What is Flow Meter Turndown Ratio?

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.

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 is chosen can handle the full range of flow from the minimum to the maximum operating points. If the operating flow rate will vary significantly or if the process is prone to notable fluctuations, selecting a flow meter with an adequate turndown ratio becomes crucial. If the process experiences a consistent flow rate, 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 that 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. In another example, 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 ratio 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.

• Generally, 20:1 to 250:1

• Generally, 10:1 to 30:1 (higher turn down is possible with advanced models, under specific conditions, or with a loss of accuracy)

• Generally, 5:1 to 10:1

• Generally, 15:1 to 100:1, the KOBOLD DUK Inline Ultrasonic Flow Meter offers 250:1 standard

• Generally, 10:1 to 30:1

• Generally, 10:1 to 50:1 (up to 100:1 may be achievable under specific conditions)

• Generally, 10:1

• Generally, 10:1 to 40:1