Brooks MFM - Appendix A: Gas Conversion Tables

Installation and Operation Manual
X-TMF-MfS-MFC-eng
PN 541-C-061-AAG
November, 2008
MF Smart Series
36
Appendix A: Gas Conversion Tables
USE OF THE CONVERSION TABLES
If a Mass Flow Meter or Controller is operated on a gas other than the gas it
was calibrated with, a scale shift will occur in the relationship between the output
signal and the mass ow rate as a result of the difference in heat capacities
between the two gases. This scale shift can be calculated by using the ratio of
the molar specic heat of the two gases, or the sensor conversion factor.
The gasconversion table is available at Brooks Instrument under document
no.: J-836-D-508.
USE OF A MFC/M FOR A DIFFERENT GAS THAN IT IS CALIBRATED FOR:
For use with a different gas type, multiply the output reading by the ratio of the
gas factor of the desired gas and the gas factor of the calibrated gas.
Actual gas Output Factor of the new gas
Flow rate = Reading x ____________________
Factor of the calibrated gas
Example:
The controller is calibrated for nitrogen: 1000mln/min full scale.
The desired gas is carbon dioxide. Multiply the output reading by 0.740 to
calculate the ow of carbon dioxide. When the output reading is 750 mln/min,
the ow rate for carbon dioxide is 750 x 0.740 = 555 ml
n
/min.
CALCULATE GAS FACTOR FOR MIX GASSES:
In order to calculate the conversion factor for a gas mixture the following
formula should be used:
Sensor 100 .
conversion = P1 . P2 . Pn .
factor for sensor + sensor + sensor
Mixture conversion conversion conversion
factor 1 factor 2 factor n
Where P1= percentage (%) of gas 1 (by volume)
P2= percentage (%) of gas 2 (by volume)
Pn= percentage (%) of gas n (by volume)
Example:
The desired gas is 20% Helium (He) and 80% Chlorine (Cl2) by volume. The
desired full-scale ow rate of the mixture is 20 ln/min.
Sensor conversion factor for the mixture is:
100 .
Mixture Factor = 20 + 80 = 0.946
1.386 0.876
N2equivalent ow = 20/0.946 = 21.14 ln/min .
NOTE:
It is generally accepted that the mass ow rate derived from this equation is only accurate to ± 5%. The table below lists the conversion factors
for various gases with respect to nitrogen. The conversion table is used to determine the ow rate for gases other than the pre-selected gas.
The ow meter is factory set to a pre-selected gas, pressure and ow range.