Aalborg DFC Digital - Manual (Page 23)

20
Q
O
= Q
a
= Q
r
x K = 1000 X 0.9926 = 992.6 sccm
where K = relative K factor to reference gas (oxygen to nitrogen)
1
d X C
p
where d = gas density (gram/liter)
C
p
= coefficient of specific heat (cal/gram)
Q
a
K
a
Q
r
K
r
where Q
a
= mass flow rate of an actual gas (sccm)
Q
r
= mass flow rate of a reference gas (sccm)
K
a
= K factor of an actual gas
K
r
= K factor of a reference gas
=
9. CALIBRATION CONVERSIONS FROM REFERENCE
GASES
The calibration conversion incorporates the K factor. The K factor is derived from
gas density and coefficient of specific heat. For diatomic gases:
= K
= K
gas
Note in the above relationship that d and C p are usually chosen at standard con-
ditions of one atmosphere and 25
F
C.
If the flow range of a Mass Flow Controller or Controller remains unchanged, a
relative K factor is used to relate the calibration of the actual gas to the reference
gas.
For example, if we want to know the flow rate of oxygen and wish to calibrate
with nitrogen at 1000 SCCM, the flow rate of oxygen is:
2