GB2463308A - Rebreather respiratory loop failure detector incorporating a carbon dioxide sensor - Google Patents

Abstract

A means to detect a wide range of failures of a rebreather by measurement of the expired carton dioxide level and application of that level to trigger alarms, provide loop shut-off or provide safety warnings. The means comprises a rebreather safety monitoring device that comprises a carbon dioxide sensor 5, a means to sample the gas in the rebreather loop between the inhale one-way valve on the rebreather mouthpiece 2 and the carbon dioxide scrubber 6 and provide that gas to the sensor and means to provide alarms based on the levels of sensed carbon dioxide in the gas. The carbon dioxide sensor may be calibrated using the carbon dioxide levels in human expired gas, may be fitted with a hydrophobic membrane and may be a dual channel infra-red absorption sensor.

Description

Rebreather Respiratory Loop Failure Detector

Background

Rebreathing equipment is used for human life support in adverse environments, such as in dMng underwater, hazardous materials handling and for manned activities in space outside of a spacecraft.

Rebreathers operate by circulating the user's expired gas through a loop comprising countetlungs, a carbon dioxide scrubber and a means to inject oxygen to make up for that lost through metabolism or vented from the loop, then back to the user to inspire.

The critical nature of the scrubber unit in the rebreather has given rise to multiple proposals for a Carbon dioxide (C02) sensor to monitor inspired gas. For example in the European Standard for rebreathers EN1 4143:2003, a detailed performance requirement is stated, and the means by which that is tested, for inspired gas carbon dioxide sensors. OSHA regulations contain reference to a similar such device. There is also a problem with inspired C02 sensors in that no company has managed to create an inspired C02 sensor that works in practice to date The limitation of these inspired C02 sensors, if they were to be reduced to practice, is that they are limited to monitoring inhaled gas to detect scrubber failure or scrubber breakthrough.

Unfortunately these two failure modes are just two out of nine failure modes in a rebreather that involve C02. The full list indudes: 1. Scrubber breakthrough 2. Scrubber bypass 3. Flooding of the loop causing an increase in breathing resistance 4. Flooding of the loop causing scrubber failure 5. Foreign material blocking any part of the breathing loop 6. One way valve failure (flapper valve failure) 7. Excessive Work Of Breathing 8. Excessive dead volume 9. The user has a physiology that causes them to retain C02 more than normal.

Serious or fatal accidents appear to have occurred due to each of these root causes, in some cases a series of accidents. The population of active rebreather users does not appear to exceed 10,000 in number, so it would appear these failure modes occur frequently. Many rebreather users also report having experienced one or more of the above failure modes but survived: this reinforces the view that these failure modes occur frequently.

It is possible to add additional sensors to a breathing loop to detect specific failure modes, but it is generally uneconomic. A good example is the one way valves, which are typically mushroom valves. These valves can tear, fail to be installed, catch on the web that holds the valve, or stick either open or shut in the presence of foreign matter. A simple method of detecting mushroom valve failure is to place an infra red LED inside the mouthpiece and sensors on either side of the mouthpiece -in the breathing hoses. The output of the sensors should show a peak not less than once every ten seconds, and should not both show a signal at the same time. The problem with this method of monitoring is that it is very specific to the mouthpiece and does not indicate any failures due to excess dead volume or work of breathing. It is a large cost for just one failure mode. It is also liable to maunction due to water ingress or detritus on the LED or sensor.

What may appear to be fixed features, such as the dead volume when the user is breathing from a mouthpiece, may not be fixed in all applications; for instance, when an oro-nasal mask is used the dead volume depends on how well the mask fits the user, and how haiti the user presses their face into the mask. That is, the range of causes for a high retained C02 is broader than simple equipment design issues.

These C02 related failure modes quickly disable the user due to an inherent positive feedback loop. Increases in retained C02 cause the user to breathe faster. This causes an increase in amounts of infra-red energy. The British company Analox Ltd even produces a range of dual channel sensors suitable for hyperbanc use. However, serious problems remain for carbon dioxide sensors to operate reliably in a rebreather, including: 1. Pressure causes a spectral broadening of the infra red absorption bands normally used to detect carbon dioxide. The broadening typically reduces the magnitude of the received signal by 300% over the range of depths used for manned underwater operations. The pressure broadening effect on C02 spectra is well known.

