# Instant Notes: 5. Flow Meters

### Rotameter

In the case of venturi and orifice meters the area of constriction remains constant and the pressure drop varies with flow rate; in the rota meter, the pressure drop remains nearly constant and the area of constriction varies. Refer to Fig.(15).

This meter may thus be considered as an orifice meter with a variable aperture, and the formula derived for orifice meter / venturi meter are applicable.

The pressure drop over the float \(\Delta P\), is given by: \[\Delta P = \frac{V_f(\rho_f - \rho)g}{A_f}\] where \(V_f\) is the volume of the float, \(\rho_f\) is the density of the float material, and \(A_f\) is the maximum cross sectional area of the float in a horizontal plane.

Because of rotameter relies on gravity, it must be installed vertically (with the flowtube perpendicular to the floor).