Electrochemical device for measuring oxygen partial pressure
Electro-galvanic fuel cell as used in a diving rebreather to measure the partial pressure of oxygen. Two oxygen cells as used by oxygen analysers for diving gas showing commonly used connectors
An electro-galvanic fuel cell is an electrochemical device which consumes a fuel to produce an electrical output by a chemical reaction. 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 medicalbreathing gases.
Electronically monitored or controlled diving rebreather systems,[1] saturation diving systems,[2] and many medical life-support systems use galvanic oxygen sensors in their control circuits to directly monitor oxygen partial pressure during operation.[3] They are also used in oxygen analysers in recreational, technical diving and surface supplied mixed gas diving to analyse the proportion of oxygen in a nitrox, heliox or trimixbreathing gas before a dive.[4]
These cells are lead/oxygen galvanic cells where oxygen molecules are dissociated and reduced to hydroxyl ions at the cathode. The ions diffuse through the electrolyte and oxidize the lead anode. A current proportional to the rate of oxygen consumption is generated when the cathode and anode are electrically connected through a resistor[1]
Function
The cell reaction for a lead/oxygen cell is: 2Pb + O2 → 2PbO, made up of the cathode reaction: O2 + 2H2O + 4e− → 4OH−, and anode reaction: 2Pb + 4OH− → 2PbO + 2H2O + 4e−.[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. This makes the current proportional to PO2. The load resistor over the cell allows the electronics to measure a voltage rather than a current. This voltage depends on the construction and age of the sensor, and typically varies between 7 and 28mV for a PO2 of 0.21 bar[1]
Diffusion is linearly dependent on the partial pressure gradient, but is also temperature dependent, and the current rises about two to three percent per kelvin rise in temperature. A negative temperature coefficient resistor is used to compensate, and for this to be effective it must be at the same temperature as the cell. Oxygen cells which may be exposed to relatively large or rapid temperature changes, like rebreathers, generally use thermally conductive paste between the temperature compensating circuit and the cell to speed up the balancing of temperature.[1]
Temperature also affects the signal response time, which is generally between 6 and 15 seconds at room temperature for a 90% response to a step change in partial pressure. Cold cells react much slower and hot cells much faster. As the anode material is oxidised the output current drops and eventually will cease altogether. The oxidation rate depends on the oxygen reaching the anode from the sensor membrane. Lifetime is measured in oxygen-hours, and also depends on temperature and humidity[1]
Applications
Gas mixture analysis
Trimix gas analyser showing startup checksTrimix gas analyser showing calibration optionsTrimix gas analyser showing 1 point oxygen calibrationTrimix gas analyser showing oxygen and helium partial pressures
The oxygen content of a stored gas mixture can be analysed by passing a small flow of the gas over a recently calibrated cell for long enough that the output stabilises. The stable output represents the fraction of oxygen in the mixture. Care must be taken to ensure that the gas flow is not diluted by ambient air, as this would affect the reading.[citation needed]
Breathing gas composition monitoring
The partial pressure of oxygen in anaesthetic gases is monitored by siting the cell in the gas flow, which is at local atmospheric pressure, and can be calibrated to directly indicate the fraction of oxygen in the mix.[citation needed]
The partial pressure of oxygen in diving chambers 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.[5][2]
Diving rebreather control systems
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. When the partial pressure is between the upper and lower set-points, it is suitable for breathing at that depth and is left until it changes as a result of consumption by the diver, or a change in ambient pressure as a result of a depth change.[citation needed]
Accuracy and reliability of measurement is important in this application for two basic reasons. Firstly, if the oxygen content is too low, the diver will lose consciousness due to hypoxia and probably die,[6] or if the oxygen content is too high, the risk of central nervous system oxygen toxicity causing convulsions and loss of consciousness, with a high risk of drowning becomes unacceptable.[6] Secondly, decompression obligations cannot be accurately or reliably calculated if the breathing gas composition is not known.[6] Pre-dive calibration of the cells can only check response to partial pressures up to 100% at atmospheric pressure, or 1 bar. 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. To compensate for the possibility of a cell failure during a dive, three cells are generally fitted, on the principle that failure of one cell at a time is most likely, and that if two cells indicate the same PO2, they are more likely to be correct than the single cell with a different reading. Voting logic allows the control system to control the circuit for the rest of the dive according to the two cells assumed to be correct. This is not entirely reliable, as it is possible for two cells to fail on the same dive.[6]
