Recalibrate Rotameters


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I have a rotameter for air measured in SCFM. I need to measure a helium flow rate.

I can't find a conversion on the net, but have come up with the actual flow rate in helium is equal to:

Q helium = Q air * (1/ Sg)^.5

Where Sg = specific gravity with respect to air
(Note: Helium Sg = 0.138)

So for example, if I'm measuring the flow of helium and the rotameter says 1 SCFM air, then I'm getting (1/.138)^.5 = 2.69 SCFM of helium.

Can anyone else confirm this or provide a different conversion?

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