GB2069341A - Breathing Apparatus - Google Patents

Abstract

Semi-closed circuit re-generative breathing apparatus for use by divers wherein the flow of gas into the breathing circuit is automatically adjusted to suit the pressure at the depth that a diver happens to be and which compensates for variation in the pressure of gas supply to the apparatus from an external source. This is met by having mass reducing means for gas admitted to the breathing circuit sensitive to external pressure with the mass being decreased as the dive depth increases, and whereby the oxygen partial pressure in the breathing circuit is maintained between 0.16 and 2.0 atmospheres. <IMAGE>

Description

SPECIFICATION Breathing Apparatus This invention is related to the control of the flow of fluids from a region of higher pressure to a region of lower pressure, and particularly relates to the control of the flow of gas or gas mixture from bottles or compressors into the breathing circuit of a semi-closed circuit regenerative breathing apparatus for use by divers.

Regenerative breathing apparatus is usually understood to mean that the gas being breathed by the diver is partially reconstituted to render it fit to be rebreathed, in contrast to open circuit breathing apparatus in which the gas once breathed is exhausted to waste. The reconstition of the breathing gas may take the form of simply adding more fresh air to the existing mixture and exhausting the excess volume to waste. This system has been in use in helmet diving gear since the earliest days of diving, although it is not usually regarded as being regenerative.

More modern regenerative breathing apparatus also incorporates a method of removing the carbon dioxide produced by the diver's metabolism by passing the breathed gas mixture through a filter incorporating a chemical which reacts with and extracts the carbon dioxide from the gas which can then be rebreathed. This flow of regenerative air or gas mixture has to be maintained in order to replace the oxygen consumed by the diver. If a gas mixture is used in which the replacement oxygen is accompanied by a gas that may be considered to be physiologically inert in that it does not participate in the diver's metabolic activity, such as nitrogen or helium, then the excess volume has to be exhausted.

As the diver is not sensitive even to a dangerous depletion of the oxygen in the breathing circuit, and as his consumption of oxygen is variable, the oxygen partial pressure in the breathing circuit of the equipment must be controlled so that it does not drop below a predetermined level even with heightened metabolism, nor exceed a predetermined level at the least possible level of metabolism.

To achieve these conditions apparatus currently available may be considered under two headings. There is, therefore fully closed circuit equipment, in which the gas in the breathing circuit is either pure oxygen or in which the oxygen partial pressure is monitored and maintained at a predetermined value, and there is semiclosed circuit equipment in which the oxygen partial pressure varies within acceptable limits by admitting either a continuous flow of mixed gases or an intermittent flow controlled by the diver's breathing rate. Both rely upon a similar arrangement of filtering apparatus to remove the carbon dioxide from the expired gas mixture.

With semi-closed circuit apparatus that admits a continuous flow of gases and the control of that flow, and with which this invention is concerned, it has been the general practice in the past to contain pre-mixed gases, one of them being oxygen, compressed in high pressure bottles and usually carried by the diver. The gas mixture from these bottles, or from a compressor if the mixture is air, is passed to a pressure reducing valve where the gas pressure is reduced to a set value-which in most equipment is more than twice the greatest ambient pressure to which the diver will use the equipment. In some equipment this value is increased as the diver's ambient pressure is increased with depth.The gas under reduced pressure is admitted to the breathing apparatus through a small aperture whose dimensions are such as to allow sufficient gas to flow, driven by the set reduced pressure to satisfy the following expression and to maintain a constant mass flow throughout the depths to which the diver may descend MF-C O= F-C Where 0= proportion by volume (at NTP) of the oxygen in the breathing circuit so that the partial pressure of the oxygen remains between 0.16 and 2.0 atmospheres.

M = proportion by volume (at NTP) of the oxygen in the mixture of gases being supplied to the breathing circuit.

F = volume of flow gas (at NTP) being admitted into the breathing circuit.

C = volume flow of oxygen (at NTP) being consumed by the diver and usually assumed to be a minimum of 0.25 litres/minute (at NTP) and a maximum continuous rate of 3 litres/minute (at NTP) for young men working as divers.

