Having sold gas measurement equipment for decades, I’m often asked, “What is a turndown ratio?" While turndown (turn down) ratio is usually noted in a flow meters specifications, it’s rarely explained in layperson’s language what it actually is.

Turndown ratio is also commonly referred to as rangeability. It indicates the range of flow a meter can accurately measure the natural gas (or whatever is being monitored). In other words, it’s simply the high end of a measurement range compared to the low end, expressed in a ratio and is calculated using a simple formula.

Turndown Ratio = maximum flow / minimum flow

To put this in perspective, if a gas flow is expected to run between 350,000 cubic feet/ day and 3,500,000 cubic feet/day the application would have a turndown ratio of 10:1. Turndown is one of the key parameters used to select the proper flow meter technology for a specific application. Other factors which help select the right flowmeter are: the product being measured, its flow, the price of the equipment, maintenance cost and accuracy.

The chart below includes turndown of various flow technologies.
 

The Sage Metering thermal gas mass meter has a wide turndown of 100:1 (with 1000:1 possible upon request) which is extremely desirable for natural gas applications because of the large swings in demand throughout the course of a day as well as seasonal fluctuations.

What is turndown ratio? One of the first parameters to consider when ascertaining which flow technology to use for your natural gas application.  For more information read "What is a thermal mass flow meter?"

* Please note that these figures are a guideline and referenced on https://controls.engin.umich.edu/wiki/index.php/FlowSensors#Thermal_Flow_meters. They may be different from manufacturer to manufacturer.