What do you mean by turndown ratio?
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 ln/min the flow meter will measure down to 0.4 ln/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).