Abstract:
A breathing apparatus includes: supply means for supplying a breathable gas at a fist super-atmospheric pressure, face-piece means which can be worn by a user, the face-piece means being connected to the supply means and including an exhalation valve and a positive pressure demand valve for establishing a second super-atmospheric pressure, lower than said first super-atmospheric pressure, within the face-piece means when worn by the user, and a flow control device for controlling the flow of gas from the supply means to the face-piece means, the flow control device having a first operating position in which the flow gas to the demand valve is restricted and a second operating position in which the flow of gas to the demand valve is substantially unrestricted, and the flow control device being biased towards its first operating position and being moved to its second operating position by the establishment of the second super-atmospheric pressure within the face-piece means.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to breathing apparatus, particularly, but not exclusively, to self-contained breathing apparatus which can be used to enable the wearer to escape from areas of irrespirable atmosphere. Such apparatus typically includes a face-piece such as a flexible air-tight hood enclosing the wearer&#39;s head and sealed around the wearer&#39;s neck, the hood being supplied with air by a regulating means from a pressurised reservoir carried by the wearer.  
           [0002]    Breathing apparatus incorporating face-pieces such as flexible hoods are well known and have been mostly of the “constant flow” type, in which air is supplied to the hood at a substantially constant rate, typically at about 40 litres per minute, which is generally accepted as being adequate for average consumption. However, as respiration is cyclic, the instantaneous flow rate required by the wearer will continually vary from zero during exhalation to about 125 litres per minute at the peak of each inhalation. It will thus be seen that a constant flow of 40 litres per minute will not satisfy the wearer&#39;s requirements unless the incoming air can be stored during exhalation in a flexible reservoir of sufficient volume to enable the wearer to inhale from that volume without restriction. In practice, the hood itself normally acts as the required reservoir and is made sufficiently large for this purpose.  
           [0003]    A typical hood of the constant flow type is shown in FIG. 1 of the accompanying drawings and comprises a flexible hood A, a transparent visor area B and a neck seal C of elasticated fabric. Air is supplied from a high-pressure source (not shown), typically a cylinder containing air at 200 bar pressure. A valve (not shown) at the cylinder controls the flow of air to a pressure reducer (again not shown), which supplies air at about 10 bar to a flow-regulating valve which supplies the hood A. Air from the flow-regulating valve enters the hood A through a flexible hose D and a connector E and is directed over the visor area B by a deflector F to reduce misting. Surplus air and exhaled air can pass to atmosphere through the space between the neck seal C and the wearer&#39;s neck during exhalation.  
           [0004]    There are a number of significant limitations inherent in the constant flow apparatus described above. Because the hood A acts as a reservoir into which the wearer exhales and from which he subsequently inhales, a significant proportion of the exhaled carbon dioxide will be re-inhaled. The effect of this is to stimulate more rapid breathing in an attempt by the wearer to reduce the carbon dioxide level in his or her lungs. As the rate of carbon dioxide production varies in dependence upon the wearer&#39;s physical condition and, most significantly, upon the rate at which he or she is working, it can be seen that, with a constant flow of fresh air into the hood  1 , there will be a definite limit to the rate at which the wearer can work before the level of carbon dioxide within the hood A reaches a level high enough to cause distress. Thus, whilst the constant flow apparatus has the merits of simplicity and a predictable duration of flow from a given size of cylinder, these merits are, in practice, obtained at the expense of an adequate air supply to cater for the high demands that may be encountered in, for example, the stressful circumstances associated with escape from a contaminated area.  
           [0005]    A further disadvantage of the constant flow hood apparatus is that the continuous inflation and deflation of the hood can cause aberration of the wearer&#39;s view through distortion of the visor. Also, the protection factor offered by the apparatus is poor, due to the fact that, during inhalation, the pressure within the hood may fall below that of the surrounding atmosphere with the result that, unless the integrity of the hood and its seal to the wearer&#39;s neck is very good, there may be some inward leakage of the surrounding atmosphere into the hood.  
