The Mott certified cylindrical metal diffuser meets the certification requirements for the ASTM D892 & D6082 foam method.
There are arguments that metal gas diffusers are much more uniform, easier to clean, and holds it specification for a much longer time than the stone diffuser. Diffusers permeability and porosity change over time so it is recommended to test them periodically preferably after each use.
Lawler Part No. FM-3MC