GB2427365A

GB2427365A - Rebreather with gradual CO2 breakthrough

Info

Publication number: GB2427365A
Application number: GB0512359A
Authority: GB
Inventors: Alex Deas
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-06-17
Filing date: 2005-06-17
Publication date: 2006-12-27
Anticipated expiration: 2025-06-17
Also published as: GB2427365B; GB0512359D0

Abstract

A rebreather which has a plurality of scrubbers 32, 33 or scrubber regions operating in parallel where the gas flow through the scrubbers or scrubber regions is unequal. Preferably the scrubbers 32, 33 or scrubber regions have independent CO2 sensors or thermal monitors. The unequal gas flow may be created by use of a membrane with perforations or varying permeability, baffles with varying shapes or positions or by a physical offset in the inlet 13 or outlet hoses. The rebreather may have a radial or wide axial scrubber with an uneven backpressure or airflow to provide gradual collapse. The scrubber may use granular material with offset inlet and outlet hoses. Alternatively a laminar scrubber may vary the gas flow using cones. The unequal gas flow may be in the ratio range from 1:5 to 1:1:1 and may indicate the scrubber life or operation.

Description

Rebreather with Gradual C02 Breakthrough Rebreathers are recirculating gas systems to enable a person to breath underwater or in hazardous environments. The rebreather removes the C02 that is exhaled and injects 02, to maintain a breathable gas mixture. To remove C02, a scrubber is used. The scrubber is an alkaline immobilised in either granules or a porous plastic extrusion. The active fhemicals are usually Calcium and Sodium Hydroxide, such as in the brand Sofnolime (RTM) though others are also used, including Potassium Hydroxide and Lithium Hydroxide. Sonfolime (RTM) is the most widely used scrubber material and comprises Calcium Hydroxide - Ca(OH)2 (75%), Water H20 (20%), Sodium Hydroxide - NaOH (3%). A general description of the reaction of sodium and calcium hydroxide with C02 is as follows: 1. The gaseous C02 reacts with water to form carbonic acid - H2C03. 2. The NaOH reacts with the carbonic acid to produce Na2C02 and H20. 3. The Na2C02 reacts with the Ca(OH)2 which has been disassociated into Calcium and Hydroxide Ions. (Ca++ and OH-) to produce CaC02 (calcium carbonate, chalk). 4. There is a net production of one extra H20 molecule for every molecule of C02 which is taken in. The chemical reactions are: C02 (g) + H20 (I) ≤> H2C03 (aq) H2C03 (aq) + 2 NaOH (aq) -> Na2C03 (aq) + 2 H20 (I) + Heat 2NaC03 (aq) + Ca(OH)2 (aq)-> 2NaOH (I) + CaC03 (s) + Heat The C02 absorption reaction sequence is highly exothermic Figures 2 and 3 show a experimental apparatus using which the temperature through the axis of a scrubber was measured in 18 locations. Sensors have been developed to measure the life of the scrubber and indicate this to the rebreather diver, by measuring the temperature differential across the scrubber. Such sensors were in common knowledge by 1985, and are included for example, in Readey et. Al, US Patent 6,003,513. Readey measures the temperature of the scrubber in eight positions. Peter Steggle disclosed first in 2002, now published on , also using 8 temperature sensors. The APD Evolution rebreather also uses a "scrubber temperature-measuring stick, which indicates on the handset which part of the scrubber is being used and so helps the user understand its depletion. It measures temperatures throughout the profile of the stack and is claimed to compensate automatically for depth, work-rate, water temperature and part-used scrubber material." The problem with these sensors is that when the scrubber is depleted, C02 levels rise very rapidly. In the experiment producing the data of Figure 3, the experiment was started at 12:38 and finished at 15:20. During the experiment the C02 concentration reading from gas monitor was 0.0% until at 15:11 a rapid growth of the C02 concentration was observed, starting at 0.006% C02, rising within 9 minutes to 5% C02.The reading continued to rise due to the release of C02 from that stored in the form of carbonic acid (H2C03). There appears to be no correlation between any of the temperature readings or combination thereof (18 sensors distributed evenly along the axis of the scrubber), and the C02 level (bottom-most trace). The temperature data can provide an indication that a scrubber is operating, or that it is in a suitable pre-dive state. This rapid rise of C02 levels is highly dangerous to a user. Scrubber sensors and C02 sensors can indicate the start of that rise, but the sensors tend to have a long time constant which means the readings lag the C02 levels considerably, and in any case, the user cannot usually remove himself from the rebreather within the short time available. It is the object of the present invention to provide a rebreather which has a deliberately gradual onset of scrubber failure and which shows a slow rise in C02 as the total mass of scrubber material is depleted. The present invention uses two scrubbers or two sections of scrubber, such that one scrubber or section of scrubber has a higher airflow than the other section. The result is that the dwell time is different for the two scrubbers or scrubber sections, and hence the onset of scrubber depletion is different. When the two scrubbers operate in parallel the C02 levels at output of the scrubbers, whether combined airflow or the output of two C02 or scrubber life sensor systems, show a rise in CO2 levels while the user then has a known time period in which to abort their dive, move onto bail out equipment, or remove themselves from the hazardous environment. 