US3463015A - Decompression monitor - Google Patents

Description

Aug. 26, 1969 GULINO ETAL DECOMPRE SSION MONITOR Filed Dec. 12, 1966 5 Sheets-Sheet 2 INVEN'IORS ROSARIO GULINO 8 KENNETH H. HALL ,w 4%w44 their ATTORNEYS Aug. 26, 1969 R. GULINO ETAL DECOMPRESSION MONITOR 5 Sheets-Sheet 5 Filed Dec. 12. 1966 FIG. 6

INVIZN'IORS ROSARIO GULINO 8 KENNETH H. HALL 5M, By, dw [Ono-LL.

their ATTORNEYS United States Patent 3,463,015 DECOMPRESSION MONITOR Rosario Gulino and Kenneth H. Hall, Ledyard, Conn.,

assignors to General Dynamics Corporation, New York,

N.Y., a corporation of Delaware Filed Dec. 12, 1966, Ser. No. 601,083 Int. Cl. G01n 33/00 [1.8. CI. 73-432 12 Claims ABSTRACT OF THE DISCLOSURE A monitor is provided for a diver indicating to him a continuous rate of ascent after exposure to high hydrostatic pressures. Flow barriers within the monitor permit passage of fluid at a rate dependent upon the ambient pressure and the length of time the diver has been down in the water, creating a mechanical analogy to the absorption of inert gas in the divers bodily tissues. As the diver ascends, the indication of the comparative pressures permits the diver to ascend at the fastest rate permissible to avoid decompression syndrome, or bends.

This invention relates to diving apparatus and, more particularly, to instruments which automatically compute decompression schedules.

When a diver descends into the sea, the extra air pressure to which he is subjected is instantly transmitted to the inside of his body. Consequently, the diver breathes the breathing mixture at a pressure higher than atmospheric and at each breath, a certain amount of nitrogen or some other inert gas is dissolved in the lungs. The body tissues, in their turn fed by the blood, are charged with the inert gas in an amount dependent upon the duration of the dive and the hydrostatic pressure. On returning to the surface, the process is reversed. The excess gas dissolved in the different tissues is carried by the blood to the lungs and then eliminated in respiration. If the rise is too rapid, the difierence between the pressure of the inert gas dissolved into the tissues and the hydrostatic pressure is such that bubbles form and it is these bubbles which cause depression syndrome or bends. More particularly, decompression syndrome results whenever the tissue ratio of any individual body tissue is exceeded, the tissue ratio being expressed as the ratio between the pressure of the gas dissolved in the tissues and the hydrostatic pressure to which the tissue is exposed.

One technique designed to calculate the safe rate of ascent by the driver involves the use of tables and equations wherein the compression and decompression history of representative body tissues are computed separately and compared to the hydrostatic pressure. While reliable, manual calculations of the exact pressure-time ascent trajectory are extremely arduous and time-consuming. Moreover, once the calculations are made, the rate of ascent is still restricted in that safety factors are applied to the results of the calculations in order to avoid any possibility of decompression syndrome.

Attempts have been made to construct decompression monitors which automatically compute the trajectory of safe ascent. One such device includes a single time constant diffusion barrier which is employed to analog all the tissues of the human body. It is apparent that the grouping of all the body tissues into a representative diffusion barrier is wholly inadequate since decompression syndrome results whenever the tissue ratio of any body tissue is exceeded.

It is an object of the present invention, accordingly, to provide a decompression monitor which provides information for the establishment and verification of the parameters used in decompression tables and equations.

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It is another object of the present invention to provide a decompression monitor which eliminates restricted ascent by indicating a minimal time ascent trajectory for preparameters used in decompression tables and equations.

It is a further object of the present invention to provide a decompression monitor which generates information for the evaluation of the pressure safety margin of a given decompression procedure.

These and other objects are accomplished by providing a decompression monitor which is responsive to several components, each of which is arranged to represent the recent pressure history of representative body tissues, the various body tissues each having a maximum permissible ratio of the partial pressure of absorbed gas to hydrostatic pressure. In order to avoid decompression syndrome resulting when the ratio of partial pressure to hydrostatic pressure exceeds the above-defined ratio for any tissue, the decompression monitor automatically computes the minimum permissible hydrostatic pressure for the most susceptible body tissue and provides an indication of the maximum further decompression or ascent permissible continuously in time.

