What is a Medical Oxygen Sensor?
Galvanic medical oxygen sensors known as electro-galvanic fuel cells are electro-chemical devices used for the analysis and monitoring of oxygen partial pressure in a number of different devices such as analyzers, anesthesia machines, ventilators, and incubators.
HOW DO GALVANIC CELLS WORK?
Galvanic cells consist of a cathode, an anode, a membrane, and an electrolyte. Oxygen molecules enter the cell through the membrane, are reduced to hydroxyl ions at the cathode, and diffuse through the electrolyte to be oxidized at the lead anode.
This principle of lead oxidation is described by the following:
- Cathodic Reaction: 02 + 2H20 + 4e - 4OH
- Anode Reaction: 2Pb +4 OH - 2Pb0 +2H20 + 4e
- Net Cell Reaction: 2Pb + 02 = 2PbO
The current generated by the cell is directly proportional to the reduction of the oxygen at the cathode. The oxygen partial pressure linearity is maintained by the diffusion membrane which limits the amount of oxygen seen by the cathode. The oxygen diffusion rate is also temperature-dependent and output is controlled by a temperature compensation resistor known as a thermistor.
Depending upon the application instrument requirements, oxygen sensor outputs are in microamps, or through the use of a load resistor, a millivolt output can be generated.
Because of the fact that the lead anode is eventually significantly oxidized, the output will be reduced to the point where the voltage output to the oxygen partial pressure will no longer be linear and the cell must be replaced.
HOW LONG DO GALVANIC CELLS LAST?
Generally speaking, sensor life is expressed in terms of months in room air. The average sensor generally contains enough fuel to last for up to 4 years. Because most devices are used in higher oxygen partial pressure environments than room air, life expectancy can be significantly reduced from the quoted specification. It is recommended that sensors be stored in a cool, dry place when not in use. Sensors can be purchased fresh when needed here, and replacement sensors should not be stored on the shelf for any length of time. Storage in a refrigerator is also not recommended because of the high humidity environment.