A test apparatus (and method of operation) for characterizing properties of a sample under test (such as porous material, for example, samples of reservoir rock) that operates in conjunction with a source of test fluid. The test apparatus includes an intake valve fluidly coupled to the source of test fluid, a reference cell fluidly coupled to the source of test fluid via the intake valve, a sample cell that holds the sample under test, an isolation valve fluidly coupled between the reference cell and the sample cell, an exhaust port, an exhaust valve fluidly coupled between the sample cell and the exhaust port, a first pressure sensor associated with the reference cell for measuring pressure within the reference cell, and a second pressure sensor associated with the sample cell for measuring pressure within the sample cell. The method of operation includes calibration procedures to compensate for systematic measurement errors.