2. Inert gases, especially helium, cause a narrowing of the spectrum, which can result in a large change in the sensitivity of absorption infra-red sensors. This effect is well known, and described in papers such as that by Golovko, "Modeling of IR absorption spectra of the mixture C02-He at moderate and high pressures", Tenth Joint lnt. Symp. on Atmospheric and Ocean Optics/Atmospheric Physics I: Radiation Propagation in the Atmosphere and Ocean, G. G. Matvienko, G. M. Krekov, SPIE Vol. 5396 SPIE, Beilingham, WA, 2004) . 0277-786X1041$1 5 doi: 10.1117/12.548204.

3. Humidity in a rebreather is normally over 70%, and condenses. This is particularly the case for sensors for inspired gas, where the sensor is downstream of the scrubber. The reaction in a carbon dioxide scrubber generates water vapour.

4. Inert gases cause the infra-red emitter to cool, which modifies the spectrum: this can change the response by orders of magnitude. The correction to the received signal used in WO 02/036294 by the present inventor can be insufficient, with some emitter types, to produce a useable output with the required sensitivity at deep depths, when the infra-red emitter is sealed to prevent helium ingress but helium migration into the compartment around the emitter can be problematic.

5. The gas leaving the scrubber is warm: typically 35C to 95C in temperature, and the entrained water vapour in particular gives off Planck radiation that can be in the detection band. This creates a noise floor that can interfere significantly with the detection of carbon dioxide.

Basic carbon dioxide sensors such as GB2394281 simply do not work in practice due to these problems. An earlier invention by the present inventor, WO 02/036294, addressed the problems by correcting the received signal for pressure and helium, using hydrophobic membrane and the scrubber heat to keep the sensor dry. Others, such as Rose in US2007/0090290, dry the gas by using an injected gas or pressure expansion to create a dry gas that avoids condensation on the sensor.

Other methods such as fluorescent, sol-gel and chemical change sensors have been proposed on some internet forums for measuring inhaled rebreather gas, but none have been brought to a state where they have worked.

Methods involving heated sensors tend not to work in a rebreather due to the condensing humidity in the loop and the large differences in heat loss caused by the use of a variable fraction of helium as the make-up gas in the loop.

Object of the Present Invention It is an object of the present invention to detect a wide range of failures that cause carbon dioxide retention in rebreathers.

It is a further object of the present invention to detect one-way valve failure in rebreathers.

It is a further object of the present invention to detect when there is a failure of other parts of the breathing loop causing an increase in retained carbon dioxide levels due to excessive work of breathing.

If is a further object of the present invention to detect failures that cause an increase in the dead volume that affects the safe operation of a rebreather.

It is a further object of the present invention to detect carbon dioxide in the presence of helium, and under pressure, in a humid environment.

Summary of the Present Invention

The present invention is carbon dioxide sensor using the level of expired C02 as a means to detect the presence of a broad range of rebreather safety hazards, including failure of rebreather one-way valves, excessive work of breathing, scrubber failure, and excessive dead volume.

These failures are implied by the present invention from the nse in the partial pressure of carbon dioxide of the user's exhaled gas, by measurement of the partial pressure of carbon dioxide and application of the amplitude of that signal to trigger alarms or provide warnings.

Brief Description of the Invention and Figures

The invention will now be described by way of example, without limitation to the generality of the invention, and with reference to the following figures: Figure 1 shows an example of a rebreather loop with a C02 sensor fitted accortling to the present invention, comprising a user (1), who breathes into a mouthpiece or oro-nasal mask (2) containing two one-way valves that regulate the flow direction around the breathing loop such that exhaled gas is channelled to an exhale countetlung (4), and then to a scrubber (6). A C02 sensor (5) suitable for rebreather applications is located between the exhale port of the mouthpiece (2) and the scrubber (6). The rebreather loop contains an injector valve (7) for dosing the loop with an oxygen bearing gas from a gas cylinder (8), it may contain a further inhale counterlung (9), a loop over-pressure valve (3) shown here on the inhale counterlung but many rebreathers may locate the over-pressure valve (3) on the exhale counterlung, a means to add a make-up gas to the loop, shown here using an Automatic Diluent Valve (ADV) (11) from a second cylinder (10), and a breathing path to the inhale one-way valve in the mouthpiece (2).