Further objects and advantages of the present invention will be apparent to those skilled in the art from a reading of the following description of the invention taken in conjunction with the following drawings, in which:

FIGURE 1 is a schematic block diagram illustrating the arrangement of a typical decompression monitor according to the invention;

FIGURE 2 is a sectional illustration of one of the body tissue simulators coupled to the pressure multiplier of the decompression monitor shown in FIGURE 1;

FIGURE 3 is a sectional view showing one of the pressure measuring units of the monitor;

FIGURE 4 is a digrammatic representation of the electrical circuit used in the permissible ascent indicator of the monitor;

FIGURE 5 illustrates one type temperature compensated flow barrier for use in the representative body tissue simulator of FIGURE 2; and

FIGURE 6 illustrates another type flow barrier for use in the representative body tissue simulator of FIGURE 2.

As represented in the block diagram of FIGURE 1, a decompression monitor according to the present invention includes a pressure multiplier 4 for applying a pressure corresponding to the product of the hydrostatic pressure and the percentage of inert gas in a breathing mixture through a pressure-transmitting conduit 5 to several body tissue simulators 6, 6', 6 comprising pressure accumulators arranged in the manner described hereinafter to simulate representative body tissues. Each tissue simulator is connected by a pressure-transmitting conduit 7, 7, 7" to a corresponding pressure detector 8, 8, 8" which responds to pressures applied from the related tissue simulator to transmit corresponding signals to a permissible ascent indicator 9. This indicator selects from the various signals applied to it, the one representing the lowest permissible ascent, and provides a corresponding visual indication. It will be understood that the monitor illustrated schematically in FIG. 1 is preferably embodied in a portable unit carried by a diver and may include more or fewer tissue simulators and pressure detectors if desired. In this regard, since each body tissue is characterized by a time constant which is the rate of gas elimination or absorption plotted against time, the time constants may be represented by exponential curves, each curve exhibiting a different gradient. Each pressure accumulator 6, 6', 6" corresponding to a particular body tissue therefore is characterized by an exponential curve, and because of the variety of tissues in the body having different characteristics, the preferred embodiment of the invention shown in FIG. 1 includes three accumulators, the characterizing curves of each having different gradients.

All of the tissue simulators have the same structure and, as illustrated in FIGURE 2, the simulator 6 comprises a pressure accumulator housing 10 having a rigid outer wall 12 which flares out into a partial pressure chamber 14 and a flexible diaphragm 16 located within the chamber 14. This diaphragm is secured at its ends between the outer wall 12 and a pair of sealing members 18 and 20 which prevent liquid leakage along the edges of a fluid flow barrier 22. The fluid flow barrier 22 comprises any arrangement permitting fluid to pass in proportion to the pressure difference across the barrier and is interposed between the diaphragm 16 and an inner accumulator chamber 24. The accumulator chamber is connected to the pressure detecting assembly of FIGURE 3 through a passageway 26 and the conduit 7 and has its volume defined by a spring biased piston 28 and the barrier 22.

A fluid, such as, for example, Dow Corning 200 Silicone fluid having a viscosity of 500 centistokes at C., fills the diaphragm 16 and passes through the barrier 22 at a rate which is proportional to the difference in pressure between the fluid in the diaphragm 16 and the fluid which has passed into the chamber 24. Moreover, the rate of liquid pressure buildup in the chamber 24 is further dependent upon the time constant of the accumulator which is determined by the viscosity of the fluid contained in the diaphragm 16, the porosity of the fluid flow barrier 22 and the volume of the chamber 24. It is necessary that the time constant of the accumulator approximate as nearly as possible the time constant of the particular body tissue which the accumulator simulates. To this end, the spring constant of the piston 28 is chosen such that the volume of the chamber 24 increases slightly with increasing pressure and the fluid flow barrier 22 is designed such that the liquid flow across the barrier remains constant over a wide temperature range. The fluid flow barriers illustrated in FIGURES 5 and 6 are of such design and maintain a uniform flow rate, as will be described further below.

In a typical breathing mixture, the percentage of inert gas present may vary between 5% and 95%. In order to take this percentage into account, the pressure chamber 14 is filled with a fluid, such as the Dow Corning Silicone mentioned above, and connected to the pressure multiplier 4 which, as mentioned above, applies a pressure through the conduit 5 to the fluid of chamber 14 corresponding to the product of the hydrostatic pressure and percentage of inert gas.

