Should I still measure ppm? With any process control, but especially a critical control point for food safety programs, it is important to develop systems to crosscheck disinfectant levels. Standard paper strips or colorimetric test kits or, if using a panel-mounted ORP probe, a recently calibrated hand-held probe should be used periodically to assure that the primary ORP readings are accurately monitoring the desired conditions. Trevor V. Suslow, Ph.D. U.C. Davis