Dissolved oxygen sensors and dissolved oxygen probes are used to measure the amount of oxygen that is dissolved water, by unit volume. The amount of oxygen that a given volume of water can hold is a function of the atmospheric pressure at the water-air interface, the temperature of the water, and the amount of other dissolved substances.  The concentration of dissolved oxygen (DO) is usually expressed in milligrams of oxygen per liter of water (mg/L) or parts per million (ppm). Some meters compare calculated oxygen content with observed concentration and report percent saturation (% sat.).