Background to the Invention

A rebreather is a breathing apparatus that absorbs carbon dioxide from a user's exhaled breath so as to allow the substantially unused Oxygen and Nitrogen in the breath to be recycled and rebreathed. By adding sufficient oxygen to compensate for the metabolic usage, removing the carbon dioxide, and rebreathing the gas, most of the volume is conserved.

Rebreather apparatus differs from open-circuit breathing apparatus, where the exhaled gas is discharged directly into the environment.

One of the essential requirements of a rebreather unit is that it include a counterlung or breathing bag which holds gas when it is not in the diver's lungs. The counterlung is designed to change in volume by the same amount as the user's tidal volume when breathing. This assists in maintaining a constant total volume of gas in the lungs and the rebreather unit throughout the breathing cycle. The volume of the counterlung should allow for the maximum likely breath volume of a user.

Tidal volume (symbol VT or TV) is the lung volume representing the normal volume of air displaced between normal inhalation and exhalation when extra effort is not applied. In a healthy, young human adult, tidal volume at rest is approximately 500 mL per inspiration or 7 mL/kg of body mass, however during exercise or when under stress, tidal volume increases significantly and can approach 3 liters to 5 liters, depending on the diver’s unique physiology and respiratory needs.

Underwater, the position of the counterlung - on the chest, over the shoulders, or on the back - has an effect on the work of breathing. This is due to the hydrostatic pressure difference between the counterlung and the diver's lungs caused by the vertical distance between the two.

The various locations of the counterlung have their own pros and cons:

• Front mounted counterlung: When horizontal the counterlung is under greater hydrostatic pressure than the diver's lungs. This makes it easier for the diver to inhale, harder to exhale.

• Back mounted counterlung: When horizontal the counterlung is under less hydrostatic pressure than the diver's lungs. The amount of pressure varies, as some counterlungs are located closer to the back than others. This makes it harder for the diver to inhale, and easier to exhale. • Over the shoulder counterlung: When horizontal, the counterlung is positioned close to the lung centroid, resulting in good breathing characteristics, however the hydrostatic pressure will vary depending on how much gas is in the counterlungs, and increases as the volume increases and the lowest part of the gas space moves downward.

The design of the counterlungs can also affect the swimming diver's streamlining due to location and shape of the counterlungs themselves. Back mounted counterlungs are generally the most streamlined while over the shoulder types offset their good hydrostatic positioning by normally being more bulky than other types.

A further problem faced with traditional rebreather systems, is that the total air volume of the breathing loop comprises the counterlung volume (movable portion) as well as the breathing hose volume (immovable portion). This large air volume requires the diver to use lead ballast in order to counteract the buoyant lift of the air volume, adding to the weight of the system.

It is an object of this invention to provide a rebreather which at least partially alleviates some of the above-mentioned problems.

Summary of the invention

In accordance with this invention there is provided a breathing hose for ducting inhaled and exhaled gas between a user and a rebreather apparatus, the breathing hose comprising a collapsible inner tube located within a non-collapsible flexible outer tube.

There is further provided for the collapsible inner tube to inflate to accommodate the exhaled breath of the user, and to deflate to accommodate the inhaled breath of the user/

The collapsible inner tube is sized to accommodate at least a portion of the breath volume of the user, and can be sized to accommodate the maximum breath volume of a user.

The collapsible inner tube functions as an in-hose counterlung.

The collapsible inner tube may be used to complement the function of a traditional counterlung in a rebreather unit, by sharing the work of maintaining a constant total volume of gas in the lungs and the rebreather unit throughout the breathing cycle.

Alternatively, the collapsible inner tube may replace the traditional counterlung in a rebreather unit. The collapsible inner tube may be in-elastic, with no bias in either the full or empty state, alternatively the collapsible inner tube may be pre-formed with a memory to create positive and/or negative pressure to counteract hydrostatic pressure imbalances, further alternatively, the inner tube may be elastic so as to bias the gas to move through the rebreather apparatus towards the inhale hose.

A hollow member may be located inside the collapsible inner tube, extending along the length of the collapsible inner tube, for preventing the complete collapse of the inner tube under hydrostatic pressure.

The hollow member may include, but is not limited to a hollow tube, or a spiral.

The in-hose counterlung may be used in conjunction with either a Closed Circuit Rebreather, or a Semi-closed Circuit Rebreather.

Brief Description of the Drawings

Preferred embodiments of the invention are described below by way of example only and with reference to the following drawings, in which;

Figure 1 is a schematic illustrating a traditional rebreather loop as is known in the art for closed circuit and semi-closed circuit rebreathers;

Figure 2 is a schematic illustrating a first embodiment of the breathing hose of the invention in use with the inner tube in a deflated state;

Figure 3 is a schematic illustrating the first embodiment of the breathing hose of the invention in use with the inner tube in an inflated state;

Figure 4 is a schematic illustrating a second embodiment of the breathing hose of the invention in use, with the inner tube in the inflated state;

Figure 5 is a schematic illustrating a second embodiment of the breathing hose of the invention in use, with the inner tube in the deflated state;

Figure 6a is a cross sectional view A-A of the breathing hose of the invention, with the breathing hose in the inflated state;

Figure 6b is a cross sectional view B-B of the breathing hose of the invention, with the breathing hose in the deflated state;

Figure 7 is a schematic illustrating an alternative embodiment of the invention; and

