Flow meter Turndown is the term used to define the flow range over which the flow meter maintains the accuracy.
All flow meter do have a maximum cut off flow rate & minimum cut of flow rate over & below which it can’t measure accurately, this range is nothing but a turndown.
Turn Down on orifice plate depends on the beta ratio & generally it is 3:1 to 5:1. To maintain the turndown ratio flow meter manufacturer do reduce the flow meter size. so that its turndown can capture application’s best possible flow range.
Flow Meter turndown is not actually your application turndown hence you should always ask for the maximum & minimum flow the flow meter can accurately measure to know flowmeter’s turndown on the application.
Flow meter could be 10:1 turndown but on the application it would be less as well hence it is very important to know the turndown with respect to application.