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This technique uses the gas displacement method o measure of measuring volume accurately. Inert gases, such as helium or nitrogen, are used as the displacement medium. The sample is sealed in the instrument compartment of a know volume, the appropriate inert gas is admitted, and then expanded into another precision internal colume. The pressures observed upon filling the sample chamber and then discharging it into a second empty chamber allow computation of the sample solid phase colume. Helium molecules rapidly dill pores as small as one angstrom in diameter: only the solid phase of the sample displaces the gas. Dividing this volume into the sample weight gives the gas displacement density.
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