Delta Class SLA6900 UHP & SLA6800 Series Metal Sealed MFC's/MFM's
Installation and Operation Manual
X-TMF-SLA6900-SLA6800-TMF-eng
Part Number: 541B100AAG
August, 2009
4-6
Section 4 Maintenance
& Troubleshooting
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 4-4. 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 (CO
2
)
The output reading is 75 sccm when Carbon Dioxide is flowing
Then 75 x 0.773 = 57.98 sccm of (CO
2
)
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)
Table 4-3 Orifice Capacities.
Orifice Size (inches)
Minimum Flow Rate (sccm)
0°C
(21.1°C)
0.0013
5.3
(5.7)
0.002
12.5
(13.5)
0.003
39.2
(42.2)
0.004
82.5
(88.9)
0.0055
190
(205)
0.007
374
(403)
0.010
748
(806)
0.014
1364
(1469)
0.020
2673
(2879)
0.032
6490
(6991)
0.048
12980
(13980)
0.062
22000
(2879)
0.078
31900
(34400)
0.093
42500
(45800)
0.120
69300
(74700)
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