Regarding variable orifice flowmeters true is
The variable orifice flowmeter is rotameter. The tube is vertically mounted. the bobbin is scored so that it rotates without sticking. Tube is gas specific and laminar flow occurs at the base and turbulent flow occurs at the top.
The tube must be slightly inclined to the vertical to avoid friction
The bobbin is scored to avoid friction
Are accurate only for laminar flow
An orifice has diameter shorter than its length