[From：来自网络] [Author：不详] [Date：12-05-28] [Hit：]

In the ultrasonic flow meter, it is necessary to calculate the speed of sound in the working fluid which is usually a gas fluid. The measured speed of sound will be compared with the calculated speed of sound (calculated as per AGA 10) to check whether the ultrasonic flow meter is still in good condition or not.

In practice, the ultrasonic flow meter is measure the fluid flow velocity and speed of sound by knowing the travel time from one transducer to another transducer, the speed of sound that is calculated will be referred as measured speed of sound. This speed of sound will be compared with the calculated speed of sound from knowing gas composition, gas pressure, and gas temperature.

If the measured speed of sound is deviate from the calculated speed of sound exceed the allocated margin then the fluid velocity measurement is in bad conditions. It can be caused by ultrasonic transducer that need to be replaced or any other cause that need to be rectified as soon as. The deviation margin itself is following some specific gas sales agreement or following the AGA 9 deviation margin.

See below illustration to get a better understanding regarding how the ultrasonic flow meter measures the speed of sound and the fluid velocity in the working fluid at the same time.

We will get the fluid velocity by substract the equation (1) with the equation (2) as follows:

We will also get the speed of sound in the fluid by adding the equation (1) with the equation (2) as follows:

Thus as shown in the above equation, the ultrasonic flow meter measure two variable at the same time, the fluid velocity and the speed of sound. Practically it is difficult to compare the ultrasonic output of the fluid velocity with the actual known fluid velocity. But it is practically to compare the ultrasonic flow meter speed of sound output with known speed of sound calculated from known gas composition by using AGA 10 calculation routine. If the comparation of the calculated speed of sound and the ultrasonic output of the speed of sound isn’t deviate too much (still within the margin), then we are confident that our ultrasonic flow meter is working properly.