A simple pocket dissolved oxygen meter using an industry-standard polarographic DO probe, offering 1 or 2 point calibration using air-saturated water or zero oxygen solution. Automatic temperature compensation, and manual salinity and barometric pressure compensation improve accuracy across the entire measurement range. The HL103 is designed for ease of use, with auto-hold functionality to freeze a stable reading for ease of recording. A visible battery indicator and auto-off functions effectively conserve and communicate battery life to the user. A fully replaceable electrode module reduces maintenance costs.