Inside there are two noble metal electrodes separated by electrolyte.  A carefully fixed voltage allows only oxygen to be reduced.  The greater the oxygen partial pressure in the sample, the more oxygen diffuses.  Current is proportional to oxygen in the sample.  Several factors affect dissolved oxygen measurement.  Temperature, salinity and barometric pressure all need to be considered.  Thermo Scientific meters automate these procedures.  Gentle stirring is recommended to prevent oxygen depletion at the membrane interface.