It is undoubtedly true that larger quantities of water are used in industry than the aggregate of all other materials. The variety of functions which water serves, as well as the circumstances under which testing of water quality is necessary, are proportionately numerous. The minimum chemical and physical requirements, including the presence and concentration of certain critical constituents, vary greatly from use to use. As a consequence, a proper selection of tests must be made on the basis of specific requirements dictated by the service. Where throughput is small or operations performed batchwise, periodic sampling and testing are usually adequate. However, the growth in complexity and size of industrial plants, particularly in the chemical, power, and process fields, has brought with it an ever-increasing rate of material flow, including water, which for closest control requires continuous automatic measurement of all significant variables. It is in this light that measuring equipment for pH, electrical conductivity, and oxidation-reduction potential and their limitations will be considered.