How does ORP relate to ppm? ORP does not relate directly to ppm because it measures the oxidizing activity of the water and not the concentration of the oxidizer (chlorine, ozone, other oxidizing disinfectants). As the concentration of chlorine increases the ORP values increase but at a slower rate of change. In a similar manner, as the pH is altered to increase the relative proportion of the antimicrobial HOCL concentration (low pH, more acid) the ORP value increases. An ORP value of 650 mV measured at pH 6.5 or 8.5 provides the same killing potential, although the ppm "free chlorine" needed is greater at the higher pH value. A ten-fold increase in ppm chlorine (10ppm to 100ppm of added NaOCL) does not result in a linear increase in mV; the ORP probes approach their saturation capacity and reach a plateau. The higher oxidation potential is reflected in higher mV values, up to the maximum capacity of the specific probe. Trevor V. Suslow, Ph.D. U.C. Davis