Dissolved oxygen refers to the measure of a percentage of oxygen that dissolves in a given medium. The measurement can be determined with a dissolved oxygen meter — a device that uses oxygen-sensing probes. The probes contain optical fluorescence sensors, galvanic sensors or polarographic sensors.
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An optical fluorescence sensor is coated with fluorescent properties. Light is exposed to this coating, producing a light and subsequent afterglow of fluorescence. The oxygen level of a medium is measured based on the duration of the afterglow.
A galvanic sensor acts as a battery and is able to generate power without external voltage. Galvanic probes contain an anode and cathode in an electrolyte. Oxygen enters the electrolyte via a membrane, which generates voltage between the anode and cathode. This difference in potential voltage is used to measure the amount of dissolved oxygen.
Polarographic sensors operate in a fashion similar to galvanic sensors, but require a connection to an external voltage source. The difference in voltage potential between the anode and cathode yields a measurement of the dissolved oxygen.