Brooks Instrument SLA7840 - Manual (Page 29)

4-5
Model SLA7840
Section 4 Maintenance
& Troubleshooting
Installation and Operation Manual
X-PR-SLA7800-RT-eng
Part Number: 541B048AAG
August, 2009
4-3 Gas Conversion Factors
If a RT 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.
Consult factory or nearest Brooks Instrument rep for 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 (CO
2
)
The output reading is 75 sccm when Carbon Dioxide is flowing
Then 75 x 0.740 = 55.5 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)
P
n
= percentage (%) of gas n (by volume)
Example: The desired gas is 20% Helium (He) and 80% Chlorine (Cl) by
volume. The desired full scale flow rate of the mixture is 20 slpm. Sensor
conversion factor for the mixture is:
Since 2 gases are in the mix;
Nitrogen equivalent flow = 20/.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 4-3
are calculated based on a gas temperature of 21°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.