Dissolved oxygen monitor accurately measures the dissolved oxygen content of a variety of samples, including drinking water and wastewater. It provides a solid ppb-level dissolved oxygen measurement. It contains an alarm unit that notifies users when minimum and maximum of dissolved oxygen concentration levels are violated. It is suitable for a wide range of applications, including monitoring the deaeration level of ultrapure water used in semiconductor manufacture, and dissolved oxygen concentration levels in sewage fermentation tanks as well as in hydroponic cultivation and desalination plants.