35
Q
O2
= 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 Cp are usually chosen at the same con-
ditions (temperature, pressure).
If the flow range of a Mass Flow Controller remains unchanged, a relative K fac-
tor 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:
NOTE: One common reason for proportional solenoid valve to be out of
adjustment: keeping control set point even very small (2% for example)
while disconnecting inlet pressure. In this case the valve becomes
overheated within 15 minutes and mechanical characteristics of the seat
insert and compression spring are compromised. Avoid this mode of
operation in the future.
For best results it is recommended that instruments are returned to the factory for
servicing. See section 1.3 for return procedures.
8.4
Technical Assistance
OMEGA
®
Engineering will provide technical assistance over the phone to quali-
fied repair personnel. Please call our Flow Department at 800-872-9436 Ext.
2298.
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