Abstract:
A method for supplying a mask through which air flows with a regulated degree of humidification (m) is provided. The method may include: providing a water reservoir configured such that the air circulates in contact with the surface of water within the reservoir is charged with humidity; providing a heating device for heating the water in the reservoir by circulating an electric current; measuring an average intensity (Iav) of the current passing through the heating device; and controlling the average intensity (Iav) relative to a reference value (Iav c ) to obtain a degree of humidification (m) of the air that is independent of the ambient temperature (Ta). An apparatus for regulating the degree of humidification of air flow, as well as a heating humidifier including such a regulation device, are also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of co-pending International Application No. PCT/EP2005/009950 filed Sep. 15, 2005, which designates the United States, and claims priority to French application number 0452060 filed Sep. 15, 2004. 
    
    
     TECHNICAL FIELD 
     The invention concerns a process for supplying a mask with air with a regulated degree of humidification, a regulation device for the degree of humidification of an air flow, as well as a heating humidifier including such a regulation device. 
     BACKGROUND 
     It is known to place a humidifier at the output of a respiratory assistance device that delivers air to a user, so as to humidify the air supplied to the user and thus avoid drying out of the respiratory tract. 
     In order to obtain sufficient air humidification over the entire range of possible air flow rates (between 0 and 2 L/s), it is prescribed to heat the air contained in a humidifier reservoir, so as to accelerate evaporation and therefore transfer the water molecules to the air delivered to the user. 
     For the comfort of the user, the humidifier should be able to permit control of the degree of humidification of the supplied air. 
     Regulation systems of such heating humidifiers are already known that seek to supply the user air having a desired humidity level, for example, an essentially constant humidity level. 
     A first known system consists of regulating only the water temperature. Hence, the degree of humidification of the air delivered to the user also depends on the temperature of the air, which can vary, especially between day and night. It follows that simple regulation of the water temperature may not permit a constant air humidification level, which may be unsatisfactory. 
     A second known system employs, on the one hand, a temperature probe for the water, and, on the other hand, a temperature probe for the air. This system has higher performance, because it accounts for ambient conditions. However, such a system requiring the use of two probes may be relatively costly and more complex to use. 
     SUMMARY 
     According to some embodiments, a method for supplying a mask through which air flows with a regulated degree of humidification (m) is provided. The method may include: providing a water reservoir configured such that the air circulates in contact with the surface of water within the reservoir is charged with humidity; providing a heating device for heating the water in the reservoir by circulating an electric current; measuring an average intensity (Iav) of the current passing through the heating device; and controlling said average intensity (Iav) relative to a reference value (Iav c ) to obtain a degree of humidification (m) of the air that is independent of the ambient temperature (Ta). 
     In some embodiments, the heating device can be supplied a rectified sinusoidal current including passing half-cycles and blocked half-cycles blocked at 0, and the average intensity of the current passing can be regulated by controlling the number of passing half-cycles during a given time interval. 
     According to one embodiment, the method may also include: 
     storing a reference power (Pc) supplied to the heating device, the reference power (Pc) selected by a user; 
     measuring and storing a peak value (Umax) of a feed voltage for the heating device; 
     calculating the reference value (Iav c ) of the average intensity of the current passing through the heating device based on the reference power (Pc) and the peak value (Umax) of the feed voltage, and storing said reference value (Iav c ) of the average intensity; 
     measuring and storing the average intensity (Iav) of the current passing through the heating device; 
     comparing the measured value of the average intensity (Iav) and the reference value (Iav c ) of the average intensity; and/or 
     controlling the number of passing half-cycles during a given time interval to reduce the difference between the measured value (Iav) of the average intensity and the reference value (Iav c ) of the average intensity. 
     According to other embodiments, an apparatus for regulating the degree of humidification (m) of an air flow circulating in contact with the water surface of water located in a reservoir and intended to be distributed to a user via a mask is provided. The apparatus may include a heating device operable to heat said water by circulating an electric current; means for measuring an average intensity (Iav) of the current passing through the heating device; and means for regulating said average intensity (Iav) relative to a reference value (Iav c ). 
     According to certain embodiments, the apparatus may also include: 
