When we mention “density”, we would immediately bring into mind the famous scientist: Archimedes. He applied the water displacement method and buoyancy method, through which we know the basic concept that “density” equals to “weight” divided by “volume”. However, as the demand of the market, the new composite materials have been successively developed, displacing the traditional manufacturing method. As the replacement of the materials, the measuring methods of materials and products have also changed. According to the ASTM, the materials and products are divided into: 1、Non-absorbent or porous materials and products containing less than 2% porosity . 2、Absorbent or porous materials and products containing more than 2% porosity. Therefore, there are different classifications for materials and products made at this stage. We can divide them into two types: 1、Apparent density： The density of impermeable product was defined as the volume does not contain closed pores, or open pores which will not be water-infiltrated. This is what we call apparent density Measuring range: The measured density contains the sample within the close pores. Typical products: rubber, plastic, hard alloy, precious metals, etc., or an external liquid can not penetrate into the porous sintered products of powder metallurgy or the small porosity of the materials and porous products. 2、Bulk density: As the “permeable density” defined that the volume contains the open pores and the close pores, in powder metallurgy we generally call it “sintered density”, in precision ceramics industry we call it “bulk density” Measuring range: The measured density contains the sample within the close pores and open pores. Typical products: powder metallurgy, precision ceramics, oxidation magnet, soil, absorptive materials, or any powder metallurgy products that outside liquid can penetrate in, or materials with big pores/porous products.