Patent Publication Number: US-2022215736-A1

Title: Apparatus for detection of drowning conditions

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority from U.S. Provisional Patent Application No. 63/133,801 filed Jan. 5, 2021, the disclosure of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates to the field of swimming accessories. More particularly, the invention relates to a method and apparatus for detection of drowning conditions. 
     BACKGROUND 
     Existing systems used to detect if a person swimming in the sea or in a pool is about to drown are not able to identify whether the person is with the nose and mouth inside the water, or if the person is not deep in the water, they are unable to make a direct measurement of head position without placing any sensor on the head. 
     Many patents and articles describe the detection of a drowning condition using systems which combine use of inertial motion sensors, respiration sensors, pulse sensors, etc. Some systems use a combination of a depth gauge (usually pressure gauge) attached to the upper part of the body combined with a timer capable of detecting extreme conditions such as swimming deep underwater for a long time. 
     Most, if not all, drowning detection systems are unable to distinguish between a state of a person drowning in a shallow depth and a state in which a person swims and the breathing is irregular and the pulse is accelerated (for example, a state in which the person swims with the mouth close to the water). The inability to distinguish between a drowning condition and a swimming condition such as that described above is due to the lack of knowledge of whether a person&#39;s mouth or nose is out of the water. There are inventions that contain a depth sensor above the neckline (such as placing the sensor on a swimming cap, or goggles, etc.). These solutions allow real-time knowledge of the position of the head relative to the surface of the water. The main disadvantage of this type of invention is the need to wear the product on the head while in the water. In this condition, the product may detach from the body and lose its effectiveness in detecting a drowning condition. 
     The object of the present invention is to provide a solution for the automatic detection of a drowning condition, including borderline states of swimming in depth in which the mouth and nose are close to the surface of the water and without sensors located on the head/neck. 
     SUMMARY 
     An apparatus for detecting drowning conditions, the apparatus including three depth/pressure sensors attached to a user&#39;s torso for identifying angles of the torso in two axes in relation to the water level and two depth/pressure sensors for assessing respiration patterns. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts the location of the depth and respiration pattern sensors according to one embodiment of the invention. 
         FIG. 2  depicts trigonometric relation between measured depths and pitch and roll angles of upper body relative to water surface plane. 
         FIG. 3  depicts geometric dimensions and angles for drown depth calculations. 
         FIG. 4  depicts different head tilt positions limits used for depth calculations. 
         FIG. 5  depicts one embodiment of respiration rate/pattern estimation using respiration pattern sensors. 
         FIG. 6  depicts a selected part of an article example of illustrating the ability to extract relevant data required for effective comparison against the data measured by the system. 
         FIGS. 7,8,9,10,11  illustrate the drowning detection system. 
         FIG. 12  is a side view of the user wearing the apparatus of  FIG. 19 , at a (−60) degrees pitch angle. 
         FIG. 13  is a side view of the user wearing the apparatus of  FIG. 19 , at a (30) degrees pitch angle. 
         FIG. 14  is a side view of the user wearing the apparatus of  FIG. 19 , at a (90) degrees pitch angle. 
         FIG. 15  is a side view of the user wearing the apparatus of  FIG. 19 , at a (−90) degrees pitch angle. 
         FIG. 16  is a rear view of the user wearing the apparatus of  FIG. 19 , at the (90) degrees pitch angle of  FIG. 14 , and of zero degrees roll. 
         FIG. 17  is a rear view of the user wearing the apparatus of  FIG. 19 , at the (90) degrees pitch angle of  FIG. 14 , and of 37.7 degrees roll. 
         FIG. 18  is a rear view of the user wearing the apparatus of  FIG. 19 , at the (90) degrees pitch angle of  FIG. 14 , and of (−37.7) degrees roll. 
         FIG. 19  is front view of a user wearing an apparatus. 
         FIG. 20  is rear view of the user wearing the apparatus of  FIG. 19 . 
         FIG. 21  is a side view of the user wearing the apparatus of  FIG. 19 , at a zero pitch angle. 
         FIG. 22  is a front view depicting sensors used for angular positions&#39; calculations. 
         FIG. 23  depicts some of the content of memory  50  of  FIG. 19 . 
         FIG. 24  is a side view of the user wearing the apparatus of  FIG. 19 , at (−90) degrees pitch. 
         FIG. 25  is a side view of the user wearing the apparatus of  FIG. 19 , at (−45) degrees pitch. 
         FIG. 26  is a side view of the user wearing the apparatus of  FIG. 19 , at 45 degrees pitch. 
         FIG. 27  is a side view of the user wearing the apparatus of  FIG. 19 , at (90) degrees pitch. 
         FIG. 28  is a side view of the user wearing the apparatus of  FIG. 19 , at a different scenario of (90) degrees pitch, being due to a different position of the nose in relation to the torso. 
         FIG. 29  is a top view of the torso wearing the apparatus of  FIG. 19 . 
     
