The valve which controls the amount of gas which can flow through a Hydro Instruments flow meter, chlorinator, or sulfonator device. Control is provided by a graduated notch or slope in the rate valve stem.  When used in a corrosive environment such as chlorine, the rate valve stem must be made of a non-reactive and non-corrosive substance such as silver.

This is a flow meter set apart from a chlorinator or sulfonator which is used to remotely measure gas flow. Remote flow meters are used in single-point, single-point automatic standby, multi-point, and multi-point automatic standby applications.