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What does turndown ratio mean?

Turndown ratio is also commonly referred to as rangeability. It indicates the range in which a flow meter or controller can accurately measure the fluid. 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

For example, if a given flow meter has a 50:1 turndown ratio the flow meter is capable of accurately measuring down to 1/50th of the maximum flow.  So, suppose a flow meter has a full scale rating of 20 l/min the flow meter will measure down to 0.4 l/min of flow.

Keep in mind that the maximum and minimum flow capability of a meter or controller is likely to be a greater span than the measurable and controllable range.  For example, a mass flow controller with a 50:1 turndown ratio may have the capability of measuring as high as 25 ln/min or as low as 0.16 ln/min but the turndown ratio will govern the actual measurable range.  In this example if the calibrated high flow is 25 ln/min, then the lowest that can be measured is 0.5 ln/min (1/50th of 25).  If the application requires that the calibrated minimum flow is 0.1 ln/min, then the maximum flow that can be measured is 5 ln/min (50 times 0.1).

A mental image of this concept may be to picture a set of 100 stairs (the overall minimum and maximum flow of an MFC), and a length of carpet that will only cover 50 stairs (turndown ratio).  You can cover (measure) any 50 of the 100 stairs, but you can not stretch the carpet to cover more than 50.