    
    
     DETAILED DESCRIPTION 
     An aspect of the present invention relates to a system for detecting a drowning condition including 3 or more pressure sensors to detect depth/location relative to the water level and to detect the upper body angular position relative to the water level. The sensors may include a piezoelectric pressure sensor, a capacitive pressure sensor, among others. In addition, the system may include one or more pressure sensors to detect respiration patterns. These sensors may include piezoelectric sensors, piezo-resistive sensors, and capacitive sensors, among others. The three pressure sensors may be located in the upper body with two of them located in the front of the body and one of them located in the back or the opposite. 
       FIG. 1  depicts the location of the depth and respiration pattern sensors according to one embodiment of the invention. Each pressure sensor may be used to measure and determine its depth (distance) relative to the water surface. The combination of the three distances of the pressure sensors from the surface defines a plane in the space where these three points are located. Calculating this plane makes it possible to know the angles of the upper body relative to the water surface and to estimate whether the mouth and nose are in the water. 
       FIG. 2  depicts trigonometric relation between measured depths and pitch and roll angles of upper body relative to water surface plane according to the trigonometric relations: 
     
       
         
           
             
               α 
               pitch 
             
             = 
             
               
                 sin 
                 
                   - 
                   1 
                 
               
               ( 
               
                 
                   
                     
                       
                         d 
                         4 
                       
                       + 
                       
                         d 
                         3 
                       
                     
                     2 
                   
                   - 
                   
                     d 
                     5 
                   
                 
                 
                   R 
                   
                     5 
                     , 
                     0 
                   
                 
               
               ) 
             
           
         
       
       
         
           
             
               α 
               
                 r 
                 ⁢ 
                 o 
                 ⁢ 
                 l 
                 ⁢ 
                 l 
               
             
             = 
             
               
                 sin 
                 
                   - 
                   1 
                 
               
               ⁡ 
               
                 ( 
                 
                   
                     
                       d 
                       3 
                     
                     - 
                     
                       d 
                       4 
                     
                   
                   
                     R 
                     
                       3 
                       , 
                       4 
                     
                   
                 
                 ) 
               
             
           
         
       
     
     The estimation of whether the mouth and nose are in the water is based on anatomical dimensions together with calculated torso&#39;s depth and angles (pitch and roll). 
       FIG. 3 . depicts relative distances between the mouth/nose and the three sensors (R h , d h , s, c) according to the trigonometric calculations: 
     
       
      
       d 
       drown_depth 
       =d 
       pitch 
       +d 
       roll  
      
     
         d   pitch   =R   N ·cos (α pitch   +h   0 ′)
 
         d   roll   =d   roll,body ·sin (α pitch )
 
     where:
 
R h —constant distance between head tilt axis point (on neck) and nose
 
d h —constant distance between head tilt axis point and origin plane (horizontal)
 
s—constant distance between head tilt axis point and origin point
 
c—constant angle between nose and vertical at head tilt axis point
 
ht p —input angle of front or back (pitch) head tilt
 
ht r —input angle of side (roll) head tilt
 
h o —front/back head tilt angle shifted to origin plane
 
h o ′—front/back head tilt angle shifted to origin point
 
R N —distance between nose and origin
 
     
       
      
       h=c+ht 
       p 
      
     
               tan   ⁡     (     h   o     )       =         R   h     ·     sin   ⁡     (   h   )               R   h     ·     cos   ⁡     (   h   )         +     d   h                       tan   ⁡     (     h   o   ′     )       =           R   h     ·     sin   ⁡     (   h   )         -   s           R   h     ·     cos   ⁡     (   h   )         +     d   h                       R   N     =           R   h     ·     cos   ⁡     (   h   )         +     d   h         cos   ⁡     (     h   o   ′     )                 d   0,roll   =R   N ·sin ( h   o ′)
 
         d   roll,body   =d   0,roll ·(1−cos (α roll   +ht   r ))
 