The sensors should be placed in the rebreather where a temperature gradient between the gas and the electronics in the back of the cells will not occur.[1]
Lifespan
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.[7]
Oxygen is one of the fuels of the cell so the more oxygen there is at the reaction surface, the more electrical current is generated. The chemistry sets the voltage and the oxygen concentration controls the electric current output. If an electrical load is connected across the cell it can draw up to this current but if the cell is overloaded the voltage will drop. 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). Storage at low oxygen partial pressure when not in use would seem an effective way to extend cell life, but when stored in anoxic conditions the sensor current will cease and the surface of the electrode may be passivated, which can lead to sensor failure. High ambient temperatures will increase sensor current, and reduce cell life. In diving service a cell typically lasts for 12 to 18 months, with perhaps 150 hours service in the diving loop at an oxygen partial pressure of about 1.2 bar and the rest of the time in storage in air at room temperature.[1]
Failures in cells can be life-threatening for technical divers and in particular, rebreather divers.[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.[1][7]
Shelf life can be maximised by keeping the cell in the sealed bag as supplied by the manufacturer until being put into service, storing the cell before and between use at or below room temperature, - a range of from 10 to 22°C is recommended by a manufacturer - and avoid storing the cell in warm or dry environments for prolonged periods, particularly areas exposed to direct sunlight.[9]
Failure modes
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. This state is called current-limited. Current limited cells do not give a high enough output in high concentrations of oxygen.[1][7] The rebreather control circuit responds as if there is insufficient oxygen in the loop and injects more oxygen in an attempt to reach a setpoint the cell can never indicate, resulting in hyperoxia. When a current limited sensor can no longer reliably activate the control system at the upper set-point in a life support system, there is a severe risk of an excessive oxygen partial pressure occurring which will not be noticed, which can be life-threatening.[1][10]
Other failure modes include mechanical damage, such as broken conductors, corroded contacts and loss of electrolyte due to damaged membranes.[1][7]
Failing high – producing an output indicating partial pressure higher than reality – 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 can result in hypoxia.[7]
Non-linear cells do not perform in the expected manner across the required range of oxygen partial pressures. 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. This gives the potential for decompression illness if the loop is maintained at a lower partial pressure than indicated by the cell output, or hyperoxia if the loop is maintained at a higher partial pressure than indicated by cell output.[7]
Testing cells in the field
Preventing accidents in rebreathers from cell failures is possible in most cases by accurately testing the cells before use.[7] 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.[11] This test is only a spot check and does not accurately assess the quality of that cell or predict its failure.[11] The only way to accurately test a cell is with a test chamber which can hold a calibrated static pressure above the upper set-point without deviation and the ability to record the output voltage over the full range of working partial pressures and graph them.[citation needed]
Managing cell failure in a life-support system
If more than one statistically independent cell is used, it is unlikely that more than one will fail at a time. If one assumes that only one cell will fail, then comparing three or more outputs which have been calibrated at two points is likely to pick up the cell which has failed by assuming that any two cells that produce the same output are correct and the one which produces a different output is defective. This assumption is usually correct in practice, particularly if there is some difference in the history of the cells involved.[12][7] The concept of comparing the output from three cells at the same place in the loop and controlling the gas mixture based on the average output of the two with the most similar output at any given time is known as voting logic, and is more reliable than control based on a single cell. If the third cell output deviates sufficiently from the other two, an alarm indicates probable cell failure. If this occurs before the dive, the rebreather is deemed unsafe and should not be used. If it occurs during a dive, it indicates an unreliable control system, and the dive should be aborted. Continuing a dive using a rebreather with a failed cell alarm significantly increases the risk of a fatal loop control failure. This system is not totally reliable. There has been at least one case reported where two cells failed similarly and the control system voted out the remaining good cell.[6][10]
