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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:
1
d X Cp
where d = gas density (gram/liter)
Cp
= coefficient of specific heat (cal/gram)
K gas =
Note in the above relationship that d and Cp are usually chosen at standard conditions of one atmosphere and 25F 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.
K =
where Qa
Qr
Ka
Kr
=
=
=
=
Qa
Qr
=
Ka
Kr
mass flow rate of an actual gas (sccm)
mass flow rate of a reference gas (sccm)
K factor of an actual gas
K factor of a 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:
QO = Qa = Qr x K = 1000 X 0.9926 = 992.6 sccm
2
where K = relative K factor to reference gas (oxygen to nitrogen)
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