What is Turndown Ratio?

Why rangeability matters in selecting a flow meter.

What is Turndown Ratio?

Turndown ratio is often also referred to as the rangeability of an instrument. 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 range to the highest range. When flow is not held to a consistent value in a system and is prone to notable variations, it is important to consider the total range of potential flow within your application and ensure the meter can handle the full range of flow from the lowest flow to the peak flow values. If your process produces a pretty consistent flow stream value, then rangeability is not a huge consideration in choosing a flow meter.

An Example of How the Ratio is Derived:

Let's say you have a flow meter that can detect a minimum rate of flow of 1 GPM and measure a maximum rate of flow of 10 GPM. That would be a turndown ratio, or a rangeability of 10:1. Let's say the minimum flow was 2 GPM and the maximum rate was 20 GPM. This also delivers a turndown ratio of 10:1. If the minimum range was 2 GPM and the maximum rate was 10 GPM, that is a turndown ratio of 5:1.

Some General Turndown Ratios:

• Positive Displacement - 10:1 to 100:1
• Ultrasonic - 250:1
• Variable Area - 10:1
• Turbine - 10:1 to 20:1
• Orifice Plate - 5:1 to 10:1