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Flowmeters and Turndown Ratio - Rangeability

Turndown ratio for flow measurement devices like orifices, venturi meters etc.

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Turndown ratio - rangeability - is often used to compare the span - the range - of flow measurement devices.

Turndown Ratio - Rangeability

Turndown ratio can be expressed as:

TR = qmax / qmin                                  (1)

where

TR = Turndown Ratio

qmax = maximum flow

qmin = minimum flow

Maximum and minimum flow is stated within a specified accuracy and repeatability for the device.

Example - Turndown Ratio for an Orifice Meter

The turndown ratio - TR - for an orifice meter with maximum flow of 12 kg/s and a minimum flow of 3 kg/s can be calculated as:

TR = (12 kg/s) / (3 kg/s)

    = 4

    - normally expressed as turndown ratio of 4:1

This is a typical turndown ratio for a orifice plate. In general a orifice plates has turndown ratio between 3:1 and 5:1.

Turndown Ratio and Measured Signal

Flowmeter - turn down ratio and measured signal span

In a flow meter based on the orifice or venturi principle the differential pressure upstream and downstream of an obstruction in the flow is used to indicate the flow. According the Bernoulli Equation the differential pressure increases with the square of flow velocity. A large turndown ratio will cramp the measurement signal at low flow rate.

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