The multiplier comprises a housing 30 and a pair of levers 32 and 34 pivotably mounted on opposite sides of the housing 30 and connected to a pair of pistons 36 and 38, the piston 36 engaging liquid at the hydrostatic pressure and the piston 38 engaging the liquid in the pressure chamber 14. Further included within the multiplier 5 are a fulcrum roller assembly 40 for slidably engaging levers 32 and 34 and a control knob 42 for positioning the assembly 40 and for imparting translational movement to the pivotably mounted arm 44 of a percentage inert gas indicator 46 which includes markings from 0 to 100.

In order to pressurize the fluid of chamber 14, as well as the partial pressure chambers of the tissue simulators 6' and 6", to a value which corresponds to the percentage of inert gas in the breathing mixture and the hydrostatic pressure, the knob 42 is adjusted until the indicator 46 affords a percentage inert gas reading which coincides with the actual percentage inert gas. For greater percentages of inert gas, as indicated by the meter 46, greater percentages of the hydrostatic pressure exerted against the piston 36 are correspondingly translated into pressure forces exerted on the liquid in the chamber 14 by the piston 38. If, for example, the diver descended to a depth of 198 feet, he would be subjected to a hydrostatic pressure of 88.1 lbs. per square inch (p.s.i.). If his breathing mixture contained 50% nitrogen, the indicator 46 would be set to read 50%, which setting would place the fulcrum roller assembly midway between the center points of the pistons 36 and 38. The force exerted against the lever 32 by the piston 36 would be translated to the piston 38 by the lever 34 in a ratio of two to one such that the pressure of the liquid in the chamber 14 would amount to 44.05 p.s.i. If the indicator 46 were set to read the pressure of the liquid in the chamber 14 would correspondingly increase to a value of 72.5 p.s.i. for the same value of hydrostatic pressure.

As indicated in FIGURE 1, the decompression monitor further includes three pressure detectors 8, 8' and 8", each being associated with a corresponding tissue simulator. FIGURE 3 illustrates in detail the structure of the pressure detector 8, the others being identical in arrangement. As shown in FIGURE 3, the pressure detector 8 comprises a balancing mechanism arranged to provide an output signal indicative of the diflerence between the hydrostatic pressure of the water at the depth to which the diver has descended and the critical hydrostatic pressure as determined by the tissue ratio of the simulated body tissue. The balancing mechanism includes a housing 50 having located therein an accumulator pressure chamber 52 connected through the conduit 7 to the chamber 24 of the accumulator and a hydrostatic pressure chamber 54 exposed to the ambient water or to a liquid at the hydrostatic pressure.

The pressure of the liquid in the accumulator chamber 52 is converted into a proportional static force by acting on a diaphragm 56 which is connected to the chamber 52 via a bellows 58. Similarly, the hydrostatic pressure is converted into a proportional static force by acting on a diaphragm 60 which is connected to the chamber 54 through a bellow 62. It is noteworthy that the opposite sides of the diaphrgam 56 and 60 are exposed to a vacuum, thus providing forces proportional to absolute pressure. Each static force is transmitted to opposite ends of a balance beam 62 by a pair of push rods 64 and 66 whose lengths are adjusted by a pair of sleeves 68 and 70, respectively, such that minimal contact is made between the push rods and the diaphragms 56 and 60 when the beam 62 is in the balanced condition and the bellows 58 and 60 are in the relaxed position. Moreover, adjustable mechanical stops 72 are provided and place along the rod 64 for limiting the travel of the rod for an unbalanced condition exceeding the range of interest.

The balance beam 62 engages a fulcrum 74 which is positioned along the beam 62 by means of a pair of adjustable screws 76 and 78 in accordance with the tissue ratio of the simulated body tissue. While the areas of the diaphragms 56 and 60 may be varied in order to change the tissue ratio, in the preferred embodiment of the invention, the areas of the diaphragms are maintained equal and only the fulcrum 74 is varied to achieve the tissue ratio of the simulated body tissue. For example, a typical body tissue may have a maximum permissible ratio of the partial pressure of absorbed gas to hydrostatic pressure of 1.80. The fulcrum 74 would be positioned along the beam 62 such that a balance condition would exist whenever the force exerted by the pressure of liquid in the accumulator chamber 52 against the diaphragm 56 is 1.8 times the force exerted by the pressurized liquid in the chamber 54 against the diaphragm 60.

