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Flow Measurement · Glossary

What is Turndown Ratio?

ratio, for example 10:1Flow Measurement

Turndown ratio is the maximum flow divided by the minimum flow a meter can measure within its stated accuracy.

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Turndown Ratio Explained

Turndown, also called rangeability, shows how wide a flow range one meter can cover. A meter with 10:1 turndown and 100 m³/h maximum measures accurately down to 10 m³/h.

DP meters have limited turndown because DP falls with the square of flow. Magnetic, coriolis and ultrasonic meters usually offer much wider turndown.

Turndown Ratio Formula

Turndown = Qmax ÷ Qmin
Qmaxmaximum measurable flow
Qminminimum flow within the accuracy statement
Example: 100 m³/h maximum and 10 m³/h minimum give 10:1.

Turndown Ratio: Typical Values

Orifice with one DP transmitterabout 3:1 to 5:1
Vortexabout 10:1 to 20:1 for liquids
Magnetic and coriolisoften 100:1 or more
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Calculators for Turndown Ratio

CalculatorTurndown Ratio CalculatorFree online calculator with formula and worked example.CalculatorFlow Meter Accuracy CalculatorFree online calculator with formula and worked example.

Frequently Asked Questions

What is Turndown Ratio?

Turndown ratio is the maximum flow divided by the minimum flow a meter can measure within its stated accuracy.

How can DP meter turndown be increased?

With stacked DP transmitters on different ranges, or a smart transmitter with good low range accuracy.

Is turndown the same as accuracy?

No. Turndown says over what range the stated accuracy applies; always check whether accuracy is % of rate or % of full scale.

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All flow measurement terms · Full glossary A to Z

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