Technological developments in diving equipment and practices over recent years now permit recreational divers regularly to achieve dive depths and durations equivalent to, or in excess of, those practised by the military. One example of such a development is the introduction of exotic gases into breathing apparatuses. These gases are employed to reduce the proportion of nitrogen used in gas mixtures. This permits diving to take place at greater depths, without increasing the risk of narcosis.

One specific development which has contributed to the above achievements is the use of closed-circuit rebreather (CCR) equipment. A CCR system adjusts the mixture of gases breathed by a diver on a breath-by- breath basis. Exhaled carbon dioxide is removed and oxygen is replenished, without expelling bubbles from the system, so that an optimum gas mixture is provided throughout a dive. A CCR system may, therefore, be more than ten times more gas efficient than open- circuit equipment ; which uses standard dive cylinders and regulators. However, although less efficient, open-circuit equipment always delivers a safe breathable gas mixture, provided a user observes the specific depth limits associated with the equipment.

There is an increased level of risk associated with rebreather systems, on the other hand, since the gas mixture delivered by such systems can easily become lethal at certain depths, without careful attention and control.

The risk of the gas mixture delivered by a CCR

apparatus becoming poisonous is well known. As such, it is standard practice to teach divers the procedures necessary to deal with all known situations involving equipment problems or failures. These include, in the first instance, monitoring the diving equipment, especially during a dive, to ensure that the equipment is still functioning correctly, before a problem has become dangerous. In particular, in most closed- circuit and semi-closed-circuit rebreather systems, a user is required regularly to monitor his gas mixtures, to check that a desired safe mix is actually being delivered. In order to be able to do this, most closed-circuit rebreathers rely on oxygen sensors, which measure the partial pressure of oxygen (pp02) in the mixture and are used to inform control electronics in the system of the oxygen content of the breathed gas. Without oxygen sensors, rebreather systems would be much more limited, if not impossible. Despite their importance to a CCR system, oxygen sensors are not particularly reliable inside rebreathers, making them a weak point in the system. In order to try to mitigate the risk of an oxygen sensor failure rendering a rebreather virtually unusable-a clear danger during a dive-it is conventional to use three oxygen sensors at the same time. Voting logic software is then applied to the signals from the three sensors to decide which of the sensors are working and which is not. On the basis of this decision, the controller processes this information for suitable presentation to the user and, if necessary, adjusts the flow of oxygen delivered to the user to maintain a desired oxygen partial pressure.

There is a number of ways in which the oxygen content information is presented to the user. This is usually either directly by display of oxygen partial pressure readings on a dial or gauge, or indirectly by display

of one indication representative of an acceptable oxygen level and another indication representative of an unacceptable level, as appropriate, on a head-up display (HUD). The direct-display gauges are typically worn on one or both wrists of the user, and there may be one or more additional gauges attached to the chest straps of the breathing apparatus. These direct- display partial pressure gauges are employed as the primary and secondary monitors in most rebreather systems. By definition, therefore, a user is dependent on the primary and secondary gauges during a dive and needs to check them regularly to be certain that he is breathing an acceptable gas mixture. An indirect- display monitor is typically included as an additional, back-up monitor, for situations in which the primary and secondary gauges fail. The indirect- display monitors may include a LED, an audible alarm, or a vibrating alarm. The electronic controller typically instructs the LED to display a green light when the oxygen content is within a certain range and a red light when the oxygen content is outside that range. In a similar manner, the alarms are usually set to sound or vibrate when the oxygen content falls outside the pre-set range.

An example of an indirect-display monitor is the Display Integrated Vibration Alarm, or DIVATM, manufactured by Juergensen Marine of Pennsylvania, USA. Figure 1 shows a side view of a DIVATM 1, mounted on a dive surface valve (DSV) 2 by means of a bracket 3. The DSV 2 incorporates a mouthpiece 4, through which gas is delivered to a user. The DIVATM 1 includes a tricolour LED 5 and a vibrator motor 6, both contained in a sealed, stainless steel housing 7.

The LED 5 flashes green when the partial pressure of oxygen is above a certain value, orange when the partial pressure is at that value and red when the

partial pressure is below the value. When the LED 5 flashes red, the vibrator motor 6 spins an eccentric weight, so that pulses of vibration are transmitted to the user's mouth via the DSV 2 and a buzzing sound may be heard by the user.

The indirect-display monitors provide an indication of the relative safety of the gas mixture being delivered, by means of a head-up display. In this way, information about the gas mixture may be viewed by the user without the need to look down. Instead, the HUD is arranged so that the display appears in the user's peripheral vision. The user is thereby able to focus predominantly on the dive site and to glance at the HUD when desired. In addition, the user will be able to notice immediately a change in the colour of light emitted by the LED (for example, from green to red) when this occurs. However, a common problem with audible alarms is either that they are too sensitive and beep too often, or that they beep too quietly and are not heard, or even that they beep too loudly. In addition, among a group of divers, it is not always clear whose alarm has been set off. Furthermore, a number of divers are deterred from using a vibrating alarm, because they do not like the idea of having their mouthpiece, which is held between their teeth, vibrated on a potentially regular basis. Finally, the LED in the HUD represents, by definition, a single point of failure in the monitor and cannot, therefore, be used as a reliable primary method of monitoring gas mixtures. A number of recreational divers may be tempted to rely on the HUD to monitor oxygen partial pressure, instead of using the wrist or chest-strap gauges, because the HUD represents less of a distraction to them. However, such use of the HUD is unsafe and has the potential to harm its user. For example, if the HUD failed, but the LED continued to

show green, the user would continue a dive unaware of whether or not, in fact, he was breathing a safe gas mixture. Unless the user referred to the direct- display partial pressure gauges, as he should, that particular dive has the potential of being the user's last.

