TECHNICAL FIELD The present invention relates in general to the field of diving and diving rebreathers, and more particularly, to the analyses of partial pressure of oxygen, PP02, sensor signals in electronic closed circuit rebreathers, eCCR.

BACKGROUND Electronically controlled rebreathing apparatuses have been on the market for a long time now, and available for a big enthusiastic audience mainly within diving. The purpose of a rebreathing system 10, as shown in fig 16, is to control the gas content of a typical breathing loop consisting of a counter bellow 1 that receives the exhaled gas. The exhaled gas needs to be cleaned from carbon dioxide, CO2, and this is usually performed by some sort of CO2-scrubbing mechanism 2. The exhaled gas also needs to be replenished to compensate for the consumed oxygen. This is usually done using a gas injection technique employing some sort of mechanically controlled valve or, if an electronic control system is present, by an electronically controlled solenoid or other means such as needle valve. The electronic control system is relying on the gas injection technique and a multitude of sensors to uphold life-supporting function for the diver.

Current control systems could incorporate:

- solenoid valve, or other means for gas injection, for controlling the gas levels in the loop;

- O2-sensors, or other means, to measure the oxygen content in the loop; - loop/ambient pressure sensors to measure pressures;

- PC02-sensors to measure pressure of carbon dioxide, PC02, in the loop; and

- advanced microcomputers to calculate decompression algorithms etc.

However, the accuracy and reliability is sometimes weak, especially for the PP02 monitoring, and often needs to include redundant sensors. Examples of semi-closed circuit and closed circuit rebreather are presented in, for example the patents US 5,503, 145 and US 6,302, 106.

The benefits compared to traditional self-contained breathing apparatuses with open circuit systems are many. Some examples are reduced gas- consumption and the ability to breathe an optimal oxygen/diluent blend at the current depth. However, there are some disadvantages of current electronic rebreathers where mainly the PP02-sensors are described as a weak link. If they are abused, aged or blocked, the signal cannot be described as reliable. The user could in worst case result in that the diver is exposed to hyperoxia or hypoxia which is what the control-system primary is developed to avoid.

The design of such a control system must therefore be of such robustness that this never happen, because of the severeness of the outcome, which could be fatal for the diver. Hypoxia could lead to unconsciousness and is mostly spoken of in situations of hypobaric environments but can also occur if breathing from a closed circuit. In the case of being underwater, it could lead to drowning. Hyperoxia occurs when a user is exposed to an excessive level of oxygen pressure and is mainly present in hyperbaric environment. Hyperoxia could cause oxygen toxicity, which may lead to a fatal cause of events.

Acceptable levels of PP02 are usually thought to be within 0.16-1 .6 bar and are described as the product of oxygen fraction, F02, and ambient pressure measured in bars. In water the pressure increases with 1 bar per 10 meters. At surface the pressure absolute is 1 bar and altitude decreases the ambient pressure. An example is while at surface sea level and breathing air (assuming average sea-level pressure being 1 bar and F02=21 %) the PPO2=0.21 bar, while at ten meters of water depth the pressure has increased to ~2 bar and the PP02 has also increased by a factor 2 and is -0.42 bar.

Even if the human body could manage these variations in PP02, there are other factors that make it important to know the actual PP02 at your actual depth. If one is heading for a higher ambient pressure than the current, it is necessary to be aware of the expected PP02 related to what gas is brought. Unfortunately, the human body also picks up diluent (the gas that is not oxygen, normally nitrogen) during the pressure increase, which must be ventilated from the tissues before returning or going to lower pressure. Otherwise hyperbaric/decompression illness could occur. Another risk with changing to a lower ambient pressure is if the FO2 in the breathing loop is low and the ambient pressure drops, a hypoxic situation might occur. Benefits with having a high PPO2 i.e. a low partial pressure of diluent could make the decompression time shorter. If the sensor system or the signal system fails, the breathing loop might become hyperoxic or hypoxic. With the above in mind it is essential to be aware of the breathing loops PPO2 and thus be able to trust the sensors in the diver ^' s life-support system.

There are currently several approaches to address the fact that PPO2-sensor reading in breathing circuits might be erroneous or unreliable due to a) unlinearity; b) current limitation: when the PPO2-sensor becomes nonlinear above a certain level of PPO2 since the output current of the sensor (or the output voltage) due to an error cannot rise over a certain level. This results in too low sensor signals for high levels of PPO2; c) erroneous signal from one or more sensors respectively whichever the sensor signal is processing, where the sensors may be blocked by moisture, aging, system fault etc; and d) erroneous calibration, where the user has calibrated the sensor-system with erroneous reference gas, loop gas or pressure.

Some manufacturers use redundancy with two or more sensors as shown in patent US 6,712,071 . If the system incorporates three sensors it is also possible to use voting logic that excludes any sensor that doesn't agree with a majority of the others two. Another approach is to look at the reaction at the sensor after flushing it with a known gas and thereby validating the output.

The validation method is currently used and patented by Poseidon (US201 10041848, US20070215157) and Arne Sieber (US20100313887) and described as follows. The PP02-sensors are calibrated at surface using 100% oxygen, which can indicate a maximum partial pressure of oxygen, PP02, to 1 .0 bar. A normal set point is 1 .2-1 .3 bar and anticipated to measure up to 1 .6 bar. A set point is the level of PP02 which the OCS is regulating towards, by injecting oxygen to increase the set point or by letting the diver's metabolism consume the oxygen, alternatively purge with diluent gas to decrease the oxygen partial pressure. The set point is often above atmospheric pressure and thus the sensors are not calibrated at the set point before the dive. What set point to choose depends on several aspects but the higher the set point the shorter decompression stops the diver has to do before surfacing, but the higher the risk of oxygen toxicity. The set point can be chosen manually or automatically depending on which the user prefers and physiological limitations This means that the actual functioning pressure of oxygen, P02, is never achieved during calibration and it cannot be determined whether the sensors are erroneous above PP02 1 .0 bar. To address this problem a solenoid injects gas at depth and at 6 meters it is possible to receive a PP02 signal equivalent to 1 .6 bar. This also determines whether a correct calibration has been done. This is only done in this depth area and normally during descent and this is also the only time when the sensor is tested in the upper range PP02>1 bar and this cannot be done at deeper depths.

