62
Reference Manual
00809-0200-4840, Rev BA
December 2014
Section 4: Configuration
4.3.23
Speed of sound (P024)
This is for entering the speed of sound of the gas above the liquid surface (ullage gas) in a closed
tank at 32
F (0
shows Speed of Sound values for ullage gases at 32
F (0
C).
A new speed of sound is then calculated for the ullage gas at the temperature and humidity level
in the tank (see calculated
Speed of Sound
). The correction for temperature effects
is made using the parameter
Temperature
(
) and assumes the entered Speed of Sound
(P024) value is valid at 32
F (0
C).
For systems monitoring aqueous liquids with air (or nitrogen) as the primary gas in the ullage
space, the entered Speed of Sound (P024) value should be 1088.6 ft/s (331.80 m/s). This is the
most accurate setting for temperatures in the range of 32 to 104
F (0 to 40
C). It takes into
account the typical variations in humidity and assumes the temperature compensation is based
on the temperature sensor measurement.
If the tank vapor space is filled with a different gas, a revised Speed of Sound (P024) value should
be entered. Gas mixtures have speed of sound values calculated as an average according to the
proportion of the gases present.
Note
The speed of sound is inversely proportional to the molecular weight of the gas
molecules. Certain triatomic gases attenuate ultrasound significantly, because of
molecular resonance. Examples are carbon dioxide, oxides of nitrogen and sulphur and
chlorine. Ultrasonics should not be used where carbon dioxide or chlorine is present in
the ullage space and only after careful consideration in the case of oxides of nitrogen
and sulphur. Operational performance of ultrasonic pulse echo systems is significantly
reduced when such gases exceed small percentages in the Ullage space (e.g. 5%).
Table 4-2. Speed of sound of ullage gases
Fast Keys
2, 2, 5, 4
Ullage Gas
Speed of Sound
ft/s (m/s)
Ullage Gas
Speed of Sound
ft/s (m/s)
Acetaldehyde
800.53 (244)
Ethylether
675.85 (206)
Ammonia
1361.55 (415)
Methane
1410.76 (430)
Argon
1010.50 (308)
Methanol
1099.08 (335)
Benzene
580.71 (177)
Nitrogen
1105.64 (337)
Carbon Dioxide
849.74 (259)
Nitric oxide
1095.80 (334)
Carbon Tetrachloride
475.72 (145)
Oxygen
1089.24 (332)
Cyclohexane
593.83 (181)
Propane
780.84 (238)
Ethane
1036.35 (316)
Sulphur hexafluoride
436.35 (133)
Ethylalcohol
846.46 (258)
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