• Operating pressures and temperatures - These have a direct bearing on the choice of material for the body, seal, and viewing glass.
• Characteristics of the medium - The choice of body, seal, and viewing glass material is directly affected by the chemical and physical characteristics of the medium being transported. The degree of opacity or transparency, viscosity, and corrosiveness must all be considered.
• Characteristics of the process - Such factors as the rate and volume of flow, direction of flow (horizontal, upward vertical, or downward vertical), and other operational considerations.

Installation - The physical make-up of the process line, accessibility, and space limitations must be considered.