Electro-galvanic fuel cell
Encyclopedia
An electro-galvanic fuel cell is an electrical device, one form of which is commonly used to measure the concentration of oxygen
gas in scuba diving
and medical equipment.
A chemical reaction
occurs in the fuel cell
when the potassium hydroxide
in the cell comes into contact with oxygen. This creates an electric current
between the lead
anode
and the gold
-plated cathode
through a load resistance. The current produced is proportional to the concentration (partial pressure) of oxygen present.
They are used in oxygen analysers in technical diving
to display the proportion of oxygen in a nitrox or trimix breathing gas
before a dive. They are also used in electronic, closed-circuit rebreather
s to monitor the oxygen partial pressure
during the dive.
The partial pressure of oxygen in diving chamber
s and surface supplied breathing gas mixtures
can also be monitored using these cells. This can either be done by placing the cell directly in the hyperbaric environment, wired through the hull to the monitor, or indirectly, by bleeding off gas from the hyperbaric environment or diver gas supply and analysing at atmospheric pressure, then calculating the partial pressure in the hyperbaric environment. This is frequently required in saturation diving
and surface oriented surface supplied mixed gas commercial diving.
Electro-galvanic fuel cells have a limited lifetime which is reduced by exposure to high concentrations of oxygen. The reaction between oxygen and lead at the anode consumes lead, which eventually results in the cell failing to sense high concentrations of oxygen. Typically, a cell used for diving applications will function correctly for 3 years if stored in a sealed bag of air but only for four months if stored in pure oxygen.
described above causes the cell to create an electrical output that has a predicted voltage which is dependent on the materials used. In theory they should give that voltage from the day they are made until they are exhausted, except that one component of the planned chemical reaction has been left out of the assembly: oxygen.
Oxygen is one of the fuels of the cell so the more oxygen there is, the more electricity is generated. The chemistry sets the voltage
and the fuel, the oxygen, sets how much electric current
it can give. If you put an electric circuit on the cell that draws current you can draw up to this current but ask for more and the voltage from the cell fades.
Failures in cells can be life threatening for technical divers and in particular, rebreather
divers. The failure modes common to these cells are: failing with a higher than expected output due to electrolyte
leaks, current limitation due to exhausted cell life and non linear output across its range. These failures are usually attributable to physical damage, contamination during manufacture or defects in manufacture.
Failing high is invariably a result of a manufacturing fault or mechanical damage. In rebreathers, failing high will result in the rebreather assuming that there is more oxygen in the loop than there actually is which results in hypoxia
.
Current limited cells do give a high output in high concentrations of oxygen. The rebreather assumes there is insufficient oxygen in the loop and injects to reach a setpoint the cell will never achieve resulting in hyperoxia
.
Non-linear cells do not perform in an expected manner across its range of oxygen partial pressures. Calibration will not pick up this fault which results in inaccurate loop contents of a rebreather. This gives the potential for decompression illness.
Preventing accidents in rebreathers from cell failures is possible in most cases by accurately testing the cells before use. Some divers carry out in-water checks by pushing the oxygen content in the loop to a pressure that is above that of pure oxygen at sea level to indicate if the cell is capable of high outputs. This test is only a spot check and does not accurately assess the quality of prediction of failure of that cell. The only way to accurately test a cell is with a calibrated test chamber which can hold a static pressure without deviation and the ability to log the results and graph them.
, NURC (NATO Underwater Research Centre) and Diving Diseases Research Centre
.
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
gas in scuba diving
Scuba diving
Scuba diving is a form of underwater diving in which a diver uses a scuba set to breathe underwater....
and medical equipment.
A chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
occurs in the fuel cell
Fuel cell
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...
when the potassium hydroxide
Potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...
in the cell comes into contact with oxygen. This creates an electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
between the lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
and the gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
-plated cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
through a load resistance. The current produced is proportional to the concentration (partial pressure) of oxygen present.
They are used in oxygen analysers in technical diving
Technical diving
Technical diving is a form of scuba diving that exceeds the scope of recreational diving...
to display the proportion of oxygen in a nitrox or trimix breathing gas
Breathing gas
Breathing gas is a mixture of gaseous chemical elements and compounds used for respiration.Air is the most common and only natural breathing gas...
before a dive. They are also used in electronic, closed-circuit rebreather
Rebreather
A rebreather is a type of breathing set that provides a breathing gas containing oxygen and recycled exhaled gas. This recycling reduces the volume of breathing gas used, making a rebreather lighter and more compact than an open-circuit breathing set for the same duration in environments where...
s to monitor the oxygen partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
during the dive.