If the probability of failure of each cell was statistically independent of the others, and each cell alone was sufficient to allow safe function of the rebreather, the use of three fully redundant cells in parallel would reduce risk of failure by five or six orders of magnitude.[6]
The voting logic changes this considerably. A majority of cells must not fail for safe function of the unit. In order to decide whether a cell is functioning correctly, it must be compared with an expected output. This is done by comparing it against the outputs of other cells. In the case of two cells, if the outputs differ, then one at least must be wrong, but it is not known which one. In such a case the diver should assume the unit is unsafe and bail out to open circuit. With three cells, if they all differ within an accepted tolerance, they may all be deemed functional. If two differ within tolerance, and the third does not, the two within tolerance may be deemed functional, and the third faulty. If none are within tolerance of each other, they may all be faulty, and if one is not, there is no way of identifying it.[6]
Using this logic, the improvement in reliability gained by use of voting logic where at least two sensors must function for the system to function is greatly reduced compared to the fully redundant version. Improvements are only in the order of one to two orders of magnitude. This would be great improvement over the single sensor, but the analysis above has assumed statistical independence of the failure of the sensors, which is generally not realistic.[6]
Factors which make the cell outputs in a rebreather statistically dependent include:[6][7]
Common calibration gas - They are all calibrated together in the pre-dive check using the same diluent and oxygen supply.
Sensors are often from the same manufacturing batch - Components, materials and processes are likely to be very similar.
Sensors are often installed together and have since been exposed to the same PO2, temperature profile over the subsequent time.
Common working environment, particularly with regards to temperature and relative humidity, as they are usually mounted in very close proximity in the loop, to ensure that they measure similar gas.
Common measurement systems
Common firmware for processing the signals
This statistical dependency can be minimised and mitigated by:[6][7]
Using sensors from different manufacturers or batches, so that no two are from the same batch
Changing sensors at different times, so they each have a different history
Ensuring that the calibration gases are correct
Adding an statistically independent PO2 measuring system to the loop at a different place, using a different model sensor, and using different electronics and software to process the signal.
Calibrating this sensor using a different gas source to the others
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. This method has the added advantage of allowing calibration at higher oxygen partial pressure than 1 bar.[6] This procedure may be done automatically, where the system has been designed to do it, or the diver can manually perform a diluent flush at any depth at which the diluent is breathable to compare the cell PO2 readings against a known FO2 and absolute pressure to verify the displayed values. This test does not only validate the cell. If the sensor does not display the expected value, it is possible that the oxygen sensor, the pressure sensor (depth), or the gas mixture FO2, or any combination of these may be faulty. As all three of these possible faults could be life-threatening, the test is quite powerful.[6]
Testing
A galvanic oxygen cell checkerHyperbaric oxygen cell testing pressure pot
The first commercially available certified oxygen cell checking device was launched in 2005 by Narked at 90,[11] 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.[13] Narked at 90 Ltd also won the Innovation Award for "an technical diving product that has made diving safer" at EUROTEK.2010 for their Oxygen Cell Checker.Archived 2021-01-23 at the Wayback Machine . The Cell Checker has been used by organisations such as Teledyne, Vandagraph,[citation needed]National Oceanic and Atmospheric Administration,[11] NURC (NATO Undersea Research Centre),[11] and Diving Diseases Research Centre.[11]
A small pressure vessel for hyperbaric testing of cells is also available in which a pressurised oxygen atmosphere of up to 2 bar can be used to check linearity at higher pressures using the electronics of the rebreather.[14]
Hypoxia is a condition in which the body or a region of the body is deprived of adequate oxygen supply at the tissue level. Hypoxia may be classified as either generalized, affecting the whole body, or local, affecting a region of the body. Although hypoxia is often a pathological condition, variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise.
