This sensor can be used to make measurements of the level of free-oxygen in air or dissolved oxygen in water. The free-oxygen in air mode is used to measure changes in oxygen levels during combustion or in reactions that produce oxygen (hydrogen peroxide decomposition). The Dissolved-oxygen mode is useful in the study of photosynthesis. The oxygen sensor is designed for use both in the school laboratory and in the field. The electrode itself is constructed of Delrin® for durability. With its integral thermistor, it provides dependable temperature-compensated measurements. The thermistor is housed in stainless steel and sealed on the electrode's outer wall providing fast, accurate readings.