     selection means allowing a user to select a desired reference power (Pc); 
     means for measuring a peak value (Umax) of the feed voltage of the heating device and an average intensity (Iav) of the current passing through the heating device; 
     means for storing the reference power (Pc) and peak value (Umax) of the feed voltage; 
     means for calculating a reference value (Iav c ) of the average intensity of the current passing through the heating device from the reference power (Pc) and the peak value (Umax) of the feed voltage; 
     means for storing the reference value (Iav c ) of the average intensity and the measured average intensity (Iav); 
     means for comparing the measured value (Iav) and the reference value (Iav c ) of the average intensity; and/or 
     means controlled by the comparison device and operable to act on the power supply of the heating device to reduce the difference between the measured value (Iav) and the reference value (Iav c ) of the average intensity. 
     In addition, according to some embodiments, the apparatus may also include a rectification device for rectification of the voltage delivered by the line voltage, and a blocking device operable to block some of the half-cycles of the voltage at 0. The rectification and blocking devices may be configured such that the heating device can be supplied a rectified sinusoidal current including passing half-cycles and half-cycles blocked at 0. 
     The blocking device may be configured to control the number of half-cycles blocked at 0 during a given time interval as a function of the difference between the measured value (Iav) and the reference value (Iav c ) of the average intensity. 
     According to yet another embodiment, a heating humidifier apparatus includes a water reservoir configured to house water; a heating device operable to heat the water in the reservoir by circulating electric current; an air input and output configured such that an air flow can circulate in contact with the surface of the water and be charged with humidity; and a control device for regulating the degree of humidification of the air flow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics of various embodiments of the invention are apparent from the following description, a description made with reference to the appended drawings, in which: 
         FIG. 1  is a perspective view of a heating humidifier according to one embodiment of the disclosure, including an outlet line associated with a mask applied to the nose and mount of the user; 
         FIG. 2  is a graphic representation showing the calculated water evaporation curve (in g/s) on contact of the air, for an ambient temperature of 25° C., as a function of the temperature difference between the water and the air, as well as the linear approximation of this curve; 
         FIG. 3  is a graphic representation showing the calculated evolution of water evaporation (in g/s) on contact of the air, for an ambient temperature of 25° C., as a function of relative humidity of the air for different values of the temperature differences between the water and air; 
         FIG. 4  shows the current signal passing through the heating element of the heating humidifier of  FIG. 1  as a function of time; and 
         FIG. 5  is a diagram illustrating functioning of the heating humidifier of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     A heating humidifier  1  is shown in  FIG. 1 , interposed between a respiratory assistance apparatus and a user  2 . For example, the respiratory assistance apparatus may be a medical apparatus for treatment of symptoms of sleep apnea, which may be used in a laboratory or at home. 
     The heating humidifier  1  may include a heating element, e.g., a metal plate  3  in contact with a resistance. 
     The resistance may comprise a screen-printed track on an insulated metal support or a resistance on a flexible film glued to a metal support. The heating element may be designed to ensure electrical insulation of 400 Veff between its conducting part (resistance) and its upper face, which may be an accessible part. The trace of this track (coil) may be such that heat transfer is distributed homogenously over the entire metal surface in contact with the water reservoir of heating humidifier  1 . 
     A water reservoir  4  may be positioned on the heated metal plate, a spring system (not shown) keeping the plate in contact with the bottom of reservoir  4  and thus improving heat transfer. 
     Reservoir  4  may be equipped with an input through which air from the respiratory assistance apparatus enters, and an output connected to a line  6 , at the end of which a mask  7  may be connected, which may be intended to be applied to the nose and/or mouth of user  2 . 
     The different elements forming the heating humidifier  1  may be housed in an enclosure made of an insulating material, for example, plastic, and equipped with selection and control devices  9  that can be operated by a user. 
     The air delivered by the respiratory assistance apparatus may pass through the heating humidifier  1  such that it is charged with humidity on contact with the water surface in reservoir  4 . The humidified air leaving the heating humidifier  1  may then be directed to user  2  via line  6  and mask  7  (see the arrows shown in  FIG. 1 ). 
     In some embodiments, the heating humidifier may be intended to function normally under the following conditions: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 atmospheric pressure: 
                 700 hPa to 1060 hPa 
               
