View Full Version : Calculating actual flow rate.

08-26-2009, 04:43 PM
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.

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.

08-26-2009, 09:05 PM
Great Info. Thanks!

08-27-2009, 05:04 PM
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.

08-27-2009, 06:04 PM
I have a flow meter that fits over the end of a torch shield ( TIG, MIG ).
Only cost about $10.

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