Straight run
Ten diameters upstream, five downstream. More than any other single factor, this is what separates a defensible clamp-on reading from a number you cannot stand behind.
The baseline requirement
Both rental meters specify ten pipe diameters of straight, unobstructed pipe upstream of the transducers and five downstream. On a 6 inch line that is 5 feet before and 2.5 feet after. On a 24 inch line it is 20 feet and 10 feet, which is where site reality starts to bite.
Not all disturbances are equal
Ten diameters is a general figure. Some upstream fittings need considerably more:
- A single elbow — 10 diameters is usually adequate.
- Two elbows in different planes — this generates swirl, which persists far longer than simple profile distortion. Allow 25 to 40 diameters, or accept that the reading is comparative rather than absolute.
- A partially open valve — the worst common case. A throttled valve is a deliberate flow disturbance. Allow 30 diameters or measure upstream of it instead.
- A pump discharge — volute swirl. At least 10 diameters, preferably 20.
- A reducer or expander — relatively benign, 10 diameters is fine.
- A tee with flow joining — 15 to 20 diameters.
What insufficient straight run actually does
It distorts the velocity profile across the pipe. An ultrasonic meter measures velocity along a chord and applies a profile correction factor to convert that to mean velocity. If the real profile is not the developed profile the correction assumes, the conversion is wrong. The error is typically a few percent but can reach ten or more downstream of a throttled valve.
Two things follow. First, the error is systematic, not random — the reading will be consistently high or low, and repeating it will not reveal the problem. Second, repeatability stays good even when accuracy is poor, which is why bad installations feel trustworthy.
Working with what the site gives you
- Measure upstream of the disturbance, not downstream. Ten diameters of approach pipe is easier to find than thirty diameters of recovery.
- Use Z mode on large pipe. A single diagonal path across the full bore averages more of the profile than a reflex path.
- Rotate the sensor position around the pipe. On a horizontal line downstream of an elbow, mounting at the 3 and 9 o'clock positions samples across the distorted profile rather than along it.
- Note the limitation in the report. A flagged reading with a stated caveat is professional. The same reading presented as certified is not.