Figure 2 shows a block diagram of a typical infra-red C02 sensor circuit comprising a signal generator or power source (12), driving an lnfra-Red light source (13) such as a silicon micromachined IR sourte or a lamp, which produces an infra-red output at a wavelength that is absorbed by C02 and at another wavelength that is not absorbed so significantly. The emitted Inf ia-Red signal is allowed to pass through sufficient length of path containing the gas (14) to be measured (typically 30 to 60mm), and the infra-red signal that has passed through the gas to be measured is detected by an inf ía-red sensor or detector (15) containing fitters, prism or grating to produce two signals, one of which is proportional to the amplitude of the signal at the wavelength absorbed by C02 and the other at the reference wavelength which is absorbed less strongly or not absorbed, which are then amplified by amplifiers (16) and (17), applied to a divider (18), the output of which is a ratio of the energy in the two signals: C02 absorption channel and reference channel. A means to correct the sensor output value is applied (19) using data from the environment such as depth or gas type, and the output is a signal proportional to the partial pressure of C02 in the infra-red light path. The amplifiers are normally a low drift chopper equivalent type, and may be AC coupled where the signal source (12) can be commutated quickly.

Figure 3 shows the waveforms in one example embodiment of a rebreather carbon dioxide sensor according to Figure 2, where the absorption of infra-red energy is used to detect the level of carbon dioxide. In Figure 3a when the Infra-Red source is switched on, the output of the

I

divider (18) shows a large negative drop, which peaks 360ms after the light source is switched on, due to thermal dynamics of the infra-red light source (13), gas thernio-dynamics and re-emission effects within the gas. The negative peak of this signal is proportional to the C02 gas concentration. In Figure 3b, when the infra-red light source is switched off, there is an instant extinction of the reference path due to no energy being absorbed by the gas at that wavelength but the C02 re-emits energy causing a positive peak in the output signal. The positive peak can be used as an alternative to the first peak on switch on, or to obtain greater accuracy out of the sensor system by averaging it with the first peak, reducing noise.

Figures 3c and 3d show the actual reference signal and actual C02 signal from the reference signal amplifier (16) and C02 signal amplifier (17) respectively, as a function of time for a series of four short light pulses. Note that the sensor amplifiers can be AC coupled, as in this case, which reduces problems of drift.

Figures 4a and 4b show two different views of a moulded plastic shell around which a hydrophobic membrane can be welded, to protect the carbon dioxide sensor from humidity. The shell sits on a circuit board using an 0-ring seal, and the other end is in contact with the scrubber canister in this example embodiment. The dimensions of the shell are in this case 75mm in diameter other dimensions can be scaled.

Figure 5 shows a block diagram of a circuit to overcome the effect of pressure and helium on the sensor, by ovenJriving the infra red emitter to maintain a constant amplitude of the decay signal in the reference channel. This is the same as for Figure 2, except there is an additional signal (21) that keeps the reference channel at a constant amplitude. A closely related embodiment uses a feedback signal taken from the correction unit (19) where the feedback signal can be checked to ensure it will result in the IR source operating within a safe operating envelope by using an additional pressure sensor (internal to the correction unit 19), and helium sensor (internal to the correction unit 19), such that the infra-red light source (13)-this will allow the IR source (13) to be driven with significantly more power than would be safe at one atmosphere pressure in air when under pressure or in the presence of a sufficient pressure of helium, as these media tend to cool the sensor such that considerably more energy may be required to maintain the infra-red light output at the wavelengths of interest (reference channel and C02 channel). It is advantageous but not essential to have the reference channel at a longer wavelength that the C02 channel, as when the Infra-Red sensor cools, the longer wavelengths seem to diminish in amplitude first -this is contrary to what would be expected but is the observation with several types of infra-red emitter in laboratory experiments.

Figure 6 is a UML description of the calibration and measurement algorithm for an example embodiment of a carbon dioxide sensor, the block diagram for which is described by Figure 5.