While the direct-display gauges provide a read-out of the oxygen partial pressure as measured by the sensors, a user needs physically to look down towards his wrist and/or chest straps in order to read the gauges. If the user is to ensure that a consistently safe gas mixture is being provided by the breathing apparatus, he must look down at the partial pressure gauges on a regular basis. Such a procedure represents a continual distraction to a user and detracts from the diving experience. In addition, the user may forget to check the gauges regularly if the dive site is particularly interesting, sensitive or hazardous, since much attention instead will be devoted to the site in question.

As described above, the oxygen partial pressure sensors are linked to an electronic controller, which processes the information and makes suitable adjustments to the gas mixture, when necessary, and presents the information to the user via the different monitors. As will be understood, software crashes are unfortunately not rare. In addition, problems such as unit leaks and mechanical or switch faults can cause electronic controller failures. In order to compensate for these eventualities, rebreather-and mainly CCR- apparatuses typically include a duplicate, or redundant, electronic controller. Even so, controller failures are still not uncommon and dual controller failures are not unknown. This places an additional onus on the user to check all gauges and monitors

regularly, not only to ensure that the gas mixture being breathed is acceptable, but also to verify that the electronic controller providing this information is actually working.

While all rebreather divers should be trained and competent in checking their gauges and monitors regularly, understanding what the information means and knowing what to do in the event of an emergency, recreational diving should be an enjoyable, stress- free experience. Managing and monitoring equipment should ideally, therefore, be minimised, so that the user may concentrate on enjoying the dive.

There is a need, therefore, for a reliable, primary means of monitoring gas mixture content, which is readily visible and does not require a user to look down regularly at gauges on his breathing apparatus straps or his wrists. There is a further need for such a means not to intrude undesirably on the user's dive experience, either by taking up a significant amount of time to check, by obscuring the user's field of view, or by distracting the user unnecessarily.

The present invention aims to address the above objectives by providing an improved diving equipment monitor.

According to a first aspect of the present invention, there is provided a breathing apparatus gas monitoring device, comprising: a first oxygen monitoring circuit, comprising a first sensor means, adapted to generate a first signal representative of a pressure of oxygen within a breathing apparatus, and a first indicator means; a second oxygen monitoring circuit, comprising a second sensor means, adapted to generate a second signal representative of a pressure of oxygen within a

breathing apparatus, and a second indicator means; and a head-up display, adapted to house the first and second indicator means generally within a line of sight of a user, both the first and second indicator means being adapted independently to generate one of a first indication representative of an acceptable oxygen pressure and a second indication representative of an unacceptable oxygen pressure, based on the first and second signals respectively.

This is advantageous because the device provides a display which may be viewed at all times during a dive, either passively or actively. By employing two oxygen monitoring circuits, with respective indicator means, the device may be used with confidence to monitor oxygen partial pressure in a rebreather breathing apparatus. This, in turn, may provide a much improved diving experience for a user, by removing the continual distractions associated with checking partial pressure gauges located on wrists or breathing apparatus straps. Further benefits include generally safer diving, in terms of affording both increased concentration on a dive site of interest and faster reactions to oxygen partial pressures which leave the acceptable, specified ranges.

Preferably, the gas monitoring device further comprises a third oxygen monitoring circuit, comprising: a third sensor means, adapted to generate a third signal representative of a pressure of oxygen within a breathing apparatus, and a third indicator means, the head-up display being further adapted to house the third indicator means generally within a line of sight of a user, and the third indicator means being adapted independently to generate one of the first indication representative of an acceptable oxygen pressure and the second indication

representative of an unacceptable oxygen pressure, based on the third signal.

By using doubly-redundant electronics, the gas monitoring device will continue to function, even in the unlikely event that two of the three oxygen monitoring circuits fail. Preferably, in addition to this, the device provides self-detection of power failure and software crashes, so that the device has no single point of failure. The advantage of such a device is that the device is both reliable and fail- safe.

In a preferred embodiment, the each sensor means further comprises a respective signal control means, adapted to receive the respective sensor signals and to provide the respective first, second or third signals. Preferably, each respective one of the signal control means is independent of the other of the signal control means. In this way, failure of one of the signal control means will not affect the performance of the other of the signal control means and the device will continue to function.

Preferably, each respective one of the oxygen monitoring circuits is independent of the other of the oxygen monitoring circuits. Preferably still, each respective one of the oxygen monitoring circuits is independent of any component external of that circuit.

The advantage of this is that the safety and reliability of the gas monitoring device is improved.

