Chemtrol® CH255 Digital PPM/pH Controller with Sensor, 110V, 230V or 24 VAC Polystyrene Cabinet. Chemtrol® CH255 PPM/pH digital controller introduces the new sensor for free chlorine control. A microprocessor-based digital controller, designed to maintain both free chlorine (PPM) and water pH in residential pools and spas, commercial pools and spas, hotels, motels, health clubs, apartments buildings, condo associations and industrial water treatment. Simplicity of installation, compatible with all common sanitizers and oxidizers, ease of use and long-term reliability.