Operation of the Present Invention The operation of the invention will be described, by reference to example embodiments without limit to the generality of the invention.

The functionality of the present invention should be apparent to a person skilled in the art of rebreathers and sensor electronics from Figures 1 to 5, the brief description above, in conjunction with the following description of the operation of the present invention.

For brevity, the examples will assume the user is a diver, and the rebreather is the closed loop type rather than semi-closed in that it uses both a make-up gas and an oxygen containing gas to maintain the loop PPO2, though the invention can be applied to the widest range of rebreathers including pure oxygen rebreathers and to semi-closed rebreathers without material modification.

A loop type rebreather will be described, though by the application of a masking gate function to the sensor signal, or by use of averaging, the invention can be applied to pendulum type rebreathers. The example rebreather will use two countertungs, but the invention is equally applicable to single counterlung rebreathers.

The partial pressure of C02 (PPCO2) in exhaled gas is normally around 0.04 ATM, but under heavy work the respiratory quotient increases, and the exhaled gas can contain 0.06 ATM of C02 without there being any fault condition in the rebreather. Where there is no scrubber fitted, the second or third breath will see the exhaled PPCO2 increase to 0.08 ATM or more.

The failure of the breathing loop can be indicated to the user using a tricolour LED. For example, using a Blue/RedlGreen LED the C02 information status may be communicated as: * If 0.OO5ATM <PPCO2 then LED off. This indicates there is no breathing detected in the loop, or there is a complete failure of the inhale one way valve.

* If 0.005 ATM < PPCO2 < 0.035 ATM, the Green led is flashing, with 500ms flashes, 250ms pauses, then repeat on an 8 second cycle. Each flash means the PPCO2 is 0.005 and, so 0.03 would be 6 green flashes; Values are rounded to the nearest 0.005.

This means that the loop should be checked, in particular for partial or complete failure of the inhale valve.

* If 0.035 ATM < PPCO2 <= 0.060 ATM, the blue led is flashing with 500ms flashes, 250ms pauses, then repeat on an 8 second cycle. Each flash means the PPCO2 is 0.01 so 0.06 would be 6 f'ashes; Values are rounded to the nearest 0.01. Values in this range are normal.

* If 0.060 ATM < ppCO2 < 0.065 ATM, there is blue solid light; This is the maximum safe level.

* If ppCO2 >= 0.065 ATM, there is red solid light and the rebreather should preferably close the breathing loop to prevent the user breathing from it while in this hazardous state, such as by an automatic loop shut off valve described by the invention US 6,817,359 with the shut off valve located preferably after the inhale one-way valve in the mouthpiece (2).

Where the C02 sensor has a high power consumption, it is of benefit to combined the C02 monitoring with a respiratory monitor, such as that described in GBO51 6751.5 to reduce the power consumption and provide a very rapid response to a change in breathing pattern: any increase in retained C02 causes an automatic change in breathing rate.

The fault modes are detected by the expired C02 monitor as follows: * Scrubber breakthrough causes a rise in the inspired C02, and a similar rise in expired C02. The body acts as an averaging mechanism, that reduces false alarm rates that are present if a pure level based inspired C02 sensor is used: scrubbers often breakdown momentarily well before they become exhausted, due to surface area saturation effects in the scrubber. For example, if a person working very hard is producing a PPCO2 of 0.065 ATM, a scrubber breakthrough would increase this above alarm thresholds. A user who is working more gently may tolerate a higher degree of breakthrough, and an inspired PPCO2 of 0.03 ATM is not unreasonable as a safety limit in that case.

* Scrubber bypass causes a similar rise in inspired and expired C02 as for scrubber breakthrough above.

* Flooding of the loop causing an increase in breathing resistance, which is only detectable either directly as water in the loop (which would require multiple sensors) or as an increase in expired C02. The expired PPCO2 will exceed 0.065 very rapidly in the event of a senous loop flood.

* Flooding of the loop causing scrubber failure. This is detected in the same manner as loop flooding, and later, as scrubber breakthrough above.

* Foreign material blocking any part of the breathing loop. This resufts in an increased Work of Breathing, causing increased retained C02, and an increase in expired C02.

