Instrument for indicating the depths and durations of decompression stops required during underwater submersions
The present invention relates to decompression meters, that is instruments which can be used for determining the depth and duration of decompression stops which must be made during an ascent from an underwater submersion, or dive, performed with breathing apparatus. 
     One known instrument for this purpose is described in Italian Pat. No. 624,174; this instrument includes a deformable chamber which in use of the instrument is exposed to the ambient pressure and which is connected by a constricted neck to a rigid (or in practice non-resilient under operating conditions) chamber filled, like the resilient chamber, with gas. The instrument is fitted with means for indicating the depth and duration of the required decompression stops in dependence on the pressure prevailing in the rigid chamber, this being possible because the neck joining the deformable chamber to the rigid chamber is such that the pressure variations in the rigid chamber simulate the behaviour of human tissue insofar as it relates to the variation of the nitrogen pressure contained therein during the decompression arising from ascent from a dive. The gas may be replaced by a liquid if the rigid chamber is provided with a resilient membrane. 
     In order to indicate the depths and durations of the required decompression stops, use is made of manometers fitted with appropriate scales, in particular a Bourdon tube manometer is used in practice. However, this makes the instrument fairly cumbersome and rather costly. 
     Also known (from Italian Patent 935,741) is an instrument for the same purpose, which includes a sliding piston sealed in a cylinder of rigid material so that it separates the interior of the cylinder into two chambers. One chamber of the cylinder contains gas and the other contains a viscous liquid and is connected by a narrow passageway or neck to a resilient chamber, also containing the viscous liquid; the plunger is subjected to the action of a spring which opposes those displacements of the cylinder which reduce the volume in the chamber of the cylinder which contains gas. For indicating the depths and durations of the required decompression stops, a manometer is supplied, this being connected to that chamber of the cylinder which contains gas. Such an instrument has a number of disadvantages: the use of a spring to resist the movement of the plunger increases the cost and bulk of the instrument, and the measurement error is proportional to the pressure because the gaskets carried by the plunger only work well in conditions of equal pressure. 
     It is possible to make a plurality of such instruments as a battery in which the neck of each instrument is constructed in such a way that each instrument provides an indication related to the behaviour of a particular tissue of the human body during submersion and the subsequent decompression upon ascending. Using the known constructional forms, however, it is not possible to obtain batteries of acceptable size and cost. This is also true of the apparatus described in Italian Pat. No. 624,174, by reason of the size of the manometers which measure the pressure in the rigid chamber, and which are provided with scales which indicate the depths at which decompression stops are required during ascent in order to avoid the risk of embolism. 
     According to the present invention there is provided an instrument for determining the depths and durations of decompression stops required to be made by a diver when ascending from an underwater submersion, of the type comprising a first chamber having a resilient wall connected by a restrictor to a second chamber having rigid walls, in which the second chamber is formed as a transparent cylinder within which is housed a coaxial pipe made from a material which is at least semi-transparent, the said semi-transparent pipe being open at one end for communication with the interior of the second chamber, a piston being slidingly and sealingly housed within the bore in the pipe so that the position of the piston along the pipe represents the pressure at any time within the said second chamber, which latter is provided with a scale against which the positions of the piston can be referred in determining the depth and duration of decompression stops required during an ascent. 
     The scale carried on or by the second chamber is preferably marked with a plurality of predetermined depths in such a way that the position of the piston along the scale indicates the level to which a diver can safely rise before the next decompression stop is required. If the diver then rises to that level and stops the piston will slowly move to indicate the level at which the next decompression stop should be made; because of the slow transfer of fluid through the restrictor the piston will not have moved far from its original position when the diver reaches the new level and the time it takes to reach the next position is the time for which the decompression stop should last. The position of the piston along the scale thus directly indicates the depth at which the decompression stop should be made, and the required duration of the decompression stop is indicated by the time taken for the piston to move to a position indicating the next level at which the next decompression stop should be made. 
    
    
     One embodiment of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is an axial section of a decompression meter constructed in accordance with the invention; 
     FIG. 2 is a diagrammatic axial section illustrating a battery of instruments such as are illustrated in FIG. 1; and 
     FIG. 3 is an axial section, on an enlarged scale, of a construction detail of the decompression meter illustrated in FIG. 1. 
    
    
     Referring now to the drawings there is shown an instrument having a chamber 1, formed of a resilient material, the interior of which is connected to a rigid chamber 3 of transparent material by a pipe 2 having two branches 2a and 2b extending at right angles to one another. The pipe 2 communicates with the resilient chamber 1 through a restrictor 4 which acts to slow down the flow of fluid between the two chambers. The restrictor 4 may be a small bar of ceramic material held in place by means of a threaded cap 12 and sealed by means of a seal 13. 