As mentioned above, the tissue ratio for any tissue varies as a function of the hydrostatic pressure, decreasing slightly at great depths, increasing slightly at small depths. In order to compensate for this variance, the balancing mechanism further includes a spring-biased pressure responsive bellows 80 exposed to liquid at the hydrostatic pressure and connected to the fulcrum adjust screw 78 through a diaphragm 82. This diaphragm imparts a force against an opposing adjustable spring bias mechanism 84 so as to move the fulcrum 74 in propor tion to the hydrostatic pressure and the setting of the bias. Thus, the tissue ratio as determined by the positioning of the fulcrum '74 decreases with increasing hydrostatic pressure and increases with decreasing hydrostatic pressure.

The relative motion of the push rods 64 and 66 may therefore be expressed in terms of the difference between the ambient hydrostatic pressure and the hydrostatic pressure required for a balanced condition. Coupled between the push rods 64 and 66 is a rotary balance beam potentiometer 86, having a movable contact 88 which provides an electrical signal indicative of the difference between the hydrostatic pressure of chamber 54 and the pressure of the liquid in the chamber 52. When the pressure of the liquid in the chamber 54 is greater than the pressure of the liquid in the chamber 52 divided by the tissue ratio, the push rod 64 exerts a translational movement against the potentiometer 86 such that the effective resistance of the potentiometer increases. When the pressure of the liquid in the chamber 54 is less than the pressure of the liquid in the chamber 52 divided by the tissue ratio, the push rod 66 exerts a translational movement against the potentiometer 86 such that the effective resistance of the potentiometer decreases. Voltage is coupled to the potentiometer through a pair of conductors 90 and 92 and the control arm 88 of the potentiometer is connected to the indicating system of FIG. 4 through a conductor 94, the potentiometer 86 being shown schematically in that illustration.

The permissible ascent indicator 9 of FIGURE 1, as diagrammatically shown in FIGURE 4, is responsive to the signals generated by the pressure detectors 8, 8', 8" and provides an indication of the maximum ascent or minimum depth permissible or the further descent and decompression required to compensate for too rapid ascent. For this purpose, the system comprises a voltmeter 100 having a scale with appropriate markings from 4 to +4, a power supply 102 connected through an on-off switch 103 across a potentiometer 104, a power supply checking network 106 and a plurality of balancing impedance networks 108, 110 and 112. The potentiometer 1041s adjusted to apply a voltage to an input terminal 114 of the voltmeter 100 which corresponds to the voltage applied to a second input terminal 116 of the meter when a balanced condition exists in the balancing impedances 108, 110 and 112.

The power supply checking network 106 is provided for checking the capacity of the power supply 102 and includes a pair of resistors 118 and 120, a rheostat 122 and three switch contacts 124, 126 and 128, linked to operate in unison. When testing the supply 102, the contacts are moved to the positions opposite to those shown in the drawing, thereby placing the meter 100 across the power supply 102. Thereupon, the voltage of the supply 102 is indicated by the meter 100 and checked for its capability. This feature is provided in the decompression monitor because changes in the voltage of the supply 102 will produce proportional changes in the unbalanced condition indication.

The balancing impedance network 108 is electrically coupled to the balance beam potentiometer 86 (shown diagrammatically in FIGURE 4), of the pressure balancing assembly of FIGURE 3 through the conductors 90 and 92 and includes a pair of potentiometers 130 and 132 which regulate the voltage developed across the potentiometer 86 for both the balanced condition of operation and null or zero adjustment of the meter 100. Similarly, the networks 110 and 112 are associated with the pressure detectors 8' and 8" of FIGURE 1, the balance beam potentiometers of these assemblies being diagrammatically shown as 86' and 86", respectively. These networks include two rheostats 134, 136 and 138, 140, respectively, which regulate the voltage developed across the potentiometers 86' and 86" for the balanced 6 condition of operation and the null or zero adjustment of the meter 100.