The PPCO2 will increase to above 0.065 ATM rapidly if there is a significant loop blockage.

* One-way valve failure (flapper valve failure). Where the inhale or exhale one-way valves fail in the mouthpiece (2), the PPCO2 will fall to an unusually low level. This should indicate a problem to the user, and can be checked for automatically during pre-dive checks: the rebreather controller or monitor can check that the expired PPCO2 is above 0.03 ATM. The location of the C02 sensor has a material bearing on the ability to detect this failure mode reliably: if the sensor is in the exhale counterlung or too close to the mouthpiece, it will generally not be detected in every case. However, it the C02 sensor is located between the exhale counteriung and the scrubber as shown in Figure 1, then this mode can be detected very reliably.

* Excessive Work Of Breathing, causes an increase in retained C02 at depth, or when the user is working hard. This shows as an increase in expired C02 levels and can be detected easily.

* Excessive dead volume, causes an increase in the expired C02 that travels to the exhale counterlung, and hence to the expired C02 sensor. This high reading will vary depending on work levels, but it will be noticeably higher and will trigger the suggested 0.065 ATM alarm level before it is a disabling factor.

* The user has a physiology that causes them to retain C02 more than normal. The user will have a high expired PPCO2 level when performing work at depth. This will require the user to slow down and take more caution than average diver.

The above failure modes may occur singularly or multiple modes may occur at the same time.

The important parameter to track is the blood 002 level, which is well represented in the expired PPCO2 level which is measured by the present invention and used as the basis of an alarm system.

A problem that plagues C02 sensors for measuring inhaled C02 is that of calibration. Even the background ambient carbon dioxide level, currently averaging around 38Oppm, undergoes significant seasonal variations. In addition to that are changes in carbon dioxide levels due to nearby machines, flames, poor building ventilation and other factors. The result is that background C02 levels vary from 200ppm to 800ppm, depending on location and the environment. This is a ratio of 4:1, so any extrapolation from that calibration point would produce an error of up to 400%. These large tolerances in the calibration gas makes inhale side C02 sensors almost unworkable in an operational context, without regular recourse to laboratory calibration using trace gases.

It is possible to calibrate an inspired C02 sensor if the user is able to provide a reference ambient carbon dioxide reading taken independently that is entered by a menu on a micro-controller display integrated with the device. However, this can cover only those environments where either an independent calibrated C02 monitor is used or C02 ambient data is available.

This calibration problem can be ovettome in the present invention by calibrating the C02 sensor to the partial pressure of C02 in the exhaled gas from a relaxed user, which is normally between 0.035 and 0.04 ATM: an error band of just 14%. By the user breathing out into the exhale one-way valve in the mouthpiece (2) with the inhale side of the rebreather disconnected, a known gas can be applied to the C02 sensor for calibration purposes, which has a very much smaller tolerance than the gas in the ambient environment, as a fraction of the full scale or range of the sensor. This process can be initiated automatically in the present invention by detecting the scrubber is open and the scrubber is removed, such as by detecting the presence of light: the inside of a scrubber canister is normally darker than the ambient light level,. The user can then be prompted to exhale into the sensor assembly to obtain a reading of a users rest expired C02 level as a calibration gas.

Attention will now turn to the practical issue of how to measure C02 in a rebreather.

The level of carbon dioxide may be measured by some new forms of sol-gel sensor directly, but these are not readily available at the present time. There are micro-miniature phase fluorometers available commercially, but these appear to suffer significant problems in a rebreather type environment at the present state of this technology, particularly aging and contamination of the sol-gel. However, when those technologies mature, a similar method as described here can be used with those sensors, including protection from humidity, power management and multi-variate analysis to improve the signal to noise ratio under fluctuations of temperature and pressure.

The method for measuring C02 that the invention uses in an example embodiment relies on the absorption of infra-red energy by carbon dioxide.

There are many possible circuits for a carbon dioxide sensor involving the measurement of the ratio of the signal strength between a measurement channel and a reference infra-red channel, where the measurement channel has a filter at an absorption peak for the desired gas, namely for carbon dioxide, and the reference channel has a nearby frequency that is absorbed to a much lesser extent by carbon dioxide. Sensors fitted with these filter combinations are readily available commercially.

