What is turndown ratio in gas measurement?
Having sold gas flow meters for decades, I’m often asked, “What is turndown ratio?” While the turndown (turn down) ratio is usually in the flow meter’s specifications, it’s rarely explained what it means.
What is the meaning of turndown ratio in flow meters?
Turndown ratio is also commonly referred to as rangeability. It indicates the flow range a meter can accurately measure natural gas (or whatever the gas). In other words, it’s simply the high end of a measurement range compared to the low end, expressed in a ratio and calculated using a simple formula.
Turndown Ratio = maximum flow / minimum flow
To put this in perspective, if 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 critical parameters used to select the proper flow meter technology for a specific application. Other factors that help determine the right flowmeter are the product measured, its flow, the equipment’s price, maintenance cost, and accuracy.
The chart below includes the 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 hugely desirable for natural gas applications because of the large swings in demand throughout a day as well as seasonal fluctuations.
The turn-down ratio is 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 and will vary from manufacturer to manufacturer.