Flow-Meter Turndown Ratio
Turndown (rangeability) from max/min accurate flow, plus a quick accuracy-band check at the low end.
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The engineering
Turndown ratio — also called rangeability — is how wide a flow band a meter can measure while still holding its rated accuracy. A 10:1 turndown on a 100 gpm meter means it stays accurate down to 10 gpm; below that the reading drifts out of spec even though the meter still shows a number.
The trap is that Q_min must be the flow where accuracy is still met, not the flow where the meter twitches. A vendor quoting 'reading down to 2% of scale' is not the same as 'accurate to 2% of scale.' For percent-of-reading meters (Coriolis, magnetic) the absolute error band shrinks with flow, giving huge turndown; for percent-of-full-scale devices (orifice plate, ~3.5:1) the fixed band swamps the signal at low flow. Size the meter so your normal operating point sits in the upper half of its range.
Where this math comes from
Turndown started as a burner term — how far down you could throttle a furnace before the flame went unstable — and instrument engineers borrowed the word for flow. Early orifice-plate installations of the 1920s–30s were notoriously poor at it: because differential pressure goes as the square of flow, dropping to a quarter of full flow cut the DP signal to a sixteenth, and 3:1 or 4:1 was the practical ceiling.
The Instrument Society of America codified how to state and compare rangeability so vendors couldn't quote it against different accuracy baselines, work that lives in the ISA recommended-practice series (RP31.1 on turbine flowmeters). The arrival of magnetic and Coriolis meters in the later 20th century blew the numbers open — percent-of-reading accuracy turned 3:1 into 100:1 or better.
- 1911Thomas WeymouthStandardizes orifice-plate flow measurement, whose square-law DP inherently limits turndown.
- 1945Instrument Society of AmericaFounded; begins standardizing how instrument performance including rangeability is specified.
- 1977Instrument Society of AmericaRP31.1 recommended practice for turbine flowmeter specification and turndown reporting.
- 1977Micro MotionCommercializes the Coriolis mass flowmeter, pushing percent-of-reading turndown past 100:1.
See the full timeline of the math behind every calculator →
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