A method and apparatus for analyzing sag in drilling fluids or solid bearing fluids wherein a cylindrical high-pressure cell assembly (80) capable of withstanding high pressure and high temperature with a coaxial cylindrical rotor (33) driven to rotate inside cell assembly (80), a sample port (12) for testing sample subtraction used for further analysis, and a sample inlet port (74) for adding testing sample. This said cell assembly (80) is supported on a pivotal cell support (90) so that it can be tilted and fixed at any angle.