               
                   
                 temperature: 
                 +5° C. to +35° C. 
               
               
                   
                 relative humidity: 
                 15% to 95% without condensation 
               
               
                   
                   
               
             
          
         
       
     
     The use temperature may be limited to 35° C. in order to meet the requirements of the standard, which imposes a maximum air temperature delivered to the user of 41° C. 
     In order to obtain a desired humidification of the supplied air, the disclosure may provide for controlling the power delivered to the heating element. 
     It is demonstrated below that regulation of the power may permit a constant humidification level to be obtained, whatever the ambient temperature, without requiring the use of a temperature probe. 
     If a water surface in contact with an air mass is considered, transfer of water to the air, under the conditions of use of the humidifier, can be described by the equation of Incropera and Dewitt:
 
 m =( S/Rw )( h/Cp Le   (1−n) )(( Ps   (Ts) / Ts )−( PV   (Ta) / Ta )  Equation 1
 
where:
         m=weight of the water transferred per unit time (in g/s)   S=water/air exchange surface (in m 2 )   Rw=425 J/kg·K (constant related to water)   h=heat transfer coefficient of water to air (depends on the considered system)   Cp=1008 J/kg·K (specific heat of water)   Le=0.846 (Lewis constant)   n=3 (coefficient, determined empirically in the case of water)   Ps (Ts) =611 exp (17.27×Ts/(237.3+Ts)) (saturation vapor pressure of water at temperature Ts)   Ts=Surface temperature of the water (in ° C.)   Pv (Ta) =Ps (Ta) ×HR (water vapor pressure at ambient temperature and humidity)   Ta=Air temperature (in ° C.)   HR=relative humidity of the air (between 0 and 1)
 
The factors that intervene in mass transfer and their effect on it are as follows:
   The surface S of air/water exchange:
           All other conditions remaining constant, especially the temperatures of the water and air, Equation 1 is equivalent to: m=cte×S. The degree of humidification is therefore a linear function of the water/air exchange surface.   
           The temperature difference between the water and air (Ts−Ta):
           If the water and air are at the same temperature, transfer of water molecules to the air occurs (natural convection), which depends only on the degree of humidity of the air. This transfer is not zero if the relative humidity of air less than 100%.   If the water is heated so as to increase its temperature a few degrees, greater transfer is produced, the degree of humidification (weight of transfer of water m) being proportional to the temperature difference between the water and air, as shown in  FIG. 2 .   The curve in  FIG. 2  may be obtained by calculation of Equation 1 using the following values (parameters close to those of one example contemplated application):   
               

     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 S = 0.01 m 2   
                 h = 20 
               
               
                   
                 ρ = 1.2 g/L (volumetric weight of air) 
                 Ta = 25° C. 
               
               
                   
                   
               
             
          
         
       
         
         
           
             
               
                 It is found, for the user, that the sensation of air humidity can be considered a linear function of the temperature difference between the water and air (linear approximation of the curve of  FIG. 2  with a correlation coefficient R=0.952). 
               
             
             Humidity of the ambient air (HR):
           The curves of  FIG. 3  were obtained by calculation of Equation 1 using the following values:   
         
           
         
       
    
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 S = 0.01 m 2   
                 h = 10 
               
               
                   
                 Ta = 25° C. 
                 Ts = 25, 35, 50, 65° C. 
               
