Transit time flowmeters operate on the principle that a sonic pulse is sent from a transmitting transducer, which propagates through the flowing medium to a receiving transducer. Depending on the velocity of the flow, the difference between the times of arrival of a pulse propagated against the flow (upstream direction) and one propagated with the flow (downstream) is used to calculate the velocity of the flow. There are two types of Transit Time meters: Time Domain Transit Time Meter and Frequency Domain Transit Time Meter.

A pulse is transmitted in a given direction and the arrival time is recorded on the other end of the path. Then, a pulse is transmitted in the opposite direction and the arrival time is also recorded. The difference between the two time measurements provides information on the motion of the fluid.

Governing equation:

Pipe size and the ability to resolve time differences are the main practical limits for the time transit flowmeter. An advantage is that a determination of velocity can be made every transmit cycle, which allows rapid response in large pipe. Therefore, it is an ideal choice for pipe diameter above 1 meter.

Frequency domain meters use the same sensor as time domain meters. The only difference lies in the processing of the signals. Here, the time information is converted into a frequency rather than reading time directly. As soon as a pulse is received it is re-transmitted immediately to form a pulse frequency (pulse repetition rate) proportional to the transit time of the pulse. Two frequencies are generated from two paths- one upstream and one downstream. The difference in these two frequencies is proportional to the flow velocity.

Governing equation:

The frequency domain meter has an advantage for meters with a diameter of one meter or less.

Applicable fluids and fluid conduit characteristics

• Acoustically transport (sound can pass through)
• Full conduit
• Continuous, non-pulsating flow
• Contain no material to deposit on the conduit wall
• Clean

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