The 366xEx family of instruments measures oxygen (O₂), hydrogen (H₂), and ozone (O₃) in areas where hazardous and flammable conditions are possible. When coupled with a choice of membrane covered, electrochemical sensors, the 366xEx is suitable for sampling dissolved concentrations from trace ppb to supersaturation, and gaseous con-centrations from ppb to percent (%) levels. These sensors can be constructed from a variety of chemically resistant materials and use an assortment of membranes with permeability and chemical resistance to optimize long-term measurement performance.