The partial pressure of oxygen in diving chamber
Diving chamber
A diving chamber has two main functions:* as a simpler form of submersible vessel to take divers underwater and to provide a temporary base and retrieval system in the depths;...
s and surface supplied breathing gas mixtures
Surface supplied diving
Surface supplied diving refers to divers using equipment supplied with breathing gas using a diver's umbilical from the surface, either from the shore or from a diving support vessel sometimes indirectly via a diving bell...
can also be monitored using these cells. This can either be done by placing the cell directly in the hyperbaric environment, wired through the hull to the monitor, or indirectly, by bleeding off gas from the hyperbaric environment or diver gas supply and analysing at atmospheric pressure, then calculating the partial pressure in the hyperbaric environment. This is frequently required in saturation diving
Saturation diving
Saturation diving is a diving technique that allows divers to reduce the risk of decompression sickness when they work at great depth for long periods of time....
and surface oriented surface supplied mixed gas commercial diving.
Electro-galvanic fuel cells have a limited lifetime which is reduced by exposure to high concentrations of oxygen. The reaction between oxygen and lead at the anode consumes lead, which eventually results in the cell failing to sense high concentrations of oxygen. Typically, a cell used for diving applications will function correctly for 3 years if stored in a sealed bag of air but only for four months if stored in pure oxygen.
Cell limitations
Oxygen cells behave in a similar way to electrical batteries in that they have a finite lifespan which is dependent upon use. The chemical reactionChemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
described above causes the cell to create an electrical output that has a predicted voltage which is dependent on the materials used. In theory they should give that voltage from the day they are made until they are exhausted, except that one component of the planned chemical reaction has been left out of the assembly: oxygen.
Oxygen is one of the fuels of the cell so the more oxygen there is, the more electricity is generated. The chemistry sets the voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
and the fuel, the oxygen, sets how much electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
it can give. If you put an electric circuit on the cell that draws current you can draw up to this current but ask for more and the voltage from the cell fades.
Failures in cells can be life threatening for technical divers and in particular, rebreather
Rebreather
A rebreather is a type of breathing set that provides a breathing gas containing oxygen and recycled exhaled gas. This recycling reduces the volume of breathing gas used, making a rebreather lighter and more compact than an open-circuit breathing set for the same duration in environments where...
divers. The failure modes common to these cells are: failing with a higher than expected output due to electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
leaks, current limitation due to exhausted cell life and non linear output across its range. These failures are usually attributable to physical damage, contamination during manufacture or defects in manufacture.
Failing high is invariably a result of a manufacturing fault or mechanical damage. In rebreathers, failing high will result in the rebreather assuming that there is more oxygen in the loop than there actually is which results in hypoxia
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
.
Current limited cells do give a high output in high concentrations of oxygen. The rebreather assumes there is insufficient oxygen in the loop and injects to reach a setpoint the cell will never achieve resulting in hyperoxia
Hyperoxia
Hyperoxia is excess oxygen or higher than normal partial pressure of oxygen.In medicine, it refers to excess oxygen in the lungs or other body tissues, which can be caused by breathing air or oxygen at pressures greater than normal atmospheric pressure...
.
Non-linear cells do not perform in an expected manner across its range of oxygen partial pressures. Calibration will not pick up this fault which results in inaccurate loop contents of a rebreather. This gives the potential for decompression illness.
Preventing accidents in rebreathers from cell failures is possible in most cases by accurately testing the cells before use. Some divers carry out in-water checks by pushing the oxygen content in the loop to a pressure that is above that of pure oxygen at sea level to indicate if the cell is capable of high outputs. This test is only a spot check and does not accurately assess the quality of prediction of failure of that cell. The only way to accurately test a cell is with a calibrated test chamber which can hold a static pressure without deviation and the ability to log the results and graph them.
Testing
The first certified cell checking device that was commercially available was launched in 2005 by Narked at 90 but did not achieve commercial success. A much revised model was released in 2007 and won the "Gordon Smith Award" for Innovation at the Diving Equipment Manufacturers Exhibition in Florida. Narked at 90 Ltd won the Award for Innovation for the Development of Advanced Diving products at Eurotek 2010 for the Cell Checker and its continuing Development. Now used throughout the world by organisations such as Teledyne/Vandegraph National Oceanic and Atmospheric AdministrationNational Oceanic and Atmospheric Administration
The National Oceanic and Atmospheric Administration , pronounced , like "noah", is a scientific agency within the United States Department of Commerce focused on the conditions of the oceans and the atmosphere...
, NURC (NATO Underwater Research Centre) and Diving Diseases Research Centre
Diving Diseases Research Centre
The Diving Diseases Research Centre is a British hyperbaric medical organisation located near Derriford Hospital in Plymouth, Devon. It is a registered charity and was established in 1980 to research the effects of diving on human physiology....
.