Turndown ratio is the ratio between maximum value to minimum value of instrument range where instrument is able to provide accurate measurement.
The turndown ratio may vary between each type of instrument. The following list is several type of flow instrument and its turn down ratio:
Orifice flow meter has 1:3 turn down ratio, for single DP transmitter.
PD meter or turbine meter has 10:1 turn down ratio, some vendors claim they have manufactured model which has higher turn down ratio.
Coriolis flowmeter can provide as high as 100:1 turn down