               
                   
                 HR varying from 10 to 90% 
               
               
                   
                   
               
             
          
         
       
         
         
           
             
               
                 It is found that the degree of humidification m is a linear function of the relative humidity of the ambient air. 
               
             
             The air flow rate (V, in L/s, considered as constant and uniform):
           Equation 1 describes exchange of water molecules in static fashion. However, in the humidifier, the reservoir is traversed by air such that during flow, the ambient air replaces the air that was just humidified, thus increasing the degree of humidification relative to the static case.   
         
           
         
       
    
     According to the first law of thermodynamics, we have:
 
 P =( Ts−Ta )/ Rth   Equation 2
 
     where P is the power supplied to the system and Rth is the heat resistance of the system, expressed in K/W. 
     The heat resistance Rth may depend only on the dimensions of the heat exchange system (its thickness e and the exchange surface S) and the heat transfer coefficients, which depend, for example, on the nature of the materials present. This heat resistance may therefore be a fixed characteristic of the system. Since the heat resistance is inversely proportional to exchange surface, one can write: Rth=cte/S. 
     For this reason, Equation 2 can be written:
 
 P /[( Ts−Ta )× S]=cte   Equation 3
 
     Thus, power regulation permits a constant product (Ts−Ta)×S to be conserved. 
     This means that for a given exchange surface, the temperature difference between water and air is a linear function of the power supplied to the system. Hence, as mentioned above, the degree of humidification may depend essentially in linear fashion on this temperature difference. All other parameters remaining constant, one can therefore write that: m≈cte×P. 
     On the other hand, for a constant power, any variation of the exchange surface due, for example, to the variable cross section of the reservoir as a function of the amount of liquid that it contains, may be automatically compensated by an inversely proportional variation of factor (Ts−Ta), which permits the above constant that relates m and P to be left unchanged. 
     As a result, power regulation may enable us to guarantee a degree of humidification independent of the ambient temperature Ta and the exchange surface of the reservoir. 
     Nevertheless, this degree of humidification remains dependent on the ambient humidity HR and the flow rate of the air traversing the apparatus (the heat transfer characteristics of the heating element and the reservoir are fixed for a given system). 
     We will now describe the manner in which the power supply to the system may be controlled, according to this disclosure. 
     The general principle is to control the power by allowing more or less half-cycles of a rectified sinusoidal signal to pass through, obtained from the line voltage. This has the advantage of permitting adaptation to all possible line voltages by compensating for low voltage with a larger number of passing half-cycles and vice versa. 
     The signal delivered to the heating element is shown in  FIG. 4 . It is a rectified sinusoidal signal with double alternation of amplitude A and period T, canceled between times XT and ZT. One can therefore write: 
     
       
         
               
               
               
             
           
               
                   
               
             
             
               
                 between 0 and XT: 
                 1 (t) = A sin (ω) 
                 with ω = 2π/2T = π/T 
               
               
                 between XT and ZT: 
                 1 (t) = 0 
               
               
                   
               
             
          
         
       
     