The voltage developed across the potentiometer 86 is applied to a rectifier 142 through its movable tap 8-8 and the conductor 94. Similarly, the voltages developed across the potentiometers 86 and 86 are applied to a pair of diode rectifiers 144 and 146 through their respective movable taps 88 and 88". The cathodes of the diodes are coupled together and to the input terminal 116 of the meter through the contact 124.

In operation, when a diver wishes to ascend safely from a certain depth, the meter 100, which can be calibrated in feet of water, pounds per square inch, fathoms, or the like, provides him with an accurate indication of how high he may ascend without exceeding the tissue ratio of the most susceptible body tissue or to what depth he must descend should the maximum permissible tissue ratio be exceeded.

The on-otf switch 103 is thrown to the on position and the power supply 102 is applied across each of the variable impedance networks 108, and 112. As mentioned above, the effective resistance of any of the balance beam potentiometers 86, 8-6 and 86" increases when the hydrostatic pressure of the liquid in its corresponding hydrostatic pressure chamber 54 is greater than the pressure of the liquid in the pressure chamber 52 divided by its specific tissue ratio, and decreases for the opposite condition. It can be readily seen, therefore, that the balancing impedance network corresponding to the pressure balance assembly having the greatest positive difference between the pressure of the liquid in the accumulator pressure chamber 52 divided by its tissue ratio and the pressure of the liquid in the hydrostatic pressure chamber 54 will develop the largest voltage signal. The occurrence of such positive differences indicates an unsafe condition requiring the diver to descend. Similarly, the balancing impedance network corresponding to the pressure balance assembly having the least negative difference between the pressure of the liquid in the tissue pressure chamber 52 divided by its respective tissue ratio and the pressure of the liquid in the hydrostatic pressure chamber 54 will develop the largest voltage signal, the negative pressure diflerence corresponding to a safe condition permitting ascent.

The voltage signals developed across the balancing potentiometers 86, 86' and 86" are applied to the rectifiers 142, 144 and 146 through their respective movable taps 88, 88 and 88". Because the largest voltage signal will back-bias the other two diodes and thereby prevent conduction in them, only the largest of the three signals developed across the impedances 86, 86' and 86" is detected at the input terminal 116 of the voltmeter 100 through the normally closed contact 124 and compared with the balanced condition voltage derived across the potentiometer 104. When the arm of the meter 100 is deflected in the positive direction, it indicates further descent to the indicated depth and decompression are required. When the arm of the meter 100 is deflected in the negative direction, it indicates the minimal depth to which the diver may ascend without exceeding the maximum permissible tissue ratio of the most susceptible body tissue.

Referring to FIGURE 5, there is shown one particular form of fluid flow barrier which may be used as the barrier 22 in the accumulator assembly of FIGURE 2. This barrier is designed to maintain constant for any given pressure difference, the flow of a fluid between the diaphragm 16 and the inner chamber 24 over a temperature range of 30 to 90 F. As noted above, the time constant and rate of pressure buildup within the chamber 24 are critical factors in effective simulation of a body tissue by the accumulator assembly. Inasmuch as the time constant is partly dependent upon the porosity of the barrier 22 and the viscosity of the fluid employed in the accumulator, the barrier of FIG. 5 serves to restrict fluid flow for increasing temperatures and liberate fluid flow for decreasing temperatures.

The flow barrier includes a steel casing 150 having a hollow inner chamber 152, two cover plates 154 and 156 having internal recesses communicating with the casing 150 through perforated corresponding steel diaphragrns 158 and 160. The cover plates and diaphragms are mounted to the casing 150 through corresponding gaskets 162 and 164, respectively, and the casing has two openings at opposite ends 166 and 168 which act as passageways between its inner chamber 152 and the diaphragm 16 and the inner chamber 24, respectively, of the accumulator. Situated within the chamber 152 is an aluminum cylinder 170 which is centered within the chamber 152 and connected to the diaphragms 158 and 160 by a pair of support screws 172 and 1'74, respectively.

In order to achieve the desired flow, the spacing between the cylinder 170 and the inner surface of the inner chamber 152 must be very small and the cylinder 170 must remain fairly well centered. This may be accomplished by heating the aluminum cylinder 170 within the steel casing 150 to a specified temperature, such as 660 R, which causes the aluminum cylinder 170 to yield such that, upon cooling to the operating temperature, the clearance between the cylinder 170 and the inner wall of the chamber 152 has the desired value. Thereafter, the casing 150 and the cylinder 170 are reheated to a temperature at which the aluminum cylinder expands sufliciently to just engage the inner surface of the chamber 152. At this temperature, the diaphragms 158 and 160 and cover plates 154 and 156 are firmly attached. Upon cooling, the aluminum cylinder 170 will remain centered and have the proper clearance between the top and bottom walls of the chamber 152.

