Transit Time Flow Measurement

Using upstream and downstream ultrasonic transit time difference to measure flow velocity.

Transit time ultrasound sensor technology transmits and receives a high frequency sound wave pulse, upstream and downstream of flowing liquid. The time taken for the acoustic pulse to traverse the distance between the transmitter and receiver is recorded. If the acoustic medium is moving away from the receiver, the pulse will take longer to reach it, and vice versa, if it is moving towards the receiver, the pulse will arrive quicker.

An upstream transit time can be measured by placing the pulse transmitter downstream, and the receiver upstream. The receiver can be mounted on the opposite side of the flow, to create a diagonal path through the flowing liquid. Alternatively the receiver can be placed upstream on the same side as the transmitter, and the pulse bounced off a reflector on the opposite side. Ultrasonic sensors can include both an acoustic transmitter and receiver in the one device, therefore it is possible to alternate operation between upstream and downstream transit time modes using the same two devices.

Since there is a direct relationship between the speed of the medium and difference in transit time, the flow velocity of the liquid can be calculated from the transit distance, upstream and upstream transit time measurements and the appropriate mathematical formula.

The performance of a transit time ultrasonic flowmeter is very dependent on the cleanliness and purity of medium being measured. Any particles or gas bubbles in the liquid will interfere with the transmitted acoustic pulse causing it to deflect and attenuate in an unpredictable way. Therefore it is only practical to use this type of ultrasonic flow meter with clean and pure liquids.

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