Gas Conversion Factors - Brooks Instrument Sla5840 Installations- Und Bedienungsanleitung

Section 4 Maintenance
& Troubleshooting
®
Brooks Model SLA5840
4-3 Gas Conversion Factors (SLA5840 Only)
4-4
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.
Actual Gas Flow Rate = Output Reading x
Example:
The RT is calibrated for Nitrogen. (Sensor factor - 1.000)
The desired gas is Carbon Dioxide (CO
The output reading is 75 sccm when Carbon Dioxide is flowing
Then 75 x 0.740 = 55.5 sccm of CO
1.000
In order to calculate the sensor conversion factor for a gas mixture, the
following forumula should be used:
Sensor Conversion Factor
Mixture
Where,
P = percentage (%) of gas 1 (by volume)
1
P = percentage (%) of gas 2 (by volume)
2
P = percentage (%) of gas n (by volume)
3
Example: The desired gas is 20% Helium (He) (Sensor Factor = 1.386)
and 80% Chlorine (Cl ) (Sensor factor = 0.876) by volume. The desired full
2
scale flow rate of the mixture is 20 slpm. Sensor conversion factor for the
mixture is:
20
Mixture Factor =
1.386
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.
Installation and Operation Manual
X-PR-SLA5840-Series-RevB-RT-eng
Part Number: 541B201AAG
Sensor Factor of the New Gas
Sensor Factor of the Calibration Gas
) (Sensor factor = 0.740)
2
2
100
100 100
P
1 +
=
Sensor Sensor
Conversion Conversion
Factor Factor
1
100
80
+
0.876
June, 2016
100
P P
2 + 3
Sensor
Conversion
Factor
2 3
= 0.945