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4 Functional principle – Ultrasonic-flow measurement
4.3 Diagnostic function Speed of Sound
The USM-GT-400 can calculate the SoS in 3 different ways.
40
4.3.1
Standard method of SoS calculation
The first calculation is realized with help of the transit time t
trasonic pulses (see above) along the measuring path with the length L . It is
straight forward to result in the speed of sound SoS or c
Calculation of SoS
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This first option is pretty fast and is almost permanently "online" available.
4.3.2
SoS calculation via gas components
The second version of SoS calculation uses pressure, temperature and composi-
tion of the gas to determine the SoS according to the specifications of the AGA 10
standard (AGA Report No. 10, Speed of Sound in Natural Gas and Other Related
Hydrocarbon Gases; January 2003; AGA - American Gas Association). The calcu-
lation is based on statistical considerations of thermodynamics; since it is very
complex, it will not be presented here. Knowing the gas composition precisely val-
ues such as density, sound velocity and other gas properties can be calculated
with very high accuracy.
Depending on the type of gas analyzer it may take 5-10 minutes to determine the
volume fractions of the individual gas components accurately. Accordingly, the
precise allocation of flow to the gas composition can be done in this time frame
only.
Diagnostic function SoS
The USM-GT-400 determines with highest accuracy the flow rate of the gas flow-
ing through it. For the payoff the gas quality, respectively the calorific value of the
gas resulting from the gas composition is of course of big interest, too. The USM-
GT-400 allows a second billing of the volume flow rate with the "right" gas compo-
sition, ie the "right" calorific value.
This temporal resolution can be achieved receiving permanently the gas composi-
tion data from a gas analyzer. A comparison of the two differently calculated SoS's
in the USM-GT-400 allows the immediate detection of any deviation; in particular,
another gas composition results in a different SoS. A confirmation of another gas
composition then provides the next comparison with the data of the gas analysis
instrument.
Manual USM GT400 · EN10 · May 2021
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