     These distances vary between people of different height, sex, race, age, etc. The standard deviation of this variable can be significantly decreased by using relevant anthropometry statistical data tailored by the overall height of the individual—a measure closely correlated to the size of the swim-suite fitted with wearable pressure sensors. 
       FIG. 4  depicts examples of the neck joint angles relative to the torso. Assuming the person is tilting his\her head towards the water surface can increase the maximal breathing depth. The head tilt added value for the ability to reach the surface with the mouth/nose will be simplified to constants representing near maximal flexion/extension and/or side rotation of the head. 
     To determine whether the given body&#39;s submersion state has the ability to breath, we should compare the maximal depth of breathing at the current calculated a angles (pitch and roll) to the measured depth of the origin point. 
     If the origin&#39;s measured depth is greater (deeper) than the maximal depth of breathing, then a submerged indication will arise for the current time-frame. 
     By measuring the time duration in which the mouth and nose are inside the water, the potential of a drowning condition may be determined. A drowning condition may be determined based on a time measurement where the nose and mouth are in the water for a cumulative time period of 40 seconds or other predetermined amount of time during the last minute, or over another continuous period of time. 
     The measurements by these 3 depth/pressure sensors allow the system to compare the measured body movements to known swimming styles patterns (such as front crawl, backstroke, breaststroke, butterfly, sidestroke etc.). All these common swimming styles are expressed in cyclical movements of the torso. 
     These movements are expressed by unique pitch and/or roll angles of the torso. Extensive academic studies have been carried out to map the statistics of the different swimming styles for performance improvement. These studies have created databases that incorporate parameters required for real-time comparison to identify swimming patterns. Therefore, if the measured body angles match with these statistical databases, then the system would conclude that the person is swimming and there is no need for floatation process activation. 
       FIG. 6  depicts an article example from “SwimBIT: A Novel Approach to Stroke Analysis during swim training based on Attitude and Heading Reference System (AHRS”. The example illustrates the ability to extract relevant data required for effective comparison against the data measured by the sensors. 
     In some embodiments, the system may include 2 pressure sensors located on the chest and/or in the area of the diaphragm whose function is to detect abnormal respiration patterns. The two sensors may be located close to each other. One of the sensors may be positioned between the user&#39;s chest and an elastic band which secures the sensor to the chest. This sensor is oriented so it can measure the total pressure based on the hydrostatic and hydrodynamic pressure inside the water and the pressure exerted by the expansion of the chest against the elastic belt. The second sensor may be positioned between the user&#39;s chest and the elastic band oriented at an angle relative to the first sensor and measure only hydrostatic and hydrodynamic pressure (does not measure pressure associated with chest expansion). Alternatively, the second sensor may be positioned on the outer side of the elastic band and measure only hydrostatic and hydrodynamic pressure (does not measure pressure associated with chest expansion). Alternatively, one of the three sensors S 3 ,S 4 ,S 5  may be used as the second sensor for respiration abnormality detection. In this case due to the distance between the sensors, pressure compensation may be needed. 
     In some embodiments, a controller may process the pressure information received from both sensors and may determine the pressure due to the chest expansion by cancelling out the hydrostatic and hydrodynamic pressure measurements of both sensors: 
         S 1 pressure   =S 1 hydrostatic&amp;hydrodynamic   +S 1 breathing    
         S 2 pressure   =S 2 hydrostatic&amp;hydrodynamic    
     In the case that S 1  is locate near S 2 , we can assume that the hydrostatic and hydrodynamic water pressure is the approximately the same for both of the sensors: 
         S 1 hydrostatic&amp;hydrodynamic   ≅S 2 hydrostatic&amp;hydrodynamic    
     From the equations above, we can conclude that: 
         S 1 breathing   =S 1 pressure   −S 2 pressure    
     In the case that S 1  is not locate near S 2 , we can first compensate the distance difference between the sensors and then cancel out the hydrostatic and hydrodynamic pressure. 
     Alternatively, the system can use one strain-gauge embedded within an elastic band, instead of the two pressure sensors described above (S 1 ,S 2 ). 