     “Alternation” is used to denote the half-period of the sinusoidal signal, i.e., the period of the rectified sinusoidal signal. 
     X is therefore the number of passing half-cycles and Z the number of total half-cycles (passing and blocked). 
     The delivered electrical power to the heating element is: 
                                                             P   =       Ueff   ⨯   Ieff     =     R   ⨯     Ieff   2                     Hence   ,     Ieff   =           1   ZT       ⨯       ∫   0   XT     ⁢         f   2     ⁡     (   t   )       ⁢     ⅆ   t     ⁢           ⁢   and   ⁢           ⁢   l   ⁢           ⁢   m   ⁢           ⁢   oy         =       1   ZT     ⨯       ∫   0   XT     ⁢       f   ⁡     (   t   )       ⁢     ⅆ   t                                 Therefore   :                       Ieff   2     =         1   ZT     ⨯       ∫   0   XT     ⁢       A   2     ⁢     1   2     ⁢     (     1   -     cos   ⁡     (     2   ⁢   wt     )         )     ⁢     ⅆ   t     ⁢           ⁢   l   ⁢           ⁢   m   ⁢           ⁢   oy         =       1   ZT     ⨯       ∫   0   XT     ⁢     A   ⁢           ⁢     sin   ⁡     (   wt   )       ⁢     ⅆ   t                                 Ieff   2     =             A   2       2   ⁢   T       ⨯     X   Z       ⁢       ∫   0   T     ⁢       (     1   -     cos   ⁡     (         2   ⁢   π     T     ⨯   t     )         )     ⁢     ⅆ   t     ⁢           ⁢   l   ⁢           ⁢   m   ⁢           ⁢   oy         =         A   T     ⨯     X   Z       ⁢       ∫   0   T     ⁢       sin   ⁡     (       π   T     ⨯   t     )       ⁢     ⅆ   t                                 Ieff   2     =             A   2     2     ⨯     X   Z       ⁢           ⁢   l   ⁢           ⁢   m   ⁢           ⁢   oy     =         2   ⁢   A     π     ⨯     X   Z                             Hence   ⁢     :     ⁢           ⁢       l   ⁢           ⁢   m   ⁢           ⁢   oy       Ieff   2         =           2   ⁢   A     π     ⨯     X   Z     ⨯       2   ⁢   Z         A   2     ⁢   X         =         4   ⁢           ⁢   A         A   2     ⁢   π       =     4     A   ⁢           ⁢   π                               And   ⁢     :     ⁢           ⁢     Ieff   2       =           A   ⁢           ⁢   π     4     ⨯   l     ⁢           ⁢   m   ⁢           ⁢   oy                   Equation   ⁢           ⁢   4               
Equation 4 can therefore be written:
 
             P   =           R   ⨯   l     ⁢           ⁢   m   ⁢           ⁢     oy   ⨯   l     ⁢           ⁢   m   ⁢           ⁢     ax   ⨯   π       4     =       U   ⁢           ⁢     max   ⨯   l     ⁢           ⁢   m   ⁢           ⁢     oy   ⨯   π       4                       car   ⁢           ⁢     R   ⨯   I     ⁢           ⁢   max     =     U   ⁢           ⁢   max       ,       because   ⁢           ⁢     sR   ⨯   I     ⁢           ⁢   max     =     U   ⁢           ⁢   max             
The peak line voltage value (Umax) being constant, one can therefore write:
 
