Underwater breathing apparatus

The present invention is directed to a closed circuit underwater breathing apparatus having a mouthpiece and an outward and return connection leading from the mouthpiece to an inflatable breathing bag and back respectively to form a breathing circuit. A carbon dioxide absorbent is arranged in the breathing circuit. In use, exhaled gases flow from the mouthpiece through the outward connection to expand the breathing bag and then, on inhalation, they return from the breathing bag through the return connection to the mouthpiece. The apparatus also includes a demand valve controlling a connection between the mouthpiece and an inlet for an attachment to a supply of breathable gas. The demand valve so operates automatically to open the connection between the inlet and the mouthpiece when inhalation demand through the mouthpiece is not completely satisfied by the gases from the breathing bag so that the inlet connected to a supply of breathable gas is opened and the operation of the demand valve allows sufficient gas from the supply of breathable gas to enter the mouthpiece to complete the satisfaction of inhalation demand.

The invention relates to a closed-circuit underwater breathing apparatus of 
the kind comprising a mouthpiece and outward and return connections 
leading from the mouthpiece to an inflatable breathing bag and back 
respectively to form a breathing circuit. A carbon dioxide absorbent is 
arranged in the breathing circuit. In use, exhaled gases flowing from the 
mouthpiece through the outward connection expand the breathing bag, then, 
on inhalation, as the gases pass from the breathing bag through the return 
connection to the mouthpiece for inhalation, the carbon dioxide absorbent 
removes carbon dioxide from the gases during their passage around the 
breathing circuit. A non-return valve system is included in the breathing 
circuit preventing reverse flow around the circuit, hereinafter called 
apparatus of the kind referred to. 
BRIEF DESCRIPTION OF THE PRIOR ART 
Apparatus of the kind referred to requires some further supply of 
breathable gas to make up the oxygen absorbed in use during breathing and 
thus satisfy inhalation demand. A known apparatus of the kind referred to 
uses for this purpose an oxygen supply connected to the breathing bag by a 
manually operable valve which when opened admits additional oxygen into 
the breathing bag. Another known apparatus of the kind referred to uses a 
supply of oxygen connected to the breathing bag by a constant mass-flow 
valve which supplies oxygen to the breathing bag at a rate substantially 
equal to that at which oxygen is used during breathing. 
SUMMARY AND OBJECTS OF THE INVENTION 
An object of the invention is to provide an improved apparatus of the kind 
referred to. 
According to the invention there is provided a closed circuit breathing 
apparatus of the kind referred to and further comprising a demand valve 
controlling a connection between the mouthpiece and an inlet for 
attachment to a supply of breathable gas, the demand valve so operating 
automatically to open the connection between the inlet and the mouthpiece 
when inhalation demand at the mouthpiece is not completely satisfied by 
the gases from the breathing bag. In use, with the inlet connected to a 
supply of breathable gas, the operation of the demand valve allows 
sufficient gas from the supply of breathable gas to enter the mouthpiece 
to complete the satisfaction of inhalation demand. 
An apparatus according to the invention has the advantage that the 
automatic response of the demand valve requires no control by the breather 
to make up the supply of oxygen. The demand valve will respond 
automatically to increases in depth. 
There may be provided a manually operable control valve between the 
mouthpiece and the outward and return connections. The control valve, in a 
first position, connecting the mouthpiece to the outward and return 
connections to allow closed circuit breathing and, in a second position, 
disconnecting the mouthpiece from the outward and return connections and 
connecting the mouthpiece to a one-way exhaust valve to allow gas from a 
supply of breathable gas connected to the inlet to be inhaled through the 
mouthpiece and exhaled through the exhaust valve. 
The apparatus may include a supply of breathable gas connected to the 
inlet. 
The supply of breathable gas may be a supply of oxygen, a first shut-off 
valve being provided for halting the supply of oxygen to the demand valve 
and a second shut-off valve being provided which controls a connection 
between the breathing bag and a supply of an oxygen/helium mixture 
included in the apparatus separate from the supply of oxygen. In use, 
initial descent from the surface can be made with the first shut-off valve 
open and the second shut-off valve closed to allow oxygen from the supply 
of oxygen to be inhaled and then, when the partial pressure of the oxygen 
is too great for breathing or at a predetermined depth, the first shut-off 
valve can be closed to discontinue the oxygen supply and the second 
shut-off valve be opened to admit the oxygen/helium mixture into the 
breathing bag at a steady rate of supply, exhalation and inhalation then 
being to and from the breathing bag, with oxygen deficiencies being 
replenished by the supply of the oxygen/helium mixture to the breathing 
bag, the sequence of operation being reversed on ascent. 
BRIEF DESCRIPTION OF THE DRAWINGS 
The following is a more detailed description of two embodiments of the 
invention, by way of example, reference being made to the accompanying 
drawings in which: 
FIG. 1 is a front elevation of a first embodiment of a closed-circuit 
breathing apparatus, 
FIG. 2 is a schematic view of the apparatus of FIG. 1, 
FIGS. 3, 4 and 5 are schematic cross-sectional views of a three-position 
value of the apparatus of FIGS. 1 and 2, each figure showing a different 
one of the three positions of the valve, 
FIG. 6 is a schematic view of a second embodiment of a closed-circuit 
breathing apparatus.