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 exploded drawing of a rebreather according to the present invention, where a frame 1 holds a cylinder of dilutent gas - the large cylinder under 3, an oxygen cylinder 5, via a bracket 7, a breathing bag 9 connected to a scrubber by an inlet connector 11 and outlet connector 13, the scrubber housing 15 holds two scrubbers.The connectors 11 and 13 are arranged such that the gas enters the scrubber container at the bottom and flows through both scrubbers in parallel, with the inlet offset from the centre of the two scrubbers or a baffle or other obstruction such that the flow through the two scrubbers have different back pressures, the scrubber unit being connected to a scrubber head 25 holding the electronics, connecting to the handset or information unit 21, the breathing loop 19, and any fail safe auto bailout valve control 17 The autobail out cuts off the gas supply to the diver from the rebreather and provides gas from the dilutent cylinder, which is either exhausted by the diver or is exhaled into the breathing bag thereby acting as an auto-flush of the system - excess gas is vented via overpressure valves on the breathing bag or scrubber head.Gas is taken from the cylinders to the rebreather via hoses 23, including any pressure sensing hose or electrical lead indicating cylinder pressure. Figure 2 shows an example of a scrubber housing according to the present invention with partial transparency of solid items, where a scrubber housing 15 has gas from the diver enter it via the breathing bag connector 13 and exit to the diver directly or through a second breathing bag via a connector which forms part of the breathing loop 19 commonly comprising connectors, hose, unidirectional valves, mouthpiece, further unidirectional valve, hose and connector. The breathing loop can run through the scrubber head into a connector 11 into the inlet of a breathing bag to minimise strain on the breathing bag from moving connectors.The scrubbers 32 and 34 operate in parallel, but the gas inlet 13 is offset in this example so the scrubbers will have differing gas flow. The scrubber head 25 holds in this example a hermetically sealed section 34 with batteries, USB connector to a PC, charger socket 30 compatible in this example with a Nokia (RTM) phone for ease of supply. Under the gas exit port into the breathing loop 19 are sensors including oxygen sensors and in this example C02 sensor surrounded by a hydrophobic material and membrane protecting the sensor, humidity sensors, moisture sensors, temperature sensors, and scrubber life sensors. Figure 3 shows the airflow through gas distribution cones (3 and 19 in Figure 2), using Ansoft 3D Modelling Software, with a different flow rate through one scrubber than the other, where the inlet air flow is offset.Figure 4 shows empirical measurements of the temperature distribution in a perfectly uniformly packed scrubber, the measurement being taken using a 8.5 digit TTI 1906 Computing Bench Reference Multimeter from 18 temperature sensors 41 distributed linearly along the axis of the centre of the test scrubber 39. The battery and electronics 30 in this case is a bench meter, supplemented with a C02 monitor 37, with the user 47, breathing in air 45 from the environment and exhaling into a test scrubber housing 43. The exit air stream 33 is controlled by a plate 35. Figure 5 shows a set of results from the experiment shown in Figure 4, with temperature from each sensor plotted against time in minutes. The measurements were taken using a 4 inch diameter, 150mm long prepacked scrubber from Extend Air Inc Similar readings can be observed using granular scrubber material. The operation of the present invention will now be described with reference to the figures and by way of examples without limitation to the generality of the invention. The key feature of the present invention is a gradual scrubber breakdown arranged by creating an non-uniform air flow through the scrubber, or by a plurality of scrubbers differentiated by different back pressure to the gas flow. The result is that the dwell time through the different regions of the scrubber is different, and hence the scrubbers break down at different times. There are various different possible implementations of the present invention. One is shown in Figures 1, where two scrubbers are arranged with gas flow from a breathing bag into the scrubber housing, offset such that one scrubber receives a higher gas flow than the other. Another method of achieving the same result is to use flow cones or baffles that create a greater back pressure in one scrubber than the other as shown in Figure 2 and 3. A third method of implementing the present invention is to arrange the gas flow across the input face of the scrubber such that one section of a large scrubber, such as a radial scrubber, receives more gas than another section. Baffles on the input of a scrubber can be located alternatively on the output of the scrubber with similar effect.A fourth implementation fits a pad to one end of the scrubber, preferably the outlet, with an array of holes which vary in location or size, such that air flow through one section of the scrubber is different to that of another: for example, more holes or perforations can be made near the centre and fewer on the periphery. A fifth implementation is to use a gas permeable membrane, with different permeability across the scrubber. In an implementation where there there are two scrubbers in operation, the gas flow through one being 66% of that through the other scrubber is an effective ratio, that is a ratio of 2:3, though ratios of 1:2 to 1:1.1 can be used depending on the life of the scrubber and the degree of soft scrubber failure that is desired. The present invention should be combined, preferably, with scrubber life sensing and C02 sensing. In this case, the user will see a rise in C02 levels, or a fall in scrubber temperature, especially comparing one scrubber with the other, and this will signal the user to abort the dive, or move to bail out equipment, long before the C02 levels rise to a dangerous level. Where two scrubbers are used, with a gas flow ratio of 1.2, the failure time of 9 minutes can be extended to more than one hour using the same scrubber type as shown in the experiment in Figure 4a and 4b. Where a single scrubber is used, that is not laminar in flow, such as it uses granular scrubber material rather than extruded material such as ExtendAir plastic scrubber cartridges, then the gas flow through the scrubber is highly turbulent and creating a region in a single scrubber that has more gas than another region, is more difficult but can be achieved by use of either multiple membranes with perforations or permeability arranged to prefer one region over another, or by a substantial offset of both inlet and outlet hoses or gas streams. With granular material, the offset of a single hose is insufficient to meet the technical requirements for irregular gas flow that are imposed by the present invention.

