Brooks Instrument 5851E - Manual (Page 43)

Brooks ® Model 5851E
4-11
Section 4 Maintenance
Installation and Operation Manual
X-TMF-5851E-MFC-eng
Part Number: 541B104AAG
November, 2008
Note: Due to possible heat capacity and density differences between
the test gas and actual process gas for which the MFC was sized, it
may be necessary to increase the inlet pressure to obtain proper
control at 100% flow.
i. Measure the valve voltage by connecting a voltmeter between Test
Point 3 (TP3) and Test Point 4 (TP4). Refer to Figure 4-3.
Valve Voltage = 1.95 x TP3 + 33.4
j1. If the flow controller output signal is 100% (5.0 V) and the valve
voltage is less than 18 V, the valve adjustment is complete.
j2. If the flow controller output signal is 100% (5.0 V) and the valve
voltage is greater than 18 V, decrease the air gap with a small
0.005 in. air gap spacer. Refer to Figure 4-2. Repeat Steps h and i.
j3. If the flow controller output signal is less than 100% (5.0 V) and the
valve voltage is greater than 18 V, first check Section 4-6 to insure
that the orifice size is correct.
k. Proceed to Section 3-4 and perform calibration procedure, if re-
quired.
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
sensor factor for the desired gas by the gas sensor factor for the
calibration gas used.
Actual Gas Flow Rate = Output Reading x
Sensor Factor of the New Gas
Sensor 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)