           [0006]    The deficiencies described above which are inherent in the constant flow hood apparatus can, to some extent, be overcome by supplying air on demand, rather than at a constant rate. In such a system, air is supplied as before from a high-pressure source, typically a cylinder containing air at 200 bar pressure. A valve at the cylinder controls the flow of air to a pressure reducer, which supplies air at about 10 bar to a positive pressure demand valve which regulates the flow of air precisely in accordance with the wearer&#39;s instantaneous requirements and, in conjunction with a spring-loaded exhalation valve, maintains a constant super-ambient pressure within the hood, effectively preventing any inward leakage.  
           [0007]    A typical demand valve will have a sensitive pressure-responsive diaphragm, one face of which is exposed to the pressure within the hood and the other face exposed to ambient pressure. Movement of the diaphragm, in response to changes in the pressure within the hood, operates a valve to control the flow of air into the hood. The diaphragm is so biased as to close the valve when the pressure within the hood is, for example, 2 millibars above the ambient pressure. The exhalation valve, which allows the escape of surplus air to the atmosphere, is so biased as to open when the pressure within the hood is, for example, 3 millibars above the ambient pressure.  
           [0008]    Thus, when the hood is sealed around the wearer&#39;s neck, a super-ambient pressure of between 2 and 3 millibars is maintained within the hood and the demand valve responds to the pressure changes caused by respiration and the admission of air into the hood in accordance with the user&#39;s requirements. The hood is kept constantly in an inflated condition, so that the visor remains in a substantially fixed position relative to the wearer&#39;s eyes and there is no flexing of the visor to distort the wearer&#39;s view.  
           [0009]    Known hoods as described above still have significant disadvantages. Thus, in order to obtain a completely air-tight seal around the wearer&#39;s neck, a diaphragm seal is used. This consists of a disc of thin elastic material, such as latex, with a central hole for the neck, and can be difficult to put over the head, particularly by someone wearing spectacles, and can be prone to deterioration and tearing. The large volume of the hood necessitates the incorporation of an inner mask, covering the wearer&#39;s nose and mouth, to reduce the volume of the breathing circuit and so maintain an acceptably low level of inhaled carbon dioxide. It is necessary to secure this inner mask to the wearer&#39;s face in the correct position by means of an elastic or adjustable harness arrangement.  
           [0010]    The edges of the inner mask, which project towards the wearer&#39;s face, are prone to catch on spectacles and thereby dislodge the spectacles, which can then be difficult to reposition within the hood. In addition, the inner mask can itself be pushed out of position when donning the hood, so that the inner mask has to be repositioned and secured to the wearer&#39;s face, in some cases by making adjustments to external straps. These are disadvantages which can adversely affect the ease and speed of donning the hood, these factors being of critical importance in emergency escape situations, particularly when the wearer has had limited training or experience in the use of the apparatus.  
           [0011]    A further disadvantage of the known apparatus is that, in order to avoid a significant loss of air through the demand valve whilst the hood is being donned, it is necessary either to don the hood before opening the air supply from the cylinder or to provide the demand valve with what is known as a “first breath” mechanism, which prevents any flow of air into the hood until it is sealed to the wearer&#39;s neck and a partial vacuum can be drawn by the wearer&#39;s inhalation in order to operate the mechanism. Either of these arrangements causes further difficulty and delay in making the apparatus operational and an inexperienced user may be reluctant to don the hood if there is no apparent air supply to the hood.  
           [0012]    A typical hood of the positive pressure type is shown in FIG. 2 of the accompanying drawings, in which like reference letters are applied to parts corresponding to those of FIG. 1. A flexible hood A has a transparent visor area B and is sealed around a wearer&#39;s neck by a neck seal C. A demand valve E is connected by a detachable coupling to an inner mask F via an adapter G. The inner mask F is held to the wearer&#39;s face by means of an elastic or adjustable harness H. Exhaled air escapes to atmosphere through a spring-loaded exhalation valve J rather than between the neck seal C and the wearer&#39;s neck.  
