Fluid flow measurement is used in almost every process from small-scale medical applications to large petroleum refineries. Industries use flow metering devices to regulate the amount of chemicals to be reacted or separated in a continuous process and accurately determine the production rate. Flow meters are also indispensable in the supply of utilities such as water and gas to your home. There are many types of flowmeters used in industry, but all flowmeters need to be calibrated.

A rotameter consists of a graduated tapered glass tube mounted vertically in a frame with the larger end up. The fluid flows upward through the tapered tube and suspends a float (or bob) in the flowing stream. The float indicates the relative flow rate of the fluid using the graduations on the glass scale. Measurements using the float are made at its largest cross-section which is at the center of a spherical float.

At a given flow rate the float maintains an equilibrium position and therefore does not move. The three forces acting on the float are: (1) the weight of the float ß , (2) the buoyant force of the fluid on the float Ý , and (3) the drag force on the float Ý .

A simplified rotameter equation is used to describe the rotameter operation.

This expression relates a volumetric flow rate q to the scale reading h, using a rotameter constant Crot. The constant is a function of the fluid properties and the physical dimensions of the rotameter and float.