Abstract:
A system is disclosed for measuring and recording a quantity of liquid in an absorbing material ( 16 ), such as the quantity of urine in a nappy, which system comprises a sensor ( 1 ) comprising a first pair of electrodes ( 4, 7 ) for measuring the quantity of liquid, which first pair of electrodes is arranged to be placed around the absorbing material in, for example, a nappy, and a data logger ( 10 ), which is arranged to apply an alternating voltage across the first pair of electrodes and, concurrently herewith, to measure the capacitance between the two electrodes of the first pair of electrodes and to calculate, based on the measured capacitance, an estimate of the quantity of liquid in the absorbing material. In an embodiment, the system further comprises a second pair of electrodes ( 6, 8 ) for estimating the presence of a substance, such as faeces, there between using a similar method.

Description:
[0001]    The present invention relates to a system for measuring the quantity of urine in an absorbing material and, in some embodiments, detecting the presence of faeces, for example in a nappy, wherein one or more capacitive sensors are used to determine the state of fullness of liquid in the absorbing material and for detecting the presence of faeces. 
       BACKGROUND OF THE INVENTION 
       [0002]    Alarms used in connection with measuring the presence of urine in nappies, underwear and sheets, etc. are known in the art in the form of various enuresis and incontinence products. Urine sensors in nappies are known in the prior art, but there are none for measuring faeces. 
         [0003]    The conventional products are characterised by their emission of an alarm as soon as a liquid is present on the sensor, a so-called instant alarm. The products typically function by making a resistive measurement across a sensor in which no current is passing between the electrodes in a normal situation (when the nappy is dry). In an alarm situation, an electrical connection is created between the electrodes via the liquid between the electrodes, following which a given current will pass from one electrode to the other. The liquid, in these cases urine, works as a simple contact between two or more electrodes. 
         [0004]    Examples of such systems are described in U.S. Pat. No. 7,700,821 B2 and U.S. Pat. No. 7,667,608 B2, and the object of such systems is precisely to produce an alarm as quickly as possible, as urine on the sensor is an undesirable situation for children who are bed-wetters or for incontinence patients. The conventional resistive instant alarms are found in countless variants and are, as noted above, included in nappies, panty liners, underwear, sheets, underlays and beds etc., all being characterised by the above technology and function. 
         [0005]    Systems also exist for detecting quantities of liquid in, for example, nappies, which systems are characterised in being embedded in the liquid-absorbing material or in measuring through the liquid-absorbing material in the nappy. Examples of such systems are described in US 2008/0278337 A1 and WO 2013/013197 A1, the object of which systems is to signal a degree of wetness in the liquid-absorbing material in a nappy. 
         [0006]    Criteria for when there is a need for changing the nappies of children and persons suffering from incontinence are usually determined through subjective assessments by the nearest relatives or caregivers based on the time elapsed since the last change, or by feeling the nappy&#39;s degree of fullness. These methods entail several disadvantages. Firstly, they result in considerable variation in the degree of utilisation of the nappies and, consequently, a number of nappies will be changed, in which unutilised absorbency remains. Secondly, the nappy&#39;s maximum absorbency may be exceeded, resulting in overflow in situations, in which the nappy wearer experiences excessive discharge of urine. Finally, the practice of subjective assessment of the criteria for when a change of nappy is required is time consuming and attention demanding for relatives and caregivers irrespective of the assessment method used. 
         [0007]    The presence of faeces in the nappy is an undesirable condition as it may cause damage to the wearer&#39;s skin. Faeces are only detected through visual checking or if the user can say so him-/herself. This is inappropriate. 
         [0008]    This practice also gives occasion for considerations of whether the high frequency of cystitis among nappy wearers is caused by a prolonged exposure to moist surroundings. Said practice makes it difficult to collect and compare data, which could help to improve, or even prevent, different side effects of wearing a nappy. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0009]    It is an object of the present invention to provide a more advanced alarm system, which does not produce an either/or alarm, but uses a more differentiated measuring method with the possibility of self-selected alarm levels, which can be adapted to the institution or wearer. In this way, the system cannot only enhance the possibilities for improving the conditions for the nappy wearer, but also the working environment and the possibilities of preventing inappropriate situations. 