Trimix is a breathing gas consisting of oxygen, helium and nitrogen and is used in deep commercial diving, during the deep phase of dives carried out using technical diving techniques, and in advanced recreational diving.
A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another. Transducers are often employed at the boundaries of automation, measurement, and control systems, where electrical signals are converted to and from other physical quantities. The process of converting one form of energy to another is known as transduction.
A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. Oxygen is added to replenish the amount metabolised by the user. This differs from open-circuit breathing apparatus, where the exhaled gas is discharged directly into the environment. The purpose is to extend the breathing endurance of a limited gas supply, while also eliminating the bubbles otherwise produced by an open circuit system. The latter advantage over other systems is useful for covert military operations by frogmen, as well as for undisturbed observation of underwater wildlife. A rebreather is generally understood to be a portable apparatus carried by the user. The same technology on a vehicle or non-mobile installation is more likely to be referred to as a life-support system.
A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common and only natural breathing gas, but other mixtures of gases, or pure oxygen, are also used in breathing equipment and enclosed habitats. Oxygen is the essential component for any breathing gas. Breathing gases for hyperbaric use have been developed to improve on the performance of ordinary air by reducing the risk of decompression sickness, reducing the duration of decompression, reducing nitrogen narcosis or allowing safer deep diving.
Gas blending for scuba diving is the filling of diving cylinders with non-air breathing gases such as nitrox, trimix and heliox. Use of these gases is generally intended to improve overall safety of the planned dive, by reducing the risk of decompression sickness and/or nitrogen narcosis, and may improve ease of breathing.
An oxygen sensor (or lambda sensor, where lambda refers to air–fuel equivalence ratio, usually denoted by λ) or probe or sond, is an electronic device that measures the proportion of oxygen (O2) in the gas or liquid being analyzed.
Gordon Smith was an inventor, machinist and tool and die maker notable for inventing the KISS SCUBA diving rebreather.
Latent hypoxia is a condition where the oxygen content of the lungs and arterial blood is sufficient to maintain consciousness at a raised ambient pressure, but not when the pressure is reduced to normal atmospheric pressure. It usually occurs when a diver at depth has a lung gas and blood oxygen concentration that is sufficient to support consciousness at the pressure at that depth, but would be insufficient at surface pressure. This problem is associated with freediving blackout and the presence of hypoxic breathing gas mixtures in underwater breathing apparatus, particularly in diving rebreathers.
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A solid oxide electrolyzer cell (SOEC) is a solid oxide fuel cell that runs in regenerative mode to achieve the electrolysis of water by using a solid oxide, or ceramic, electrolyte to produce hydrogen gas and oxygen. The production of pure hydrogen is compelling because it is a clean fuel that can be stored, making it a potential alternative to batteries, methane, and other energy sources. Electrolysis is currently the most promising method of hydrogen production from water due to high efficiency of conversion and relatively low required energy input when compared to thermochemical and photocatalytic methods.
Scuba gas planning is the aspect of dive planning and of gas management which deals with the calculation or estimation of the amounts and mixtures of gases to be used for a planned dive. It may assume that the dive profile, including decompression, is known, but the process may be iterative, involving changes to the dive profile as a consequence of the gas requirement calculation, or changes to the gas mixtures chosen. Use of calculated reserves based on planned dive profile and estimated gas consumption rates rather than an arbitrary pressure is sometimes referred to as rock bottom gas management. The purpose of gas planning is to ensure that for all reasonably foreseeable contingencies, the divers of a team have sufficient breathing gas to safely return to a place where more breathing gas is available. In almost all cases this will be the surface.
