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#1
08-26-2009, 04:43 PM
 Mu-T WeldingWeb Apprentice Join Date: Jul 2009 Posts: 12
Calculating actual flow rate.

In the field, sometimes all the correct tools are not available, and we have to use what is at hand. This is the formula I use when the input gas pressure on a flow meter is not what it was calibrated for.

Variables:
AFR = Actual flow rate (CFH)
IFR = Indicated flow rate (CFH)
AP = Actual pressure (PSI) + psia constant at 1atm (14.7)
CP = Calibrated pressure (PSI) + psia constant at 1atm (14.7)
sqrt(x) = Square root of x

AFR = IFR x sqrt(AP/CP)

For example, a flow meter with a calibrated pressure value of 30, but with an actual pressure of 50, and an indicated flow rate of 35cfh, has an actual CFH of 42.108.

42.108 = 35 x sqrt((50+14.7)/(30+14.7))

I hope at some point in time this information will help someone.
#2
08-26-2009, 09:05 PM
 OAdoctor WeldingWeb Tradesman Join Date: Mar 2009 Location: Loganville, GA Posts: 232
Re: Calculating actual flow rate.

Great Info. Thanks!
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#3
08-27-2009, 05:04 PM
 pulser WeldingWeb Craftsman Join Date: Nov 2005 Location: Santa Fe, NM Posts: 1,708
Re: Calculating actual flow rate.

Good point, the floating ball type flow meters are calibrated for a particular pressure, and operating at a different pressure will give incorrect readings. A mass flow meter is not affected by pressure.
#4
08-27-2009, 06:04 PM
 LarryO WeldingWeb Craftsman Join Date: Feb 2009 Location: Melbourne, Australia Posts: 1,537
Re: Calculating actual flow rate.

I have a flow meter that fits over the end of a torch shield ( TIG, MIG ).

Found what i get at the gun is always different at the reg.

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