Mechanical variable area flow meters (known as rotameters) provide direct viewing of flow rate for basic flow measurement of either water / liquids or gases (eg air, nitrogen, hydrogen, oxygen, argon, helium) by aligning the float with marks on the rotameter flow tube.
Where VA flow meter calibration has been carried out for defined media conditions, deviations in the quoted density, pressure or temperature of gases, or in the density or viscosity of liquids, will result in measurement errors.
Variable Area / Rotameter Description, Pros and Cons
Advice on Selecting Variable Area / Rotameter Flow Meters
Variable Area Flow Meters & Rotameters - Parameter Comparison