Choosing an ultrasonic flow meter
Including, honestly, the cases where you should not use one at all.
Transit-time versus Doppler
These measure fundamentally different things and are not interchangeable.
- Transit-time sends a pulse diagonally with the flow and another against it, and derives velocity from the time difference. It needs a clear acoustic path, so the liquid must be clean. Both of our rental meters are transit-time.
- Doppler bounces a signal off particles or bubbles in the stream and reads the frequency shift. It requires suspended solids or entrained gas to work at all — on clean water it has nothing to reflect from. It is also less accurate, typically 2 to 5%, because it measures the velocity of whatever it happened to reflect off rather than the bulk flow.
The practical rule: clean liquid needs transit-time, dirty liquid needs Doppler. We rent transit-time only, so slurry, sludge and sewage are outside what we can supply.
Clamp-on versus inline
| Clamp-on ultrasonic | Inline meter | |
|---|---|---|
| Installation | Strap on, minutes | Cut pipe, shutdown |
| Typical accuracy | 1–2% in the field | 0.2–0.5% |
| Dirty fluids | No (transit-time) | Yes, mag meters |
| Pressure drop | None | Small to moderate |
| Moving parts in flow | None | Depends on type |
| Cost to deploy | Low | High |
| Suits temporary work | Yes | No |
Clamp-on wins decisively on installation cost and on any application where cutting the pipe is impractical. Inline wins on absolute accuracy, on dirty fluids, and on permanent installations where the one-time installation cost amortises.
Where clamp-on is the wrong answer
- Custody transfer and legal metrology. Clamp-on measurement is not certifiable for billing where regulation requires a verified meter.
- Partially full pipes. The pipe must run full. Gravity sewers and open channels need an area-velocity meter.
- Very low flow in an oversized pipe. Below roughly 0.1 m/s the reading degrades regardless of instrument quality.
- Heavily scaled or tuberculated pipe. The meter cannot see the effective bore and will compute flow from the diameter you entered.
- Lined pipe where nobody knows the lining. Cement lining thickness has to be entered. If the records are gone, you are guessing.
- Above 80 °C, or in a classified area. Outside our rental fleet entirely.
Rent or buy
Rent if you need the measurement a handful of times a year, if the job is a one-off survey, or if you are evaluating whether to buy. Buy if it will be in the van every week — the crossover is roughly six to eight rental weeks a year. Rental payments can be credited against a purchase.
Not sure a clamp-on will work on your line?
Describe the pipe and the fluid. If it is outside what these meters can do, we will say so.