This manual mainly describes the use of the CyberScan DO 300/310 waterproof handheld dissolved oxygen/temperature meter, including preparation, calibration, measurement, locking function, memory function, advanced setting function, probe maintenance and maintenance, troubleshooting guide, error information, specifications, accessories, Appendix 1: Dissolved oxygen and instrument principle, Appendix 2: Pressure and altitude relationship, Appendix 3: The instrument's factory settings, Appendix 4: Setting functions, warranty and return terms.