         [0010]    Such a system is provided in the present invention, which relates to system for measuring and recording a quantity of liquid in an absorbing material, such as the quantity of urine in a nappy, which system comprises a sensor comprising a first pair of electrodes for measuring the quantity of liquid, which first pair of electrodes is arranged to be placed around the absorbing material in, for example, a nappy, and a data logger, which is arranged to apply an alternating voltage across the first pair of electrodes and, concurrently herewith, to measure the capacitance between the two electrodes of the first pair of electrodes and to calculate, based on the measured capacitance, an estimate of the quantity of liquid in the absorbing material. 
         [0011]    Such a system is advantageous because it is able to continuously measure and pass on information on the quantity of liquid in a given absorbing item, such as a nappy. Greater awareness of when the nappy needs changing can thus be obtained. 
         [0012]    To ensure that the system is intact and that the capacitance measurements are correct, it will be advantageous, concurrently with the capacitance measurements, to check the so-called “dissipation factor”, which is an expression of an undesirable resistive connection between the two electrodes between which the capacitance is measured. This can be done, for example, by measuring the ohmic resistance between the two electrodes. 
         [0013]    In an embodiment of the invention, the sensor further comprises a second pair of electrodes, and the data logger is further arranged to be able to apply an alternating voltage across the second pair of electrodes and, concurrently herewith, to measure the capacitance between the two electrodes of the second pair of electrodes and to estimate, based on the measured capacitance, whether a substance, for example faeces, is present adjacent the two electrodes of the second pair of electrodes. 
         [0014]    In an embodiment of the invention, the data logger further comprises a contact surface, which is arranged to be placed in electrical contact with a body of a person, and the data logger is further arranged to be able to apply an alternating voltage between the two electrodes of the second pair of electrodes on the one side and this contact surface one the other side and, concurrently herewith, measure the capacitance between the two electrodes of the second pair of electrodes on the one side and the contact surface on the other side and to estimate, based on the measured capacitance, whether a substance, for example faeces, is present between the two electrodes of the second pair of electrodes and the body. 
         [0015]    In these embodiments, the invention can measure and pass on an alarm when faeces are present in the nappy, such that greater awareness can be obtained of when the nappy needs changing. 
         [0016]    In an embodiment of the invention, a temperature-measuring instrument is arranged in connection with the contact surface of the data logger. 
         [0017]    Such temperature-measuring instrument will, for example, be able to be used for checking that the surface is in fact in contact with the body as desired. It will also be able, in certain cases, to be used for measuring the person&#39;s body temperature. 
         [0018]    In an embodiment of the invention, the data logger is provided with a gyroscope and/or an accelerometer. 
         [0019]    The use of a gyroscope and/or an accelerometer makes it possible to determine the spatial orientation of the system (and of a person wearing the system), which makes it possible to adjust the measurement algorithm accordingly and obtain better utilisation of the filler capacity. 
         [0020]    In an embodiment of the invention, the data logger is provided with a GPS tracking module for localising the data logger. 
         [0021]    In an embodiment of the invention, the data logger is provided with a GSM modem for telecommunication with the data logger. 
         [0022]    One segment to be helped by the invention is persons with impaired cognitive functions including Alzheimer&#39;s disease/dementia. In cases, where the consequence of this disease is a combination of incontinence and confusion, an embodiment with a GPS tracking system in the data logger can be a major advantage. In these cases, where the user has a requirement for both nappy and tracking, both can be combined in one unit. A combination of data logging and a GPS tracking system can also take advantage of the fact that a GPS tracking module and a GSM module can be switched off until the user is outside the local wireless network to which the data logger is normally connected, thus saving electricity. 
         [0023]    In an embodiment of the invention, the alternating voltage applied has an amplitude of between 0.5 Vac and 15 Vac, preferably between 0.5 Vac and 10 Vac, most preferably between 1 Vac and 6 Vac. 
         [0024]    In an embodiment of the invention, the alternating voltage applied has a frequency of between 1 kHz and 100 kHz, preferably between 5 kHz and 25 kHz, most preferably between 8 kHz and 12 kHz. 
         [0025]    Amplitudes and frequencies within these intervals have proved to produce good measurement results and, furthermore, they mean that persons using the invention neither register nor are affected by the alternating voltage applied. 
         [0026]    In an embodiment of the invention, the alternating voltage is applied for short periods at predefined intervals of between 5 seconds and 10 minutes, preferably between 5 seconds and 1 minute. 
