Oxygen availability determines the rate of many biological and chemical processes and is required for aerobic respiration. It is the absolute amount of oxygen (measured as partial pressure in kilopascals) that nearly always determines oxygen availability, but we think of oxygen as a percent of the total number of molecules in the air (20.95 %). The best example of this is the oxygen on top of Mount Everest, which is 20.95 %, but most climbers need supplemental oxygen to get to the top.
There are two types of oxygen sensors: those that measure gaseous O2 and those that measure dissolved oxygen in a solution. The Apogee sensor measures gaseous O2 as a percentage. Gas sensors read out in percent because this value does not change with temperature or pressure.
There are multiple techniques for measuring gaseous oxygen. Three widely used approaches for environmental applications are galvanic cell sensors, polarographic sensors, and optical sensors. The Apogee sensor is a galvanic cell type that operates by electrochemical reaction of oxygen with an electrolyte, which produces an electrical current. The electrochemical reaction consumes a small amount of oxygen in the reaction in order to produce the current flow and subsequent mV output. The current flow between the electrodes are proportional to the oxygen concentration being measured, and an internal bridge resistor is used to provide the mV output. The mV output responds to the partial pressure of oxygen in air.
Typical applications of oxygen sensors include:
- measurement of O2 in laboratory experiments
- monitoring gaseous O2 in indoor environments for climate control
- monitoring of O2 levels in compost piles and mine tailings
- monitoring redox potential in soils
- determination of respiration rates through measurement of O2 consumption in sealed chambers
- measurement of O2 gradients in soil/porous media
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Apogee oxygen sensors can be used in conjunction with carbon dioxide sensors to help improve the characterization of soil respiration. Typically, soil oxygen sensors use a galvanic cell to produce a current flow that is proportional to the oxygen concentration being measured. These oxygen sensors are buried at various depths to monitor oxygen depletion over time, which is then used to predict soil respiration rates. Apogee oxygen sensors are equipped with a built-in heater to prevent condensation from forming on the permeable membrane, as relative humidity can reach 100 percent in soil.