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How It Works
An acoustic flow meter sends ultrasonic pulses both upstream and downstream through a fluid. The downstream pulse arrives faster because it travels with the flow, while the upstream pulse is slowed. The transit-time difference between the two signals reveals the fluid's average axial velocity.
The angle θ between the acoustic path and the pipe axis, and the path length L between transducers, are fixed by the installation geometry. This calculator rearranges the master equation to solve for any of the five variables.
Example Problem
A 12-inch pipe has transducers mounted with a path length of 0.5 m at an angle of 0.7854 rad (45°). The downstream travel time is 0.00032 s and the upstream travel time is 0.00034 s. What is the axial velocity?
- cos(0.7854) ≈ 0.7071
- V = 0.5 / (2 × 0.7071) × (1/0.00032 − 1/0.00034)
- V ≈ 0.3536 × (3125 − 2941.2) ≈ 64.97 m/s
Frequently Asked Questions
How accurate are acoustic flow meters?
Modern transit-time meters typically achieve ±0.5–1% accuracy in clean liquids. Accuracy drops in fluids with high particle content or gas bubbles, which scatter the signal.
What angle should the transducers be mounted at?
Most installations use 45° (0.7854 rad) because it balances sensitivity to flow speed with signal strength. Angles between 30° and 60° are common depending on pipe size and fluid.
Can acoustic flow meters measure gas flow?
Yes, but gases attenuate ultrasonic signals more than liquids, so the path length must be shorter and the transducer power higher. Natural gas pipelines routinely use multipath ultrasonic meters.
Related Calculators
- Pipe Hydrostatic Pressure Calculator — external water pressure on buried pipes.
- Plastic Pipe Design Calculator — pressure class and dimension ratios for plastic pipes.
- Pipe Flow Calculator — compute Reynolds number and flow rate for circular pipes.
- Doppler Effect Calculator — calculate frequency shifts caused by relative motion between source and receiver.
- Speed Converter — convert between m/s, ft/s, mph, and other velocity units.
U.S. Department of the Interior Bureau of Reclamation. Water Resources Research Laboratory: Water Measurement Manual. Washington DC, 2001.