Brooks Instrument SLA5840 - Manual (Page 26)

4-4
Brooks ® Model SLA5840
Section 4 Maintenance
& Troubleshooting Installation and Operation Manual
X-PR-SLA5800-RT-eng
Part Number: 541B120AAG
August, 2009
4-3 Gas Conversion Factors
If an RT is operated in flow control mode 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 4-1.
To change to a new gas, multiply the output reading by the ratio of the
sensor factor for the desired gas to the sensor factor for the calibration gas
used.
Example:
The RT is calibrated for Nitrogen. (Sensor factor - 1.000)
The desired gas is Carbon Dioxide (CO
2
) (Sensor factor = 0.740)
The output reading is 75 sccm when Carbon Dioxide is flow
Then 75 x 0.740 = 55.5 sccm of CO
2
1.000
In order to calculate the sensor conversion factor for a gas mixture, the
following forumula should be used:
Where,
P
1
= percentage (%) of gas 1 (by volume)
P
2
= percentage (%) of gas 2 (by volume)
P
3
= percentage (%) of gas n (by volume)
Example: The desired gas is 20% Helium (He) (Sensor Factor = 1.386)
and 80% Chlorine (Cl
2
) (Sensor factor = 0.876) by volume. The desired full
scale flow rate of the mixture is 20 slpm. Sensor conversion factor for the
mixture is:
To calculate the sensor Nitrogen equivalent flow of the gas sensor mixture,
use the actual gas flow rate calculation above.
Nitrogen equivalent flow - 20 x 1.000 - 21.16 slpm Nitrogen
Sensor Conversion Factor
Mixture
=
100
100 100 100
P
1
P
2
P
3
Sensor Sensor Sensor
Conversion Conversion Conversion
Factor
1
Factor
2
Factor
3
+ +
Mixture Factor = = 0.945
100
20 80
1.386 0.876
+
Actual Gas Flow Rate = Output Reading x
Sensor Factor of the New Gas
Sensor Factor of the Calibration Gas