         [0027]    Application of the alternating voltage at said intervals has been found to produce good measurement results at the same time as electricity is saved. 
         [0028]    In an embodiment of the invention, the data logger is further arranged to be able to transmit signals, which are representative of the calculated estimations of a quantity of liquid or of the presence of a substance, wirelessly to one or more external monitoring stations. 
         [0029]    This allows for remote monitoring of the calculated estimations of the quantity of liquid and the presence of, for example, faeces. More specifically, this means that the invention can be arranged to emit a signal when a predefined optimal, uniform and objective criterion, for example for nappy changing, is met. 
         [0030]    In an embodiment of the invention, the signals, which are representative of a quantity of liquid, represent an absolute quantity of liquid, such as for example 300 ml. 
         [0031]    In an embodiment of the invention, the signals, which are representative of a quantity of liquid, represent a relative quantity of liquid, such as for example 20% of the absorbing material&#39;s total capacity. 
         [0032]    Depending on the application of the invention, it may be most advantageous to define the system, such that either absolute or relative values are estimated and form the basis for determining whether a predefined criterion is met. 
         [0033]    In an embodiment of the invention, the sensor with the one or two pairs of electrodes is arranged to be placed so that it extends along two opposite surfaces of the absorbing material, such as the two surfaces of a nappy facing towards the wearer and away from the wearer, respectively. 
         [0034]    The fact that the sensor only extends on the surfaces of the absorbing material, for example a nappy, means that the sensor can be retrofitted without changing the original design of the product/material onto which it is fitted. The invention can be fitted on an arbitrary liquid-absorbing product/material, including, for example, all types of nappies, sanitary towels, panty liners etc. 
         [0035]    In an aspect of the invention, it provides a method for measuring and recording the quantity of liquid in an absorbing material, for example a filler in a nappy, which method comprises the steps of:
       applying an alternating voltage across the two electrodes of a first pair electrode of electrodes in a sensor, which first pair of electrodes is arranged to be placed on the underside, i.e. on the surface facing towards the centre of the earth, of an absorbing material,   simultaneously measuring of the capacitance between these two electrodes of the first pair of electrodes in the sensor, and   calculating, based on the capacitance measured, an estimate of the quantity of liquid in the absorbing material.       
 
         [0039]    In an embodiment of the invention, the method further comprises the steps of:
       applying an alternating voltage across the two electrodes of a second pair of electrodes in the sensor, which second pair of electrodes is arranged to be placed opposite an area, in which a substance, the presence of which is desired to be estimated, such as for example faeces, can be expected to occur,   simultaneously measuring of the capacitance between these two electrodes of the second pair of electrodes in the sensor, and   calculating, based on the capacitance measured, an estimate of the presence of a substance, for example faeces, adjacent the two electrodes of the second pair of electrodes.       
 
         [0043]    In an embodiment of the invention, the method further comprises the steps of:
       applying an alternating voltage between the second pair of electrodes on the one side and a contact surface arranged to be in electrical contact with a body of a person on the other side,   simultaneous measuring of the capacitance between the second pair of electrodes on the one side and the contact surface on the other side, and   calculating, based on the capacitance measured, an estimate of the presence of a substance, for example faeces, between the second pair of electrodes and the body.       
 
     
    
     
       THE FIGURES 
         [0047]    A few embodiments of the present invention are described in more detail below with references to the figures, of which 
           [0048]      FIG. 1  illustrates schematically a sensor for retrofitted use according to an embodiment of the invention as seen from the outside, 
           [0049]      FIG. 2  shows the sensor of  FIG. 1  as seen from the rear, 
           [0050]      FIG. 3  shows the sensor of  FIG. 1 , wherein a part of the outer layer is removed to make internal layers of the sensor partly visible, 
           [0051]      FIG. 4  shows the sensor of  FIG. 2 , wherein a part of the rear side layer is removed to make internal layers of the sensor partly visible, 
           [0052]      FIG. 5  shows a cross-section of the sensor of  FIGS. 1 and 3  as indicated by the A-A line therein, 
           [0053]      FIG. 6  shows a cross-section of the sensor of  FIG. 1  as indicated by the B-B line therein, 
           [0054]      FIG. 7  illustrates schematically a nappy as seen from the inside with the sensor of  FIGS. 1-6  fitted therein, 
           [0055]      FIG. 8  shows a cross-section of the nappy and sensor of  FIG. 7  along the C-C line indicated therein with an indication of electrical fields, 
           [0056]      FIG. 9  shows a cross-section of the nappy and sensor of  FIG. 7  along the D-D line indicated therein with an indication of electrical fields, 
           [0057]      FIG. 10  illustrates schematically an associated data logger (transmitter) as seen from the front, 
           [0058]      FIG. 11  shows the data logger (transmitter) of  FIG. 10  as seen from the side, 
           [0059]      FIG. 12  shows the data logger (transmitter) of  FIG. 10  as seen from the rear, and 
           [0060]      FIG. 13  illustrates schematically a perspective view of a complete functioning unit according to an embodiment of the invention with a nappy, a sensor and a data logger. 