During the dive a similar flush with diluent can be done to rinse humidity from the sensor and to determine the step answer and the linearity of the sensor, especially if the expected PP02 from the diluent is lower than actual. The method used in Poseidon mkVI rebreather system is based on checking the response of the sensor with diluent flush, see the White Paper by Poseidon "A New Approach to Closed Circuit Rebreather Gas Monitoring: Why Two Oxygen Sensors can be Better than Three". However, since this method uses a gas with lower PPO2 than the setpoint it does not determine the validity of the sensor at that specific setpoint of PPO2.

Previous patent US6, 302, 106 (entitled Rebreather system with optimal PPO2 determination) describes a method for injecting oxygen and diluent at a certain rate to achieve a perfect blend with optimal PPO2 for the particular ambient pressure using a digital signal-processing unit. This, however, has no bearing on sensor validation.

In patent US20030188744 (entitled Automatic control system for rebreather) it is shown that an automatic control system for a rebreather with improved life-supporting characteristics can be designed. However, the system analysis of the oxygen sensor signal is not significantly different from others and described by having a microcontroller that takes the median of the two closest signals as being the true oxygen value. The result is used to maintain an accurate PPO2 in the breathing circuit. The system of US20030188744 do not detect whether multiple sensors are presenting an incorrect signal.

The patent WO 2012025834 (entitled Rebreather control parameter system and dive resource management system) incorporate a method for controlling the PPO2-setpoint in the breathing loop of an underwater rebreathing apparatus related to ambient pressure. The user has the possibility to control the settings during the dive. In this method, the diver can specify some input values for a control parameter like a minimum and maximum value of partial pressure of oxygen and a concentration of oxygen in a gas supply. The maximum operating value of partial pressure of oxygen is calculated as a function of ambient pressure and concentration of oxygen in the gas supply. However, this system does not integrate the characteristics of the PP02- signal and cannot be said to increase accuracy of the control system or signal management.

Thus, a method and system that can validate the 02 sensor during the whole usage, at high PPO2 >1 .0 bar is therefore highly sought after.

SUMMARY OF THE INVENTION

With the above description in mind, then, an aspect of some embodiments of the present invention is to provide a method and a system for determining the validity of an oxygen, 02, sensor, an oxygen control system, OCS, in rebreathers, which seeks to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination. With the suggested method, it is possible to address issues described above without increasing the mechanical complexity of the system, in the case where an electronically controlled system already is present. According to some aspects the disclosure provides for a method for determining the validity of an oxygen, 02, sensor, wherein the 02 sensor is comprised in a breathing apparatus used for diving purposes

According to some aspects, the method comprises the steps of retrieving one or more partial pressure of oxygen, PPO2, signals from the O2-sensor, comparing the PPO2 signal to a calculated PPO2 signal, determining the validity of the retrieved PPO2 signal based on the comparison, determining the validity of the 02 sensor based on the validity of the PPO2 signal and wherein the current operation of the breathing apparatus is updated based on the validity of the 02 sensor. By performing these steps, a way to provide validity control of each 02 sensor included in breathing apparatus separately is provided.

According to some aspects, the method comprises measuring the time interval to reach a determined set point of the PPO2 signal and wherein the validity of the PPO2 signal is based on the time to reach the determined set point.

According to some aspects, the step of comparing comprises determining whether the retrieved PPO2 signal is coherent and reasonable or if the PPO2 signal is deviating from the calculated PPO2 signal based on the comparison. According to some aspects, wherein the PPO2 signal is deviating from the calculated PPO2 signal, the method comprises changing a first PPO2 set point to a second PPO2 set point, measuring the time interval to reach the second PPO2 set point by measuring at least one of a breathing volume of the breathing apparatus, an oxygen injection rate, an oxygen injection volume, an oxygen injection flow and an oxygen consumption, calculating an incline of a graph presenting the values of the PPO2 signal collected over the time interval for the PPO2 set point change and determining the validity of the PO2 signal based on the calculated incline of the graph.

According to some aspects, the method comprises detecting at least one of an oxygen injection rate, oxygen injection volume, oxygen injection flow, breathing loop volume or oxygen consumption of the breathing apparatus, comparing the current amplitude of the PPO2 signal to an amplitude of a calculated PPO2 signal related to at least one of the detected oxygen injection rate, oxygen injection volume, oxygen injection flow, breathing volume and oxygen consumption of the breathing apparatus and determining the validity of the PPO2 signal based on the comparison.

According to some aspects, the method comprises initiating a test cycle, which is performed by altering the current PPO2 set point to either a lower or higher value, when each value is reached the PPO2 set point is changed to the opposite side of the current PPO2 set point, registering the time to perform the test cycle and determining the validity of the PPO2 signal based on the registered time.

According to some aspects, the breathing apparatus comprises an oxygen control system, OCS, wherein the method comprises loading the OCS with data related to the breathing apparatus and the dive to be performed, wherein the OCS is configured to predict expected characteristics of the PPO2 signal from an 02 sensor of the OCS, based on the loaded data, calculating S420, by OCS, characteristics of the PPO2 signal, both previous and actual, during the dive performed by the diver, comparing S430, by OCS during the dive, the calculated characteristics of the PPO2 signal with the expected characteristics of the PPO2 signal and determine whether previous and actual calculated characteristics of the PPO2 signal are valid or if the PPO2 signal has changed characteristics based on the comparison. According to some aspects, a PPO2 set point is calculated for the OCS and the method comprises comparing at least one of the calculated PPO2 value, an amplitude of the PPO2 signal, a sawtooth pattern of a graph of the calculated PPO2 values and a speed of change of PPO2 set point with expected values of PPO2 values, amplitude of the PPO2 signal, sawtooth pattern of a graph of PPO2 values and speed of change of PPO2 set point and determining the validity of the PPO2 signal based on the comparison.

According to some aspects, the method comprises collecting volume information from a solenoid injection of 02, wherein the total breathing volume of the OCS is calculated by adding the collected volume information to the existing breathing volume.

