Brooks Instrument 5800S Series - Manual (Page 28)

Installation and Operation Manual
X-TMF-5800S-MFC-eng
PN 541-C-051-AAG
November, 2008
Models 5800-S
28
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 flow 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 specific 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 flow of carbon dioxide. When the output reading is 750 mln/min,
the flow rate for carbon dioxide is 750 x 0.740 = 555 mln/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 flow 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
N2 equivalent flow = 20/0.946 = 21.14 ln/min .
NOTE:
It is generally accepted that the mass flow 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 flow rate for gases other than the
pre-selected gas. The flow meter is factory set to a pre-selected gas, pressure and flow range.
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