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3098 Technical Manual Principles of Operation
B-1
Appendix B
Principles of Operation
B1.1 Theory - Specific Gravity Measurement
By definition:
Gas Specific Gravity =
Molecular Weight of Gas
Molecular Weight of Standard Air
i.e. G
M
M
G
A
= (1)
where M
A
is taken as 28.96469
Relative Density =
Density of Gas
Density of Air
i.e.
ρ
ρ
ρ
r
=
G
A
(2)
at the same conditions of temperature and pressure.
The relative density is numerically equal to specific gravity when the supercompressibility factors of both the
gas and the standard air at the measurement conditions are taken into consideration. Therefore:-
G
Z
Z
GG
AA
=
ρ
ρ
(3)
Now let the density of the gas sample measured be
ρ
1
, again by definition :-
ρ
1
11
11
=
PM
ZRT
(4)
By comparing the density of the sample gas with the density
ρ
2
of a fixed quantity of a reference gas
contained in a fixed volume,
where
ρ
2
22
22
=
PM
ZRT
(5)
since conditions of constant volume and quantity exist for the reference gas, its density and molecular weight
are constant and from equation 5.