8, 2013 #2


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    Staff Emeritus
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    It may be that the various instruments use different methods to measure gas flow, which is why the various factors are so different.
  4. Apr 9, 2013 #3
    A good point, and there are indeed two common systems, mainly, pressure-based flowmeters which use an orifice and a pressure gauge, and vertical tube flowmeters that have a floating weight in a transparent tube. In the welding world they are used interchangeably. But if there was is a significant difference between the readings the two produce, it seems odd that conversion charts and recommended flow charts such as I quoted don't ever specify what kind of flowmeters they apply to.
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