Dissolved oxygen meters are analytical instruments used to measure the amount of oxygen dissolved in a unit volume of water. It is an important indicator of the degree of usefulness of a sample of water for a specific application.  Air consists of 21 percent oxygen and approximately 78 percent nitrogen by volume. Oxygen dissolves poorly, and can only exist in water in low concentrations. Even so, dissolved oxygen (DO) is essential for the respiration of a wide variety of animals and bacteria in the aquatic environment.  The accurate measurement of dissolved oxygen is critical to many other applications including water treatment plants, sewage treatment works, effluent activated sludge process, river monitoring, fish farming, and any other field where water quality is important. Dissolved oxygen is also one of the key measurements in biotechnical processes, and is essential to maintain quality of the finished product.