Any individual failure of a component, either in one of the oxygen monitoring circuits or outside of the oxygen monitoring circuits, will not cause a corresponding failure in the other, or each, of the oxygen monitoring circuits. Of course, where a single power supply for the device is employed and fails,

such a failure will be evident to a user, who will then be able to respond appropriately. In this way, the user will not continue unknowingly to use a failed device.

In a preferred embodiment, the gas monitoring device is independent of any component external of the device. Such a device is advantageously not affected by external failures ; for example, failures in a rebreather electronic controller. Instead, the device is a stand-alone device, which is self-reliant and may be used as a separate means for monitoring oxygen partial pressures.

Preferably, the head-up display is a primary means of monitoring breathing apparatus gas. This is so, because of the improved reliability of such a gas monitoring device and its fail-safe nature. Therefore, instead of using partial pressure gauges as the primary means of monitoring breathing apparatus gas, the HUD may be used, offering less distraction to the user. The partial pressure gauges may be used as back- up monitors, either to corroborate the information displayed by the HUD of the gas monitoring device or in case the head-up display stops functioning correctly.

In a preferred embodiment, the gas monitoring device further comprises a LED power supply, adapted to provide power to both or all of the LEDs. Preferably, the LED power supply is independent of any other power supply. Power supply redundancy advantageously provides a further fail-safe mechanism to the gas monitoring device of the present invention.

In a preferred embodiment, the LED power supply provides a supply of less than approximately 3V.

Preferably still, the LED power supply provides a supply of 2.3V. In this way, resistive losses may be reduced in the power supply circuits for the LEDs, which, in turn, advantageously increases battery life.

In a preferred embodiment, there is provided a closed- circuit rebreather apparatus, comprising: a gas supply, including at least an oxygen gas supply, and adapted to supply a gas mixture; a rebreather electronic controller, adapted to receive sensor signals and to control a flow rate of the oxygen gas supply in response to the sensor signals; and the gas monitoring device of the present invention.

Preferably, the gas monitoring device is functionally independent of the rebreather electronic controller.

In this way, failure of the rebreather electronic controller will not cause a consequential failure of the gas monitoring device of the present invention.

The gas monitoring device of the present invention, therefore, may be contained within a conventional electronic controller unit, but remains functionally independent of the controller (in terms of circuit components and the like). Of course, the gas monitoring device may alternatively be housed separately from the electronic controller. In either case, the gas monitoring device is not affected by failures of the controller.

Preferably, the closed-circuit rebreather apparatus further comprises an oxygen gas supply manual control means, adapted to change the flow rate of the oxygen gas supply on manual initiation, either in combination with or in substitution for the rebreather electronic controller, and in response to the indications displayed by both or each of the indicator means respectively. A user may, therefore, use the head-up display of the gas monitoring device to determine when

the oxygen partial pressure is again within an acceptable range, after manually adjusting the flow of oxygen into the breathing apparatus. This is so, especially when the rebreather electronic controller (s) has failed, such that the partial pressure gauges no longer work. Conventionally, a user would have to resurface, by bailing out to open circuit gases and ascending in a controlled manner.

With the gas monitoring device of the present invention, however, the user may continue a dive, controlling oxygen flow manually and monitoring the partial pressure with the head-up display.

According to a second aspect of the present invention, there is provided a method of monitoring a gas in a breathing apparatus comprising: a first oxygen monitoring circuit, comprising a first sensor means, and a first indicator means; a second oxygen monitoring circuit, comprising a second sensor means, and a second indicator means; and a head-up display, adapted to house the first and second indicator means, the method comprising the steps of: generating, from the first sensor means, a first signal representative of a pressure of oxygen within the breathing apparatus; generating, from the second sensor means, a second signal independently of the first sensor means, the second signal being representative of a pressure of oxygen within the breathing apparatus; receiving, at the first indicator means, the first signal; receiving, at the second indicator means, the second signal; generating, from the first indicator means, one of a first indication representative of an acceptable oxygen pressure and a second indication representative of an unacceptable oxygen pressure, based on the first signal; generating, from the second indicator means, one of the first indication representative of an acceptable oxygen pressure and

the second indication representative of an unacceptable oxygen pressure, based on the second signal; and presenting the respective first and/or second indications generated by the first and second indicator means generally within a line of sight of a user.

Preferably, the breathing apparatus further comprises: a third oxygen monitoring circuit, comprising: a third sensor means, and a third indicator means, the head-up display being further adapted to house the third indicator means, and the method further comprises the steps of: generating, from the third sensor means, a third signal representative of a pressure of oxygen within the breathing apparatus; receiving, at the third indicator means, the third signal; generating, from the third indicator means, one of the first indication representative of an acceptable oxygen pressure and the second indication representative of an unacceptable oxygen pressure, based on the third signal; and presenting the first or second indication generated by the third indicator means generally within a line of sight of a user.

In a preferred embodiment, the breathing apparatus further comprises: a gas supply, including at least an oxygen gas supply, and a rebreather electronic controller, and the method further comprises the steps of: delivering, from the gas supply, a gas mixture, including oxygen; and receiving, at the rebreather electronic controller, sensor signals and controlling a flow rate of the oxygen gas supply in response to the sensor signals, each respective one of the oxygen monitoring circuits performing the method steps independently of the rebreather electronic controller.