Another aspect of the disclosure provides for an oxygen control system, OCS, comprised in a breathing apparatus used for diving purposes, for determining the validity of an oxygen, 02, sensor, comprised in the OCS. According to some aspects the OCS is configured to perform the above discussed method.

The features of the above-mentioned embodiments can be combined in any combinations.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the present invention will appear from the following detailed description of the invention, wherein embodiments of the invention will be described in more detail with reference to the accompanying drawings, in which:

Fig 1 a shows a graph of the actual PPO2 value, the OCS believed PPO2 value and a functioning OCS variation;

Fig 1 b shows a graph of characteristics of a linear and an erroneous PPO2 sensor in the case of fig 1 a; Fig 2a show a graph of the actual PPO2 value, the OCS believed PPO2 value and a functioning OCS during a PPO2 increase during descent or set point change;

Fig 2b shows a graph of characteristics of a linear and an erroneous PPO2 sensor in the case of fig 2a; Fig 3a shows a graph of how the amplitude will differs for a functioning sensor and an erroneous calibrated sensor that was calibrated with air for PPO2=0.21 bar and 80% oxygen at PPO2=1 bar;

Fig 3b shows a graph of characteristics of a linear and an erroneous PPO2 sensor in the case of fig 3a; Fig 4a shows a graph of how the characteristics of the PPO2-reading will deviate between a functioning and an erroneous calibrated sensor; Fig 4b shows a graph of characteristics of a linear and an erroneous PP02 sensor in the case of fig 4a;

Fig 5a shows a graph of a sensor characteristic that presents a maximum output for a functioning and erroneous sensor; Fig 5b shows a graph of characteristics of a linear and an erroneous PP02 sensor in the case of fig 5a;

Fig 6a shows a graph of the deviation between an erroneous and functioning OCS when V02 is 2-2.5 l/min;

Fig 6b shows a graph of characteristics of a linear and an erroneous PP02 sensor in the case of fig 6a;

Fig 7 shows a graph of the amount of oxygen added in a functioning and erroneous OCS;

Fig 7b shows a graph of characteristics of a linear and an erroneous PPO2 sensor in the case of fig 7a; Fig 8 shows a graph of the actual values of PPO2 for a functioning OCS;

Fig 9a shows a graph of any possible readings independent of VO2 for a specified system volume;

Fig 9b shows a graph of the depth in the case of fig 9a;

Fig 10a shows a graph of PPO2 values when performing a test-cycle with a PPO2 set point change;

Fig 10b shows a graph of characteristics of a linear and an erroneous PPO2 sensor in the case of fig 10a;

Fig 13 shows a graph of characteristics of the OCS sensor signal for oxygen consumption between 0.3-4 l/minute; Fig 14 shows a graph of the saw tooth pattern of a functioning OCS and an erroneous OCS;

Fig 15 shows a graph of an erroneous and a functioning PPO2-sensor;

Fig 16 shows a rebreathing apparatus, rebreather, according to an embodiment of the present disclosure;

Fig 17 shows a flowchart illustrating the method according to an embodiment of the present disclosure;

Fig 18 shows a flowchart illustrating the method according to an embodiment of the present disclosure; Fig 19 shows a flowchart illustrating the method according to an embodiment of the present disclosure;

Fig 20 shows a flowchart illustrating the method according to an embodiment of the present disclosure;

Fig 21 shows a flowchart illustrating the method according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

Embodiments of the present invention relate, in general, to the field of diving equipment and particularly to the field of analyzing the output of PPO2 sensor signals in systems used for diving purposes. A preferred embodiment relates to rebreathers, such as a closed circuit rebreather. However, it should be appreciated that the invention is as such equally applicable to other similar diving systems such as semi-closed circuit rebreather and other diving and breathing applications where determining if an oxygen sensor reading is correct or not is important. However, for the sake of clarity and simplicity, most embodiments outlined in this specification are related to an electronic closed circuit rebreather, eCCR. Embodiments of the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference signs refer to like elements throughout.