 P=cte×Iav  
 
     The principle regulation is therefore as follows: the reference power Pc may be regulated by the user of the heating element according to his preferences. The reference value of Iav (Iav c ) may then be determined by the system, knowing Umax. The values of Pc, Iav c  and Umax may be digitized by a microcontroller. 
     The average real current traversing the heating element, Iav, may be measured with a simple measurement resistance mounted in series with the heating element. The microcontroller may then compare Iav to Iav c  and determine if the number of half-cycles traversing the heating element must be increased or reduced such that the value of Iav is stable and equal to Iav c . For example, if Iav is greater than Iav c , the number of half-cycles traversing the heating element must be reduced. 
       FIG. 5  is now referred to, which schematically illustrates the heating humidifier, as well as the means of supply and the associated controls. 
     The humidifier  1  may comprise an electronic card intended to be fed with line voltage  10 , via an IEC C8 plug. A specific cord, furnished with the heating humidifier, may permit it to be connected to all available networks (varying according to country). 
     At the input of the electronic card, a filtering/protection device and device for rectification of the line voltage  10  may be provided. 
     A bipolar switch  11   a  may thus be provided, permitting cutoff of the electrical input on the electronic card, two fuses  11   b  of  2 A that protect the electronic card, a filter  11   c , represented by a capacitor, to avoid electromagnetic compatibility problems, and a varistor  11   d , mounted in parallel at the line voltage input, to shunt any overvoltages carried by the line voltage. 
     The line voltage  10  may undergo double alternation rectification, so that the resistance of the heating element is supplied by a current, as shown in  FIG. 4 . The rectified voltage may also serve to create a power supply  12  of two DC voltages V 1  and V 2 . A ballast transistor  12   a , a voltage reference  12   b  and a regulator  12   c  may thus be provided. 
     Voltage V 2  may be used for the control  16  of a control transistor  17  of the heating element, and to supply an operating amplifier (see below), whereas voltage V 1  may supply a microcontroller  13 . 
     Regulation of the power delivered to the heating element may be carried out by microcontroller  13 , housed in the heating humidifier  1 . 
     The microcontroller  13  may be present in a housing. It may include an ROM memory of the flash type, a RAM and EEPROM. It may include a 10-bit analog/digital converter, integrated with 4 multiplexed analog paths, an integrated analog comparator and an internal 4 MHz oscillator. It may also include a feed voltage drop detector and a watchdog. 
     The reference power Pc may be controlled by the potentiometer  14 , accessible from the outside of the humidifier housing  1 , so that the user  2  can choose the desired power, i.e., the desired level of humidification. The potentiometer  14  may be graduated from 1 to 5: position  5  (cursor at the maximum stop) delivers the maximum heating power and position  1  (cursor at the minimum stop) delivers a power equal to 20% of the maximum power, the other positions delivering a power proportional to the gradation. Use of the humidifier  1  without heating may require, if necessary, cutoff of the power supply to the humidifier with a switch. 
     The potentiometer  14  may be supplied by V 1 , which may guarantee that the analog input is never at a voltage greater than V 1 . Its value may be chosen to be comparable to the input impedance of microcontroller  13 . The signal of potentiometer  14 , freed of any parasitic signals by a filter capacitive  21 , may be directly digitized by microcontroller  13 . 
     The feed voltage (Umax) of the heating element may also be measured by microcontroller  13 , which may digitize an analog voltage taken at the center point of a resistance bridge supplied by the voltage coming from the double-alternation rectification. The resistance bridge may be designed such that the voltage applied to the analog input of microcontroller  13  is always less than V 1 , whatever the line voltage  10  used to supply humidifier  1 . 
     The current passing through the heating element (Iav) may be measured by measuring the voltage at the terminals of the small power resistor. The obtained signal on this resistor may first be filtered by a capacitor  18  to eliminate the parasitic signals, and may then be amplified by an operating amplifier  19 , whose gain may be calculated, so that the signal remains less than V 1 , whatever the operating mode of the installation. 
     The amplifier output may then pass through a low-pass filter  20  (resistance/capacitance) with a low cutoff frequency to recover an analog signal that is an image of the average current value (Iav) passing through the heating element. This signal may finally be digitized by microcontroller  13 . 
     Finally, the temperature T of the heated plate  3  may be measured by means of a thermistor of the CTN type, placed in the center of the lower face of plate  3 . This thermistor may be included in a linearization bridge, including two resistors and fed by V 1 . The voltage from this bridge may then be digitized by microcontroller  13 . 
     The heating element  3  may be controlled by a MOSFET transistor  17  of the N-channel type with enrichment. It may permit the current to pass through the heating resistor when a voltage VGS is applied to its gate. The importance of this component is to present an extremely low resistance to the passing state (RDSon), which may avoid a power loss at the level of the heating element and may limit the increase in temperature of the transistor. 
     In order to obtain the best possible resistance RDSon, the voltage VGS should not be less than 10 V. Hence, the digital control signal from the microcontroller may be limited. 
     The operating amplifier, fed by V 2 , may be used as a comparator, with a nominal commutation threshold of 2.96 V, to convert the signal of the microcontroller to a signal with a maximum value V 2 , which is applied to the gate of the MOSFET. The low output impedance of the amplifier may also permit a rapid commutation time of the transistor to be obtained. 
     The calibrated safety thermostat  15  may be mounted in series with the heating resistor. This thermostat  15 , which may be in direct contact with the metal plate  3 , may permit interruption of heating if the temperature T exceeds the safety vale. When the thermostat  15  is triggered, the electronic card may continue to function normally, but the resistance is no longer supplied. Only a mechanical action permits the thermostat to be reset. 
     Apart from the heating element, all the electronic components may be positioned directly on the printed circuit. The heating element may be connected on the electronic card by connectors that may permit simplification of assembly and reduce the assembly time. The card may be prescribed to be installed simply in the lower half-shell of the humidifier  1  and held by means of plastic clips. 
     The regulation program is now described, whose purpose may be to deliver a heating power P equal to a reference power Pc, controllable by user  2 , whatever the line voltage  10  (85 to 264 Vac). Passage or blocking of the half-cycles may be carried out by an electronic control device capable of cutting off supply of the heating element, as previously indicated. 
     The values of Pc, Umax, Iav and temperature T of plate  3  may be digitized by microcontroller  13 , via the analog/digital converter, and these values may then be used by the program. 
     Initially, the microcontroller  13  may allow a specified number of half-cycles out of  100  to pass through the heating element. The program may occur as follows. 
     In order to be synchronized relative to the wave cycles of the line voltage, the microcontroller  13  detects zero passage of the rectified wave. The line voltage is therefore not abruptly cut off. 
     The microcontroller  13  measures the value Umax of the line voltage, awaiting zero passage, and adds this value to the previously measured one. Once zero passage is detected, the microcontroller  13  decrements the counter of the number of passing half-cycles (X) and the counter of the total number of half-cycles (Z). If the number of passing half-cycles is equal to zero, then the microcontroller  13  blocks the half-cycles. Otherwise, the microcontroller  13  allows the half-cycles to pass. 
     The microcontroller  13  may then measure:
         the current value passing through the heating element (Iav) and adds this value to the one previously measured; and   the temperature T of plate  3 , and adds this value to the previously measured one.       

