Galvanic oxygen sensor
A galvanic oxygen sensor is a specific type of fuel cell . It is an electrical device that is used to measure the concentration of oxygen in scuba diving and in medical equipment.
If the potassium hydroxide contained in it comes into contact with oxygen, a chemical reaction occurs in the fuel cell. This results in an electrical current between a lead - anode and a gold cathode through a load resistor flows. This current is proportional to the oxygen concentration present.
Galvanic sensors are used in oxygen meters when diving to check the oxygen content in Nitrox or Trimix breathing gas mixtures before a dive . They are also used in electronic rebreather devices , so-called rebreathers , to monitor the partial pressure of oxygen during a dive.
Galvanic oxygen sensors have a finite lifespan that is reduced when exposed to high concentrations of oxygen. When oxygen and lead react at the anode, lead is consumed, which at some point leads to measurement errors at high oxygen concentrations. Normally, a cell that is used for diving will work correctly for about 3 years if it is kept in an airtight bag. On the other hand, the period is reduced to only 4 months if you store them in pure oxygen.
disadvantage
The form of fuel cell described is comparable to a battery , because it has a limited service life depending on the area of application. The chemical reaction causes the cell to generate a current with a calculable voltage that depends on the material used. Theoretically speaking, a cell should deliver this voltage from the day of its manufacture until it is used up, unless the oxygen component of the intended reaction is removed from the aggregate.
Oxygen is one of the fuels in the cell, so the more oxygen is available, the more electricity is generated. The chemical properties of all materials determine the voltage, while the amount of available oxygen determines how much electrical current can be produced. If you connect a circuit with a corresponding consumption to such a cell, you can use a certain electrical power. However, as soon as the consumption is increased, the performance of the cell decreases.
Malfunction of the sensors can be life-threatening for divers, especially for users of rebreathers. The usual types of failure of galvanic sensors are:
- Excessive readings due to leaks in the electrolyte container,
- limited amperage when cells are used up,
- non-linear performance within the measuring range.
All malfunctions are usually caused by mechanical damage, contamination during manufacture or defects in the manufacturing process.
Excessive measured values are without exception caused by manufacturing errors or mechanical damage. In a rebreather they result in an overestimation of the oxygen content in the circuit, causing the device to replace too little oxygen, which usually leads to hypoxia .
Sensors with limited amperage generate a lower reading despite high oxygen levels. In the rebreather, oxygen is constantly added due to the underestimation of the oxygen content in the circuit in order to achieve a value that the sensor can no longer display. The result is usually hyperoxia known as the Paul Bert effect .
In the case of non-linear performance, the sensor does not behave as expected in the intended measuring range of the oxygen partial pressure. During calibration , the error is usually not recognized, which leads to an incorrect fill level in the rebreather. This can potentially cause decompression sickness .
Accidents due to malfunction of the sensors in rebreather diving devices can in most cases be prevented by properly checking the cells before each use. Some divers perform tests in the water by increasing the partial pressure of the oxygen in the circuit to a value above that of pure oxygen at sea level to determine whether the sensor can still process high values. However, this test is only selective and does not allow any reliable statement about the possible faultiness of a sensor. The only way to properly check a sensor is therefore in a calibrated test chamber that can both maintain a static pressure without deviations and has the ability to record and graphically evaluate measured values .
Verification
One of the first commercially available devices for checking galvanic oxygen sensors was launched by Narked at 90 in 2005, but without great success. A heavily revised model followed in 2007 and won the "Gordon Smith Award" for innovation at the diving equipment trade fair in Florida. The devices are now used worldwide by organizations such as NOAA (National Oceanic and Atmospheric Administration), NURC (NATO Underwater Research Center) and DDRC (Diving Diseases Research Center).
credentials
- ↑ Narked at 90 ( Memento of the original from April 7, 2008 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ NOAA (National Oceanic and Atmospheric Administration)
- ↑ NURC (NATO Underwater Research Center) ( Memento from March 13, 2007 in the Internet Archive )
- ↑ DDRC (Diving Diseases Research Center)
Oxygen measurement for divers
Helge Weber: reference book, recommended by the Rebreather Advisory Board eV. Books on Demand GmbH 2006, ISBN 978-3-8334-6795-0
Oxygen measurement for OC and Rebrather divers
Helge Weber: reference book, recommended by the Rebreather Advisory Board eV. Books on Demand GmbH 2009, ISBN 978-3-8370-3307-6
Web links
- Scientific technical workshops - manufacturer of oxygen sensors. (PDF file; 1276 kB)
- Analytical Industries - manufacturer of oxygen sensors for rebreather diving equipment.
- Teledyne Analytical Instruments - manufacturer of oxygen sensors for rebreather diving equipment .
- SECO Controls ( Memento of January 3, 2008 in the Internet Archive ) (Web archive) - Indian manufacturer of zirconium dioxide-based oxygen sensors and monitoring devices.