To address the problem of an erroneous PP02 signal coming from a faulty 02-sensor of the eCCR there are currently multiple methods on the market as previously described in the background section. The method, according to an embodiment of the present invention, is depending on an oxygen control system, OCS, to collect and analyze one or multiple PP02-sensor signals independent of type of 02-sensor. The method according to the present invention is based on analyzing the characteristics of the retrieved PPO2- signals. This is performed by analyzing the behavior of the PPO2-signal retrieved by the system and/or determining the validity of the PPO2 signal compared to any one or more of the ambient pressure change, solenoid injection volume, rate or flow, pressure decrease in an oxygen or diluent cylinder, gas injection from automatic diluent valve, breathing frequency of the diver, heart rate of the diver, flow rate in breathing loop etc. According to another aspect, the determination of the validity of the PPO2 signal compared to the expected output from the 02-sensor of the eCCR can be done by analyzing a change of a PPO2-setpoint. While using the inputs as described above it is possible to determine whether the sensors are giving a coherent and reasonable signal or if they are deviating from what is expected. The deviations can be described as not following a predicted pattern of output, see figure 8. The graph presented in figure 8 shows possible PPO2 values for a fully functional eCCR, with PPO2 setpoint at 1.3/0.7 bar, FO2 diluent at 7%, total volume 18.6 L, maximum solenoid injection 8.4 L/m. In this case the expected PPO2-signal is predetermined or calculated. The PPO2-sensor values are depending on the PPO2 setpoint which normally is set to 1 .2-1 .3 bar. The setpoint can be chosen manually or automatically depending on which the user prefers and physiological limitations. If the setpoint is changed, manually or automatically by the OCS (normally 0.7 bar at shallow depths <10 m), the time for the change will to a large degree be determined by the system volume and the oxygen injection rate, injection volume and injection flow and the oxygen consumption. The incline of the graph presenting the PPO2-values during setpoint change are expected to be linear, and steep with a functional oxygen control system (OCS) of the eCCR, whereas the graph presenting the PPO2 values of an erroneous OCS could fade in the upper region (>1 bar). The amplitude of the PPO2 signal at the current setpoint is related to the oxygen injection rate, volume, flow and the breathing volume of the eCCR. The amplitude of the PPO2 signal is also dependent on configuration of the OCS and on the oxygen consumption. By knowing these factors, it is possible to see that a correct OCS will provide different amplitude of the PPO2 signal than an erroneous OCS, if the predetermined settings are similar, see figure 1 a-b, 3 a-b and 5 a-b. Figure 1 a-b graphically presents possible PPO2 values for a functional eCCR with PPO2 set point at 1 .3/0.7 bar, FO2 diluent at 21 %, total system volume of 10-18 L, maximum solenoid injection of 9.4 L/min. Figure 3a-b and figure 5a-b graphically presents possible PPO2 values for a functional eCCR with PPO2 set point at 1 .3/0.7 bar, FO2 diluent at 21 %, total system volume of 10-18 L, VO2 at 0.3-3.9 L, maximum solenoid injection of 9.4 L/min. If the OCS is unable to accept higher PPO2 signal than a certain current (i.e. current limitation), see figure 5 a-b, this is also revealed as a lower amplitude than expected. When the OCS is unable to follow expected PPO2 values during a pressure drop or an increase, see figure 2 a- b, 4 a-b and 6 a-b, the OCS will interpret this in a similar way as a setpoint change. Figure 2a-b and 4 a-b graphically presents possible PPO2 values for a functional eCCR with PPO2 set point at 1 .3/0.7 bar, FO2 diluent at 21 %, total system volume of 10-18 L, VO2 at 0.3-3.9 L, maximum solenoid injection of 9.4 L/min. Figure 6a-b graphically presents possible PPO2 values for a functional eCCR with PPO2 set point at 1.3/0.7 bar, FO2 diluent at 21 %, total system volume of 10-18 L, VO2 at 2-2.5 L, maximum solenoid injection of 9.4 L/min. The incline of the graph of PPO2-values during setpoint change are expected as correlated to the depth change and diluent compensation with functional OCS, whereas an erroneous system has a different time to reach the new setpoint and thereby also a different incline of the graph of PPO2 values. If the OCS is unable to find a definite reliable and trustworthy PPO2 signal it is possible to initiate a manual or automatic full or half test cycle. This is performed by altering the actual setpoint to either a lower or higher value. When this value is reached the setpoint is changed to the opposite side or level of the original setpoint. By registering the time it takes to perform this test-cycle it can be determined if the system is erroneous or not. A fully functional system will typically have a shorter duration and less oxygen injection than an erroneous for the test-cycle, see figure 10 a-b. Figure 10 a-b graphically presents possible PPO2 values for a functional eCCR with PPO2 set point at 1 .3/0.7 bar, FO2 diluent at 21 %, total system volume of 10-18 L, VO2 at 2-2.5 L, maximum solenoid injection of 9.4 L/min. As mentioned in the description of the test-cycle above it is also possible to retrieve information on the gas injection rate, the gas injection volume and gas injection amount. The characteristics of larger amount injected oxygen is graphically presented in figure 7a-b where it is shown how identical diver's profiles, oxygen consumption rates and setpoint changes lead to different amount of oxygen injected.

The intelligence of the OCS relies on the accuracy of the oxygen, O2, sensors. This includes more information than the actual sensor-reading. By looking at the dynamics of the signal it can be determined if the diver needs to be warned or if the system is correct. The work-flow, as shown in figure 21 , for determining the dynamics of the signal from the O2 sensor can be described as follows: In step S410 the OCS is loaded with data to predict the expected characteristics or values for a PPO2 sensor signal from an O2 sensor of the OCS, i.e. the PP02-levels, amplitudes, sawtooth pattern and PP02-setpoint change speed, for example a linear sensor. This means that the behavior of the signal is known and pre-determined or calculated. The data is related to the breathing apparatus, i.e. total breathing volume and the dive to be performed, i.e. planed diving depth, time.

In step S420, the calculations of, both historically, actual and future PP02- values of the PP02 signal must be determined during the dive performed by a diver, since the OCS doesn't know the upcoming dive profile.

In one aspect, the OCS is also loaded data related to the oxygen fraction, FO2, in a gas-supply of the breathing apparatus.

In one aspect, additional information regarding max and min breathing loop volume is also of interest as optional input to increase accuracy of the expected 02 sensor signal output.

In step S430, depending on the set point for PPO2 that is chosen for the OCS at the actual depth, the PPO2-levels, amplitudes, sawtooth pattern and PPO2-setpoint change speed of the PPO2 signal will be compared to the expected the PPO2-levels, PPO2-amplitudes, PPO2-sawtooth pattern and PPO2-setpoint change speed.

In one aspect, additionally, the OCS also records data to compare whether previous and actual data values are valid or if the OCS related data has changed.

By collecting information from solenoid injection volume and rate as shown in figure 7, and from the saw tooth pattern of the PPO2 signal from the 02 sensors, as shown in figure 14, it is possible to approximate the system breathing volume. By determining the graphical area of possible PPO2 values it is possible to decide an optimal solenoid injection flow rate. It is important to determine the volume of the system in order to be able to predict the sensor-output. This can be done by assuming the total variation of the system volume, i.e. approximation of the volume of the lungs of the diver and the rebreathers loop volume, and use it as a one compartment model. One method is to measure the flow in the breathing volume loop 3, as shown in figure 16. This flow, together with a time-value, can be used as an estimation of the system volume, as long as there is no oxygen consumption between the oxygen injection and the oxygen sensor. The amount of injected oxygen and the flow are in this case the main determinants of the oxygen variation. One method is to estimate the flow in the breathing loop 6 (i.e. divers ventilation) without a flow-sensor is to determine the oxygen consumption and the known relationship between the oxygen consumption and the user's ventilation.

In one example: The sensor is measuring a specific PPO2 value which can be translated to FO2 with an ambient pressure sensor. The volume between the O2-injection and the sensor is fixed (for example the volume of canister). This volume does not change during the dive. If a flow sensor is measuring x l/min over the area where it is mounted one can calculate the expected increase of one or more of PPO2 and FO2 when injecting a known amount of oxygen.