       
    
    
       [0061]    The figures are reproduced as examples of how the present invention can be implemented and should be regarded as non-limiting to the scope of protection as laid down in the claims. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0062]      FIG. 1  shows an example of a complete sensor  1  according to an embodiment of the invention with a skin-friendly and moisture-proof electrically insulating outer layer  2 , wherein the opening indicated through the outer layer  2  and a first non-liquid absorbing insulating layer (dielectric)  9  (not shown in  FIG. 1 ) provides electrical access from the outside to a first electrode  7  for capacitive liquid detection. The outer layer  2  can, for example, consist of a plastic film. 
         [0063]      FIG. 2  shows the same sensor  1  from the rear side, which is covered by a rear side layer  3 , wherein an opening through the rear side layer  3  provides electrical access to a second and innermost electrode  4  for capacitive liquid sensing, and wherein two other openings through the rear side layer  3  and a second non-moisture absorbing insulating layer (dielectric)  5  (not shown in  FIG. 2 ) provide electrical access to a first  6  and a second  8  electrode for capacitive faeces detection. The rear side layer  3  of sensor  1  is preferably an adhesive moisture-proof and electrically insulating layer, such as for example double-sided tape. 
         [0064]      FIG. 3  shows the sensor  1  with the outer layer  2  shortened, thus exposing parts of the dielectrics  5 ,  9 , the electrodes  6 ,  8  for capacitive faeces detection and the rear side layer  3 . In fact, the outer layer  2  covers the entire sensor  1 , but the inner layers  5 ,  6 ,  8 ,  9  are shown in this way for explanatory reasons. The electrodes  6 ,  8  for capacitive faeces detection together constitute a capacitive sensor for detection of faeces. 
         [0065]      FIG. 4  shows the sensor  1  from the rear side, this time with the rear side layer  3  shortened, thus exposing parts of second electrode  4  for capacitive liquid detection and of the second dielectric  5 . In fact, the rear side layer  3  covers the entire sensor  1 , but the inner layers  4 - 9  are shown in this way for explanatory reasons. The second dielectric  5  has also been shortened. In fact, it covers the entire sensor  1  as indicated in  FIG. 3 . The electrodes  4 ,  7  for capacitive liquid detection and the second dielectric  5  together constitute a capacitive sensor for detection of the quantity of liquid. The lengths of the electrodes  4 ,  7  for liquid detection correspond to the length of the liquid absorbing material for which measurements are desired. In  FIG. 4 , the electrodes  6 ,  8  for capacitive faeces detection, the first dielectric  9  and the outer layer  2  are also shown. The lengths of the electrodes  6 ,  8  for capacitive faeces detection are adjusted to fit the area in the nappy, in which faeces can be expected to occur. 
         [0066]      FIG. 5  shows a cross-section of the sensor  1  as indicated by the A-A line in  FIGS. 1 and 3 . It illustrates schematically an example of a possible design of sensor  1  comprising an outer layer  2  and a rear side layer  3  as described above. The dielectrics  5 ,  9  consist of electrical insulating materials, such as for example foam tape, polyethylene foam, paper or an elastic liquid polymer applied in a printing process, which polymer subsequently solidifies. The four electrodes  4 ,  6 ,  7 ,  8  are made from electrically conductive materials, such as copper, aluminium, iron, graphite coating or a conductive polymer, and can be installed as tapes with an adhesive or applied in a printing process directly on the carriers constituted by the dielectrics  5  and  9 . 
