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Appendix A Series 820 Instruction Manual
A-4 IM-82-C
For our purposes, we want the ratio of the flow rate, Q
1
, for an ac-
tual gas to the flow rate of a reference gas, Q
2
, which will produce
the same output voltage in a particular mass flow meter or controller.
We get this by combining equations (1) and (5):
Q
1
/Q
2
= K
1
/K
2
= (N
1
/ r
1
C
p1
)/(N
2
/r
2
C
P2
) (6)
Please note that the constant b cancels out. Equation (6) is the funda-
mental relationship used in the accompanying tables. For convenience,
the tables give ÒrelativeÓ K-factors, which are the ratios K
1
/K
2
, instead
of the K-factors themselves. In the tables, the relative K-factor is
K
actual
/KN
2
where the reference gas is the commonly used gas, nitrogen
(N
2
). The remaining columns give Cp and r, enabling you to calculate
K
1
/K
2
directly using Equation (6). In some instances, K
1
/K
2
from the
tables may be different from that which you calculate directly. The value
from the tables is preferred because in many cases it was obtained by
experiment. Sierra calibrates every transducer with primary standards
using the actual gas or a molecular equivalent reference gas. The cali-
bration certificate accompanying the transducer cites the reference gas
used.
Example 1:
A transducer is calibrated for nitrogen (N
2
), and the flow rate is
1000 sccm for a 5.000 VDC output signal. The flow rate for carbon
dioxide at a 5.000 VDC output is:
Q
CO2
/Q
N2
= K
CO2
/K
N2
, or
Q
CO2
= (0.74/1.000)1000 = 740 sccm
Example 2:
A transducer is calibrated for hydrogen (H
2
), and the flow rate is
100 sccm for a 5.000 VDC output signal. The flow rate for nitrous
oxide (N
2
O) is found as follows:
Q
N2O
/Q
H2
= K
N2O
/K
H2
, or
Q
N2O
= (0.71/1.01) 100 = 70.3 sccm
Note that the K-factors relative to nitrogen must be used in each case.
Example 3:
We want a transducer to be calibrated for use with dichlorosilane
(SiH
2
Cl
2
) at a 100 sccm full scale flow. We wish to use the pre-
ferred reference gas Freon-14 (CF
4
). What flow of CF
4
must we
generate to do the calibration?
Q
SiH2CL2
/Q
CF4
= K
SiH2CL2
/K
CF4
100/Q
CF4
= 0.869
Q
CF4
= 100/0.869 = 115 sccm