To determine the oxygen consumption, rate a number of methods can be employed. In one method the pressure drop in the oxygen cylinder related to temperature is measured. In another method the oxygen consumption from breathing frequency could be estimated. In yet another method the oxygen consumption is determined from the heart rate of the diver. In yet another method the oxygen consumption rate is determined from how much oxygen that is injected into the breathing loop 3 by the oxygen injection system. In yet another method the time it takes for the PPO2-value to return to the same level as previous of an oxygen injection is determined.

The results related to the proposed method and system will now be described referring to figures 1 -15. As previously discussed, the disclosure provides a method 100 and system 10 for determining the validity of an oxygen, 02, sensor, wherein the 02 sensor is comprised in a breathing apparatus used for diving purposes.

General valid values (predicting PP02)

According to the present invention, it is possible to determine whether a signal is correct or erroneous depending on predetermined data given by the user, system/breathing apparatus specific data and sensor data. During current operation, it is possible to do a prediction of the PPO2 signal or PPO2-value. It is also possible to analyze the PPO2-data recorded during the actual dive. By recording the PPO2-data it can be determined whether the sensor presents any differences, that could be found from the historical PPO2-data and whether the OCS signal deviates.

General valid values (predicting PP02)

According to the present invention, it is possible to determine whether a signal is correct or erroneous depending on predetermined data given by the user, system/breathing apparatus specific data and sensor data. During current operation, it is possible to predict the PPO2-signal or PPO2-value. It is also possible to analyze the PPO2-data recorded during the actual dive. By recording the PPO2-value it can be determined whether the sensor presents any differences, that could be found from the historical PPO2-data and whether the OCS signal deviates over time.

By picking or collecting a new value sample of a PPO2 signal with a predetermined interval it can be determined whether the sensor reading is incorrect, by looking at the decrease of PPO2, related to ambient pressure. If the slope is flat the oxygen consumption is low, if the slope is steep the consumption is high. This gives a specific characteristic depending on how accurate the oxygen consumption is determined. The pattern is shown in figure 13 for oxygen consumption between 0.3-4 l/minute (with FO2 diluent of 7%). Figure 13 shows how new sample values of a PPO2 signal are collected every second minute (120 second), from this point the characteristics of the OCS sensor signal should stay within the plotted area for consumptions between 0,3-4 l/min.

When predicting the PP02 in the breathing loop it is thereby possible to find an optimal injection rate for the solenoid injection. An optimal solenoid injection is presented as a small area/amplitude of possible values when using the prediction method.

PP02-signal characteristics

The variation in amplitude between a functioning and erroneous sensor is depending on system volume, oxygen consumption and solenoid or manual injection quantity of oxygen. The amplitude difference is different if the PP02- sensor is functioning compared to if there is a nonlinear- or current limit- problem or has an erroneous calibration. If the sensor is static the amplitude change is obviously very low. The amplitude changes from low value to higher value correlates to the solenoid injection timing and opening time as well as maximum flowrate. It also correlates to ambient pressure increase where automatic diluent addition should be considered. The decrease of PP02-sensor signal correlates to a pressure decrease or an oxygen consumption. These characteristics can be analyzed and separate a fully functioning sensor from an erroneous sensor. Test-cycle

In one aspect, a potentially erroneous calibration, linearity problem, static behavior and maximum output limitation is determined by analyzing the behavior of the sensor when changing setpoint for a short period. All of the above described methods for finding erroneous sensor-reading will become more accurate if additional sensors are present, such as flow-rate in the breathing loop, pressure from cylinders, the heart rate of the diver etc.

If the OCS cannot detect any specific characteristics or if the signals are within a value where it can have multiple interpretations, it is possible to perform a test-cycle. The test-cycle is performed by changing the setpoint to a different value from the current setpoint. When the preset level for high or low PP02 test value is reached another setpoint is chosen on the opposite level from the origin. The time and characteristics for the signal to travel along this test-cycle is used to determine whether the signal is correct or erroneous as shown in figure 10. The method used is to compare how long time the test-cycle should take compared to the actual time for the test cycle. If the duration is longer than expected an error can be determined. The characteristics are unlinear for an erroneous sensor-signal as compared to a linear response for a functioning sensor. Oxygen injection pattern and quantity

The injection of oxygen correlates to the consumption of oxygen and/or a setpoint change and ambient pressure change. When oxygen is injected by the control system it is possible to determine how much oxygen is added. The injection should give a corresponding signal from the PP02-sensor and OCS similar to a sawtooth-pattern as shown in figure 14. Figure 14 shows the sawtooth pattern of possible PP02 values for a fully functioning OCS (solid) and an erroneous OCS (dashed) where PPO2 setpoint is 1 .3/0.7, FO2 diluent is 7%, the total system volume is 16-18 L, VO2 is 1 .5 L and maximum solenoid injection is 9.4 L/m. The amplitude is clearly lower for the erroneous sensor. The PPO2-sensor characteristics are shown in figure 15. Figure 15 shows the characteristics of an unlinear sensor (dashed) and a functioning sensor (solid) which is linear.

The quantity of oxygen that is injected is also of interest. If the expected amount of oxygen to achieve a certain setpoint is different from the actual amount of oxygen, then there is an error that can be related to an erroneous PPO2-sensor signal. As is shown in figure 7a-b where the dotted area shows the amount of injected oxygen for and the solid area is less and is showing the injected amount of oxygen for a functioning system. Method to adapt an erroneous signal to ensure accurate PP02-control

If the signal is proven to be erroneous it is also possible to adjust and adapt the values through a signal processing unit that can determine a correction for the erroneous signal. If the correction is found adequate the user doesn't have to abort, but can continue the usage. The signal is adapted in accordance to how the sensor error is analyzed. An alternative action from the OCS is to decrease the setpoint to a level where it can be trusted and instruct the user to abort the usage. If it is suggested that the sensors are reliable new iterations to determine the validity of the readings should continue.

