Brooks Instrument 5851E - Manual (Page 46)

4-14
Brooks ® Model 5851E
Section 4 Maintenance Installation and Operation Manual
X-TMF-5851E-MFC-eng
Part Number: 541B104AAG
November, 2008
Q
GAS
x
Q
GAS
x
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:
20
1.386
80
0.876
100
Mixture Factor = = 0.945
+
Nitrogen equivalent flow = 20/.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 4-3
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 pressure and/or temperature.
4-6 Orifice Sizing
The Orifice Sizing Nomograph, Table 4-4, 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
Flow Rate: 2,000 sccm
Outlet Pressure: 30 psig
Inlet Pressure:
50 psig
1. Determine Nitrogen equivalent flow rate (refer to Table 4-3).

GAS
Q
NITROGEN
=

NITROGEN
OR
SG
GAS
Q
NITROGEN
= SG
NITROGEN