Since mercury does not wet most substances and will not spontaneously penetrate pores by capillary action, it must be forced into the pores by the application of external pressure. The required equilibrated pressure is inversely proportional to the size of the pores, only slight pressure being required to intrude mercury into large macropores, whereas much greater pressures are required to force mercury into small pores. Mercury porosimetry analysis is the progressive intrusion of mercury into a porous structure under stringently controlled pressures. From the pressure versus intrusion data, the instrument generates volume and size distributions using the Washburn equation.. Clearly, the more accurate the pressure measurements, the more accurate the resulting pore size data.
The penetrometer consists of a sample cup bonded to a metal-clad, precision-bore, glass capillary stem. The sample is placed in the sample cup; during analysis, mercury fills the cup and capillary stem. As pressure on the filled penetrometer increases, mercury intrudes into the sample’s pores, beginning with those pores of largest diameter. The mercury moves from the capillary stem resulting in a capacitance change between the mercury column inside the stem and the metal cladding on the outer surface of the stem. The AutoPore detects very slight changes in capacitance (equivalent to a difference of less than 0.1 microliter of mercury) so extraordinary resolution is achieved.
Micromeritics also offers a large selection of penetrometer bulbs, volumes, stems, and closures designed to fit most sample forms, shapes, porosity, and quantity. The better the match between the sample, its porosity, and the measurement range of the sample cell, the more precise the results.