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.).
Dissolved oxygen sensors function via a thin organic membrane that covers an electrolyte and two metal electrodes. When water enters the device, oxygen diffuses through the membrane at a rate proportional to its partial pressure. The greater the partial pressure of the oxygen, the greater the volume of oxygen that will diffuse through the membrane. Dissolved oxygen meters measure the current as oxygen is reduced at the cathode and more oxygen diffuses through the membrane. Since the diffusion current is directly proportional to the concentration of dissolved oxygen, the dissolved oxygen sensor converts the current measurement into concentration units. The term “dissolved oxygen sensors” refers to the entire sensor assembly, including the electrodes, electrolyte solutions, membranes, and thermistor thermometers.
There are two fundamental techniques for measuring dissolved oxygen — galvanic and polarographic. Both probes use an electrode system where the dissolved oxygen reacts with the cathode to produce a current. The two probe types differ in that galvanic probes do not require an external potential (voltage), whereas polarographic probes do. If the difference in potential between the cathode and anode is less than 0.5 volts, the system is galvanic. Galvanic probes are more stable and more accurate at lower dissolved oxygen levels than polarographic probes. Galvanic probes often operate several months without electrolyte or membrane replacement, resulting in lower maintenance cost. Polarographic probes need to be recharged every several weeks of heavy use. Polarographic probes may be of Ross or Clark polarographic types.
The measurements from dissolved oxygen sensors are used to monitor processes where oxygen content affects reaction rates, process efficiency, or environmental conditions. They are used widely in applications where maintaining a consistent oxygen level is important for reactions to be maintained, or in some cases, minimized. These include such as wastewater treatment, wine production, and bio-reactions including aquaculture (fish farming) and environmental testing.