The end thrust of the cylinder 170 due to a pressure differential across the barrier is counteracted by maintaining the screws 172 and 174 against the cover plates 154 and 156, respectively, when inserted into their respective receptacles. In this manner, the clearance between the heads of the screws 172 and 174 and the cover plates 154 and 156 is on the order of the expected difierence in expansion in the axial direction between the drum 170 and the steel casing 150. Thus, any axial movement may be minimized.

The rationale behind the design of the barrier of FIG- URE lies in the fact that liquids become more fluid with increasing temperatures. Normally, this would cause the flow between the diaphragm 16 and inner chamber 24 to increase, assuming a constant pressure differential across the barrier 22. The aluminum cylinder 170, however, expands in size more than the steel casing 150 for the same change in temperature causing the annular space existing between the cylinder and the top and bottom walls of the inner chamber 152 to decrease in size with increasing temperature. This tends to reduce the flow of the liquid. By proper design, the decrease in viscosity is compensated by the decrease in the annular space between the cylinder 170 and the top and bottom walls of the inner chamber 152 such that the flow does not change with variations in temperature.

Referring to FIGURE 6, there is shown an arrangement for employment as the fluid flow barrier 22 in the accumulator assembly of the tissue simulator 6 and also as the fluid flow barrier in the accumulator assembly of either the tissue simulator 6 or the tissue simulator 6". To this end, there are provided a cylindrical steel housing 180 having a constant diameter opening 182 formed therein and a pair of cover plates 184 and 186, each cover plate similarly having a central recess formed therein, bolted to opposite ends of the housing 180 by a plurality of bolts 188. Interposed between the cover plates 184 and 186 and the housing are a pair of gaskets 190 and 192, preferably formed of metal, which form a liquid tight seal between the cover plates 184 and 186 and the housing 180. The housing 180 includes a passageway 194 which couples the opening 182 to conduit 196 leading to the diaphragm 16 of the partial pressure chamber 14 and a pair of passageways 198 and 200 which respectively couple the opening 182 to a conduit 202 leading to the accumulator chamber 24 of the tissue simulator 6 and to a conduit 204 leading to the accumulator chamber 24' of the second tissue simulator 6'. In the alternative, the conduit 204 may couple the passageway 200 to the accumulator chamber 24 of the third tissue simulator 6".

Situated Within the opening 182 and extending slightly beyond the ends of the housing is a generally cylindrical member 206, preferably formed of metal. The member 206 is centered within the opening 182 by a pair of spring clips 208 and 210 mounted in the recesses formed in the cover plates 184 and 186, respectively. The member 206 includes large diameter portions 212 and 214, of varying thickness, which separate a smaller diameter portion 216 from a pair of smaller diameter portions 218 and 220, respectively. The small diameter portions 218 and 220 may have formed therein flattened grooves which receive spring clips (not shown) for forcing member 206 against the bottom wall of housing 180, providing passageways of predetermined controlled size from portion 216 to the smaller diameter portions 218 and 220, respectively. With the spring clips 208 and 210 unbiased, the smaller diameter portion 216 is substantially aligned with the passageway 194 and the smaller diameter portions 218 and 220 are substantially aligned with the passageways 198 and 220, respectively.

When the pressure multiplier 4 (FIGURES l and 2) applies a pressure against the fluid in the partial pressure chamber 14, liquid flows from the diaphragm 16 through the conduit 196 and the passageway 194 and into a central channel defined by the area between the wall of the opening 182 and the thickness of the smaller diameter portion 216. From the central channel the fluid passes across the passageways formed by the large diameter portions 212 and 214 and wall 182 and into two side channels, the side channels being defined by the area between the wall of the opening 182 and the thickness of the smaller diameter portions 218 and 220. The fluid then passes into the accumulator chambers 24 and 24 through the passageways 198 and 200 and the conduits 202 and 204, respectively. It can be seen that the rate at which the fluid will pass from the central channel to the two side channels is dependent upon the difference in pressure between the fluid in the diaphragm 16 and the fluid which has already passed into the chambers 24 and 24'. Moreover, the rate is dependent upon the thickness of the large diameter portions 212 and 214 which provide constricted passageways between the central channel and the two side channels. Accordingly, by varying the thickness of the large diameter portions 212 and 214, the cylindrical member 206 acts as two separate flow barriers, each barrier having a ditferent time constant, in the same manner as described in connection with FIGURE 5 above.