The unique challenge for a sensor operating in a rebreather is to overcome the thermal noise, and variable effects of pressure and inert gases. The following description will therefore focus on an example embodiment using a circuit that includes pressure and helium compensation functions.

An example embodiment of a suitable carbon dioxide sensor is shown in Figure 2, along with its related operating waveforms, mechanics and algorithm description shown in Figures 3 to 6 inclusive. The circuit implementing that block diagram and its operation would be apparent to a person skilled in the art of rebreathers and sensor electronics from those figures, but there are some particular features that are important, to which the reader's attention will now be drawn.

In Figures 2 and 5, the C02 sensor drives an infrared source (13) from a signal generator (12), across a gas path (14), to detect the received signal using a detector (15) with amplifiers (16) and (17), divider (18), and correction block (19). The light path (14) should be protected from ambient light and condensation, such as by using a black tube for the path, surrounded by an external hydrophobic membrane. It is important there is a sufficient length of the light path (14) to contain sufficient C02 molecules to produce a marked absorption response. In C02 sensors designed for use in office type environments the light path often uses mirrors to cause the light to reflect backwards and forwards to provide a longer light path than the dimensions of the physical measurement chamber. In a rebreather the path should ideally be straight, as the effects of and condensation on the minor will be very pronounced. Path lengths of 35mm have been found to be sufficient, given careful low noise design of the circuit stages.

The divider (18) may be realised digitally by sampling the outputs of amplifiers (16) and (17), and applying an analogue to digital conversion process. Alternatively, it is possible to implement the ratio circuit using analogue differential amplifiers.

A thermal connection of the sensor light path to the scrubber has been found to be of benefit in raising the temperature of the gas path, to further exclude moisture and reduce the risk of condensation.

The optical filters in the detector (15) pass a wavelength of light that is inside the carbon dioxide absorption band. Sensors are available commercially with filters centred at 4.260 nm (2349cm1) for the detector C02 channel, with a second wavelength of lR light that is outside the C02 absorption band for use as the reference channel of the detector (15) at 3.900nm or the carbon monoxide band at 4.700nm. Other C02 spectral bands include 1 5um (667 cm1), but producing a stable IR source at that wavelength in portable equipment is difficult.

The reference channel should preferably avoid the water absorption spectra with wave numbers from 1000 to 2000, and avoid the absorption spectra for other gases that may be present.

The correction unit (19) adjusts the dMder output to compensate for variations in the environment parameters that affect the divider output. The list of the compensated parameters can include ambient temperature, thermal shifts due to gas law effects, pressure, humidity, type of the gas, and circuit parameters. In some cases, it can be advantageous to generate the IR source power control feedback signal (21) from the correction unit, where limits can be introduced to provide an extra degree of safety to avoid applying too much power to the lR source (13).

The examples of the detector responses are shown in figures 3.a to 3.d. A step input applied to the lR source generates one pulse shown in figure 3a in each detector output channel. Each pulse of the IA source generates two pulses per detector channel shown in figure 3.b. The response of the lR source pulse sequence measured in the reference and the C02 channel of the detector are shown in figures 3.c and 3.d. The amplifiers (16) and (17) are normally chopper type DC amplifiers, but may include a high pass filter to remove the effects of DC offset and drift, as was used in the circuit that generated these waveforms.

It is critically important to keep liquid water from condensation or from cleaning processes out of the light path (14) between the IR source (13) and the detector (15).

Liquid water strongly absorbs IR radiation at the same wavelengths as C02, so even very small water droplets anywhere in the sensing cell will generally cause erroneously high gas concentration readings. To protect the light path and electronics against water, the shell shown in Figure 4 is fitted with a gas permeable hydrophobic membrane, such as Zitex Al 05 or GE PTFE based hydrophobic membrane. These membranes can be welded to suitable plastics from which the shell can be moulded, such as PVDF (Kynar), and low off-gassing of Polypropylene: PP that is free of plastisizing and softening agents, to form a chamber that prevents water ingress to the C02 sensing area, but allows C02 to pass freely.

