What is turndown, and what can you expect from different types of flowmeters?
The turndown (also known as rangeability) of an instrument is the ratio between the maximum and minimum flow that the instrument is capable of measuring. If your process has a maximum flow of 10m3/h and a minimum flow of 1 m3/h, the required flow meter must have a turndown of 10:1.
The outer limits of this range are stated within a specified accuracy for a flowmeter device. For instance:
“This instrument has a turndown of 10:1 with a measurement accuracy of +/- 1% or better.”
Most flow measurement technologies cannot achieve respectable measurement linearity from the maximum rated flow all the way to zero flow. And a lot of flowmeter manufacturers do not communicate at which accuracy the given turndown is specified. So in all cases its best to have an accuracy curve over the total range of the flowmeter to match the accuracy with your needs.
[illustratie accuracy curve. Bijvoorbeeld van Vögtlin.]
To get an idea of the typical turndown for a flowmeter principle we give an overview below. Within a flowmeter principle you can find flowmeters with very different turndown. It varies heavily depending on brand, type, materials and the like. That’s why we added the min and max column. These versions are usually very exotic and costly.
Min turndown Max turndown
Thermal mass 50:1 100:1
Positive Displacement 10:1 80:1
Turbine 10:1
Orifice plate 3:1
Ultrasonic 50:1
Coriolis 100:1
Vortex 10:1
“Typical turndown” in this table should be read as “most models in the market of this flowmeter type have a turndown of…”.
“Minimal turndown” in this table should be read as “most models in the market of this flowmeter type have a better turndown than…”.
“Maximal turndown” in this table should be read as “in exceptional cases the market offers a flowmeter with this turndown…”.
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.
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