Rotameters, often called variable-area flowmeters or flow tubes, consist of a vertical glass or polymeric tube having a tapered or conical gas passage (hence, variable area) over the length of the tube. As shown in the diagram, a float located in the gas passage is used to indicate the gas flow.
Floats are either spherical or machined and are made from a variety of materials having different densities (specific gravities). In order of increasing density, float materials are typically nylon, glass, sapphire, stainless steel, WC/Co, or tantalum. The choice of float material is made primarily based on the required gas flow. For a given tube diameter and length, increasing the float density increases the flow for a given position or reading on a scale. Readings are taken the widest point of the float. The widest point of a spherical float is the equator; for a machined float it is usually a knife-edge near the top.
Volume 5A, Thermal Spray Technology → Thermal Spray Processes and Coatings → Process Control and Control Equipment → Gas Flow Measurement and Control