           [0013]    It is an object of the present invention to provide an improved form of breathing apparatus.  
           [0014]    In British Patent Specification No. 2 074 455, there is described a breathing apparatus which is so constructed that, if it is allowed to open fully, i.e. when no super-ambient pressure exists within the face-piece, the valve will close off the incoming air supply altogether. Thus, there will be no flow of air to the wearer at all while the face-piece is being donned and adjusted.  
           [0015]    In U.S. Pat. No. 4,345,592, there is described a breathing apparatus which includes a device to completely close of the supply of air to the demand valve if the flow through the demand valve exceeds a predetermined rate, such as would be the case if the air supply were turned on prior to donning the face-piece or if the face-piece were removed with the air supply still turned on.  
           [0016]    In U.S. Pat. No. 4,250,876, there is described a breathing apparatus which includes a demand valve provided with a two-position manually operated switch to either apply or remove a spring bias from the valve diaphragm such that, in one position, the demand valve functions in a positive pressure mode and, in the other position, the valve will operate in a negative pressure mode.  
           [0017]    It is a more specific object of the present invention to provide a breathing apparatus which offers significant advantages as compared with the breathing apparatus described in the above specifications.  
         SUMMARY OF THE INVENTION  
         [0018]    According to the present invention there is provided a breathing apparatus which includes:  
           [0019]    supply means for supplying a breathable gas at a first super-atmospheric pressure,  
           [0020]    face-piece means which can be worn by a user, the face-piece means being connected to the supply means and including an exhalation valve and a positive pressure demand valve for establishing a second super-atmospheric pressure, lower than said first super-atmospheric pressure, within the face-piece means when worn by the user, and  
           [0021]    a flow control device for controlling the flow of gas from the supply means to the face-piece means,  
           [0022]    the flow control device having a first operating position in which the flow of gas to the demand valve is restricted and a second operating position in which the flow of gas to the demand valve is substantially unrestricted, and  
           [0023]    the flow control device being biased towards its first operating position and being moved to its second operating position by the establishment of the second super-atmospheric pressure within the face-piece means.  
           [0024]    The face-piece means may be a hood, a full face-piece helmet or a mouth-piece having a periphery sealable around a wearer&#39;s mouth.  
           [0025]    Other preferred features of the invention are set out in the subsidiary claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1, which has been referred to above, is a schematic side view of a constant flow hood,  
         [0027]    [0027]FIG. 2, which has also been referred to above, is a schematic side view of a positive-pressure type hood,  
         [0028]    [0028]FIG. 3 is a schematic view of a breathing apparatus in accordance with the present invention incorporating a positive-pressure demand valve and a first form of air flow control valve,  
         [0029]    [0029]FIG. 4 is a sectional view of the air flow control valve of FIG. 3 in a first operating position,  
         [0030]    [0030]FIG. 5 is a view similar to FIG. 4, but showing the air control valve of FIG. 3 in a second operating position,  
         [0031]    [0031]FIG. 6 is a sectional view of a second form of air flow control valve,  
         [0032]    [0032]FIG. 7 shows a combined pressure-reducing valve and air flow control device when in an unpressurised state,  
         [0033]    [0033]FIG. 8 shows the valve of FIG. 7 when connected to a pressurised reservoir,  
         [0034]    [0034]FIG. 9 shows the valve of FIG. 7 in its restricted flow mode, and  
         [0035]    [0035]FIG. 10 shows the valve of FIG. 7 in its full flow mode.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]    The breathing apparatus shown in FIG. 3 includes a hood  1  of flexible and impervious material and a clear visor area  2 . The hood  1  is gathered around the neck where there is attached a neck seal  3  of fabric-reinforced elastic material, which can make an air-tight seal with a wearer&#39;s neck. A positive pressure demand valve  4  is incorporated into the construction of the hood  1  and has a deflector to guide incoming air, i.e. air entering the hood  1  from the valve  4 , over the visor area  2  and so reduce misting. A spring-loaded exhalation valve  6  maintains a super-ambient pressure within the hood  1  and allows the escape of surplus air to atmosphere. Air is supplied to the demand valve  4  at a substantially constant pressure via a flexible hose  7  from a pressure-regulating valve  8  attached to a high pressure air reservoir or cylinder  9 . A manually operated stop valve  9   a  controls the egress of air from the cylinder  9 .  
