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3098 Technical Manual Introduction
1-5
1.3 Definition of Terms
1.3.1 Specific Gravity (G)
This is the ratio of the molecular weight of a gas (or gas mixture) to that of the molecular weight of dry air; the molecular
weight of dry air is normally assumed to be 28.96469 (see Table 1).
i.e
G
=
M
M
G
A
where
M
G
= Molecular weight of gas (or gas mixture)
and
M
A
= Molecular weight of dry air
1.3.2 Standard (Base or Normal) Density (
ρ
s)
This is the absolute density of a gas at STANDARD (BASE or NORMAL) conditions of temperature and pressure and is
commonly used for STANDARD VOLUME flow determination from MASS flow measurement.
i.e
ρ
s
PM
ZRT
=
where P = Absolute pressure (bars)
T = Absolute temperature (degrees Kelvin)
M = Molecular weight
Z = Supercompressibility factor
R = Gas constant (taken as 0.0831434)
1.3.3 Relative Density (
ρ
r)
This is the ratio of the weight of a volume of gas (or gas mixture) to the weight of an equal volume of dry air (see Table 1),
where the weights of both gas (or gas mixture) and air are taken under identical conditions of temperature and pressure.
NOTE: Except for the effects of Boyle’s Law deviation upon both the gas (or gas mixture) and the air, G and
ρ
r are
synonymous.
i.e
G
=
M
M
G
A
=
ρ
ρ
GG
AA
Z
Z
=
ρ
r
G
A
Z
Z
where
ρ
G
= Density of the gas or gas mixture
ρ
A
= Density of air
Z
G
= Supercompressibility factor of the gas or gas mixture
Z
A
= Supercompressibility factor of air