DETAILED DESCRIPTION OF THE INVENTION 
Referring first to FIGS. 1 and 2, the apparatus comprises a mouthpiece 10 
connected by a control valve 11, whose operation is described in more 
detail hereinafter, to one end of an outward connection pipe or flow 
conduit 12 whose other end is connected to an inflatable rubber breathing 
bag 13. The outward connection pipe 12 includes a non-return valve 14 (see 
FIG. 2) which permits flow only in a direction from the mouthpiece 10 to 
the breathing bag 13. 
An outlet from the breathing bag 13 leads into a canister of a carbon 
dioxide absorbent 15 carried on the breathing bag 13. A return connection 
pipe or flow conduit 16 leads from the canister 15 to the control valve 11 
and includes a non-return valve 17 which permits flow only in a direction 
from the canister 15 to the control valve 11. 
The mouthpiece 10 includes a demand valve 18 of known type which is 
described in more detail hereinafter. The demand valve 18 has an inlet 
flow conduit 19 connected to a cylinder 20 of a breathable gas under 
pressure the breathable gas being oxygen or air, or an oxygen/nitrogen 
mixture, or an oxygen/helium mixture. In the case of an oxygen/helium 
mixture, the oxygen may be present in the mixture to give the same partial 
pressure as the partial pressure of oxygen in atmospheric air at the 
required operating depth. 
Referring now to FIGS. 3 to 5, the demand valve 18 includes a chamber 21 
divided by a diaphragm 22. A needle valve 23 is connected to the diaphragm 
22. A part of the chamber 21 on that side of the diaphragm 22 opposite the 
valve 23 is connected by a tube 24 to the mouthpiece and the pressure of 
the air in this part of the chamber on the diaphragm is counteracted by a 
spring 25, such that under normal breathing conditions the needle valve 23 
closes the inlet 19. However, when the pressure of air in the mouthpiece 
falls, the pressure of air in this part chamber reduces, thus allowing the 
spring 25 to move the diaphragm 22, so moving the needle valve 23 and 
opening the inlet flow conduit 19 to admit oxygen into the mouthpiece from 
the supply of oxygen 20. 
The control valve 11 is manually operable and has three working positions 
which are shown in FIGS. 3, 4 and 5. The first position is shown in FIG. 3 
and, in this position, the interior of the mouthpiece 10 is connected to 
the outward connection pipe 12 and the return connection pipe 16 through 
the respective non-return valves 14, 17. In the second position, shown in 
FIG. 4, the interior of the mouthpiece 10 is connected to a one-way 
exhaust valve 26, and is disconnected from the outward connection pipe 12 
and the return connection pipe 16. In the third position, shown in FIG. 5, 
the mouthpiece 10 is connected directly to the exterior atmosphere through 
a port 27 and is disconnected from the outward and return connection pipes 
12, 16, and from the one-way exhaust valve 26. The use of this valve 11 
will be described in more detail hereinafter. 
In use, the apparatus is donned by a diver above the surface of the water 
in which he is to dive. The mouthpiece 10 is placed in the mouth with the 
control valve 11 in its third position so that the diver can breathe 
atmospheric air directly and thus not waste any of the supply of 
breathable gas. The cylinder 20 of breathable gas is, however, open to 
supply breathable gas under reduced pressure to the inlet 19 of the demand 
valve 18. 
On entering the water the diver moves the control valve 11 to either the 
first position (FIG. 3) or second position (FIG. 4). 
If the first position of the control valve 11 is chosen, exhaled gases pass 
from the mouthpiece 10 through the non-return valve 14 and the outward 
connection pipe 12 to inflate the breathing bag 13. The construction of 
the non-return valve is such that any water or moisture in the mouthpiece 
is prevented from passing down the outward connection pipe 12 to the 
breathing bag 13. This can be important where such water or moisture would 
react with the carbon dioxide absorbent in the canister 15. The demand 
valve 18 does not operate because the pressure of gas in the mouthpiece 11 
is greater than the operating pressure of the demand valve 18. 
On inhalation, gases are withdrawn from the breathing bag 13 and passed 
through the canister 15 where the carbon dioxide in the gases is removed. 
The carbon dioxide depleted gases then pass from the canister 15 through 
the return connection pipe 16 and its associated non-return valve 17 to 
the mouthpiece 18, from which they are inhaled. No gas will be drawn from 
the outward connection pipe 12 because of the non-return valve 14. The 
demand valve 18 will not operate provided there is a sufficient supply of 
gas from the breathing bag 13 to satisfy completely the inhalation demand 
of the diver. However, when there is any deficiency caused by the oxygen 
in the gas being used in the diver's body and the diver's demand exceeds 
the supply from the breathing bag 13, the pressure in the mouthpiece 10 
will be reduced below the operating level of the demand valve 18. Thus the 
demand valve 18 will operate to connect the interior of the mouthpiece 10 
to the cylinder 20 of breathable gas and the deficiency will be made up 
with breathable gas. 
