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The method used is the same that for the liquids namely the measurement of the transit time
difference.
PARTICULARITIES OF GAS MEASUREMENT
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1) Probes : it is necessary for the measurement that the ultrasounds of the emitting probe reach the receiving probe with a sufficient energy. So it would be proper to select the probes which permit the best adaptation of acoustic impedance to the gas to measure, the acoustic impedance of an material is in this case the product of the sound velocity of propagation by the density of the material. Then, the probes are usually equiped with a front side adapted in λ/4 ( λ : wavelengh to the probe frequency). As they are in direct contact with the gas, they must be thought out to resist to a possible chemical aggression and to the pressure. Adapted supports permit, even under high pressure, their installation or their withdrawal under pressure.
2) Acoustic transmission : the quantity of emitted energy in the gas increases with the acoustic impedance of the latter. The emitted energy is all the more great because the pressure goes up. As an example, the acoustic transmission will be 40 times better in 60 bars natural gas than in the air under atmospheric pressure.
This is the explanation for the dimensions restrictions of pipes under low-pressure. Moreover the attenuation proper to the gas rapidly decreases with the frequency used.
3) Beam deviation : When , in the liquids the flow speed rarely exceeds 5m/s with a speed of sound 1480m/s, we frequently find in gas flow speed in the region of 20m/s for speed of sound of 400m/s. The acoustic beam is subjected in these conditions to a deviation which leads a waste of energy on the receiving probe. In order to overcome that phenomenon, the probes will have a radiation lobe sufficiently wide.
4) Noises of acoustic origins : in the industrial applications, we can find organs of regulation or of reduction in pressure, generators of acoustic noises and of which the frequencies can reach 100Khz. Their interference with emitted signals is prejudicial to the measurement quality . In this case, it is important to send away the measurement point from the disruptive organs.
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The instruments of the Usea range :
• MINISONIC G for the single chord measurements
• UF 322 G for the multichords applications
Can be used on diameters from 300mm max to atmospheric pressure, of 1m for high pressure gas and for speeds velocity under 25m/s.
Some samples
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