Brooks Instrument 5850E - Manual (Page 45)

4-13
Section 4 Maintenance &
Troubleshooting
Installation and Operation Manual
X-TMF-5850E-MFC-eng
Part Number: 541B102AAG
September, 2009
Brooks ® Model 5850E
j2. If the flow controller output signal is 100% (5.0V) and the valve
voltage is greater than 11.5 Volts, decrease the air gap with a small
0.005 inch air gap spacer. Refer to Figure 4-2. Repeat Steps h and i.
j3. If the flow controller output signal is less than 100% (5.0V) and the
valve voltage is greater than 11.5V. This condition indicates that the
inlet pressure is too low and or the orifice size is too small. First
check Section 4-6 to insure that the orifice size is correct.
k. Proceed to Section 3 and perform 3-4 Calibration Procedure, if
required.
4-5 Gas Conversion Factors
If a mass flow 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. 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 by the gas factor for the calibration gas.
Actual Gas Flow Rate = Output Reading x
Factor of the New Gas
Factor of the Calibration Gas
Example:
The controller is calibrated for nitrogen.
The desired gas is carbon dioxide
The output reading is 75 sccm when carbon dioxide is flowing
Then 75 x 0.773 = 57.98 sccm
In order to calculate the conversion factor for a gas mixture, the following
formula should be used:
P
1
Sensor
Conversion
Factor
1
P
3
Sensor
Conversion
Factor
3
P
2
Sensor
Conversion
Factor
2
Sensor Conversion Factor
Mixture
=
100
+ +
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: