A method and apparatus for monitoring swelling changes consists of a cylindrical cell assembly (80) capable of withstanding high pressure and high temperature with a wafer holder (48) containing a wafer (42) of solid sample. A sensor rod (56) moves in response to expansion or contraction of the sample wafer (42) and its movement is measured by either an LVDT sensor (66) or a magnetometer (116). Heat is provided via a heater (30) and pressure is controlled via a pressurization inlet (16) or pressurization fluid (68).