     Based on the resulting measurement of chest expansion pressure, the controller may determine the respiration pattern of the user. 
       FIG. 5  depicts one example of performing computations of the respiration pattern. The controller may use estimation methods such as FFT or other methods of spectral analysis. Noise may be filtered out and the maximum respiration frequency and amplitude may be determined. 
     Conclusions regarding respiratory pattern regularity can be achieved using these measured parameters (frequency and amplitude of respiration) by comparing the measurements to predetermined thresholds of normal respiration. Mostly, in drowning conditions, we can find that the body is entering and exiting the water in an attempt to breathe, so we can find increase in the respiration rate. Further distress would usually result in water swallowing causing rapid convulsions and spasm—which also increase the appeared respiratory movement frequency. Normal respiration rate is between 10-40 breathing per minute. A distress situation (like drowning) will increase that rate to above 40 breathing per min. 
     The decision-making algorithm may then use one or more of the following parameters previously calculated to determine whether a drowning condition is taking place: 
     a. The duration of time the nose and mouth are immersed in water (continuous time or a fragmented period of time within a defined moving time frame) 
     b. The assessment whether a person&#39;s movements resemble any of known swimming styles&#39; patterns. If a resemblance is detected, then the assessed drowning risk may be decreased. 
     c. Respiratory regularity based on the frequency and amplitude of the chest/diaphragm movement. Based on the result, the system may provide for automatic real-time detection of a drowning condition and may further provide for activation of a rescue mechanism, for example, by opening a valve in a gas container to inflate a flotation device. The floatation device may be an existing product. 
       FIG. 7  schematically illustrates a drowning detection system, according to an embodiment of the present invention. Shown in the figure is a front view of a person wearing a drowning condition detection shirt in a deflated mode ( 10 ). The shirt includes several sensors on the front ( 112 ,  114 ,  116 , and  118 ) and one sensor on the back ( 120 ), being physical sensors or combined from physical sensors. 
       FIG. 8  schematically illustrates the drowning detection system, according to an embodiment of the present invention. Shown in the figure is a front view of a person wearing the drowning condition detection shirt in an inflated mode ( 130 ). In this mode, inflator chamber ( 128 ) is filled with air after the system has detected a drowning condition. 
       FIG. 9  schematically illustrates the drowning detection system, according to an embodiment of the present invention. Shown in the figure is a side view of a person floating in water ( 132 ). The system detects that it is not a drowning condition and the inflator chamber is not filled with air. Sensors ( 116 ,  118 , and  120 ) detect the position and depth of the upper body (Lying on the back position) and the system determines that the mouth and nose are not immersed in water and therefore it is not a drowning condition, and does not activate inflation of the inflator chamber. 
       FIGS. 10 and 11  schematically illustrates the drowning detection system, according to an embodiment of the present invention. Shown in the figure is a back view of the shirt ( 136 ) including the inflation chamber ( 128 ). The shirt includes a pocket ( 122 ) on the back of the shirt which holds a pressurized air tank ( 124 ) that is connected by means of a hose ( 126 ) to the inflator chamber ( 128 ). 
       FIGS. 12,13,14,15,16,17,18  illustrate the examples of different depth and angles of user&#39;s torso and the way the measurements of the depth/pressure sensors enable the calculation of pitch and roll angles for each body position as described above. 
       FIG. 19  is front view of a user wearing an apparatus for detecting drowning according to one embodiment of the invention. 
     An apparatus  10  for alerting and avoiding drowning includes according to one embodiment of the invention, sensors such as S 3  and others, which may be attached to a shirt  18  worn by the user; a controller  14 , for calculating depth, angle of the user and additional parameters according to the sensors&#39; measurements, and for determining risk of drowning accordingly; and a device  16  for being operated in case of the determined risk, such as to alert and/or to lift the user. 
       FIG. 20  is rear view of the user wearing the apparatus of  FIG. 19 . Apparatus  10  may include a depth sensor S 5  disposed at the top of the center of the back  22 B of the user. 
