Apogee Oxygen Sensors are galvanic cell sensors that have a lead anode, a gold cathode, an acid electrolyte and a Teflon membrane. The current flow between the electrodes is proportional to the oxygen concentration being measured. An internal bridge resistor is used to provide a mV output. Being a galvanic cell type Oxygen Sensor, a small amount of oxygen is consumed in the reaction in order to produce the current flow and subsequent mV output. The oxygen consumption was measured to be 2.2 μmol O2 per day when the O2 concentration was 20.95 percent (3240 mmol) at 23°C.
The mV output responds to the partial pressure of oxygen in air. The standard units for partial pressure are kPa. However, gas sensors that respond to partial pressure are typically calibrated to read out in mole fraction of the gas in air, which has units of moles of oxygen per mole of air. These units can be directly converted to percent O2 in air or ppm O2 in air.
Gas sensors read out in percent because this value does not change with temperature or pressure. The concentration of oxygen in our atmosphere is 20.95%, and this value, to 4 significant digits, has not changed for decades. This means that we are surrounded by calibration gas for this sensor (provided you are not breathing on the oxygen sensor when it is being calibrated. Our exhaled breath is about 17 percent oxygen).
Apogee oxygen sensors can be used in conjunction with carbon dioxide sensors to help improve the characterisation 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.
Dr. Wendy Yang’s Global Change Ecology Lab, at the University of Illinois at Urbana-Champaign, is currently using the Apogee SO-110 oxygen sensor, with the optional diffusion head, for quasi-continuous measurements of bulk soil oxygen concentration in managed and natural ecosystems. Their quasi-continuous field measurements of soil oxygen has allowed them to correlate soil oxygen concentrations with process rates and determine how soil oxygen concentrations relate to precipitation and soil temperature, which can drive high biological oxygen demand to induce anoxia. The Apogee SO-110 allows Dr. Wendy Yang’s Global Change Ecology Lab to collect long-term data sets in the field without fear of compromising the sensors under harsh conditions, due to the sensors ability to withstand cold winter temperatures and wet conditions.
Oxygen is a major control for reduction-oxidation reactions in soil and can lead to the production or consumption of methane and nitrous oxide, both potent greenhouse gases. Processes such as methanogens and nitrous oxide reduction to dinitrogen were once thought to be restricted to flooded or saturated soils such as those found in wetlands, however, Dr. Wendy Yang’s lab has documented the importance of these processes in unsaturated soils from upland ecosystems. Their measurements have allowed them to correlate soil oxygen concentration with process rates and determine how soil oxygen concentrations relate to precipitation and soil temperate, even under saturated soil conditions. They are currently investigating these relationships in agricultural fields in the Midwest to better understand how current land management and historical soil drainage patterns mediate soil greenhouse gas emissions.