The light power of the IR source depends on the thermal conductivity of the ambient gas. In Figure 5, the feedback from the reference signal amplified by the amplifier (16) is used to control the source generator (12) to increase the amplitude of the source power supply when the reference signal is less than the set level and decrease the power supply when the signal is more than the set level.

The lR source (13) can be of any of several different types, including infra-red bulbs and silicon micro-machined inf ia-red sources. If helium or high pressure gas is in contact with the inf ia-red source, it will cool relative to the temperature the source operates at in normal atmospheric conditions (standard pressure, temperature, dry). This cooling will change the spectrum of the emitted mt ia-red light: in most cases, the longer wavelengths will be attenuated. If there is no energy being emitted in the absorption band, then the entire sensor will not work. It is important therefore to maintain the spectrum of the IR source (13) constant, or reasonably constant. There are two methods by which this can be achieved: 1. Protection of the IR source (13) from pressures and inert gases, such as by sealing the IR source in a chamber maintained at one atmosphere with a sapphire window through which light passes down the measurement path (14). This solution is expensive and prone to fail, particulariy through helium ingress.

2. Increasing the power to the IR source (13) to maintain the spectrum of the emitted light.

This can be done effectively, but generally requires enough power that it ft were applied to the IA source (13) at one atmosphere in air, the operating life of the emitter would be reduced very drastically. For example, one particular silicon micro-machined emitter that is commercially available and provides a strong IA source across the spectrum coveting 3.900nm to 4.700nm wavelengths, has a power supply of 3V0 at one atmosphere in air, with an absolute maximum supply rating of 3V3. Application of more than 3V3 at one atmosphere pressure in air will destroy the device, almost instantly. In 60 bar of helium, a power supply voltage of 7V3 has to be applied to obtain the same spectrum: the device is not damaged by this, because its operating temperature is the same as with 3V0 in air.

The feedback channel (21) allows the power to the IR source (13) to be increased in this manner. To avoid sudden failure of the emitter due to a water droplet or dust attenuating the reference channel, it is advantageous in some embodiments to generate the source power feedback signal (21) from the correction unit, where the partial pressure of helium (PPHe) and ambient pressure can be measured such that the feedback signal is always within a safe operating envelope: this is an alternative feedback path to that shown in the embodiment in Figure 5 As many gases absorb well in the IA area, it is often necessary to compensate for interfering components by correction block (19). The correction or compensating factors can be obtained by charactetising the sensor in the range of environments it may be exposed to, and isolating each of the parameters affecting the accuracy of the result, using normal processes for Multi-Variate Analysis to create a set of Sunogate Models or Response Surface Models that can be applied to the output of the divider (18) to produce a C02 signal that has an improved signal to noise ratio. The description of a typical calibration and measurement algorithm using UML (Unified Modelling Language) is given in Figure 6, including a parameter obtained by Multi-Variate Analysis, and the Surrogate Model equation this produced.

It is beneficial to chop or modulate the source generator (12) so that thermal background signals can be offset from the desired signal.

The power consumption of the C02 sensor assembly generally requires management, by switching the circuit on periodically for a short period to take a measurement, then switching it off.

It can be seen from the waveforms in Figures 3 that there is a minimum time required to obtain the maximum amplitude of signal, typically 360ms, which means that to take a single on-off measurement will generally require a second. This is not a response time: the response time for the C02 sensors can be a few tens of milli-seconds using chopper methods or source modulation to remove background thermal drift, but it is a time that the circuit must be on to obtain a measurement with the maximum resolution. The power consumption of a C02 sensor is difficult to reduce below 1 7OmA at 3V (510mW), and if active helium compensation is used in the correction unit (19), this figure can double as helium is measured by measuring the thermal capacity of the gas -such as by heating a resistor and measuring the time ft takes to cool by a predetermined percentage. Ha to one Watt of power is a large amount of power for a piece of

portable equipment.

There are several methods by which the mean power can be reduced. If the sensor is used just once every minute, then this power reduces by a factor of 60, but the effect of a flood in a rebreather, for example, can cause a very rapid escalation of retained C02 levels which may fall between the sample points from the time ft starts to the time where the user suffers a disabling injury. A preferred method of reducing the power consumption is to use a long interval between periodic measurements, but trigger an immediate measurement it there is a change in the user's breathing rate. The breathing rate can be measured easily using a differential sensor in the mouthpiece with a 0.3Hz low pass fiRer to remove noise from speech, clicks and fricative breathing noise.

