The measurement of particle density can be done in a number of ways: The powder is placed inside a pycnometer of known volume, and weighed.
A similar method, which does not include pore volume, is to suspend a known mass of particles in molten wax of known density, allow any bubbles to escape, allow the wax to solidify, and then measure the volume and mass of the wax/particulate brick.
Another method based on buoyancy is to measure the weight of the sample in air, and also in a liquid of known density.
After the valve is opened, the final pressure in the system allows the total gas volume to be determined by application of Boyle's law.
A continuous trace of pressure versus volume can then be generated, which allows for a complete characterization of the sample's porosity.