The Clark-type electrode is the most used oxygen sensor for measuring oxygen dissolved in a liquid. Dissolved Oxygen (DO) is really a physical distribution of oxygen molecules in water. There are two main sources of Dissolved Oxygen in water: atmosphere and photosynthesis. Waves and tumbling water mix air into water where oxygen readily dissolves until saturation occurs. Oxygen is also produced by aquatic plants and algae as a by-product of photosynthesis.
The electrochemical method of measuring Dissolved Oxygen requires a cathode, anode, electrolyte solution and a gas permeable membrane. The material of the membrane is specially selected to permit oxygen to pass through. Oxygen is consumed by the cathode which will create a partial pressure across the membrane. Oxygen will then diffuse into the electrolyte solution.
Thus, a Dissolved Oxygen Meter actually measures the pressure of oxygen in water. It can be used to measure Dissolved Oxygen in any medium.