It will be appreciated by a person skilled in the art that the monitor and alarms may be implemented using a micro-controller or gate array, and that various functions within the circuitry would be camed out by that digital logic. Where a rebreather controller contains a suitable micro-controller or gate array, the carbon dioxide monitoring system may be fully integrated within the rebreather controller or combined with another monitor, such as the partial pressure of oxygen (PPO2) monitor that is normally fitted to such equipment.

Claims (17)

1. Claims 1. A rebreather safety monitoring device comprising: a carbon dioxide sensor, a means to sample the gas in a rebreather breathing loop from a location between the inhale one-way valve on the rebreather mouthpiece and the carbon dioxide scrubber and prcvide that gas to the carbon dioxide sensor, a means to provide alarms or warnings based on the level of the expired carbon dioxide.
2. 2. A monitor according to Claim 1, wherein the carbon dioxide sensor is calibrated using the carbon dioxide level in human expired gas.
3. 3. A monitor according to any of Claimsi to 2, wherein the carbon dioxide sensor is fitted with a hydrophobic membrane.
4. 4. A monitor according to any of Claims 1 to 3, wherein the carbon dioxide sensor is a dual channel infra-red absorption sensor.
5. 5. A monitor according to any of Claims 1 to 4, wherein the carbon dioxide sensor is powered up with a low periodic duty cycle, but where the alarm system is powered even when the carbon dioxide sensor is powered down.
6. 6. A monitor according to any of Claims 1 to 5 wherein the carbon dioxide reading is powered up and a reading taken when a change in the user's respiratory rate is detected.
7. 7. A monitor according to any of Claims 1 to 6 that is augmented by a pressure sensor that is applied to compensate the carbon dioxide reading for changes in ambient pressure.
8. 8. A monitor according to any of Claims 1 to 7 that is augmented by a helium sensor that is applied to compensate the carbon dioxide reading for changes in the partial pressure of helium.
9. 9. A monitor according to any of Claims 1 to 8, wherein the carbon dioxide sensor is a dual channel inf ta-red absorption sensor with means to drive the inf ia-red light source with greater power when under ambient pressure or in the presence of gases having a high thermal capacity, than in air at one atmosphere pressure, thus stabilising the spectrum of the emitted light under a range of pressures or in the presence of helium.
10. 10. A monitor according to any of Claims 1 to9, wherein the carbon dioxide sensor is a dual channel infra-red absorption sensor with the infra-red emitter isolated from the effects of pressure and helium such that the emitted spectrum does not change by more than 50% under the range of operating pressures or helium the monitor covers.
11. 11. A monitor according to any of Claims 1 to 10, wherein the alarm level is used to drive shut a valve that closes the breathing loop.
12. 12. A monitor according to any of Claims 1 toll, wherein the alarm level is used to drive shut a valve that closes the breathing loop at the mouthpiece and switches the user to an alternative gas source.
13. 13. A monitor according to any of Claims 1 to 12, wherein the absence of carbon dioxide, or the presence at a partial pressure lower than that normally expired by a human, is applied to trigger a warning or alarm level.
14. 14. A monitor according to any of Claims 1 to 13, wherein the presence of carbon dioxide at a partial pressure higher than that normally expired by a human is applied to trigger a warning or alarm level.
15. 15. A monitor according to any of Claims 1 to 14, wherein the circuitry is integrated with a partial pressure of oxygen monitor or measurement device or controller.
16. 16. A monitor accorJing to any of Claims 1 to 15, wherein the carbon dioxide sensor is coupled thermally to the scrubber such that it operates at more than 3 degrees Celsius above the ambient temperature.
17. 17. A monitor accotIing to any of Claims 1 to 16, that is calibrated automatically when the scrubber canister is opened and the scrubber removed, as indicated by the presence of light on a light sensor, with a human interface that allows the background level of carbon dioxide to be indicated to the calibration system.