The construction of the float decides heavily, the performance of the rotameter. In general, a float should be designed such that:
(a) it must be held vertical
(b) it should create uniform turbulence so as to make it insensitive to viscosity
(c) it should make the rotameter least sensitive to the variation of the fluid density.
A typical construction of the float is shown in fig. The top section of the float has a sharp edge and several angular grooves. The fluid passing through these grooves, causes the rotation of the float. The turbulence created in this process reduces the viscous force considerably.