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Series 820 Instruction Manual Appendix A
IM-82-C A-5
Calculating Dual Gas Mixtures
Equation (6) is used for gas mixtures, but we must calculate N/rC
p
for the mixture. The equivalent values of r, C
p
, and N for a dual
gas mixture are given as follows:
The equivalent gas density is:
m
¥
r
= ( / ) + ( / )
m
¥
1T
r
1
m
¥
2
m
¥
T
r
2
Where:
¥
T
m = m
¥
1
m
¥
2
+ = Total mass flow rate (gm/min),
( )
1
= Refers to gas #1, and
( )
2
= Refers to gas #2
The equivalent specific heat is:
C
p
= F
1
C
p1
+ F
2
C
p2
Where:
1
m
¥
r
m
¥
1
r
1
m
¥
2
m
¥
T
r
2
F = ( )/( ) and
T
r
F = ( )/( )
2
The equivalent value of N is:
m
¥
N = ( / ) N + ( / ) Nm
¥
1T1
m
¥
2
m
¥
T2
The equivalency relationships for r, C
p
, and N for mixtures of
more than two gases have a form similar to the dual-gas relation-
ship given above.
IMPORTANT NOTE ABOUT K-FACTORS:
Please note that if you have a transducer calibrated for a gas such as
methane and wish to use the K-factors to measure a gas such as air,
that the inaccuracy of the measurement can range from ±5 to 10%.
The use of K-factors is, at best, only a rough
approximation and
should not be used in applications that require a better than ±5 to
10% accuracy.
It should also be noted that certain gases, in similar Òfamilies,Ó will
work exceptionally well with K-factors; however, those instances
are only true when similar thermal properties of the gas are present.