Sharp JX-9400 Informations techniques Page 154

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with
MM ¼
P
i
M
i
n
i
P
i
n
i
ð7:3Þ
being the average molar mass of the mixture. For dry air,
MM ¼ 28:96 g/mole.
The relative change in density is then:
¼
MM
MM
þ
p
p
T
T
ð7:4Þ
As long as the tracer gas is present only in trace concentration, the temperature
has the largest effect on the density.
Conversion formulae for concentration
The con version bet ween units of tracer (or contaminant) concentration
requires the knowledge of the densities of tracer (or contaminant) and air, or
of their molecular masses. Therefore, formulae are given below instead of
tables. Concentration of a gas in a mixture can be expressed in several ways:
The mass concentration, C
m
, is the ratio of the mass of the considered gas, x, and
the mass of the mixture:
C
m
¼
m
x
P
i
m
i
ð7:5Þ
The molar concentration, C
M
, is the ratio of the number of molecules, or of
moles, of the considered gas to the total number of molecules, or moles, in
the mixture:
C
M
¼
n
x
P
i
n
i
ð7:6Þ
The volume concentration, C
V
, is the ratio of the volume of the considered gas at
the considered temperature and pressure and the volume of the mixture at the
same temperature and pressure. It is also the ratio of the partial pressure of the
considered gas and the total pressure:
C
V
¼
V
x
V
¼
p
x
p
ð7:7Þ
Table 7.1 Examples of coherent units
Airflow rate Injection rate Concentration
kg/s kg/s Mass concentration
m
3
/h m
3
/h Volume concentration
m
3
/h cm
3
/h ppm
Common Methods and Techniques 133
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