Unlinear Sensor

For an unlinear PPO2-sensor the amplitude for setpoints, in the range of inaccuracy, will differentiate from a correct sensor. In figure 1 a-b it is shown how the amplitude will differ. In figure 1 a-b, top graph, fig 1 a, shows the actual PPO2 value (dotted), the oxygen control system OCS believed PPO2 value (dotted fat) and a functioning OCS (solid) variation. By analyzing the max-min value of the PPO2 amplitude, the OCS can be analyzed to have an erroneous behavior. The bottom graph, fig 1 b, shows the erroneous sensor (unlinear) characteristics. The system volume is allowed to vary between 10-18 liters and the oxygen consumption VO2 is set to 0.3-4 l/min. For this type of error, we expect a high level of OCS error identification.

During a PPO2-increase during descent or setpoint change the characteristics of the PPO2-reading will vary depending on whether you have a functioning OCS or not. In figure 2 a-b it is shown how the characteristics deviate, however only slightly for this error.

In figure 2 a-b an unlinear sensor is show, which will not necessarily be detected during setpoint change. The difference between the characteristics are shown in the figure where the actual PPO2 value (dotted), the oxygen control system OCS believed PP02 value (dotted fat) and a functioning OCS (solid) are presented. The predicted data is more or less overlapping.

Erroneous Calibrated Sensor

For an erroneous calibrated PP02-sensor the amplitude for setpoints, in the range of inaccuracy, will differentiate from a correct sensor. In figure 3 a-b it is shown how the amplitude will differ for a sensor that was calibrated with air for PPO2=0.21 bar and 80% oxygen at PP02=1 bar. As shown in figure 3 a- b, similar to a nonlinear sensor the characteristics will be different between a functioning sensor and an erroneous calibrated sensor. A PP02-increase during descent or setpoint change the characteristics of the PP02-reading will vary depending on whether you have a functioning OCS or not. In figure 4 a-b it is shown how the characteristics deviate. As shown in figure 4, during PPO2-increase the characteristics of the reading will deviate between functioning and erroneous calibrated sensor. Static Sensor

If the sensor is blocked by humidity or other reasons there will be little or no PPO2 sensor fluctuations in the sensor output. This means that there will be no PPO2-signal variations, which should occur as long as there is some sort of oxygen consumption. In figure 14 the dynamic is shown as a sawtooth pattern. If the signal doesn't have any dynamics an error can be expected. This type of error is not commonly analyzed in other systems as the normal OCS only reacts to a PPO2 below setpoint and then injects oxygen. This means that as long as the PPO2 is above setpoint OCS believes the system is ok, but the truth is that a system must have a variation in the PPO2 as long as the diver is consuming oxygen and oxygen is injected. The simplest method would be to take the interval between different oxygen injections. If the numbers of injections are fewer than expected, something is wrong with the PPO2 signal or OCS. Output Limitation

If the PP02-sensor has any limitation of the maximum output values this can be detected in similar way as previously described, i.e. low amplitude and little or no PP02-sensor fluctuation, which is shown in figure 5 a-b. Figure 5 a-b, with a sensor-characteristic that presents a maximum output for the sensor, the amplitude variation will be very small. This is shown with very small amplitude for erroneous sensor and relative large amplitude for functioning sensor. The rise in PP02-setpoint will not affect this type of error.

Increase accuracy If one can determine or at least approximate the OCS volume and the oxygen consumption it can be used to improve the accuracy of the OCS failure detection. In figure 6 a-b it is graphically shown that by determining the VO2 with +/- 0.25 bars the sensor failure is detected immediately during descent. Figure 6 a-b graphically shows the deviation between erroneous and functioning OCS when VO2 is better approximated. VO2 is to 2-2.5 l/min in the case shown in figure 6 a-b.

Also factors like dive depth and FO2 in diluent will affect the accuracy or possibility to detect an erroneous OCS. It is also claimed that the O2-injection can be used to determine an erroneous sensor. In figure 7 a-b a method is used to analyze the amount of oxygen injected from the number of solenoid injection and from that notice the difference between a correctly calibrated sensor and not. With a method, as shown in Figure 7 a-b, to determine the amount of oxygen added (either by solenoid injection rate or gas cylinder pressure decrease) one can see an obvious difference. For the erroneous calibrated sensor, an abnormal amount of oxygen is needed to increase the PPO2. In this case diluent of FO2 21 %, VO2 = 2-2.5 L/min, system volume 10-18 L is used. In one method, the PP02 value validity is determined by looking at the overall possible PP02 for a functioning oxygen control system OCS. If the OCS is functioning correctly the signal should follow a determined profile for PP02 related to the dive profile. In figure 8 it is shown how this profile could look. With the current described method, it is also possible to find an optimal maximum injection for the solenoid. The smaller area of possible values of PP02 values, the better tuned solenoid injection volume and rate.

Figure 8 graphically shows the actual values of PP02 for a functioning oxygen control system OCS. The PPO2 is held at near perfect settings and a value outside of this indicates a probable error in the overall system.

If the OCS retrieves a new PPO2-value at specific times it is possible to determine any possible future signal from that time, independent on oxygen consumption. The sooner a new value is retrieved and considered valid the better validity. If the PPO2 reading is outside of these determined levels, the OCS is erroneous and a warning is presented to the user. In figure 9 a-b any possible PPO2 values independent of VO2 for a specified system breathing volume is graphically presented. In the case presented in figure 9a-b, the OCS is expected to be fully functioning when a new reading of the PPO2 values, with VO2 = 0.3 - 4 L/min and FO2 Diluent of 7%, is performed each 2 minutes (120 seconds) and the OCS predicts any possible readings after that.

The prediction is defined by the PPO2 decrease from maximum (4L/min) and minimum (0.3 L/min) oxygen consumption. These boundaries create the area of valid PPO2 values, which is shown in figure 9 a-b. Test-cycle

A test-cycle is performed whenever the user finds it necessary (manual) or if the OCS finds the PPO2-values unreliable. This is performed by changing the setpoint, let the system adjust and then make another setpoint change opposite of the previous setpoint, see figure 10 a-b. The time it takes to perform this test cycle will reveal erroneous oxygen control systems.

