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Series 830/840/860 Instruction Manual Appendix A
IM-83/84/86-H A-5
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 control-
ler. We get this by combining equations (1) and (5):
Q
1
/Q
2
= K
1
/K
2
= (N
1
/ ρ
1
C
p1
)/(N
2
/ρ
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 conven-
ience, the tables give “relative” 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 ρ, 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 ob-
tained by experiment. Sierra calibrates every transducer with primary
standards using the actual gas or a molecular equivalent reference gas.
The calibration certificate accompanying the transducer cites the refer-
ence 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 car-
bon 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