         [0067]      FIG. 6  shows a cross-section of the sensor  1  as indicated by the B-B line in  FIG. 1  with the outer layer  2  and the rear side layer  3  as described above. The electrodes  6 ,  8  for capacitive faeces detection as well as the second dielectric  5  are also shown as described previously. 
         [0068]      FIG. 7  shows an arbitrary type of nappy seen from the inside. The surface layer  15  of the nappy is shown partly open for explanatory reasons to illustrate the liquid absorbing material  16  inside the nappy. In fact, the surface layer  15  covers the entire nappy. Sensor  1  is seen from the rear edge of the nappy across the area within the nappy, in which faeces can be expected to occur. Thus, the sensor is arranged to measure the presence of faeces. 
         [0069]      FIG. 8  shows a cross-section of the nappy and sensor  1  as indicated by the C-C line in  FIG. 7 . When an alternating voltage is applied across the electrodes  4 ,  7  of the capacitive liquid sensor, an electrical field  17  is formed within the liquid absorbing material  16 . When an alternating voltage is applied across the electrodes  6 ,  8  of the capacitive faeces sensor, an electrical field  18  is formed between the surface of the sensor and the body of a person wearing the nappy across the area of the nappy, in which faeces can be expected to occur. 
         [0070]      FIG. 9  shows a cross-section of the nappy and sensor  1  as indicated by the D-D line in  FIG. 7 . The sensor  1  may be fastened to the surface of the nappy, using, for example, glue. The electrical field  18 , which is formed when an alternating voltage is applied across the electrodes  6 ,  8  of the capacitive faeces sensor, is also seen. 
         [0071]      FIG. 10  is a schematic front view of a data logger  10  according to an embodiment of the invention, wherein a housing  11  contains the electronics (measuring circuit, accelerometer, GPS, GSM modem, power supply and radio module, etc.) and a battery, a rubber hinge  12  is installed for fastening the housing  11  across the front edge of the nappy by means of, for example, a magnet contact, and a body sensor  13  creates an electrical connection to the body of a person wearing the nappy. The data logger  10 , the nappy and the sensor  1  together constitute a complete functional liquid and faeces detection unit  19 . 
         [0072]      FIG. 11  shows the data logger  10  of  FIG. 10  as seen from the side with the housing  11 , the rubber hinge  12 , the body sensor  13 , and a plug  14  which creates electrical connections to the four electrodes  4 ,  6 ,  7 ,  8  of sensor  1 . 
         [0073]      FIG. 12  is a rear view of the same data logger  10 , wherein the plug  14  functions as a contact part for the four electrodes  4 ,  6 ,  7  and  8  of sensor  1 . Thus, the plug  14  creates the electrical connection to the four electrodes  4 ,  6 ,  7 ,  8  through the openings designed for this purpose at the top of sensor  1  as seen in  FIGS. 1 and 2 , such that the data logger  10  can easily be installed together with sensor  1 . 
         [0074]      FIG. 13  shows an arbitrary nappy, onto the outside of which is fitted a sensor  1 , and data logger  10 , viewed in a perspective. The sensor  1  is centred in the nappy&#39;s longitudinal direction. These three components together constitute a liquid and faeces detection unit  19  functioning separately in the complete system. 
         [0075]    The unit  19  functions by the data logger  10  applying an alternating voltage of, for example, 2 Vac at a frequency of, for example, 10 kHz across the two electrodes  4 ,  7  for capacitive liquid detection, concurrently measuring the capacitance between the electrodes  4 ,  7 . Because the liquid sensor (consisting of the two electrodes  4 ,  7  and the second dielectric  5 ) creates an electrical field  17  into the liquid absorbing material  16  in the nappy, the capacitance measured will depend on the dielectric properties of the liquid absorbing material  16 . This utilises the physical differences between a dry and a damp/wet liquid absorbing material  16 . The dielectric constant for water (and urine) is approximately 80 times higher than the dielectric constant for air. The effect on the electrical field  17  of this difference in dielectric constants is utilised in this design. The relationship between the capacitance measured and the quantity of liquid in the nappy is approximately linear, always with a positive slope (the more liquid present, the higher the capacitance measured). Advantageously, the width of the second electrode  4  can be approximately 50% of the width of the first electrode  7  as indicated roughly in  FIG. 5 . 