Equipment controlled in the manner described suffers from two principal defects. Firstly, because the reduced gas pressure is still relatively high, the orifices through which the gas mixture has to flow are small and easily obstructed by particles of dust or accretions of ice settling out from any included water vapour in the gas. Secondly the mass of gas supplied to the diver is either fixed or even increases with depth. Thus more gas is supplied to the diver than he needs as the depth increases and a gas that is richer in oxygen than is necessary has to be supplied to be safe yet economical throughout the depths to which the diver may go.Alternatively if a gas mixture that is rich in oxygen is supplied in order to achieve a low consumption of the supplies, the diver is limited in the depth he can dive safely due to the increasing partial pressure of oxygen in the breathing circuit and as this partial pressure approaches 2.0 atmospheres.

The object of the present invention is to provide semi-closed circuit breathing apparatus for divers in which the partial pressure of oxygen in the gas being breathed by the diver is automatically controlled and maintained between predetermined upper and lower limits over the full depth at which the diver is required to operate.

According to the present invention, semi closed circuit breathing apparatus for divers comprises means to reduce the mass of breathable gas admitted to the breathing circuit of the apparatus, said means being sensitive to the external pressure to which the diver is subjected, and whereby as the depth of the dive increases, and hence the greater the pressure, the mass flow of breathable gas into the breathing circuit is reduced, said apparatus operating in accordance with the expression O MF-C D F-C where D = the absolute depth in atmospheres at any point in the dive so as to maintain the oxygen partial pressure in the breathing circuit between 0.16 and 2.0 atmospheres at maximum or minimum rates of oxygen consumption.

The invention is based on the recognition that the expression MF-C O= F-C explained earlier needs to be modified so that instead of providing predetermined flow rates to produce a safe partial pressure at a particular depth, the flow rates are continuously adjusted in accordance with the depth reached by the diver.

Secondly because of the very low gas pressures used, the apertures throughout the apparatus are sufficiently large to avoid being blocked by dust particles or ice derived from water vapour contained in the gases of gas mixture. Thirdly, the mechanism will only occlude the passage of gas while a driving pressure is maintained and the gas that exerts this pressure is constantly leaking away into the breathing circuit through a large aperture. Once the driving pressure diminishes the residual force caused by this pressure is exerted to open the valve occluding the gas supply, so that the apparatus fails safe.

In one embodiment of the invention, these operating principles are achieved by means of a valve in supply of pressure gas to the breathing circuit, having a chamber to which the pressure gas is supplied, there being flexible means closing the chamber, the opposite side of which is subjected to the pressure at the depth to which the valve means is taken, the flexible means being connected to an inlet valve to the chamber, and having an outlet hole connected to the breathing circuit, said outlet hole having compressible means associated therewith and capable of controlling the size of the bore at the outlet hole.

Thus, the chamber may be closed by a flexible diaphragm on which is mounted a system of flexible and compressible components overlaid by a rigid plate, the rigid plate being connected to the valve through which the gas or mixture of gases is admitted to the chamber. Thus when the plate is moved either by the diaphragm or the compressible components, or both together, the valve is operated to control the flow of gas or gas mixture in the chamber. The excess of pressure between the chamber and the other side of the diaphragm is not more than 10% greater than the absolute ambient pressure in the latter at any point during the dive.

In addition to the opening and closing of the valve the plate, compressible components and diaphragm being pierced by a hole that allows the gas or gas mixture to pass from the chamber into the breathing circuit provide control over the diameter of the hole in accordance with the pressure to which the compressible components are subjected, and thereby exert still further control over the mass of gas entering the breathing circuit.

It is a major advantage of this invention that the flow of fresh gas or gas mixture can be regulated automatically so that the proportion of oxygen by volume (at NTP) in the breathing system can be made to follow the expression: O MF-C D F-C as described above, and so achieve the maximum economy of safe gas useage possible in breathing apparatus employing the semi-closed circuit principle of operation.

