Triboelectric charging has a large impact in numerous areas of science and industry. Recently, several studies of granular insulator systems have shown bipolar triboelectric charging occurs, which is dependent on the size distribution of the particles. A bench-top fluidized bed apparatus has been constructed to investigate this phenomenon. To study under highly controlled conditions, the fluidized bed is operated under vacuum (approximating 50 torr) with a nitrogen flow on the order of one liter per minute. Particles composed of a range of insulating materials, including silica-based and polymer materials, are examined. The particles are in the size range of 100- 1000 microns, and both mono-disperse and polydisperse particle size distributions are examined. The magnitude and polarity of the charging is related to the type of material and the particle size distribution.