What is ORP and why would I want to use it?

ORP is a measurement of the overall level of oxidizing or reducing substances present in the liquid. ORP is expresses as a millivolt (mV) signal. Positive voltage indicates an excess of oxidizing agents in the liquid. Conversely, negative voltage indicates an excess of reducing agents in the liquid. ORP is not specific to a single ion within the liquid but is often used to measure the status of a chemical reaction.