From the above described illustrative embodiment, it can be seen that the decompression monitor contains several new features, among which are the use of a balancing system to compare the partial pressure of the dissolved gas with the ambient pressure; the use of a pressure sensitive bellows to vary the tissue ratio as a function of the hydrostatic pressure; the simulation of a gaseous absorption by tissues of many different time constants separately; the automatic selection of the particular time constant most likely to produce decompression syndrome; the indication of the direction of magnitude of the hydrostatic pressure deviation from the critical hydrostatic pressure; its adaptability to any type gas and percentage thereof in the breathing mixture; and the employment of a temperature compensated flow barrier to maintain a constant pressure buildup over a variable temperature range.

It is understood that the above-described invention is merely illustrative and susceptible to considerable modification within the skill of the art. For example, Bourdon tubes or diaphragms may be used instead or with the bellows shown in the pressure balance assembly of FIGURE 3 and electrical balancing may be used rather than mechanical balancing. Also, a mechanical system may be used to perform the sensing action of the diodes in the indicating system of FIGURE 4. Such a sensor might consist of a spring-loaded arm which is actuated by the motion of the balance beam having the greatest imbalance. Moreover, in the barrier of FIGURE 6, the end clips 208, 210 may be eliminated, leaving only the clips pressing member 206 against the bottom wall. Accordingly, all such variations and modifications are included within the spirit and scope of the invention.

We claim:

1. A decompression monitor for automatically computing a decompression schedule, comprising pressure accumulator means arranged to represent the gas absorption characteristics of a body tissue in response to applied pressures and exposure duration thereto, including a fluid flow barrier through which fluid flows at a rate proportional to the diflerence between the pressures on one side of said barrier and on the other side of said barrier, the one side being exposed to fluid at the partial pressure of an inert gas to which the body is exposed and the other side being exposed to fluid at a pressure proportional to the partial pressure of the inert gas to which the body is exposed and the exposure duration thereto, further comprising sensing means responsive to the pressure accumulator means and to the ambient pressure for providing a signal proportional to the difference between the ambient pres sure and representations of a critical pressure value as determined by the pressure accumulator means, indicating means responsive to said signal for providing an indication of the difference, and pressure multiplying means responsive to the ambient pressure and to a setting representing the percentage of the inert gas in the breathing mixture for applying a force to the fluid exposed to said one side of the barrier which is proportional to the product of the ambient pressure and the percentage of inert gas.

2. A decompression monitor for automatically computing a decompression schedule, comprising pressure accumulator means arranged to represent the gas absorption characteristics of a body tissue in response to applied pressures and exposure duration thereto, including a fluid flow barrier through which fluid flows at a rate proportional to the difference between the pressures on one side of said barrier and on the other side of said barrier, the one side being exposed to fluid at the partial pressure of an inert gas to which the body is exposed and the other side being exposed to fluid at a pressure proportional to the partial pressure of the inert gas to which the body is exposed and the exposure duration thereto, further comprising sensing means responsive to the pressure accumulator means and to the ambient pressure for providing a signal proportional to the difference between the ambient pressure and representations of a critical pressure value as determined by the pressure accumulator means, and indicating means responsive to said signal for providing an indication of the difference, wherein the fluid flow barrier is responsive to temperature changes in inverse relation to the change of viscosity of the fluid with temperature to provide a uniform flow rate with temperature for a given pressure difference between the one side of said barrier and the other side of said barrier.

