Brooks Instrument 5851i - Manual (Page 40)

4-12
Installation and Operation Manual
X-TMF-5851i-MFC-eng
Part Number: 541B109AAG
September, 2009
Brooks ® Model 5851 i
Section 4 Maintenance
& Troubleshooting
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 0.778 = 58.35 sccm
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:
Nitrogen equivalent flow = 20/.901 = 22.20 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 temperature and/or pressure.
4-6 Use of Orifice Sizing Nomograph
The Orifice Sizing Nomograph, Figure 4-5, is used to calculate the control
valve's orifice size when changing any or all of the following factors from
the original factory calibration:
gas
operating pressure (inlet and outlet)
flow range
The flow controller's orifice is factory-sized to a preselected gas, operating
pressure and flow range. Note that the orifice is marked with its size in
thousandths of an inch. When changing the aforementioned factors,
calculate the new orifice size by following the procedure and example
outlined in the following paragraphs.
Example: Determine the orifice size for the following conditions:
Gas: Hydrogen