         [0037]    An air flow control device  10  is situated between the pressure-regulating (pressure-reducing) valve  8  and the demand valve  4 . The air flow control device  10  has a first operating position, shown in FIG. 4, in which it restricts the flow of air into the hood  1  through the demand valve  4  to approximately 35 litres per minute, thus preventing significant loss of air whilst the hood  1  is being donned. When the hood  1  has been sealed around the wearer&#39;s neck, the pressure within the hood  1  will rise, thus closing the demand valve  4 . This, in turn, will cause a rise in pressure at the inlet to the demand valve  4 , and this rise in pressure will be sensed by the air flow control device  10 , which will then adopt its second operating position, as shown in FIG. 5, in which the supply of air or the demand valve  4  is substantially unimpeded, the air flow control device  10  remaining in this second position throughout use  
         [0038]    The first form of air flow control device  10  shown in FIGS. 4 and 5 comprises a housing  11  defining a cylinder  12  within which a piston  13  is movable. At one end face  14  of the cylinder  12 , an inlet port  15  opens axially into the cylinder  12 , and an outlet port  16  is provided at a position spaced radially outwardly from the inlet port  15 . A transfer orifice  17  provides a restricted flow communication from the inlet port  15  to the outlet port  16 . The piston  13  is urged towards the end face  14  of the cylinder  12  by a spring  18 . The piston  13  carries a first sealing element  19  which is adapted to seal the inlet port  15  when the piston  13  is pressed against the end face  14  of the cylinder  12  by the spring  18 . A second sealing element  20  extends around the periphery of the piston  13  to form a seal with the wall of the cylinder  12 . The cylinder  12  is provided with a vent opening  21  in fluid connection with the ambient atmosphere, so that the face of the piston  13  remote from the end face  14  of the cylinder  12  is exposed to the ambient pressure.  
         [0039]    In operation, the air flow control device  10  is connected between the pressure-regulating valve  8  and the demand valve  4 , as shown in FIG. 3. When the apparatus is not in use, the valve  9   a  is closed and no pressurised air is applied to the air flow control device  10 . The piston  13  is urged by the spring  18  towards the end face  14  of the cylinder  12  and the first sealing element  19  seals the inlet port  15 .  
         [0040]    When the apparatus is to be used, the user first opens the valve  9   a  so that compressed air is supplied from the cylinder  9  to the pressure-regulating valve  8  and thence to the air flow control device  10 . Compressed air at a substantially constant pressure of about  10  bar enters the air flow control device  10  at the inlet port  15  which is effectively closed off by the first sealing element  19 . The spring  18  applies a force to the piston  13  which is greater than the force applied by the air pressure acting on the area of the inlet port  15  and thus holds the piston  13  in the position shown in FIG. 4. The transfer orifice  17  allows a controlled continuous flow of air through the air flow control device  10  to the demand valve  4  and, as the demand valve  4  is normally open, this allows the air to flow freely into the hood  1 . The pressure at the outlet port  16  of the air flow control device  10  is thus substantially the same as the ambient pressure so that the area of the piston  13  surrounding the inlet port  15  is thus exposed only to the ambient pressure and the piston  13  remains in its first operating position, as illustrated in FIG. 4. The user then dons the hood  1 , and adjusts the neck seal  3  so as to provide an air-tight seal. The flow of air into the hood  1  reassures an inexperienced user of the reliability of the apparatus.  
