Flow meter accuracy and turndown calculator
An accuracy quoted as a percentage of full scale becomes a large percentage of reading at low flow. That is what turndown really measures.
m3/h
m3/h
%
%
m3/h
Full-scale spec as a share of reading
3.3333%
- Absolute error, full-scale spec
- 0.500 00 m3/h
- Absolute error, reading spec
- 0.030 00 m3/h
- Turndown ratio
- 20.00
- Flow as a share of full scale
- 15.00 %
- Full-scale error at minimum flow
- 10.000 %
- Daily volume uncertainty
- 12.000
- Which specification is tighter here
- 2 The percentage-of-reading specification
Show the working
- Full scale flow = 100
- Actual flow = 15
- Accuracy as a share of full scale = 0.5
- Or as a share of reading = 0.2
- Minimum measurable flow = 5
- Absolute error, full-scale spec = 100 * 0.5/100 = 0.5
- Full-scale spec as a share of reading = 15 > 0 ? 0.5/15*100 : 0 = 3.333 33
- Absolute error, reading spec = 15 * 0.2/100 = 0.03
- Turndown ratio = 5 > 0 ? 100/5 : 0 = 20
- Flow as a share of full scale = 100 > 0 ? 15/100*100 : 0 = 15
- Which specification is tighter here = 0.5 <= 0.03 ? 1 : 2 = 2
- Full-scale error at minimum flow = 5 > 0 ? 0.5/5*100 : 0 = 10
- Daily volume uncertainty = max(0.5, 0.03) * 24 = 12
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- full-scale error is constant in absolute terms and grows as a share of reading as flow falls
- turndown = maximum flow / minimum measurable flow
A meter with 0.5% of full scale accuracy is 5% wrong at 10% of range. Always ask which basis a specification uses.
Coriolis and magnetic meters usually specify on reading; orifice plates on full scale.