Dissolved oxygen (DO) meters measure the amount of dissolved oxygen present in water. The amount of DO helps to determine, for example, drinking water quality, the health of lakes or streams, and aquatic organism sustainability. A probe, also called a sensor or electrode, is inserted into the sample to perform the measurement, and the results appear on the meter display. Applications include sewage and water treatment plants, pollution control, fish farms, and various types of manufacturing.YSI manufactures instruments, sensors, and software for environmental water quality monitoring and testing. The company, founded in 1948, is headquartered in Yellow Springs, OH.