Brooks Instrument 5851E - Manual (Page 48)

4-16
Brooks ® Model 5851E
Section 4 Maintenance Installation and Operation Manual
X-TMF-5851E-MFC-eng
Part Number: 541B104AAG
November, 2008
Where:
Q
NITROGEN
= Nitrogen equivalent flow rate (sccm)
Q
GAS
= Desired flow rate of the gas (sccm)

NITROGEN
= Density of Nitrogen at 70°F

GAS
= Density of the gas (taken at customer temperature)
SG
GAS
= Specific gravity of the gas (taken at customer
temperature)
Refer to Table 4-3 for densities.
Example:
Q
gas
= 2,000 sccm

gas
= .269

Nitrogen

gas
Q
Nitrogen
=

Nitrogen
= 2,000 x .269
= 538 sccm Nitrogen
In order to calculate the orifice conversion factor when using a gas mixture,
the following formula must be used:
Orifice
Conversion
Factor 1
P
1
Orifice
Conversion =
Factor
Mixture
+ P
2
Orifice
Conversion
Factor 2
100
Orifice
Conversion
Factor n
+ P
n
Where P
1
= percentage by volume of gas 1
P
2
= percentage by volume of gas 2
P
n
= percentage by volume of gas n
Example: Find the Nitrogen equivalent for 20 slpm of a 20% Helium and
80% Chlorine gas mixture.
Orifice
Conversion
= 20(.378) + 80(1.598)
Factor 100
Mixture
Q
Nitrogen
= Q
gas
(orifice conversion factor)
= 20 x 1.439
= 28.78 slpm Nitrogen
2. If inlet and outlet pressures are given in gauge pressure (psig) add 14.7
to convert to absolute pressure (psia).
Q
GAS
x