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)
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)
- 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
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.
Flow metering principles - Orifice, Venturi, Flow Nozzles, Pitot Tubes, Target, Variable Area, Positive Displacement, Turbine, Vortex, Electromagnetic, Ultrasonic Doppler, Ultrasonic Time-of-travel, Mass Coriolis, Mass Thermal, Weir V-notch, Flume Parshall and Sluice Gate flow meters and more.
Instrumentation and process control systems - design and documentation.
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A limited comparison of flowmeter principles - regarding service, rangeability, pressure loss, typical accuracy, upstream pipe diameters, viscosity and relative costs.
An introduction to electromagnetic flowmeters and their principles.
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An introduction to the different types of fluid flowmeters - Orifices, Venturies, Nozzles, Rotameters, Pitot Tubes, Calorimetrics, Turbine, Vortex, Electromagnetic, Doppler, Ultrasonic, Thermal, Coriolis.
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