     The microcontroller  13  may then wait until the line voltage wave has reached its zero passage to restart the cycle. 
     When the total number of half-cycles equals zero, the microcontroller  13  may: calculate the average current passing through the heating element (Iav), calculate the line voltage value Umax, acquire the reference value (Pc), calculate the safety current value, compare the measured current value (Iav) to that of the reference (Iav c ) and the safety value, and calculate the average temperature value. 
     If Iav&lt;Iav c , an increase in Iav occurs and, for this purpose, the microcontroller will allow another alternation to pass through the heating element. Otherwise, if Iav≧Iav c , the microcontroller will allow one less alternation to pass through the heating element. 
     In the event that the safety current or a temperature T greater than, for example, 70° C., is surpassed, the microcontroller may block passage of the half-cycles. Moreover, if the current exceeds the maximum admissible current, the microcontroller may be placed in a waiting routine and will do nothing, except refresh the watchdog. If the temperature exceeds 70° C., the microcontroller may be placed in a routine, where it will continuously measure the temperature and refresh the watchdog. If the microcontroller measures the temperature less than 65° C., it may resume the principal loop. 
     To summarize, the processing phase may comprise:
         decrementation of the counters (all half-cycles)   current measurement (all half-cycles)   measurement of the plate temperature (all half-cycles)   calculation of the average current (every 100 half-cycles)   calculation of the peak line voltage (every 100 half-cycles)   calculation of safety current (every 100 half-cycles)   calculation of the average current (every 100 half-cycles)   measurement of the reference Pc (every 100 half-cycles)   updating of the variables (every 100 half-cycles)   comparison of the average current with the reference current and the safety current (every 100 half-cycles)       

     Although the disclosed embodiments have been described in detail, it should be understood that various changes, substitutions and alterations can be made to the embodiments without departing from their spirit and scope.