         [0076]    The electrodes  6 ,  8  for capacitive faeces detection detect the presence of faeces in the nappy. The sensor functions by the data logger  10  applying an alternating voltage of, for example, 2 Vac at a frequency of, for example, 10 kHz between the two electrodes  6 ,  8  for capacitive faeces detection, concurrently measuring the capacitance between the electrodes  6 ,  8 . These electrodes  6 ,  8  are electrically and capacitively insulated from the surroundings by means of the two dielectric layers  5 ,  9  as indicated in  FIGS. 3-5 . As indicated in  FIGS. 3 and 4 , the first dielectric  9  is shorter than the two electrodes  4 ,  7  and the second dielectric  5 , which creates an opening to the electrodes  6 ,  8  at one end thereof. This opening provides increased capacitive sensitivity in the area, in which faeces are expected to occur. Thus, the capacitive faeces sensor consisting of the two electrodes  6 ,  8  and the two dielectrics  5 ,  9  forms an electrical field  18  with increased sensitivity in the area, in which the first dielectric  9  is not present. The dielectric constant for faeces is many times higher than the dielectric constant for air. The effect on the electrical field  18  of this difference in dielectric constants is utilised in this design. 
         [0077]    In a variant of the invention, the capacitive faeces sensor consists of the two electrodes  6 ,  8  and the two dielectrics  5 ,  9  as described above in combination with a body sensor  13 . In this embodiment, the data logger  10  applies an alternating voltage of, for example, 2 Vac at a frequency of, for example, 10 kHz between the body sensor  13  on the one side and the two electrodes  6 ,  8  on the other side, concurrently measuring the capacitance between the body sensor  13  on the one side and the electrodes  6 ,  8  on the other side. The alternating voltage applied between the body sensor  13  and the two electrodes  6 ,  8  forms an electrical field between the body of a wearer of the complete unit  19  and the designed sensitive area with electrodes  6 ,  8 , in which faeces are expected to occur. The electrical field and, consequently, the capacitance measured between the body and the electrodes  6 ,  8 , is depend on the presence of faeces, because the dielectric constant for faeces is many times higher than the dielectric constant for air. The effect of this difference on the electrical field between the body and the electrodes  6 ,  8  is utilised in this design. 
         [0078]    Increased reliability of diagnosing the presence of faeces in, for example, a nappy is obtained through a combination of measurements between the electrodes  6 ,  8  and the body via the body sensor  13 . 
         [0079]    Advantageously, the application of alternating voltages to the electrodes  4 ,  6 ,  7 ,  8  and the corresponding measurements of capacitance can be done at predefined time intervals of, for example, 10 seconds or 1 minute and temporally shifted in relation to each other in the sense that not more than one measurement is taken at any given time. 
         [0080]    The capacitances measured between the two electrodes  4 ,  7  for capacitive liquid detection are equated to estimates of the state of fullness of liquid, whereas the capacitances measured between the two electrodes  6 ,  8  for capacitive faeces detection (and in some embodiments) the body of a wearer of a nappy, is equated to estimates of whether or not faeces are present in the nappy. 
       LIST OF REFERENCE NUMBERS 
       [0000]    
       
         
           
               1 . Complete retrofitted sensor 
               2 . Moisture-proof and electrically insulating outer layer of the sensor 
               3 . Adhesive, moisture-proof and electrically insulating rear side layer of the sensor 
               4 . Second and innermost electrode for capacitive liquid detection 
               5 . Second non-moisture absorbing insulating layer (dielectric) 
               6 . First electrode for capacitive faeces detection 
               7 . First electrode for capacitive liquid detection 
               8 . Second electrode for capacitive faeces detection 
               9 . First non-liquid absorbing insulating layer (dielectric) 
               10 . Complete data logger (transmitter) for fitting on sensor and nappy 
               11 . Housing for data logger containing printed circuit board, accelerometer, GPS, GSM modem radio and battery, etc. 
               12 . Rubber hinge for fastening the data logger to the nappy and for the body sensor 
               13 . Body sensor for creating electrical connection to the wearer&#39;s body 
               14 . Plug for electrical contact with the four electrodes in the sensor 
               15 . Surface layer of an arbitrary standard nappy 
               16 . Liquid-absorbing material within an arbitrary nappy 
               17 . Electrical field from the capacitive liquid sensor 
               18 . Electrical field from the capacitive faeces sensor 
               19 . Assembled functional unit comprising nappy, sensor and data logger