Preferably, the method further comprises the step of

manually changing the flow rate of the oxygen gas supply, either in combination with or in substitution for the rebreather electronic controller, and in response to the indications displayed by both or each of the indicator means respectively.

Other preferred features are set out in the dependent claims which are appended hereto.

The present invention may be put into practice in a number of ways and some embodiments will now be described, by way of example only, with reference to the following figures, in which: Figure 1 shows a prior art indirect-display monitor; Figure 2 shows a block circuit diagram of a gas monitoring device according to a first embodiment of the present invention; Figure 3 shows a block circuit diagram of a gas monitoring device according to a second embodiment of the present invention; Figure 4a shows a conventional LED power supply circuit; Figure 4b shows a LED power supply circuit according to a third embodiment of the present invention ; Figure 5 shows a schematic graph of a working range of an oxygen sensor embodying the present invention; and Figure 6 shows a calibration graph of the oxygen sensor of Figure 5.

Referring to Figure 2, a gas monitoring device

according to a first embodiment of the present invention is shown. The device includes a first oxygen monitoring circuit 10, a second oxygen monitoring circuit 20 and a head-up display (HUD) 30. The first oxygen monitoring circuit 10 includes a first oxygen sensor 12, a first signal processor 14, a first microcontroller 16 and a first visual output 18. The second oxygen monitoring circuit 20 includes a second oxygen sensor 22, a second signal processor 24, a second microcontroller 26 and a second visual output 28. The HUD 30 houses the first and second visual outputs 18,28, such that the outputs are located within the peripheral vision of a user and may be observed by the user throughout a dive.

The first oxygen sensor 12 is disposed in a breathing loop of a breathing apparatus (not shown) and is, in use, exposed to oxygen gas. The oxygen sensor 12 is designed to generate a voltage proportional to a partial pressure of the oxygen to which the sensor is exposed. The voltage. generated by the sensor 12 is then amplified by the signal processor 14, to ensure that the amplitude of the voltage is sufficient for subsequent processing. The voltage signal is then passed on to the microcontroller 16. In the present embodiment, the microcontroller 16 is a single chip computer, conventionally used for control applications. The microcontroller 16 includes an analogue-to-digital converter, which converts the analogue voltage signal into a digital signal, which may then be processed with the use of an appropriate software program. Depending on the ability of the microcontroller's analogue-to-digital converter to handle relatively small sensor signals, the signal processor 14 may not be required. That is, the analogue-to-digital converter may be capable of resolving the voltage signal directly from the sensor

12, without the need first to have the voltage signal amplified.

The microcontroller 16 calibrates the voltage signal received from the oxygen sensor 12 and the visual output 18 indicates whether or not the resulting signal is within a pre-specified, acceptable range. In this embodiment, the visual output 18 is a LED. The LED may consist of one bicolour LED, or two single colour LEDs. In this way, one colour is displayed by the visual output 18 when the gas mixture is measured to be acceptable and another colour is displayed when the mixture is outside the acceptable range. This may, for example, be a green light for a satisfactory oxygen content in the mixture and a red light for any other mixture.

The second oxygen monitoring circuit 20 functions in a similar manner to the first circuit 10. That is, a separate oxygen sensor 22 generates a voltage signal, when exposed to oxygen in the breathing apparatus. The voltage signal, which is proportionate to the pressure of the oxygen, is then processed by the signal processor 24 (where necessary) and the microcontroller 26. The resulting signal causes the visual output 28 to display one colour when the detected pressure is deemed to be within an acceptable range and another colour when outside this range.

The head-up display (HUD) 30 houses the first and second visual outputs 18,28, such that the visual outputs may be viewed at the same time by a user. In this embodiment, the LEDs are mounted within accommodating apertures in the HUD 30. The HUD 30 is designed to be mounted in use on the breathing apparatus at a location forward of a user and generally within visible range. In this embodiment,

therefore, the HUD 30 is designed to be mounted on a mouthpiece of the breathing apparatus. The term 'mouthpiece'in this specification means that part of the breathing apparatus between a supply hose and a return hose of a breathing loop. The mouthpiece, therefore, may include a dive/surface valve, hose connectors, and a mouth part. Alternatively, the HUD 30 may be fixed to one of the hoses connected to the mouthpiece. Alternatively still, the HUD 10 may be fixed to a headband or the like, or to a diving mask, with the HUD being arranged appropriately within a user's peripheral vision. In either case, a suitable fixing bracket is used to hold the HUD 30 in place throughout a dive. In this way, the LEDs may be monitored passively within a user's peripheral vision, for colour changes or the like and a user may then concentrate his attention on a dive site, or other matters, as required.