3. A decompression monitor for automatically computing the decompression schedule of an underwater diver comprising a plurality of pressure accumulators for representing characteristics of different body tissues, each tissue being characterized by a maximum permissible ratio between the partial pressure of dissolved gas to hydrostatic pressure, each accumulator being adapted to exponentially develop a pressure proportional to the percentage inert gas in the mixture breathed by said diver, the hydrostatic pressure and exposure duration thereto, a plurality of sensors, each sensor selectively coupled to a pressure accumulator and responsive to the developed pressure therein and to the hydrostatic pressure for providing a force proportional to the difference between the hydrostatic pressure and a critical hydrostatic pressure value as determined by the ratio between the developed pressure and the tissue ratio of the represented body tissue, and indicating means responsive to the forces developed by the plurality of sensors for providing an indication of the difference between said hydrostatic pressure and the critical hydrostatic pressure for the pressure accumulator having the greatest positive difference between the critical hydrostatic pressure and the hydrostatic pressure or the smallest negative difference between said critical hydrostatic and hydrostatic pressures.

4. A decompression monitor as set forth in claim 3, wherein each of said accumulators includes a fluid flow barrier through which fluid flows at a rate proportional to the difference between the fluid pressure on one side of said barrier and the fluid pressure on the other side of said barrier, the one side exposed to fluid at the partial pressure of the inert gas to which the diver is exposed and the other side being exposed to fluid at a pressure proportional to the partial pressure of the inert gas to which the diver is exposed and the exposure duration thereto.

5. A decompression monitor as set forth in claim 4, which further comprises a pressure multiplier responsive to the hydrostatic pressure and percentage inert gas in the breathing mixture for applying a force to said liquid exposed to said one side of said barrier proportional to the product of said hydrostatic pressure and percentage inert gas.

6. A decompression monitor as set forth in claim 4, wherein said fluid flow barriers are temperature responsive, expanding the flow area for decreasing temperatures and constricting the flow area for increasing temperatures.

7. A decompression monitor as set forth in claim 4, wherein each of said plurality of sensors comprises a bellows arrangement responsive to the hydrostatic pressure and the pressure developed in its selected accumulator for converting said pressure into proportional static forces, an adjustable balance for converting the static force exerted by said developed pressure into a force exerted by the critical hydrostatic pressure, and a bellows for varying the force exerted by the critical hydrostatic pressure as a function of hydrostatic pressure.

8. A decompression monitor as set forth in claim 4, wherein each fluid flow barrier of said accumulators comprises an aluminum cylinder situated within the chamber of a hollow steel casing, said cylinder sized so as to substantially decrease the annular spacing between said cylinder and the top and bottom walls of said chamber with increasing temperature and increase said spacing with decreasing temperature.

9. A decompression monitor as set forth in claim 3 wherein each of said accumulators includes a pair of fluid flow barriers through which fluid flows at a rate proportional to the difference between the fluid pressure on one side of the barriers and the fluid pressure on the other side of the barriers, the one side of the barriers being exposed to fluid at the partial pressure of the inert gas to which the diver is exposed and the other side of the barriers being exposed to fluid at a pressure proportional to the partial pressure of the inert gas to which the diver is exposed and the exposure duration thereto.

10. A decompression monitor as set forth in claim 9 which further comprises a pressure multiplier responsive to the hydrostatic pressure and percentage inert gas in the breating mixture for applying a force to the liquid exposed to the one side of the barriers proportional to the product of the hydrostatic pressure and the percentage inert gas.

11. A decompression monitor as set forth in claim 10 wherein each of the accumulators further include a partial pressure chamber responsive to the force applied by the pressure multiplier for supplying liquid to one side of the fluid flow barriers at a pressure proportional to the product of the hydrostatic pressure and the percentage inert gas and includes a pair of accumulator chambers exposed to the other side of the fluid flow barriers for developing a pressure proportional to the product of the hydrostatic pressure and percentage inert gas and the exposure duration thereto.

12. A decompression monitor as set forth in claim 11 wherein the fluid flow barriers of each accumulator comprise a housing having an opening formed therein, a generally cylindrical member resiliently supported within the opening, said cylindrical member having large diameter portions of varying thickness separating a smaller diameter portion leading to the partial pressure chamber from another smaller diameter portion leading to one of the accumulator chambers and still another smaller diameter portion leading to another one of the accumulator chambers, each of the large diameter portions providing a constricted passageway between the smaller diameter portions for enabling fluid to flow at a controlled rate from the partial pressure chamber to both accumulator chambers.

References Cited FOREIGN PATENTS 735,170 5/1966 Canada.

DONALD O. WOODIEL, Primary Examiner US. Cl. X.R.

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