danw,
If you were to guess/estimate, what would the difference be? Searching the web I've seen 1% but this is the info I was after from the manufacturer and I can live with a difference.
Bob,
If I knew, I'd tell you. But the magnitude of error in flow measurement is largely undocumented. In 30 years of reading, I've only encountered two cases where the potential error is documented/reported.
1. I found several references that documented that orifice plates installed backwards always read low on the order of 10-35% from actual flow rate.
2. Siemens actually tested one model magmeter flow tube on a flow stand to prove it could qualify for the European standard for potable water distribution with an error less than ±2% from the reference flow rate. The test involved multiple non-allowable piping configurations - a Tee close coupled, two in-line 90° elbows with 2 Tees, Four 90° elbows. Highest error was actually less than 1%.
When I saw the report I was astounded because I had never seen any report of the magnitude of potential error for any magmeter when the minimum upstream/downstream rule was violated. I had no idea if the error was 5%, 10%, or 50%. To think that it was a mere ±2% or thereabouts.
Siemens tested only one flow tube model and would not state that other flow tube models would perform similarly. But given that 95% of all magmeters are pulsed DC excitation and use Faraday's law, I suspect that almost any magmeter would have close to the same performance. By the way, Toshiba announced their "Mount Anywhere" magmeter meaning there is no restriction on upstream/downstream piping (can't have a valve upstream) about the same time as Siemens made their announcement.
Clamp-on
I can tell you that the Siemens clamp-on ultrasonic transit time flow meter (doppler is a joke) has a selectable (blind) correction factor for several piping anomalies that are not considered "fully developed flow" (1 elbow, 2 elbows in-line, 2 elbows out-of-phase, expander, reducer, header inlet upstream; no correction for a valve or intrusions).
The amount of correction is undocumented, one just enables or disables any one of the selections. "Trust me, I'll still love you in the morning."
I tend to go with Tom Jenkins in that 1% is probably too little error from a close upstream pump, but it's my gut feeling, without any documentation. I've had three instances of confirming a magmeter reading within a fraction of a percent with sufficient upstream/downstream with a Siemens clamp-on. But clamp-on ultrasonics recommends 'multi-path', that is, multiple sets of transducers (installed at 90° radially) to correct for flow profile pertubations. Multi-path gets expensive and it wouldn't be an option if there were a better way to do it. There is, sort of, trust the flow profile correction factor if it's a piping anomaly.
But for a pump? I've never mounted one real close to a pump so I just do not know.