     A tube 5 of semi-transparent material (for example, a polypropylene known by the trade name of &#34;Moplen&#34; made by Soc. Montedison) is housed within the rigid chamber 3, with a very small amount of radial play. This has been exaggerated in the drawings in order clearly to show the individual elements forming the structure. The tube 5 has an opening 5b at one end through which it communicates with the interior of the rigid chamber 3. The other end of the tube 5 is closed and is secured to the wall of the chamber 3, for example by a weld 5c. The wall thickness of the tube 5 is about 1 mm., and within the bore of the tube 5 is mounted, for sliding movement, a piston 6 of elastomeric material (for example this may be of synthetic rubber) the outside diameter of which is less than the internal diameter of the tube 5. The piston 6 has a recess 18 at one end defined by a skirt 7 extending in bell form and having a cylindrical annular rim 7a which is coaxial with the piston 6. The diameter of the annular rim 7a of the skirt 7 is a little larger (see FIG. 3) than the internal diameter of the tube 5, so that when the piston is located within the tube 5 the rim 7a presses outwardly to form a seal between the piston 6 and the tube 5. The rim 7a is delimited by two annular edges 7b, 7c, and the outward pressure of the skirt 7 ensures that one of the edges 7b, 7c of the rim 7a is always in contact with the internal wall 5a of tube 5 even though the other edge may be slightly displaced by the burr that inevitably forms during the sliding movement of the piston 6, particularly when moving towards the left of the drawing when there will be a tendency for the skirt 7a to roll up. Around the periphery of piston 6, a short distance from the other end of the piston 6, there is machined a groove 8 in which is housed a coloured ring 9, also made, for example, of &#34;Moplen&#34; (Registered Trade Mark), which serves as another sealing ring for the plunger and which is visible from outside the tube 5 so that it forms a reference index, indicating the position of the piston 6. 
     Since the two chambers 1 and 3, the tube 5 and the pipe 2, are full of a gas, for example air, any variation in pressure within the rigid chamber 3 causes a displacement of piston 6 along the tube 5 in such a way as to establish a balance position between the pressure inside the tube 5 and the pressure in the chamber 3 outside the tube 5. 
     The system thus operates so as to maintain substantially constantly equal the pressures on the opposite sides of the piston 6 and this is a very favourable condition for the seal formed by the lip 7 of the piston 6, because the friction is less than in known instruments and does not vary appreciably with the variations of the pressure in the chamber 3. Moreover, this also avoids distortion of the thin pipe 5, which distortion could upset the proper operation of the instrument. 
     Due to the transparency of the chamber 3 and to the semi-transparency of the tube 5, the coloured ring 9 carried on the piston 6 will be visible from outside the chamber and can indicate, by reference to a scale 10, the depths and durations of the required decompression stops. The rigid chamber 3 is sealed at one end by a plug 11 which permits the introduction of the tube 5 into the chamber 3. 
     The branch pipe 2b communicates with a compensation chamber 14 which is closed by a plug 15 and into which are inserted discs 16 of inert and incompressible material for the purpose of varying the overall volume of fluid in the two chambers 1 and 3. By varying the number of discs 16 it is possible to calibrate the instrument. 
     By appropriately selecting the length and diameter of the restrictor 4 it is possible to adapt the behaviour of the instrument under variations in pressure to correspond to that of various tissues of the human body. It is thus possible to form a battery of instruments A, A&#39;, A&#34; . . . , as illustrated in FIG. 2, each of which can be used, in conjunction with a depth meter, to determine the decompression stops required for each tissue of the human body represented. The battery of instruments may have a plurality of resilient chambers 1, one for each instrument, or may incorporate a common resilient chamber 1&#39;. In the battery of instruments shown in FIG. 2 there is one, indicated by the reference B, which does not have a restrictor 4 in the passage 2. The position of the piston 6 in the instrument B thus directly represents the depth of submersion of the instrument, and it is provided with a scale 17 directly reading depth. A diver ascending from a dive can determine the decompression stops required by reference to the instrument B and the instruments A, A&#39;, A&#34;. The piston 6 which indicates the greatest depth at any one time must be considered when determining when to make a decompression stop. For example if the instrument A&#34; indicates the greatest depth, the next decompression stop will be made when the instrument B indicates the same depth as the instrument A&#34;. If the diver wants to make a further ascent he must choose again the indication of an instrument which now indicates the greatest depth, and which may be another of the instruments A, A&#39;, A&#34; (for example the instrument A). The instrument B will inform the diver in each case when he has reached the depth at which he should effect a decompression stop.