Figure 8 is a schematic illustrating a further alternative embodiment of the invention

Invention. Detailed Description of the Invention

Figure 1 depicts a traditional rebreather 10 of loop configuration as is known in the art. The rebreather 10 comprises a mouthpiece 12 (or full face mask, not shown) through which the diver breathes, connected to an inhale hose 14 and an exhale hose 16. These hoses 14, 16 are generally of corrugated synthetic rubber to allow greater flexibility while retaining a high resistance to collapse. The hoses are designed to provide low resistance to flow of the breathing gas. The hoses 14, 16 are connected to counterlungs 18, 20, which hold gas when it is not in the diver's lungs. The rebreather also includes a scrubber 22 containing carbon dioxide absorbent to remove the carbon dioxide exhaled by the diver. As will be appreciated, because the counterlungs are required to allow for the maximum likely breath volume of a user, they are generally bulky and can negatively affect the swimming diver's streamlining, depending on their shape and where they are located.

Referring to figures 6a and 6b, the breathing hose of the invention comprises a collapsible inner tube 62 located within a non-collapsible flexible outer tube 60. The collapsible inner tube 62 is designed to inflate on exhalation of breath from the lungs of the user into the hose, and subsequently the rebreather unit, and to deflate on inhalation of gas from the hose and rebreather unit into the lungs of the user. In this manner, the collapsible inner tube 62 provides an in-hose counterlung function.

Referring to figures 2 to 8, the breathing hose of the invention is depicted in situ as part of a rebreather unit.

Referring to figures 2 and 3, and in one embodiment of the invention, the breathing hose of the invention is employed in conjunction with a traditional counterlung. In this embodiment the rebreather unit 40 comprises a mouthpiece 42 (or full face mask, not shown) through which the diver breathes, connected to an inhale hose 44 and an exhale hose 46. Each inhale hose 44, and exhale hose 46 comprises a collapsible inner tube 48, 49 housed within a non-collapsible flexible outer tube 50, 51 . The hoses 44, 46 are connected to counterlungs 52, 54 which in turn are connected to a scrubber unit 56. It will however be noted that the size of the counterlungs 52, 54 in this embodiment are smaller than the counterlungs 18, 20 in the traditional rebreather unit as depicted in figure 1 . This is due to the fact that the collapsible inner tubes 48, 49 act as in-hose counterlungs, inflating to accommodate at least a portion of the breath volume of the user, and so the external counterlungs 52, 54 are not required to be sized to accommodate the maximum breath volume of the user. The volume of the counterlung can therefore be reduced by the volume of the breathing hoses, for example, typical exhalation and inhalation hoses have an inner diameter of around 40mm and a length of around 600mm each, providing a total internal volume of about 1 .5 litres. In this example, the combined counterlung volume could therefore be reduced by 1 .5 litres. Further, increasing the inner diameter of the breathing hoses from 40mm to 60mm would increase the volume of the breathing hoses from 1 .5 litres up to 3.4 litres, allowing a counterlung volume reduction of 3.4 litres in the process.

In figure 2, the inner tubes 48, 49 are in the collapsed/ deflated state, following an inhalation of breath by the user, and in figure 3 the inner tubes, 48, 49 are inflated to accommodate the exhaled breath of the user.

Referring to figures 4 to 8, the diameter of the breathing hose 60, and subsequently that of the inner tube 62, has been increased. This results in the inner tube 62 having a larger volume. In such an embodiment, it is envisaged that the inner tube can replace the external counterlung, which has been removed entirely. For example, increasing the inner diameter of the breathing hoses from 40mm to 80mm will increase the volume of the breathing hoses from about 1 .5 litres up to over 6 litres, potentially replacing the counterlung altogether. This allows for a significant improvement in the diver’s streamlining.

Referring to figures 4, 5, and 6a and 6b, a hollow member 70 may be located within the collapsible inner tube 62, to prevent the complete collapse of the inner tube 62 under hydrostatic pressure.

Referring to figure 8, the in-hose counterlung of the invention may be employed in combination with a type of semi closed circuit rebreather, which allows for the system volume to be lower than the lung tidal volume, by injecting additional breathing gas from an external air tank 64 into the system as required, by means of a demand valve 66 ideally located within the mouthpiece 68. The demand valve could alternatively be positioned anywhere in the breathing loop.

In all of the above embodiments, because the counterlung is located in-hose, just past the mouthpiece or mask, the vertical distance between the diver’s lungs and the counterlung is significantly reduced. This results in a reduction in the work of breathing.

A second major benefit of the invention is that the overall bulk is reduced, leading to a more streamlined, efficient diver profile.

In addition, as previously discussed, , the total air volume of the breathing loop of a traditional rebreather system comprises the counterlung volume (movable portion) as well as the breathing hose volume (immovable portion). This large air volume requires the diver to use lead ballast in order to counteract the buoyant lift of the air volume. The in-hose counterlung invention eliminates the immovable portion and therefore reduces the overall volume of the breathing loop, requiring less lead ballast and leading to a smaller, lighter and more streamlined system. Numerous modifications are possible without departing from the scope of the invention, for example, the collapsible inner tube may be in-elastic, with no bias in either the full or empty state, alternatively the collapsible inner tube may be pre-formed with a memory to create positive and/or negative pressure to counteract hydrostatic pressure imbalances, further alternatively, the inner tube may be elastic so as to bias the gas to move through the rebreather apparatus towards the inhale hose.