Oxygen is the most important gas for plant and animal life on earth.As oxygen is soluble in water it makes life in rivers , lakes and oceans possible.These waters are continuously exposed to the atmospheric air, the amount of soluble oxygen in water is therefore determined by the partial pressure of oxygen and the temperature. In order to determine the state of pollution in these systems or to help micro-organisms to perform well, the dissolved oxygen (DO) content has to be monitored. Accurate and continuous measurement of the DO ensures high quality products and saves a lot of money.