Flowmeters  Turndown Ratios
Turndown ratio (Rangeability) can be used to compare flow measurement devices like orifices, venturi meters etc.
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 = q_{max} / q_{min} (1)
where
TR = Turndown Ratio
q_{max} = maximum flow
q_{min} = 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
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.
Related Topics

Flow Measurement
Flow metering principles  Orifice, Venturi, Flow Nozzles, Pitot Tubes, Target, Variable Area, Positive Displacement, Turbine, Vortex, Electromagnetic, Ultrasonic Doppler, Ultrasonic Timeoftravel, Mass Coriolis, Mass Thermal, Weir Vnotch, Flume Parshall and Sluice Gate flow meters and more. 
Process Control
Instrumentation and process control systems  design and documentation.
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