Scuba gas management is the aspect of scuba diving which includes the gas planning, blending, filling, analysing, marking, storage, and transportation of gas cylinders for a dive, the monitoring and switching of breathing gases during a dive, efficient and correct use of the gas, and the provision of emergency gas to another member of the dive team. The primary aim is to ensure that everyone has enough to breathe of a gas suitable for the current depth at all times, and is aware of the gas mixture in use and its effect on decompression obligations, nitrogen narcosis, and oxygen toxicity risk. Some of these functions may be delegated to others, such as the filling of cylinders, or transportation to the dive site, but others are the direct responsibility of the diver using the gas.
Rebreather diving is underwater diving using diving rebreathers, a class of underwater breathing apparatus which recirculate the breathing gas exhaled by the diver after replacing the oxygen used and removing the carbon dioxide metabolic product. Rebreather diving is practiced by recreational, military and scientific divers in applications where it has advantages over open circuit scuba, and surface supply of breathing gas is impracticable. The main advantages of rebreather diving are extended gas endurance, low noise levels, and lack of bubbles.
Gas blending is the process of mixing gases for a specific purpose where the composition of the resulting mixture is specified and controlled. A wide range of applications include scientific and industrial processes, food production and storage and breathing gases.
A mixed oxidant solution (MOS) is a type of disinfectant that has many uses including disinfecting, sterilizing, and eliminating pathogenic microorganisms in water. An MOS may have advantages such as a higher disinfecting power, stable residual chlorine in water, elimination of biofilm, and safety. The main components of an MOS are chlorine and its derivatives, which are produced by electrolysis of sodium chloride. It may also contain high amounts of hydroxy radicals, chlorine dioxide, dissolved ozone, hydrogen peroxide and oxygen from which the name "mixed oxidant" is derived.
The following index is provided as an overview of and topical guide to underwater diving:
A Diving rebreather is an underwater breathing apparatus that absorbs the carbon dioxide of a diver's exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. Oxygen is added to replenish the amount metabolised by the diver. This differs from open-circuit breathing apparatus, where the exhaled gas is discharged directly into the environment. The purpose is to extend the breathing endurance of a limited gas supply, and, for covert military use by frogmen or observation of underwater life, to eliminate the bubbles produced by an open circuit system. A diving rebreather is generally understood to be a portable unit carried by the user, and is therefore a type of self-contained underwater breathing apparatus (scuba). A semi-closed rebreather carried by the diver may also be known as a gas extender. The same technology on a submersible or surface installation is more likely to be referred to as a life-support system.
1 2 IMCA D 022 (May 2000), The Diving Supervisor’s Manual"Archived copy"(PDF). Archived from the original(PDF) on 2011-10-17. Retrieved 2011-11-06.{{cite web}}: CS1 maint: archived copy as title (link)
↑ Goble, Steve (2003). "Rebreathers". South Pacific Underwater Medicine Society Journal. 33 (2): 98–102. Archived from the original on 2009-08-08. Retrieved 2009-03-20.{{cite journal}}: CS1 maint: unfit URL (link)
↑ Lang, M.A. (2001). DAN Nitrox Workshop Proceedings. Durham, NC: Divers Alert Network. p.197. Archived from the original on October 24, 2008. Retrieved 2009-03-20.{{cite book}}: CS1 maint: unfit URL (link)
↑ IMCA D030 Rev. 1, (August 2005); Surface Supplied Mixed Gas Diving Operations"Archived copy"(PDF). Archived from the original(PDF) on 2011-10-17. Retrieved 2011-11-06.{{cite web}}: CS1 maint: archived copy as title (link)
↑ Vann RD, Pollock NW, Denoble PJ (2007). NW Pollock, JM Godfrey (eds.). "Rebreather Fatality Investigation". Proceedings of the American Academy of Underwater Sciences. Diving for Science 2007 (Twenty-sixth annual Scientific Diving Symposium). Dauphin Island, Ala.: American Academy of Underwater Sciences. ISBN978-0-9800423-1-3. Archived from the original on November 22, 2008. Retrieved 2009-03-20.{{cite journal}}: CS1 maint: unfit URL (link)
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