Flow through molecular contamination monitor with SAW devices

A molecular contamination monitor 10 comprises a housing 11 having a fluid flow passage with an entry 32 and exit 46, and a measurement SAW device 56 located in a diffusion chamber 19. The structure of the monitor 10 permits molecular contaminants to diffuse to a detection surface of SAW 56, but prevents particulate contaminants from reaching the detection surface. The structure is preferably a nozzle 38A and a tapered funnel 22A, which guide particulates along the flow passage. A chemically selective membrane may be provided (122, figure 7). The monitor may include a reference SAW 58 not exposed to the fluid. Environmental sensors may compensate for changes in temperature and humidity. Optionally the monitor includes a flow controller (figure 10), a sample dilution feature (figure 11), multiple fluid entry ports with a mixing manifold (figure 13), a particulate contamination counter (figure 12) and a sample pre-concentrator (figure 15).