Electro-galvanic oxygen sensor
One form of electro-galvanic fuel cell based on the oxidation of lead is commonly used to measure the concentration of oxygen gas in underwater diving and medical breathing gases.
[1] The cell current is proportional to the rate of oxygen reduction at the cathode, but this is not linearly dependent on the partial pressure of oxygen in the gas to which the cell is exposed: Linearity is achieved by placing a diffusion barrier between the gas and the cathode, which limits the amount of gas reaching the cathode to an amount that can be fully reduced without significant delay, making the partial pressure in the immediate vicinity of the electrode close to zero.
As a result of this the amount of oxygen reaching the electrode follows Fick's laws of diffusion and is proportional to the partial pressure in the gas beyond the membrane.
[citation needed] The partial pressure of oxygen in diving chambers and surface supplied breathing gas mixtures can also be monitored using these cells.
[5][2] The breathing gas mixture in a diving rebreather loop is usually measured using oxygen cells, and the output of the cells is used by either the diver or an electronic control system to control addition of oxygen to increase partial pressure when it is below the chosen lower set-point, or to flush with diluent gas when it is above the upper set-point.
As the set points are commonly in the range of 1.2 to 1.6 bar,[6] special hyperbaric calibration equipment would be required to reliably test the response at the set-points.
This equipment is available, but is expensive and not in common use, and requires the cells to be removed from the rebreather and installed in the test unit.
[1] Oxygen cells behave in a similar way to electrical batteries in that they have a finite lifespan which is dependent upon use.
The chemical reaction 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.
When the lead electrode has been substantially oxidised, the maximum current that the cell can produce will drop, and eventually linearity of output current to partial pressure of oxygen at the reactive surface will fail within the required range of measurement, and the cell will no longer be accurate.
[7] There are two commonly used ways to specify expected sensor life span: The time in months at room temperature in air, or volume percentage oxygen hours (Vol%O2h).
[8] The failure modes common to these cells are: failing with a higher than expected output due to electrolyte leaks, which is usually attributable to physical damage, contamination, or other defects in manufacture,[7] or current limitation due to exhausted cell life and non linear output across its range.
[9] When new, a sensor can produce a linear output for over 4 bar partial pressure of oxygen, and as the anode is consumed the linear output range drops, eventually to below the range of partial pressures which may be expected in service, at which stage it is no longer fit to control the system.
The maximum output current eventually drops below the amount needed to indicate the full range of partial pressures expected in operation.
[1][7] Failing high – producing an output indicating partial pressure higher than reality – is invariably a result of a manufacturing fault or mechanical damage.
Two-point calibration against diluent and oxygen at atmospheric pressure will not pick up this fault which results in inaccurate loop contents of a rebreather.
Continuing a dive using a rebreather with a failed cell alarm significantly increases the risk of a fatal loop control failure.
[6] Factors which make the cell outputs in a rebreather statistically dependent include:[6][7] This statistical dependency can be minimised and mitigated by:[6][7] An alternative method of providing redundancy in the control system is to recalibrate the sensors periodically during the dive by exposing them to a flow of either diluent or oxygen or both at different times, and using the output to check whether the cell is reacting appropriately to the known gas as the known depth.
A much revised model was released in 2007 and won the "Gordon Smith Award" for Innovation at the Diving Equipment Manufacturers Exhibition in Florida.
