ORP,or Oxidation-reduction potential, is a measurement that indicates the degree to which a substance is capable of oxidizing or reducing another substance. ORP is measured in millivolts (mv) using an ORP meter. A positive ORP reading indicates that a substance is an oxidizing agent. ORP can be used for water system monitoring with the benefit of a single-value measure of the disinfection potential, showing theactivity of the disinfectant rather than the applied dose. For example, E. coli, Salmonella, Listeria and other pathogens have survival times of under 30 s when the ORP is above 665 mV, compared against >300 s when it is below 485 mV.