Our carbonation and Inline Oxygenation Kit stones feature 0.5micron pores and they have a "wetting pressure". A "wetting pressure" is the minimum amount of pressure required to push gas through the stone. The wetting pressure varies based on stone to stone but is usually in the range of 2-5psi. So, for example, the wetting pressure on this stone is 5 psi. That means you need to apply 6 psi on the stone to get 1psi of gas to come out of the stone.
You can easily test this by submerging the stone into a jar of water and slowly adding 1 psi of pressure at a time. Look for an even disbursement of bubbles around the stone to dial in this wetting pressure.