Because a very low pressure differential is used to drive the gas through the hole from the chamber into the breathing circuit, the velocity of the gas flowing through the hole is low so that the drop in temperature of the flowing gas is sufficiently low as to suppress considerably any tendency for the formation of ice from water vapour in the gas or gas mixture that might settie out to occlude the passageway. Also because of this low pressure differential, the aperture can be kept large so as to eliminate the possibility of any ice particles that may form, or dust particles originating in the high pressure gas supply equipment, from blocking the passageways and no filters are required. In addition, the partial pressure of the oxygen in the breathing system can be kept at a value below 2.0 atmospheres by means of the diver's continual minimum consumption of oxygen, and accordingly there is no limit to the depth to which this equipment can be used due to oxygen poisoning. Furthermore because the gas supply increases with a decrease in depth, the diver does not have to put an extra supply of gas into the breathing circuit to ensure that the oxygen partial pressure remains above the minimum safe value during the ascent. The use of a balanced valve to control the supply of high pressure gas into the system requires small forces to operate it and so minimizes the size of the sensitive components for any given degree of accuracy required.

One embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic sectional side elevation of semi-closed breathing apparatus according to the invention; and Figure 2 is an enlarged view of part of the construction of Figure 1.

In the drawings semi-closed circuit regenerative breathing apparatus for use by divers comprises a chamber 1 closed by a flexible diaphragm 2 secured to a sealing ring 3 engaging formations 4 in the inner wall of the chamber. The diaphragm 2 has an outlet hole 5 which although not shown is connected directly to the breathing circuit for the diver. Mounted on the diaphragm 2 surrounding the hole 5 is a resilient ring 6 having a central orifice 7 in register with the hole 5, and surrounding the resilient ring 6 is a further ring 8 of a readily compressible material such as foamed neoprene, and to the outside of the ring 8 is a further ring 9 again of resilient material.To the sides opposite those in contact with and secured to the flexible diaphragm, the rings 6 and 8 are overlaid by and secured to a rigid plate 10 having a central hole 11 in register with the orifice 7.

Secured to the plate 10 is a connecting means 12 engaging attachment means 1 3 on a movable valve member 14. The valve member 14 has a valve seat 1 5 adapted to cooperate with the open end of a cylinder 16, the movable valve member 14 being guided by a piston 1 7 extending to the cylinder 1 6. The cylinder 1 6 is connected to a source of pressure gas by a pipe 1 8 extending through and being sealed with respect to a hole in the wall of the chamber 1.

Thus, when pressure gas is first admitted to the chamber 1 via the pipe 1 8 and the cylinder 1 6 there is no difference of pressure gas across the diaphragm 2, and in the absence of any deflection of the diaphragm the valve seat 1 5 remains clear of the open end of the cylinder 1 6. Any pressure exerted by the incoming gas will act upon the piston 1 7 and the valve seat 1 5 with the same static pressure, reduced by the velocity head of the gas flowing across its surface out of the cylinder 1 6 and into the chamber. There will then exist a resultant force tending to close the valve seat on to the cylinder, and the extent to which the valve is open will decrease until the forces acting on the piston and on the valve seat are equal.

As gas continues to flow into the chamber 1, the pressure therein will begin to rise relative to the pressure outside the chamber and acting on the opposite side of the diaphragm 2, so that a flow of gas will occur through the outlet hole in the diaphragm and into the breathing circuit.

Because of the difference of pressure between the chamber 1 and the opposite side of the diaphragm 2, the diaphragm will be deflected outwardly with respect to the chamber that movement tending to close the valve seat 1 5 on to the cylinder 1 6 to reduce or cut-off the flow of gas into the chamber 1. Additionally and because of the foamed construction of the ring 8, its closed cells are at an initial pressure of approximately 1 atmosphere so that any pressure in the chamber above 1 atmosphere will cause the ring 8 to be compressed. Because of the constraint imposed on the ring 8 by its attachment to the diaphragm 2 and to the plate 10, compression of the ring 8 will preferentially be in a direction normal to the plane of the plate and diaphragm.Despite the compression of the gas in the enclosed cells, the inside and outside periphery of the ring 8 may bulge, because the total load on the constrained surfaces of the ring 8 exceeds the total load on the unconstrained periphery, producing an internal hydrostatic pressure that is higher than the pressure in the chamber. This excess pressure is balanced by the elastic strain induced in the bulging of the elastic material of the ring 8.

Under the action of the forces exerted by the pressure in the chamber acting on the ring 8 and transmitted to the plate 10 and the diaphragm 2, the ring 6 is also compressed and the deformation by bulging effectively changes the diameter of the orifice 7.

