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50 Micro Motion 3098 Gas Specific Gravity Meter
Velocity of sound effect
The velocity of sound effect on the sensor is such that:
The velocity of sound in a gas can be determined as follows:
For an ideal gas at 20°C, equation A2 can be simplified to:
Therefore, substituting into equation A1 and simplifying:
It follows that the VOS factor is substantially related to the molecular weight or base density, the main
additional influence being due to unrelated differences in the specific heat ratios. From equation A1,
the VOS factors (V
F
) for the calibration gases and sample gases can be calculated and tabulated, see
the example illustrated in Table A-2.
A1
where ρ = Line density
ρ
i
= Indicated density assuming infinite VOS
K
3
= VOS coefficient, typically 4.41x10
3
τ = Sensor periodic time, typiclly 515 μs
c = Velocity of sound in the gas in meters per second
ρρ
i
1
K
3
cτ()
2
-------------–
⎝⎠
⎜⎟
⎛⎞
=
A2
where γ = Ration of specific heats
P = Line pressure in bars
ρ = Line density
c γ
P
ρ
----
=
A3
where Μ = Molecular weight of the gas
c 1562
γ
M
-----=
A4
where can be referred to as the the
velocity of the sound factor, V
F
.
For
example
ρρ
i
1
M
γ
-----
6.95
4–
×10–
⎝⎠
⎛⎞
=
1
M
γ
-----
6.95
4–
×10–
⎝⎠
⎛⎞
ρρ
i
V
F
=