         [0041]    When the hood  1  is sealed around the wearer&#39;s neck, the incoming flow of air to the hood  1  through the transfer orifice causes the pressure within the hood  1  to rise. The rise in pressure is sensed by the demand valve  4  and eventually acts on the diaphragm of the demand valve  4  to close the demand valve  4 , thus effectively closing off the outlet port  16  of the air flow control device  10 . In consequence, the continuous flow of air through the transfer orifice  17  to the outlet port  16  causes the pressure in the ducting between the air flow control device  10  and the demand valve  4  to rise to the same pressure as that obtaining at the inlet port  15 . This pressure acts, via the outlet port  16 , upon the area of the piston  13  surrounding the inlet port  15  so that the entire face of the piston  13  which is adjacent to the end wall of the cylinder  12  is exposed to the inlet port pressure of approximately 10 bar. This produces a force on the piston  13  sufficient to overcome the opposing force applied by the spring  18  and the piston  13  is moved by this force away from the inlet port  15 . The piston  13  is held continuously away from the inlet port  15  for as long as sufficient pressure is applied to the inlet port  15 .  
         [0042]    This movement of the piston  13  to the second operating position allows substantially unrestricted flow through the flow control device as and when the demand valve  4  opens in response to the users requirements. The piston  13  will remain in its second operating position until either the air supply is disconnected by the user closing the valve  9   a  or the air in the cylinder  9  is exhausted and the pressure at the inlet port  15  falls below a predetermined level. The force of the spring  18  will then overcome the force exerted on the face of the piston  13  by the inlet pressure and the piston  13  will return to the first operating position and seal the inlet port  15 .  
         [0043]    The construction of the air flow control device may differ from the specific arrangement shown in FIGS. 4 and 5. For example, in the air flow control device shown in FIG. 6, the piston  13  is replaced by a flexible diaphragm  30 , the surface of which acts as the first sealing means. Alternatively, the inlet port  15  may be closed by a separate valve member operated by a piston or diaphragm. In a further alternative arrangement, the transfer orifice  17  may be constituted by a discontinuity in the sealing surfaces which close the inlet port so as to produce a controlled amount of leakage when the piston  13  is in its first operating position.  
         [0044]    The air flow control device  10 , although described in relation to the preceding embodiments as an independent assembly situated between the pressure-regulating valve  8  and the demand valve  4  may, in practice, be advantageously incorporated into the construction of the pressure regulator  8  or the demand valve  4 , the latter design being of advantage when the breathing apparatus is supplied by a long hose from a remote source of compressed air.  
         [0045]    FIGS.  7  to  10  illustrate a combined pressure-reducing valve and air flow control device which comprises a valve body  50  having an inlet  51  connectable to a reservoir of high-pressure gas, such as a compressed air cylinder. A typical pressure at the inlet  51  may be 200 bar but pressures of up to 300 bar are possible.  
         [0046]    The pressure-reducing valve comprises a piston  52  movable within a cylindrical bore  53  of the housing  50 . The piston  52  is connected to one end of a hollow stem  54 , the other end of which carries a sealing element  55  which co-operates with a high-pressure inlet port  56  supplied by the inlet  51 . In the arrangement shown in FIGS.  7  to  10 , a spring  57  urges the piston  52  to the right (as viewed in the drawings), so that the sealing element  55  is urged away from the inlet port  56 .  
         [0047]    The end of the cylindrical bore  53  is closed by a second piston  58  having a central through-bore  59 . The through-bore  59  contains an O-ring which seals on one end of a control rod  60 . The other end of the control rod  60  is slidingly received in a bore  60   a  in the housing  50 . A third piston  61  has a central opening and an annular skirt  62  which surrounds the central opening and extends towards the second piston  58 , the end surface of the annular skirt  62  forming a seal with the second piston  58 . The central opening in the third piston  61  slidably engages the control rod  60  adjacent its one end, and is engageable with an O-ring seal provided on the control rod  60 . The annular skirt  62  is formed with a bleed orifice  63 , providing restricted fluid communication between the interior of the annular skirt  62  and an outlet port  64  in the housing  50 . A spring  65  urges the third piston  61  towards the second piston  58 . At the extreme right-hand end of the pressure-reducing valve (as viewed in FIGS.  7  to  10 ) there is a locking pin  66  which extends through a transverse bore intersecting the bore  60   a . The locking pin  66  limits the extent of movement of the control rod  60  to the right.  
