Apparatus, systems, and methods of measuring relative pressure based on the displacement of a Cartesian diver are provided. Within the system, various designs of divers, as well as a variety of measurement methods implementing such systems and apparatus, are illustrated. The systems and methods described herein are directed to precise pressure measurements traceable to the SI through the vertical displacement of a ludion within a two fluid system. The system is selectively optimized to ensure the buoyancy and mass of the ludion as well as the density of the gases and fluids involved are known and controlled.