This document provides a general description, functional description, and specifications for the HI 9023C, HI 9023CN, HI 9024C, HI 9025C, HI 9110, and HI 9210N portable waterproof pH meters manufactured since 1978. It also includes operational guides, pH calibration procedures, pH values at various temperatures, troubleshooting guides, electrode conditioning and maintenance instructions, taking redox measurements (only applicable to HI 9023C and HI 9025C), battery replacement and auto-off instructions, accessories, and electrode application reference guide.