         [0048]    The combined pressure-reducing valve and air flow control device shown in FIGS.  7  to  10  has four operating positions, the first of which is shown in FIG. 7. This corresponds to the state in which no pressure is present at the inlet  51 . The spring  57  urges the piston  52  to the right, drawing the sealing element  55  away from the inlet port  56 . The third piston  61  is urged to the left by the spring  65 , so that the annular skirt  62  contacts the second piston  58  and, in turn, urges it to the left so as to contact the piston  52 . The control rod  60 , which is frictionally engaged by the O-ring seal in the second piston  58 , is drawn to the left, away from the locking pin  66 .  
         [0049]    [0049]FIG. 8 shows the positions of the valve components when high pressure is present at the inlet  51 . The high-pressure gas enters the inlet  51 , and passes through inlet port  56 , where its pressure is reduced to an intermediate pressure of, for example,  10  bar. A pair of transverse bores  70  admit the intermediate pressure gas to the centre of the hollow stem  54 , and the space between the piston  52  and the second piston  58  is then raised to the intermediate pressure. This causes the second piston  58  to be moved to the right, until it contacts the end of the bore  53 . Simultaneously, as the pressure in the space between the pistons  52  and  58  increases, the force on piston  52  overcomes the force of the spring  57 , and piston  52  moves to the left. The sealing element  55  is thus moved towards the inlet port  56  and eventually closes the inlet port  56 . In this position, the control rod  60  experiences the intermediate pressure on its left-hand end and is thus moved to the right, into contact with the locking pin  66 . The movement of the piston  58  also moves the piston  61  to the right, slightly compressing the spring  65  and ensuring effective sealing contact between the annular skirt  62  and the piston  58 .  
         [0050]    When it is required to provide breathable gas to a hood or other face-piece, the pin  66  is removed and the control rod  60  is then free to move to the right until its movement is arrested by a flange  60   b  contacting the face of the piston  61  between the annular skirt  62  and the central opening. In this position, which is shown in FIG. 9, the O-ring on the control rod  60  seals the central opening in the piston  61 . This movement opens the central bore  59  in the piston  58 , admitting fluid to the interior of the annular skirt  62 . Simultaneously, the pressure in the space between the pistons  52  and  58  decreases and the spring  57  moves the piston  52  to the right, opening the inlet port  56  to admit more high-pressure gas. The bleed orifice  63  in the skirt  62  allows a restricted flow of gas to the outlet port  64 , and thence to a demand valve of the breathing apparatus. The demand valve will be in an open condition, and thus the pressure sensed at the outlet port  64  will be substantially atmospheric pressure. The size of the bleed orifice  63  is so chosen that it provides the required volumetric flow rate when the pressure loss across the bleed orifice  63  corresponds to the difference between the intermediate pressure and atmospheric pressure.  
         [0051]    The intermediate pressure acting within the annular skirt  62  produces insufficient force to overcome the force exerted by the spring  65 , assisted by atmospheric pressure, acting on the right-hand face of the piston  61  so that the annular skirt  62  of the piston  61  is kept in sealing contact with the second piston  58 . Thus, in the initial flow state after removal of the locking pin  66 , the space between the pistons  52  and  58  is provided with gas at an intermediate pressure of, for example, 10 bar, while the flow control valve components  58 ,  60  and  61  provide a regulated volumetric flow rate to the outlet port  64  via the central bore  59  and the bleed orifice  63 .  