By performing a test-cycle with a PP02-setpoint change, as shown in figure

10 a-b, it is possible to determine the validity of the sensor signal. In this case the test-cycle is expected to take maximum 4 minutes but could in this case take up 10 minutes for an erroneous sensor.

Figure 1 1 a-b shows an over-view of the different sensor signals for functioning and erroneous calibrated sensor for a fictive dive profile. Figure

1 1 a-b graphically presents possible PP02 values for a fully functional eCCR with PP02 set point at 1 .3/0.7 bar, F02 diluent at 21 %, total system volume of 10-18 L, V02 at 2-2.5 L, maximum solenoid injection of 9.4 L/min.

Figure 12 a-c presents PP02 values collected from a live, simulated oxygen consumption in a breathing simulator type ANSTI at the depth of 30 m with 02 consumption of 1.78 L/m. The OCS is an electronic rebreather with PPO2-control system running on full automatic. The other data presented are from calculated predictions from the algorithm, with knowledge of the oxygen injection flow rate, in this case approximated to 7.5 l/min. With this information, the oxygen consumption, total loop volume, ventilation, O2- cylinder pressure drop and total injected oxygen can be approximated and compared.

The oxygen control system OCS described here is used to determine erroneous PPO2-signals that could occur while using a rebreathing device. By analyzing the characteristics of the signals it is possible to separate a a characteristic of a fully functioning O2 sensor from an erroneous 02 sensor. Not only by looking at the actual PPO2-signal, but also at the amount of oxygen injected, related to oxygen consumption. The system is able to determine the error and both give the diver a warning and/or adjust the OCS so that a correct PPO2 in the breathing loop can be achieved. The proposed method will now be described referring to figures 16 - 21 . As previously discussed, the disclosure provides a method 100 and system 10 for determining the validity of an oxygen, 02, sensor, wherein the O2 sensor is comprised in a breathing apparatus used for diving purposes. Figure 16 shows a breathing apparatus according to some aspect of the disclosure. The figure illustrates a rebreathing apparatus 10, eCCR, comprising a counter bellow 1 , a CO2 absorber 2, a breathing volume 3, an oxygen control system 4, OCS, an output valve 5 and a breathing valve 6.

Figure 17 - 21 are flow diagrams depicting example operations which may be taken by the system 10 of figure 16. The operations of the flow diagram will be described together with the device of figure 16.

The method comprises retrieving S1 10 one or more partial pressure of oxygen, PPO2, signals from the O2-sensor and comparing S120 the retrieved PPO2 signal to a calculated PPO2 signal. In one aspect, the step of comparing S120 comprises determining S121 whether the retrieved PPO2 signal is coherent and reasonable or if the PPO2 signal is deviating from the calculated PPO2 signal based on the comparison. In one aspect, wherein the PPO2 signal is deviating from the calculated PPO2 signal, the method comprises changing S121 1 a first PPO2 set point a second PPO2 set point; measuring S1212 the time interval to reach the second PPO2 set point by measuring at least one of a breathing volume of the breathing apparatus, an oxygen injection rate, an oxygen injection volume, an oxygen injection flow and an oxygen consumption; calculating S1213 an incline of a graph presenting the values of the PPO2 signal collected over the time interval for the PPO2 set point change; and determining S1214 the validity of the PO2 signal based on the calculated incline of the graph.

According to some aspects, the method comprises determining S130 the validity of the retrieved PPO2 signal based on the comparison. In one aspect, the method comprises measuring the time interval to reach a determined set point of the PP02 signal and wherein the validity of the PP02 signal is based on the time to reach the determined set point.

According to some aspects, the method comprises determining S140 the validity of the 02 sensor based on the determined validity of the PPO2 signal and wherein the current operation of the breathing apparatus is updated S150 based on the validity of the 02 sensor.

According to some aspects, the method comprises detecting S210 at least one of an oxygen injection rate, oxygen injection volume, oxygen injection flow, breathing loop volume or oxygen consumption of the breathing apparatus, comparing S220 the current amplitude of the PPO2 signal to an amplitude of a calculated PPO2 signal related to at least one of the detected oxygen injection rate, oxygen injection volume, oxygen injection flow, breathing volume and oxygen consumption of the breathing apparatus and determining S130 the validity of the PPO2 signal based on the comparison. According to some aspects, the method comprises initiating S310 a test cycle, which is performed by altering the current PPO2 set point to either a lower or higher value, when each value is reached the PPO2 set point is changed to the opposite side of the current PPO2 set point, registering S320 the time to perform the test cycle and determining S130 the validity of the PPO2 signal based on the registered time.

According to some aspects, wherein the breathing apparatus comprises an oxygen control system, OCS, the method comprises loading S410 the OCS with data related to the breathing apparatus and the dive to be performed, The data is related to the breathing apparatus, i.e. total breathing volume and the dive to be performed, i.e. planed diving depth, time. The OCS is configured to predict expected characteristics for the PPO2 signal from an 02 sensor of the OCS, based on the loaded data. Examples of characteristics are the levels, amplitudes, sawtooth pattern and setpoint change speed. The method comprises calculating S420, by OCS, characteristics of the PPO2 signal, both previous and actual, during the dive performed by the diver, comparing (S430), by OCS during the dive, the calculated characteristics of the PP02 signal with the expected characteristics of the PP02 signal and determine whether previous and actual calculated characteristics of the PP02 signal are valid or if the PP02 signal has changed characteristics based on the comparison.

According to one aspect, wherein a PP02 set point is calculated for the OCS, the method comprises comparing at least one of the calculated characteristics of the 02 sensor, PPO2 value, amplitude of the PPO2 signal, saw tooth pattern of a graph of the calculated PPO2 values and speed of change of PPO2 set point with the corresponding expected characteristics of the 02 sensor and determining S130 the validity of the PPO2 signal based on the comparison.

According to one aspect, the method comprises collecting volume information from a solenoid injection of 02 wherein the total breathing loop volume of the OCS is calculated by adding the collected 02 injection volume information to the existing breathing loop volume. According to one aspect, the method comprises determining maximum and minimum loop volume of a breathing loop volume of the breathing apparatus.

