Disclosed herein are improved therapeutic devices and methods and improved porous structures and measurement apparatus for use with therapeutic devices. In many embodiments, a porous structure is measured based on diffusion of the gas through the porous structure. The gas measurement may comprise an amount of gas measured to determine a resistance of the porous structure to diffusion. The diffusion of the gas through the porous structure can be used to determine release of a therapeutic agent through the porous structure, such that targeted amounts of therapeutic agent can be released for extended times and such that therapeutic device reservoir volume and porous frit structure can be tuned to release the therapeutic agent for an extended time above a target amount for the extended time.