As described above, conventional oxygen partial pressure meters are controlled by a single electronic controller unit. The controller is typically responsible for detecting oxygen sensor signals, deciding which sensors are working, calibrating the signals, determining whether the signals fall within a tolerable range, adjusting the flow of oxygen as appropriate, and transmitting signals to both the direct-display and indirect-display meters, for suitable display by them. Such an electronic controller unit is susceptible to failure in a number of ways, either in software or hardware, or mechanically (leaks, bad connections, switch <BR> <BR> malfunctions etc. ). The gas monitoring device of this embodiment is independent of such a breathing apparatus electronic controller. As a result of bypassing the controller, each oxygen sensor 12,22 is read directly by its respective monitoring circuit

10,20. In this way, a controller unit failure will have no impact on the functioning of each oxygen monitoring circuit 10, 20,. or the gas monitoring device itself.

By arranging the gas monitoring device in this way, the reliability of the device is independent of any component external of it. The duplicated, or redundant, oxygen monitoring circuit 20 provides the gas monitoring device with a fail-safe mechanism.

Should one of the circuits 10,20 fail, therefore, the gas monitoring device will still continue to function, by means of the other of the circuits. As such, the gas monitoring device may be used as the primary means of checking that the oxygen content of a gas being breathed is at an acceptable level. Accordingly, it is no longer necessary continually to check the conventional, direct-display oxygen partial pressure gauges, located on a user's wrist or breathing apparatus straps. With the inconvenience of having frequently to look down and read partial pressure gauges removed, the enjoyment of a dive may be greatly increased. In addition, there is a number of safety benefits associated with the gas monitoring device of the present invention. For example, the device provides an early warning, should a problem with the gas mix arise. The device also removes the need for a user to rely on himself in checking gauges, since the fail-safe, multiply-redundant electronics in the device perform the task of monitoring the oxygen partial pressure. The device is, therefore, particularly suited to any diving situation where the user needs to concentrate on specific, intensive tasks, as well as to recreational diving activities.

However, should either or both of the visual outputs 18, 28 in the HUD 30 stop displaying the'acceptable

gas mixture'light, this will let a user know that he needs to return to checking his oxygen partial pressure gauges every few minutes, in the traditional manner. Should one of the oxygen monitoring circuits 10,20 detect an unacceptable oxygen level, or fail, the HUD 30 will provide a warning of this in real time. The warning is in the form of one of the oxygen monitoring circuits 10,20 generating a different output from the other of the circuits. Of course, should both circuits 10,20 detect the unacceptable oxygen level, the visual outputs 18,28 will be in the same state, but the user will recognise that the change in displayed colour is indicative of a problem with the oxygen supply. Although, in this embodiment, the two possible states of the visual outputs 18,28 <BR> <BR> have been described as being one colour (e. g. , green) for an acceptable gas mixture and another colour <BR> <BR> (e. g. , red) for an unacceptable mixture, alternatives are envisaged. For example, the first state may be represented by a light being switched on and the second state by the light being switched off. In this case, only one colour of LED is required.

Alternatively still, a detected mixture within a pre- specified, acceptable range may be indicated by a green light, an unacceptable mixture by a red light and a circuit failure by either light switching off.

This alternative will help the user to determine whether a change in visual output display is due to a circuit failure or an unacceptable gas mixture.

When one or each visual output 18, 28 indicates that the oxygen partial pressure is outside the acceptable set point range, a user may control the supply of oxygen to the breathing apparatus manually, using a suitable valve (not shown). The valve may be the same gas supply valve used by the rebreather electronic controller, which valve may be overridden manually, or

the valve may be a separate, additional valve, only for manual use. Furthermore, manual initiation of the valve may result in either a temporary additional discharge of oxygen into the breathing apparatus (for example, for the duration of the manual intervention), or a generally fixed increased or decreased flow rate of oxygen through the valve. In either case, the user may increase or decrease the flow of oxygen into the breathing apparatus, as required. One potential cause of the one or each visual output displaying an indication of an unacceptable gas mixture is a failure of the rebreather electronic controller, or controllers (when a second, redundant rebreather controller is also used). Such a failure would result in the oxygen gas supply no longer being controlled automatically, so that rises or falls in the oxygen partial pressure would not be prevented. A rebreather electronic controller failure may lead to the partial pressure gauges becoming unusable, since the readings displayed by the partial pressure gauges are typically received from the rebreather electronic controller. As such, a diver using a conventional rebreather apparatus would typically have to bail out to open circuit gases and resurface. However, the gas monitoring device of the present invention may continue to be employed in such situations, since a failure of the electronic controller will not cause a corresponding failure in the oxygen monitoring circuits 10,20 or the gas monitoring device as a whole. In this way, the user may manually adjust the oxygen flow into the breathing apparatus until the visual outputs 18, 28 in the HUD 30 display an indication of an acceptable oxygen partial pressure once again. Typically, more than one set point for an acceptable oxygen partial pressure range is used during a dive. In order to avoid the possibility of the user being confused as to which of the oxygen

partial pressure ranges the visual output indications are referring to, different acceptable and/or unacceptable indications may be used for each set point. For example, an acceptable indication for one set point, predominantly used for diving at depth (and, therefore, the most often needed), may be represented by a solid light of a certain colour. For a set point used in shallow water (and, therefore, only used when beginning a dive or resurfacing), an acceptable indication may be represented by a flashing light of the same colour. As such, a user will understand the visual output display, by knowing which colour is indicative of an acceptable partial pressure and which display styles represent the shallow water set point and the deep water set point. Arranging the shallow water set point visual output display style as a flashing light and the deep water set point visual output display style as a solid light reduces the distraction caused to the user. This is because the shallow water set point is used for a relatively short period of time, so the relative distraction caused by flashing lights would be limited to the beginning and the end of a dive. Of course, all suitable variations of light colour and display style are envisaged for this purpose.

