4-4
Brooks
®
Model SLA5840
Section 4 Maintenance
& Troubleshooting
Installation and Operation Manual
X-PR-SLA5840-Series-RevB-RT-eng
Part Number: 541B201AAG
October, 2017
4-3 Gas Conversion Factors (SLA5840 Only)
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 flowing
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
÷
0.945 = 21.16 slpm Nitrogen
It is generally accepted that the mass flow rate derived from this equation
is only accurate to ±5%. The sensor conversion factors given in Table 4-1
are calculated based on a gas temperature of 80° C and a pressure of one
atmosphere. The specific heat of most gases is not strongly pressure,
and/or temperature dependent. However, gas conditions that vary widely
from these reference conditions may cause an additional error due to the
change in specific heat caused by pressure and/or temperature.
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
+
+
Actual Gas Flow Rate = Output Reading x
Sensor Factor of the New Gas
Sensor Factor of the Calibration Gas
Mixture Factor
=
= 0.945
100
20
80
1.386
0.876
+
