The TSI Model 3789 is a water-based Condensation Particle Counter (CPC) designed for detecting and counting airborne nanoparticles. It uses distilled water as a safe and environmentally friendly working fluid. The instrument operates by growing nanoparticles to a size where they can be optically detected and counted. A key feature of this model is its high sensitivity, with a user-selectable minimum detectable particle size (50% counting efficiency) down to 2.2 nanometers. Its applications include nanotechnology research, process monitoring in semiconductor manufacturing, particle formation and growth studies, and air quality monitoring.