Figure 3 shows a second embodiment of the gas monitoring device of the present invention. Items in common with the first embodiment are referred to with the same reference numerals. In this embodiment, the gas monitoring device includes a first, second and third oxygen monitoring circuit 10,20, 40. The HUD 30 houses the visual output 18,28, 48 of each circuit 10,20, 40. In this way, the HUD 30 has three light- transmitting apertures, which accommodate three LEDs, such that the LEDs are simultaneously visible in use.

The HUD 30 may then be mounted generally within a

user's peripheral vision, so that the LEDs may be monitored passively by the user during a dive.

In this embodiment, each one of the oxygen monitoring circuits 10,20, 40 functions independently of the other two circuits. This provides the oxygen monitoring device with a fail-safe arrangement, in that the device will withstand the failure of up to two of the oxygen monitoring circuits 10,20, 40 and still be capable of alerting a user to a problem with the gas mixture by means of the remaining circuit. If there is a fault with one or two of the oxygen monitoring circuits 10,20, 40, or if the oxygen content of the breathed gas falls outside an acceptable range, the user needs to be made aware of this. Once aware, the user can continue a dive using the traditional method of checking his partial pressure gauges every few minutes. Alternatively, the user may decide to resurface in order to check his equipment thoroughly and to determine the cause of the problem. The user may be alerted to the problem by one or more of the LEDs either being switched off, or not matching the state of the other LED or LEDs.

Well-designed electronic circuits generally have a high reliability and failures are, therefore, unusual.

However, circuit failures do sometimes occur. As will be understood, the probability of two oxygen monitoring circuits 10,20, 40 failing together during any particular dive is relatively small. The probability of all three circuits 10,20, 40 failing is even smaller. For example, if the probability Pf of a single circuit 10,20, 40 failing is 0.1, the probability Pf1 of one of the three circuits in the gas monitoring device failing is 0.243. The probability Pf2 of two of the three circuits 10,20, 40 failing is 0.027. Finally, the probability Pf3 of all

three of the circuits 10,20, 40 failing is 0.001.

As will be understood, a probability of failure as high as Pf=O. 1 would be unacceptable in commercial diving equipment, so the probability Pf3 of all three circuits failing would, in practice, be far lower than in the above example. Furthermore, the design of the gas monitoring device is such that there is no known failure mode which will cause all three, independent oxygen monitoring circuits 10,20, 40 to fail at the same time, from the same failure, without alerting a user to such an event. For those situations where a failure, either in hardware or software, is likely to affect all three of the oxygen monitoring circuits 10,20, 40, the effects of such a failure are designed to be obvious to the user. As such, a user may rely on the HUD 30 as a fail-safe, primary means for monitoring the oxygen content of the gas being breathed. Since the probability Pf3 of all three oxygen monitoring circuits 10,20, 40 failing together during any particular dive is negligibly small, a user may continue to use the HUD 30 to monitor oxygen gas levels, all the time the three LEDs are in the same state. As soon as one or more of the LEDs changes state, either to off or to a different colour (e. g., green to red) or display. style (e. g. , constant to flashing), the user can begin to use the back-up gauges on his wrists or breathing apparatus straps.

Power is provided to the gas monitoring device of the present invention by means of one or more power supplies. Preferably, the one or more power supplies are derived from a single battery, by means of one or more DC-DC converters (not shown). In one embodiment, the device includes one power supply 50 (not shown) for both or all of the oxygen monitoring circuits 10,20, 40. This supply 50 is substantially a 5V supply, which provides power to each of the signal processors

14,24, 44, the microcontrollers 16,26, 46, and the visual outputs 18,28, 48. In order to ensure that there is no single point of weakness in the gas monitoring device, in another embodiment, the power supply 50 is duplicated.

In a third embodiment, the device includes a power supply 52 (not shown) to the signal processor 14,24, 44 and microcontroller 16,26, 46 of each oxygen monitoring circuit 10,20, 40, which is independent of a power supply 54 to the visual outputs 18,28, 48. A separate power supply 52a, b, c is provided for each respective oxygen monitoring circuit 10,20, 40, to supply power to the signal processor 14,24, 44 and the microcontroller 16,26, 46 of the respective circuit. Preferably, each of the power supplies 52a, b, c is substantially a 5V supply. The power supply 54 to the visual outputs 18,28, 48 is a single supply. In this embodiment, the power supply 54 provides power to both or all of the LEDs housed in the HUD 30. In this way, failure of any of one the power supplies 52a, b, c will not lead to failure of the gas monitoring device itself, since the remaining supplies will continue to power the other circuit or circuits 10,20, 40. Should such a failure occur and result in one LED being switched off, a user may decide whether then to continue to rely on the HUD 30 or to begin checking the back-up partial pressure gauges. Similarly, should the LED power supply 54 fail, the user will be alerted to this by all of the LEDs going out, as a result of power loss. Either way, the user will be alerted to a failure of one or more of the power supplies 52a, b, c, 54 and be able to decide how to respond to this, before being placed in serious danger. Although failures are relatively unusual, as described above, should one occur, the user will not continue to rely unknowingly on the HUD 30.

