Dissolved oxygen (DO) meters are used to measure the amount of dissolved oxygen in a liquid. Oxygen makes its way into water through a variety of processes, including aeration, as a byproduct of photosynthesis, and from surrounding air. Any healthy aquatic system which can support aerobic life must contain certain amounts of oxygen; healthy bodies of water typically register at least 5 mg/L of dissolved oxygen. DO levels less than 5 mg/L can result in stressed aquatic organisms, and levels which remain as low as 1 mg/L for even a few hours can result in widespread fish kills. Conversely, dissolved oxygen saturation can also be harmful to aquatic life.
Dissolved oxygen meters consist of a DO probe connected to a meter/analyzer and are similar in construction to pH meters. The probe is comprised of two electrodes suspended in a potassium chloride (KCl) electrolyte solution, all of which is enclosed with glass and/or a semipermeable membrane. The electrodes are connected to the meter, which provides a small DC current to the electrodes via wiring. When the sensor is submerged in a liquid, oxygen from the liquid crosses the membrane and reacts with the cathode, causing a measurable current change; this change is converted into a millivolt output and is finally displayed by the meter.