This paper presents a method for measuring the open porosity of porous solids when the mass density of the solid phase constituent is unknown. In this case, the determination of the open porosity requires the determination of the volume of the solid phase. The proposed method is based on the measurement of the apparent (in-air) and true (in-vacuum) masses of a porous solid, where an in-air "missing mass" is found and related to the volume of the solid phase through the Archimedes' principle. The precision of the method in relation with the used experimental set-up is studied. The error on the measured open porosity is estimated only in terms of the bulk volume of the test sample and the balance readability. Experimental tests are performed to validate the accuracy of the method and its application to low and high porosity materials. Also, it is shown that the "missing mass" method can be used to accurately evaluate the air density without the knowledge of the atmospheric conditions (temperature, static pressure, and relative humidity).
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