Figures 4a and 4b show LED power supply circuitry for use with the gas monitoring device of the present invention. Figure 4a shows a first, 5V power supply arrangement 56a and Figure 4b shows a second, 2.3V power supply arrangement 56b. The layout of the first and second arrangements 56a, b is identical. A resistor 58a, b (R2) is connected in series with a LED 60a, b and a transistor 62a, b. The base of the transistor 62a, b is connected to one of the microcontrollers 16,26, 46, via another resistor 64a, b (R1), and receives control signals from the microcontroller to switch the LED 60a, b on or off.

The potential difference applied across the resistor 58a, b, the LED 60a, b, and the transistor 62a, b determines the current which flows through the LED circuit. When a 5V control signal is applied from the microcontroller 16,26, 46 to the transistor 62a, b, the transistor is switched on. The potential difference applied across the LED circuit then drives a current through the circuit, causing the LED 60a, b to emit light as required. A typical LED requires a current of approximately 20mA and has a forward voltage of approximately 2V. The typical voltage drop Vce across a transistor when saturated (that is, when used as a switch and in the'on'mode) is approximately 0.2V. In the first arrangement 56a, therefore, the voltage drop across the resistor 58a needs to be approximately 2.8V, if the required 20mA current is to flow.

Consequently, the power dissipated through the resistor 58a as wasted heat is 56mW. Since the LED 60a requires only approximately 40mW of power, the majority of power used in the circuit is wasted in the resistor 58a. Using a 5V power supply, therefore, clearly wastes a substantial amount of power in the LED switching circuit. The undesirable effect of this on battery life will be readily understood.

In the second arrangement 56b, the loss through the resistor 58b is reduced, if not substantially minimised. The potential difference applied across the LED switching circuit in this arrangement 56b is 2.3V.

Therefore, for similar specifications and requirements for the LED 60b and transistor 62b, the voltage drop across the resistor 58b needs to be 0. 1V, in order to ensure that a 20mA current flows through the circuit.

In this arrangement, the value of the resistor 58b is 5Q and the power dissipated through the resistor is 2mW. The overall demand for power in the second arrangement 56b is more than half that of the first arrangement 56a, resulting in a substantial extension of battery life for the power supply 54.

As will be understood, depending on the type or types of LED and transistor used in the switching circuits of the oxygen monitoring circuits 10,20, 40, the above calculations may vary. Accordingly, the supply voltage may need to be different from that in the second arrangement 56b. Since, in this embodiment, the power supply 54 is a dedicated LED power supply, the circuit considerations relate only to the LED switching circuit and not to the requirements of the microcontrollers 16,26, 46 or signal processors 14,24, 44, for example. Therefore, the principal of minimising losses through the resistor 56b and reducing the LED switching circuit voltage, to increase the efficiency of the circuit, may readily be applied to any suitable arrangement.

As will be understood, each microcontroller 16,26, 46 is responsible for receiving sensor signals from its respective oxygen partial pressure sensor 12,22, 42 and controlling its respective visual output 18,28, 48 appropriately. Each microcontroller 16,26, 46 is

therefore programmed to calibrate the sensor signals received and to determine whether or not the calibrated signals lie within an acceptable range.

Generally, there are two safe ranges for the oxygen partial pressure in a rebreather breathing apparatus.

A first set point is used at the water's surface and for shallow diving and a second set point is used at greater depths. The reason for this is that a rebreather unit is not normally able to maintain an optimal partial pressure, required for a dive, at the water's surface or in shallow waters. Therefore, there are two partial pressure set points within the overall measurement range of each of the oxygen sensors 12,22, 42 which will lead to the visual outputs 18,28, 48 displaying an indication of an acceptable gas mixture. For any other oxygen partial pressure detected by the sensors 12,22, 42, an indication of an unacceptable gas mixture will be displayed.

The function of each of the oxygen monitoring circuits 10,20, 40 is the same, so only the first oxygen monitoring circuit 10 will now be described. From this description, the functional details of the second or third oxygen monitoring circuits 20,40 will be understood mutatis mutandis. For any particular oxygen partial pressure, the oxygen sensor 12 will generate a corresponding voltage. This voltage will lie within a working range of outputs of the sensor 12. Any voltage which is generated by the sensor 12 and falls outside this working range (for any particular partial pressure) is representative of a failure of the sensor. Figure 5 shows a schematic graph of oxygen sensor voltage against oxygen partial pressure, illustrating the working range of the sensor. The sensor 12 may generate voltages which correspond to any line which lies within this working range.

Calibration of the sensor 12 allows the

microcontroller 16 to determine accurately whether or not the oxygen partial pressure is within an acceptable range for breathing.