Thus, the net effect of the features described above, acting together, provides control over the flow of gas into the breathing circuit from the gas supply that is dependent only upon the ambient pressure and not upon the pressure of the gas supply itself.

The ring 9 effectively acts as a stop member such that as the plate 10 approaches the diaphragm 2 on compression of the rings 6 and 8, it prevents further approach of the plate 10 towards the diaphragm 2 at the greater depths of the dive, so as to control the flow of gas from the chamber into the breathing circuit in conjunction with the action of the other components defined above and whereby the flow of gas into the breathing circuit satisifies the expression O MF-C D F-C It will also be recognised that in addition to providing automatic control over the flow of gas into the breathing circuit in accordance with the depth reached by a diver, there is also automatic compensation for any fall in the pressure of gas supply to the chamber. If, as is usual, the gas supply is from bottles carried by the diver, then as the supply of gas is used up and the pressure falls, the apparatus continues to work satisfactorily because, as has been explained above, the safe operation of the apparatus of the invention is not dependent upon the pressure of the gas from the supply.

Claims (9)

Claims

1. Semi-closed circuit breathing apparatus for divers comprising means to reduce the mass of breathable gas admitted to the breathing circuit of the apparatus, said means being sensitive to the external pressure to which the diver is subjected, and whereby as the depth of the dive increases, and hence the greater the pressure, the mass flow of breathable gas into the breathing circuit is reduced, said apparatus operating in accordance with the expression O MF-C D F-C where O=proportion by volume (at NTP) of the oxygen in the breathing circuit so that the partial measure of the oxygen remains between 0.16 and 2.0 atmospheres.

M=proportion by volume (at NTP) of the oxygen in the mixture of gases being supplied to the breathing circuit.

F=volume of flow of gas (at NTP) being admitted into the breathing circuit.

C=volume flow of oxygen (at NTP) being consumed by the diver and usually assumed to be a minimum of 0.25 litres/minute (at NTP) and a maximum continuous rate of 3 litres/minute (at NTP) for young men working as divers.

D=the absolute depth in atmospheres at any point in the dive so as to maintain the oxygen partial pressure in the breathing circuit between 0.16 and 2.0 atmospheres at maximum or minimum rates of oxygen consumption.

2. Apparatus as in Claim 1, wherein the apertures throughout the apparatus and through which gas passes are sufficiently large to avoid being blocked by dust particles or ice derived from water vapour contained in the gases of the mixture.

3. Apparatus as in Claim 1 or Claim 2, wherein a reduction in the passage of gas to the apparatus occurs whilst a driving pressure is maintained, the gas exerting that pressure being constantly leaked into the breathing circuit the means to reduce the mass of breathable gas opening as the driving pressure diminishes and whereby the apparatus is fail safe.

4. Apparatus as in any of Claims 1 to 3, wherein a valve is provided in the supply of pressure gas to the apparatus, said gas being supplied to a chamber and there being means within the chamber connected to the valve adapted to open or close the valve in accordance with the ambient pressure within the chamber.

5. Apparatus as in Claim 4, wherein the chamber has flexible means closing the chamber, the opposite side of said flexible means being subjected to the pressure at the depth to which the apparatus is taken, the flexible means being connected to the inlet valve to the chamber and having an outlet hole connected to the breathing circuit, and there being means associated with the outlet hole capable of controlling the size of the bore at the outlet hole.

6. Apparatus as in Claim 5, wherein the chamber is closed by a flexible diaphragm on which is mounted a system of flexible and compressible components overlaid by a rigid plate, the rigid plate being connected to the valve through whcih the gas or mixture of gases is admitted to the chamber.

7. Apparatus as in Claim 5 or Claim 6, wherein a relatively unflexible component is mounted on the flexible diaphragm to limit the extent to which the rigid plate can be moved towards the flexible diaphragm on compression of the system of flexible and compressible components.

8. Apparatus as in Claims 5 to 7, wherein the system of flexible and compressible components and the rigid plate have orifices cooperating with the outlet hole through the flexible diaphragm.

9. A semi-closed circuit breathing apparatus for divers substantially as hereinbefore described with reference to the accompanying drawings.