The TSI Fast Mobility Particle Sizer (FMPS) Spectrometer, model 3091, is an instrument designed for real-time measurement of nanoparticle size distributions. Unlike the Scanning Mobility Particle Sizer (SMPS) which scans through voltages sequentially, the FMPS uses a different electrical mobility measurement technique with multiple electrometers to measure the entire particle size spectrum simultaneously. This allows for a much faster measurement time, providing a full size distribution in as little as one second. The instrument measures particles in the size range from 5.6 to 560 nanometers. It is particularly useful for applications where particle concentrations and size distributions change rapidly, such as engine exhaust measurements, atmospheric process studies, and monitoring of nanoparticle synthesis.