Turndown ratio is often referred to as the rangeability of a flow meter. It is defined as the ratio of the maximum capacity of measurement to the minimum requirement for measurement. It is expressed as a multiplication factor from the lowest end of the meter’s flow range to the highest, where it can remain within the accuracy specifications.
When flow is not held to a consistent rate in a system and is prone to notable fluctuations, it is important to consider the total range of potential flow rates that could occur within the system and ensure that the flow meter that you choose can handle the full range of flow from the lowest value to the highest peak value. If the process experiences a consistent flow rate value, then rangeability is not a huge consideration in choosing the flow meter for that application.
For a very simple example, let’s say you have a flow meter than can detect a minimum flow rate of 1 GPM and measure a maximum flow rate of 10 GPM. Comparing these minimum and maximum values will identify the turndown ratio. In this example, the turndown ratio would be 10:1. Another similar example would be a flow meter that has a minimum flow rate requirement of 2 GPM, and the maximum flow rate is 20 GPM. This also is a turndown ratio of 10:1. If a flow meter had a minimum flow range of 2 GPM and a maximum flow range of 10 GPM, that is a turndown ratio of 5:1.
Different flow meter technology types generally offer a different turndown capability as a technology type. Within that specific technology type, the turndown ratios can vary considerably. Flow meter technology types that employ a mechanical means of measurement will typically have a lower turndown ration than flow meter technologies that do not have moving parts. Below are some generalizations.
Always consult your flow meter’s specification data sheet to verify the actual turndown ratio for the flow meter.
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