The diver can continue to dive with the control valve 11 in this first 
position and breathing on the closed circuit so formed within the 
limitations of depth imposed by the breathable gas. The demand valve 18 
will respond automatically to increases of depth to make up the supply of 
breathable gas. 
If the second position of the control valve 11 is chosen, exhaled air will 
pass through the one way exhaust valve 26 into the surrounding water. On 
inhalation, there will immediately be a pressure drop in the mouthpiece 10 
since the mouthpiece is no longer connected to the breathing bag 13. Thus, 
the demand valve 18 will operate to connect the mouthpiece to the supply 
of breathable gas and the diver will inhale the breathable gas from the 
cylinder 20 as with a conventional open-circuit apparatus. When the diver 
reaches a depth at which the partial pressure of oxygen in the supply of 
breathable gas is too great for breathing, which normally occurs at a 
depth of 9-12 meters, the diver moves the control valve 11 from the second 
position (FIG. 4) to the first position (FIG. 3). The apparatus will then 
operate on closed-circuit in the manner described above with reference to 
the drawings, and will allow the diver increased endurance down to depths 
limited by the breathing mixture used. 
The demand valve 18 will respond immediately to increases in the depth of 
the diver and supply extra breathable gas to maintain the contents of the 
breathing bag 13 without any action on the part of the diver. The control 
valve 11 can be moved from the first position to the second position 
easily should there by a malfunction of the closed circuit part of the 
apparatus, so that the diver immediately operates on open circuit, drawing 
breathable gas from the cylinder 20. 
The advantage of closed-circuit under-water breathing apparatus over 
open-circuit under-water breathing apparatus is that whereas, in the 
open-circuit apparatus, only a part of the oxygen breathed in at each 
inhalation is used in the diver's body and the remainder exhaled, in a 
closed-circuit apparatus there is no such wastage of oxygen, and thus the 
diver is given greatly increased endurance under water, which is limited 
by the saturation of the carbon dioxide absorbent 15 or by the quantity of 
breathable gas in the cylinder 20. 
In diving to depths greater than 50 meters, an oxygen/helium mixture is 
required which has less than 20% oxygen. Such a mixture is not breathable 
on the surface as the partial pressure of the oxygen in the mixture is 
insufficient, although the mixture can be breathed from depths of about 20 
meters downwards. Referring now to FIG. 6, the second embodiment shows the 
use of an oxygen/helium mixture in the apparatus disclosed above with 
reference to FIGS. 1 to 5, parts common to FIGS. 1 to 5 and to FIG. 6 
being given the same reference numerals and not being described in detail. 
The apparatus includes a cylinder 20 of an oxygen/helium mixture, which is 
connected through a constant mass flow valve 29 and a shut-off valve 30 to 
the breathing bag 13 and through depth sensitive valve 31 and a further 
shut-off valve 32 to the inlet to the demand valve 18. In this apparatus, 
oxygen fills the cylinder 20 of breathable gas, and a shut-off valve 33 is 
provided between the oxygen cylinder 20 and the demand valve 18. 
As the diver wearing the apparatus enters the water the oxygen/helium 
shut-off valves 30 and 32 are closed and the control valve 11 is either in 
the first position for closed-circuit breathing from the breathing bag 13 
or in the second position for open-circuit breathing of pure oxygen from 
the cylinder 20. These forms of breathing can be used until the diver 
reaches a depth of about 10 meters. At this depth, the oxygen supply is 
terminated by the shut-off valve 33 and the further oxygen/helium shut-off 
valve 32 is open to connect the oxygen/helium mixture to the demand valve 
18. With the valve 11 in the second position, this mixture is breathed on 
open circuit until a depth of about 20 meters is reached. Below depths of 
20 meters, the further shut-off valve 32 is closed and the first 
oxygen/helium shut-off valve 30 is opened and the valve 11 is moved to the 
first position so that breathing takes place through the breathing bag 13 
with deficiences in the supply of oxygen being made up from the 
oxygen/helium mixture. The constant mass flow valve 29 delivers the 
oxygen/helium mixture to the breathing bag 13 at a rate which 
substantially equals the rate at which oxygen is used up by the diver. 
As the diver ascends and decompresses, this procedure is reversed. This has 
the advantage that for the last 10 meters of his decompression ascent, the 
diver is breathing pure oxygen. The use of oxygen in decompression greatly 
speeds up the process of removal of either nitrogen or helium from the 
bloodstream due to the very much lower partial pressure of these gases 
existing in the breathing circuit. The further cylinder 28 may be filled 
with air or an oxygen/nitrogen mixture or other breathable gas mixtures, 
depending on depth requirements. 
It will be appreciated that one or more of the shut-off valves 30, 32 or 33 
may be operated automatically by hydrostatic pressure at a pre-determined 
depth. The shut-off valve 32 may, for example, remain open with the 
connection between the cylinder 28 and the demand valve 18 being 
controlled by the pressure sensitive valve 31. 
It will also be appreciated that the constant mass flow valve 29 may be 
replaced by any device which maintains a correctly constituted breathable 
mixture in the breathing bag 13 for the depth at which the diver is 
working.