Brooks Instrument 5851EM - Manual (Page 44)

4-6
Brooks ® Models 5965, 5851EM
Section 4 Maintenance
& Troubleshooting Installation and Operation Manual
X-TMF-5965-5851EM-MFC-eng
Part Number: 541B122AAG
September, 2009
4-3 Gas Conversion Factors
If a mass flow controller is operated on a gas other than the gas it was
calibrated with, a scale shift will occur in the relation between the output
signal and the mass flow rate. This is due to the difference in heat
capacities between the two gases. This scale shift can be approximated by
using the ratio of the molar specific heat of the two gases or by sensor
conversion factor. A list of sensor conversion factors is given in Table 4-5.
To change to a new gas, multiply the output reading by the ratio of the gas
factor for the desired gas by the gas factor for the calibration gas used.
Example:
The controller is calibrated for Nitrogen.
The desired gas is Carbon Dioxide
The output reading is 75 sccm when Carbon Dioxide is flowing
Then 75 x 0.773 = 57.98 sccm
In order to calculate the conversion factor for a gas mixture, the following
formula should be used:
Where,
P
1
= percentage (%) of gas 1 (by volume)
P
2
= percentage (%) of gas 2 (by volume)
P
n
= percentage (%) of gas n (by volume)