One of the outputs of mercury porosimetry is Differential intrusion in ml/g/um. this is ml of Hg per g of sample per um of what? What is that micrometer? The characteristic length of the sample, the average pore diameter or radius?. Also, why is there a "log differential intrusion", for which the unit is ml/g. The um have vanished but what has been log-ed? Thx.