Rotameters are also known as variable area flow meters.  They are typically tapered glass tubes positioned vertically in the fluid flow. A float within the tube rides upward in relation to the amount of flow. When there is no liquid flow, the float rests freely at the bottom of the tube.  As liquid enters the bottom of the tube, the float begins to rise. 

Rotameters can be configured to either measure liquid or gas flow.  Measuring the flow of liquids and gases is a critical need in many industrial plants. In some operations, the ability to conduct accurate flow measurements is so important that it can make the difference between making a profit and taking a loss. In other cases, inaccurate flow measurements or failure to take measurements can cause serious (or even disastrous) results.

Important parameters to consider when specifying rotameters include liquid volumetric flow rate, gas volumetric flow rate, operating pressure, and fluid temperature.  Liquid volumetric flow rate applies only to those rotameters that are liquid volumetric flow sensors or meters.  It is expressed as the range of flow in volume/time.  Gas volumetric flow rate applies only to those rotameters that are gas volumetric flow sensors or meters.  It is the range of flow in volume/time.  The operating pressure is the maximum head pressure of the process media the meter can withstand.  The maximum temperature of the media that can be monitored is usually dependent on construction and liner materials.  Pipe diameter is also important to consider, especially when specifying specific mounting options.  Mounting options for rotameters include insertion types, in-line flanged, in-line threaded, and in-line clamp.  Insertion flow meters are inserted perpendicular to flow path. They usually require a threaded hole in the process pipe or other means of access.  In-line flanged flow meters are inserted parallel to the flow path, usually inserted between two pieces of existing flanged process pipes.  In-line threaded flow meters are inserted parallel to the flow path, and threaded into two existing process pipes.  NPT is the most common thread type.  In-line clamp flow meters are inserted parallel to the flow path, and clamped between two existing process pipes.