Brooks Instrument 5861E - Manual (Page 27)

4-7
Model 5861E
Section 4 Maintenance Installation and Operation Manual
X-TMF-5861E-MFM-eng
Part Number: 541B107AAG
November, 2008
4-5 Use of the Conversion Tables
If a mass flowmeter is operated on a gas other than the gas it was
calibrated with, a scale shift will occur in the relationship 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 sensor
conversion factor. A list of sensor conversion factors is given in Table 4-3.
To change to a new gas, multiply the output reading by the ratio of the gas
factor for the desired gas to the gas factor for the calibration gas.
Actual gas = Output x factor of the new gas
flow rate reading factor of the calibrated gas
Example:
The flowmeter is calibrated for 100slpm Nitrogen.
The desired gas is Carbon Dioxide.
The flowmeter output reading is 75 % (3.750 volts).
What is the actual carbon dioxide flow?
Actual CO
2
= 75 slpm x .773
flow rate 1
= 58.35 slpm
In order to calculate the conversion factor for a gas mixture, the following
formula should be used:
Where,
P1 = percentage (%) of gas 1 (by volume)
P2 = percentage (%) of gas 2 (by volume)
Pn = 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:
N
2
equivalent flow = 20/.946 = 21.14 slpm