       FIG. 21  is a side view of the user wearing the apparatus of  FIG. 19 , at a zero pitch angle. 
     Controller  14  ( FIG. 19 ) determines no risk of drowning in case the nose  20  of the user will be calculated to be disposed above the water level  12 . 
     Referring again to  FIG. 19 , apparatus  10  may include depth sensors S 3  and S 4  disposed at the top of the front  22 A of the torso  22 C of the user, and being distanced ( 24 ) from one another. 
     Depth sensors S 3 , S 4 , and S 5  are sufficient for perfectly determining depth and angles of torso  22 C, since three location sensors are geometrically sufficient to determine any plane in the space. 
     Accordingly, sensors S 3 , S 4 , and S 5  may approximately determine whether nose  20  is disposed above water level  12 . 
       FIG. 22  is a front view depicting sensors used for angular positions&#39; calculations. 
     A virtual depth sensor may be obtained from the measurements of sensors S 3  and S 4  of  FIG. 19  for measuring the depth at the center  61  of the top of front  22 A of torso  22 C. 
     Apparatus  10  may include a pitch sensor and a roll sensor, for measuring pitch and roll of the user. 
     The virtual roll sensor may be obtained from the measurements of sensors S 3  and S 4  of  FIG. 19  or by other sensor. 
     The virtual pitch sensor may be obtained from the measurements of the virtual depth sensor ( 61 ) combined with measurement of S 5 , or by measurements of sensors S 3  combined with measurement of S 5 , or by measurements of sensors S 4  combined with measurement of S 5  or by sensors S 3  and S 4  and S 5  of  FIG. 19  or by another sensor. 
     A first approximation for detecting drowning may be obtained by measuring depth and pitch measurements only. 
     Referring again to  FIG. 21 , the zero angle pitch  30 A may be determined in case depth D 0  measured by the virtual depth sensor or by depth sensor S 3  (or S 4 ) is equal to D 5  measured by depth sensor S 5 . 
     In this zero angle pitch case, controller  14  ( FIG. 19 ) determines nose  20  is disposed above water level  12  in case the depth D 0  is smaller than the distance  28  between by the virtual depth sensor or by depth sensor S 3  and nose  20 . 
       FIG. 23  depicts some of the content of memory  50  of  FIG. 19 . 
     Memory  50  includes groups of depths D 0 ′ for by the virtual depth sensor or by depth sensor S 3  or S 4 , depths D 5 ′ for depth sensor S 5 , and pitches  30 A′ for which the nose is disposed above water level  12 , and the measurements are compared to the groups of memory  50 , which if found determines no risk of drowning. 
     Thus, a first group  52 A compared for the measurements of  FIG. 21  is of zero angle pitch, nose  20  is disposed above water level  12  in case the depth D 0 ′ is smaller than the distance  28  between by the virtual depth sensor or by depth sensor S 3  or S 4  and nose  20 . 
       FIG. 24  is a side view of the user wearing the apparatus of  FIG. 19 , at (−90) degrees pitch. 
     (−90) degrees pitch  30 A, i.e., lying on the back, may be determined in case depth D 5  measured by depth sensor S 5  is larger than depth D 0  measured by the virtual depth sensor or by depth sensor S 3  or S 4 , and the difference  34  therebetween is the maximal of all measured cases, being the thickness  32  of the torso. 
     In this (−90) degrees pitch case, controller  14  ( FIG. 19 ) determines nose  20  is disposed above water level  12  in case depth D 0  is of the water level  12 . This may be a second group  52 B. 
       FIG. 25  is a side view of the user wearing the apparatus of  FIG. 19 , at (−45) degrees pitch. 
     (−45) degrees pitch  30 A, i.e., lying on the back tilted, may be determined in case depth D 5  measured by depth sensor S 5  like the (−90) degrees pitch is larger than depth D 0  measured by the virtual depth sensor or by depth sensor S 3  or S 4 . However, difference  34  between D 0  and D 5  is smaller than thickness  32  of the torso. 
     The exact angle of pitch may be trigonometrically calculated according to the relation as described in  FIGS. 12,13,14,15 . 
     Group  52 C may be for the (−45 degrees) or other pitch case around (−45) degrees, controller  14  ( FIG. 19 ) determines nose  20  is disposed above water level  12  in case depth D 0  is smaller than the distance  28  between by the virtual depth sensor or by depth sensor S 3  or S 4  and nose  20 , multiplied by a fraction (sinus) calculated obtained from the (−45 degrees) or other determined angle, thus depth D 0  is smaller than distance  28  of the zero angle case of  FIG. 21 . 
       FIG. 26  is a side view of the user wearing the apparatus of  FIG. 19 , at 45 degrees pitch. 
     A 45 degrees pitch  30 A, i.e. lying on the stomach tilted, may be determined in case depth D 5  is smaller than depth D 0 . 
     The exact angle of pitch may be trigonometrically calculated according to the relation between difference  34  and thickness  32 . 
