Gordon England Surface Engineering Forum

Formula for Plasma Gas Flow Conversion

M1 = Molecular weight of calibrated gas
M2 = Molecular weight of gas flowing
P1 = Pressure of calibration gas in absolute units
P2 = Pressure of gas flowing in absolute units
T1 = Temperature of calibration gas in absolute units
T2 = Temperature of gas flowing in absolute units
F = Direct reading of flowmeter
Q = Standard volume gas flow rate

Molecular Weights of Gases:	Air	29
	Nitrogen	28
	Argon	40
	Helium	4
	Hydrogen	2

7MC Flowmeter Calibrated as Follows:

Primary gas flowmeters	Nitrogen @ 50 psi, 70^oF
Secondary hydrogen flowmeters	Hydrogen @ 50 psi, 70^oF
Secondary helium flowmeters	Nitrogen @ 50 psi, 70^oF

Simplify Formula
Assuming T1 = T2 = room temperature

The formula is reduced to:
formula

Looking back at the formula note:

- to convert psi to absolute pressure, add 14.7
- to convert ^oC to K add 273.15
- to convert ^oF to Rankine add 460

Note that this formula is only valid if there are no obstructions between the gas source and the flowmeter (valves etc.) In the cases of the 7MC system, the formula can be used to calculate the flows through the primary and helium flowmeters only.