Claims

1. A rebreather having a plurality of scrubbers or scrubber regions operating in parallel, where the gas flow through the different scrubbers or regions is designed to be unequal.

2. A rebreather having a radial scrubber or wide axial scrubber, where the backpressure or airflow through the scrubber is uneven to ensure that one region of the scrubber will fail before a different region, and provide a gradual collapse in scrubber performance instead of sudden collapse.

3. A rebreather according to claims 1 or 2 where the scrubbers or scrubber regions have independent C02 sensors.

4. A rebreather according to any of claims 1 to 3 where the scrubbers or scrubber regions have independent thermal monitors indicating scrubber life or operation.

5 A rebreather according to any of claims 1 to 4 where the difference in gas flow is created by use of a membrane with perforations or of varying permeability and is arranged across the inlet or outlet of the scrubber or is in contact with the scrubber material.

6. A rebreather according to any of claims 1 to 5 where the difference in flow rate through the scrubbers is achieved by a physical offset in the inlet or outlet gas hoses or connections sensor with respect to one scrubber compared to the other.

7. A rebreather according to any of claims 1 to 6 where the difference in flow rate through the scrubber or scrubber regions is achieved by the use of baffles that differ in location or design between one scrubber section and another.

8. A rebreather with a scrubber using granular scrubber material with inlet and outlet hoses offset by a similar amount to create uneven gas flow or preferential flow through different regions of the scrubber.

9. A rebreather with a scrubber using laminar scrubber material that achieves different gas flow through different regions by the design of different gas flow cones. 10 A scrubber device according to any of claims 1 to 9 for mounting in a rebreather.