     In the 45 degrees or other pitch case around 45 degrees, controller  14  ( FIG. 19 ) determines nose  20  is disposed above water level  12  in case depth D 5  is smaller than distance  28  between by the virtual depth sensor or by depth sensor S 3  or S 4  and nose  20 , multiplied by a fraction (sinus) calculated obtained from the (−45 degrees) or other determined angle, thus depth D 5  is smaller than distance  28 . 
       FIG. 27  is a side view of the user wearing the apparatus of  FIG. 19 , at 90 degrees pitch. 
     90 degrees pitch  30 A, i.e. lying on the stomach not tilted, may be determined in case depth D 0  larger than depth D 5 , and the difference  34  therebetween is the maximal of all measured cases, being the thickness  32  of the torso. 
     In these 90 degrees pitch case, controller  14  ( FIG. 19 ) determines nose  20  is disposed above water level  12  in case depth D 5  is of the water level  12 . 
     The above-mentioned measurements have assumed typical positions of nose  20  in relation to the torso. 
     Depth and pitch measurements alone are not sufficient to ensure the above determinations, for example that for the 90 degrees pitch that nose  20  is disposed above water level  12  in case depth D 5  is of the water level  12 , since nose  20  might be disposed differently in relation to the torso. 
       FIG. 28  is a side view of the user wearing the apparatus of  FIG. 19 , at a different scenario of 90 degrees pitch, being due to a different position of the nose in relation to the torso. 
     In these 90 degrees pitch case, i.e. lying on the stomach not tilted, where nose  20  is disposed lower than of  FIG. 27 , controller  14  ( FIG. 19 ) must determine nose  20  is disposed below water level  12  even in case depth D 0  is of the water level  12 . 
     The above-mentioned depth, angle and motion measurements are configured for estimating whether the nose is disposed above or below water level  12 . 
       FIG. 29  is a top view of the torso wearing the apparatus of  FIG. 19 . Since normal swimming allows immersing the nose in the water for short periods, the above-mentioned measurements alert upon exceeding pre-determined periods. 
     A respiration sensor may accompany the drowning determination. 
     The respiration sensor may include a depth sensor S 1  attached to the user&#39;s chest; and an elastic belt  42  for pressing sensor S 1  to the user&#39;s chest; and a depth sensor S 2  not being pressed by belt  42 . 
     Depth sensor S 2  senses water pressure  44  only, whereas depth sensor S 1  senses water pressure  44 A in addition to chest pressure  44 B, thus allowing calculating chest pressure  44 B by subtraction of S 2  measurement from S 1 . 
     In the figures and/or description herein, the following reference numerals (Reference Signs List) have been mentioned:
         Numeral  10  denotes the apparatus for determining drowning risk according to one embodiment of the invention;   D 0 : depth measured by the virtual depth sensor or by depth/pressure sensor S 3  or S 4 ;   D 0 ′: depth of by the virtual depth sensor or by depth/pressure sensor S 3  or S 4  written in memory  50 ;   D 5 ′: depth of depth/pressure sensor S 5  written in memory  50 ;   S 1 ,S 2 : depth/pressure sensors, functioning as a respiration sensor;   S 3 ,S 4 ,S 5 : depth/pressure sensors, functioning as well for measuring pitch and roll angles;     12 : water level;     14 : controller;     16 : device such as an operable inflating device, which may include an inflatable balloon, a gas tank, and a hammer for releasing the cap of the gas tank;     18 : shirt including sensors S 1 ,S 2 ,S 3 ,S 4 ,S 5  and controller  14  and other elements;     20 : user&#39;s nose;     22 A: front of torso  22 C;     22 B: rear of torso  22 C;     22 C: torso;     24 : distance between depth sensors S 3  and S 4 ;     28 : actual distance between by the virtual depth sensor or by depth sensor S 3  or S 4  and nose  20 ;     28 ′: distance between the virtual depth sensor or the depth sensor S 3  or S 4  and nose  20  written in memory  50 ;     30 A: pitch measured by pitch sensor such as by the combination of by the virtual depth sensor or by depth sensor S 3  combined with S 5 , or by the combination of depth sensors S 3 ,S 4  and S 5 ;     30 A′: pitch written in memory  50 ;     30 B: roll measured by the combination of depth/pressure sensors S 3  and S 4 ;     30 B′: roll written in memory  50 ;     32 : thickness of torso  22 C;     34 : depth difference between depths D 0  and D 5 ;     40 : water;     44 A: water pressure;     44 B: chest pressure applied by breathing;     50 : memory;     52 A: set/group of parameters written in memory, the combination thereof indicating nose  20  being disposed above the water level;     52 B, 52 C, 52 D, 52 E, 52 F: other groups like  52 A;     61 : center between sensors S 3  and S 4 ;     63 : user&#39;s toes.