The microcontroller 16 may calibrate the oxygen sensor 12 at any suitable time. For example, calibration of the sensor 12 by the microcontroller 16 may be performed when the sensor is calibrated by the rebreather apparatus itself. The two calibration procedures are independent of each other, apart from the use of the same oxygen sensor 12. Calibration may be initiated by means of a switch, which informs the software running in the microcontroller 16 when to calibrate. This is usually achieved by flooding the breathing apparatus with oxygen, so that the sensor 12 generates a voltage at substantially 100% oxygen.

However, this may also be achieved by sampling the sensor voltage at any known partial pressure produced by the rebreather breathing apparatus. Once the sensor voltage has been obtained, the microcontroller 16 derives a linearisation constant, which is then used to multiply future incoming voltage values from the sensor 12, to obtain the voltage values representative of actual oxygen partial pressure.

Figure 6 shows a schematic graph of the response of the oxygen sensor 12 following calibration. In this case, the surface/shallow set point is 0.7 bar (7 x 104 Pa) and the set point at depth is 1.3 bar (1.3 x 105 Pa). When the calibrated voltage signal from the sensor 12 is measured to be within a predetermined tolerance range corresponding to a partial pressure of 0.7 or 1.3, depending on the applicable set point, the microcontroller 16 sends a control signal to the visual output 18. The visual output 18, such as a LED, is thereby switched on in a state representative of an acceptable partial

pressure, such as by displaying a solid green light.

For any calibrated voltage signal outside the tolerance range, the visual output 18 will be in a different state. For example, the LED may display a solid red light. In any case where the visual output 18 is in a state other than that which is representative of an acceptable oxygen partial pressure, a user will know either that there has been a circuit failure or that there is a problem with the oxygen level in the gas mixture. In either case, the user will then begin to check the rebreather gauges and to carry out diagnostics and/or emergency procedures, in accordance with normal rebreather training. Of course, the present invention provides two or more oxygen monitoring circuits 10,20, 40 and corresponding visual outputs 18,28, 48. Therefore, if one LED, for. example, goes red or switches off, the user has the choice whether to carry on using the remaining one or two monitoring circuits or to start using the rebreather gauges. This choice may depend on whether or not the remaining visual outputs 18,28, 48 tally with the partial pressure readings on the rebreather gauges.

As will be understood, the set points used by the oxygen monitoring circuit 10 may be any values within the normal working limits for rebreather diving.

Depending on the specific requirements of a dive, the oxygen set points may need to be higher or lower than the above values.

In addition to the above hardware considerations, it is preferable to address the possibility of the software running on each microcontroller 16,26, 46 failing. The possibility of such a failure represents a further point of weakness for each respective one of the oxygen monitoring circuits 10,20, 40. Software

failure may result from noise, poor hardware design or poor software programming. To deal with this problem, software failures can be detected by using a'watchdog' timer. Each timer may be included in its respective microcontroller 16,26, 46, or may be external of it. In any case, each watchdog timer works by counting down for a predetermined period of time. If the watchdog timer is not reset by the software running in its respective microcontroller 16,26, 46 before the timer finishes counting down, the timer resets the microcontroller. By rebooting the microcontroller 16,26, 46 in this way, the program is restarted. If the software continues to run properly, it ensures that the watchdog timer never times out. The calibration linearisation factor described above is stored in non- volatile memory and will, therefore, be reused if the program is restarted.

The gas monitoring device of the present invention does not rely on the ability of each microcontroller 16,26, 46 to be restarted following a software crash.

This is so, because, with two or more oxygen monitoring circuits 10,20, 40, the device will continue to function if one, or possibly two, of the circuits fails. While software crashes do not occur frequently, implementing the watchdog timer mechanism makes each circuit 10,20, 40 even more reliable.

I Although the HUD 30 has been described above as housing the two or three visual outputs 18,28, 48, the HUD may additionally house the microcontrollers 16,26, 46 and, where necessary, the signal processors 14,24, 44. In either case, the oxygen monitoring circuits 10,20, 40 remain independent of each other and the HUD 30 enables both or all of the visual outputs 18,28, 48 to be viewed at the same time and substantially within a user's peripheral vision.

As will be understood, the visual outputs 18,28, 48 may be any device which may be viewed passively by a user, in the manner set out above. The first visual output state, representative of an acceptable partial pressure, may be any available colour. For example, if the visual outputs 18,28, 48 are LEDs, the colour may be any one of those possible with commercially available LEDs (such as, green, red, orange, white, blue etc. ). In addition, the first state may be displayed as a solid/constant light, or as an intermittent/flashing light. The second visual output state, representative of an unacceptable partial pressure, may be any one of the above possibilities other than that employed as the first state. In addition, the second state may be indicated by switching the respective visual output 18,28, 48 off.

It is preferable, however, for the first state, representative of an acceptable partial pressure, to be indicated in a non-distractive manner. In this way, flashing or intermittent lights would be employed in the alert modes of the gas monitoring device, corresponding to the second visual output state.

While the functionality of each of the oxygen monitoring circuits 10,20, 40 above has been described as being identical, the component specifications and circuit configurations do not necessarily need to be identical. That is, so long as both or all of the oxygen monitoring circuits 10,20, 40 function with the same effect, any suitable circuit or circuits may be employed.