Reference Manual
00809-0100-4840, Rev. BA
July 2008
Rosemount 3100 Series
F-16
P024 (Speed of Sound) P024 Description
P024 is for entering the speed of sound of the gas above the liquid surface
(ullage gas) in a closed vessel, at a temperature of 32 °F (0 °C).
The Speed of Sound (D914) is calculated for the ullage gas at the
temperature and humidity level in the tank. The correction for temperature
effects is made using P025, assuming the value for the speed of sound
entered in P024 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 value entered in P024 should be 331.80
(metres/second). This setting is most accurate over the temperature range 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 value should be
entered into P024. Gas mixtures have speed of sound values calculated as
an average according to the proportion of the gases present.
Table F-2 shows Speed of Sound values for ullage gases at 32 °F (0 °C).
Minimum/Maximum value
0.0/999999
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 F-2.
Speed of sound of ullage gases
Ullage Gas Speed of Sound Ullage Gas Speed of Sound
Acetaldehyde 244 Ethylether 206
Ammonia 415 Methane 430
Argon 308 Methanol 335
Benzene 177 Nitrogen 337
Carbon Dioxide 259 Nitric oxide 334
Carbon Tetrachloride 145 Oxygen 332
Cyclohexane 181 Propane 238
Ethane 316 Sulphur hexafluoride 133
Ethylalcohol 258
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