According to one aspect, the method comprises measuring one or more of the pressure drop in an oxygen cylinder of the OCS related to temperature, a breathing frequency of the user, breathing minute ventilation of the user, heart rate of the user, the oxygen injection volume or the time it takes for the PPO2 value to return to the same level as before an oxygen injection to determine the oxygen consumption. According to one aspect, an oxygen control system, OCS, comprised in a breathing apparatus used for diving purposes, for determining the validity of an oxygen, 02, sensor, comprised in the OCS, is configured to perform the above discussed method is provided. Embodiments and aspects are disclosed in the following items:

Item 1 . A method for determining the validity of an oxygen, 02, sensor reading, wherein the sensor is comprised in a breathing apparatus used for diving purposes, the method comprises the steps of analyzing the behavior of the retrieved PPO2-signal and/or determining the validity of the signal compared to an ambient pressure change, solenoid injection, pressure decrease in oxygen or diluent cylinder, gas injection from automatic diluent valve, breathing frequency, heart rate and flow rate in a breathing loop wherein it is determined, while using the retrieved PPO2-signal, whether the PPO2-sensors are giving a coherent and reasonable PPO2-signal or if they are deviating from what is expected based on the behavior and validity.

Item 2. The method according to item 1 , wherein the step of analyzing further comprises analyzing the change of a PPO2-setpoint to determine the PPO2 validity compared to a sensor output. Item 3. The method according to item 1 , wherein the deviations can be described as not following a predicted pattern of output, the expected PPO2- signal is predetermined and the PPO2-sensor values are depending on a PPO2 set point wherein if the set point is changed, a time for the change will to a large degree be determined by a system volume, an oxygen injection rate, injection volume, injection flow and an oxygen consumption, wherein the method comprises determining an incline of the graph presenting the PPO2- values during set point change.

Item 4. The method according to item 3, wherein the amplitude of the PPO2- signal at a current set point is related to an injection rate, injection volume, injection flow and a system volume and dependent on configuration, also on the oxygen consumption, wherein the method comprises knowing these factors, to see that a correct system will have a different amplitude than an erroneous, if the predetermined settings are similar. Item 5. The method according to any of item 1 -4, wherein if the OCS is unable to find a definite reliable and trustworthy PPO2 signal it is possible to initiate a manual or automatic full or half test cycle, which is performed by altering the actual set point to either a lower or higher value, when this value is reached the set point is changed to the opposite side or level of original set point, wherein the method comprises registering the time it takes to perform this cycle it can be determined if the system is erroneous or not.

Item 6. The method according to item 1 , wherein the breathing apparatus comprises an oxygen control system, wherein the method comprises loading the OCS with data to predict the expected characteristics for the PPO2 signal; and calculating, both historically, actual and future, PPO2 values during the dive performed by a diver, wherein the OCS is analyzing the data during the dive and comparing whether previous and actual data values are valid or if the OCS has changed characteristics. Item 7. A oxygen control system, comprised in a breathing apparatus used for diving purposes, for determining the validity of an oxygen, 02, sensor reading, wherein the OCS comprises sensors for measuring pressure of oxygen, PPO2, and the OCS is configured to perform the steps of analyzing the behavior of the retrieved PPO2-signal and/or determining the validity of the signal compared to an ambient pressure change, solenoid injection, pressure decrease in oxygen or diluent cylinder, gas injection from automatic diluent valve, breathing frequency, heart rate and flow rate in a breathing loop, wherein it is determined, while using the retrieved PPO2-signal, whether the PPO2-sensors are giving a coherent and reasonable PPO2- signal or if they are deviating from what is expected based on the behavior and validity.

Item 8. The system, according to item 7, wherein the system is configured to perform the step of analyzing further comprises analyzing the change of a PPO2-setpoint to determine the PPO2 validity compared to a sensor output. Item 9. The system according to item 7, wherein the deviations can be described as not following a predicted pattern of output, the expected PP02- signal is predetermined and the PP02-sensor values are depending on a PP02 set point wherein if the set point is changed, a time for the change will to a large degree be determined by a system volume, an oxygen injection rate, injection volume, injection flow and an oxygen consumption, wherein the system is configured to perform the step of determining an incline of the graph presenting the PP02-values during set point change.

Item 10. The system according to item 7, wherein the amplitude of the PP02- signal at a current set point is related to an injection rate, injection volume, injection flow and a system volume and dependent on configuration, also on the oxygen consumption, wherein the system is configured to perform the step of knowing these factors, to see that a correct system will have different amplitude than an erroneous, if the predetermined settings are similar. Item 1 1 . The system according to any of item 7-10, wherein if the OCS is unable to find a definite reliable and trustworthy PPO2 signal it is possible to initiate a manual or automatic full or half test cycle, which is performed by altering the actual set point to either a lower or higher value, when this value is reached the set point is changed to the opposite side or level of original set point, wherein the system is configured to perform the step of registering the time it takes to perform this cycle it can be determined if the system is erroneous or not.

Item 12. The system according to item 7, wherein the system is configured to perform the step of loading the OCS with data to predict the expected characteristics for the PPO2 signal and calculating, both historically, actual and future, PPO2 values during the dive performed by a diver, wherein the OCS is analyzing the data during the dive and comparing whether previous and actual data values are valid or if the OCS has changed characteristics.

Item 13. The system according item 7, wherein, depending on the set point for PPO2 that is chosen for the OCS at the actual depth, the system is configured to perform the step of comparing the PP02-levels, PP02- amplitudes, PP02-sawtooth pattern and PP02-setpoint change speed to the expected values.

Item 14. The system according to item 7, wherein the system is configured to perform the step of collecting information from solenoid injection volume and rate and a saw tooth pattern of a graph of PP02 values collected from the 02 sensors and wherein the total volume of the OCS is approximated.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" "comprising," "includes" and/or "including" when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should be regarded as illustrative rather than restrictive, and not as being limited to the particular embodiments discussed above. The different features of the various embodiments of the invention can be combined in other combinations than those explicitly described. It should therefore be appreciated that variations may be made in those embodiments by those skilled in the art without departing from the scope of the present invention as defined by the following claims.