         [0052]    [0052]FIG. 10 shows the positions of the valve components when a back pressure is sensed at the outlet port  64 , for example, when a wearer dons a face-piece or hood having a demand valve to which the outlet port  64  supplies breathable gas. In this condition, when the demand valve closes due to pressure within the hood or face-piece, the pressure in the space between the piston  58  and the area of the piston  61  outside the annular skirt  62  increases, and eventually equalises with the intermediate pressure within the annular skirt  62 . The stiffness of spring  65  is so chosen that, when the entire leftward-facing area of the piston  61  is exposed to this intermediate pressure, the force on the piston  61  moves the piston  61  to the right, until the piston  61  contacts the end surface of its associated bore.  
         [0053]    This movement of the piston  61  separates the annular skirt  62  from the face of the piston  58 , and thus provides a substantially unrestricted flow path for gas to flow to the outlet port  64  at the intermediate pressure established by the pressure-reducing valve components  52 ,  55  and  56 . When the demand valve opens to admit gas to a face-piece or hood, the pressure acting on the right-hand face of the piston  52  reduces, and thus the spring  57  moves the sealing element  55  away from the inlet port  56  to increase the rate of flow of gas therethrough. Likewise, when the demand valve closes, the pressure acting on the piston  52  increases and the sealing element  55  is urged towards the inlet port  56  to reduce or arrest the flow of gas.  
         [0054]    To increase the usefulness of the combined pressure-reducing valve and air flow control device shown in FIGS.  7  to  10 , a charging port  70  is provided. The charging port  70  can be attached to a high-pressure source of gas in order to replenish a cylinder attached to the inlet  51 . Replenishment will take place with the pin  66  in position, and with the valve components in their positions shown in FIG. 8. When gas pressure at the charging port  70  exceeds the gas pressure obtaining in a reservoir connected to the inlet  51 , a non-return valve  71  will be lifted and gas will flow from the charging port  70  into the reservoir. Disconnection of the supply from the charging port  70  causes the non-return valve  71  to close, preventing leakage of gas from the reservoir.  
         [0055]    As a further safety feature, the combined pressure-reducing valve and air flow control device shown in FIGS.  7  to  10  may be provided with an over-pressure relief arrangement such as a bursting disc or other pressure-limiting device, in fluid communication with the inlet  51 .  
         [0056]    It is envisaged that the combined pressure-reducing valve and air flow control device shown in FIGS.  7  to  10  may be provided in association with a hood or other face-piece and a reservoir of breathable gas in an “escape set”, preferably packaged in a protective container for emergency evacuation of personnel from a building or vessel. The container may be a flexible protective fabric bag, or a substantially rigid casing. The container may be attached to a wall of the building or vessel.  
         [0057]    In an advantageous form of “escape set”, the pin  66  may be attached by a lanyard to the container so that, when the escape set is removed from the container, the pin  66  is withdrawn and the restricted flow of gas to the face-piece is automatically established without the user having to operate any valve manually. Thus, while the user is donning the hood or face-piece, only a limited flow of gas is permitted and the reservoir is prevented from becoming prematurely depleted. Once the user has donned the hood or face-piece, operation of the demand valve will cause the flow control valve elements  58 ,  60  and  61  to assume the positions shown in FIG. 10, allowing the full flow of gas to the user. The restricted gas flow provided while the user is donning the hood or face-piece provides reassurance to the user that, when he or she has the hood or face-piece fully in place, a supply of breathable gas will become available.  
         [0058]    It will be appreciated that the present invention is not limited to breathing apparatus for escape purposes, but may be applied with equal advantage to breathing apparatus for other purposes and utilising other forms of hood, face-piece or helmet, or a mouth-piece having a flexible seal extending around its periphery for sealing engagement around the mouth of a wearer of the face-piece. For example, the breathing apparatus may be provided for use in hazardous industrial environments such as paint spray shops, in which each worker has a hood or face-piece provided with a breathable gas through a compressed air hose from a central source of supply. The connection to the supply hose is generally made with a coupling which closes the supply hose when disconnected, to prevent loss of gas. The workers can thus connect their hoods or face-pieces to the supply and don the hood or face-piece while a restricted flow of gas is supplied via the control valve. The wearer than has confidence in the apparatus, and minimum loss of breathable gas occurs during this phase of operation.