fig1 shows schematically an embodiment of a beverage brewing machine with a flow through heater . the beverage brewing machine comprises a water reservoir 1 in which the liquid 10 to be heated is stored . usually , in beverage brewing machines this liquid is water , but alternatively , the liquid may be milk . in the embodiment shown in fig1 , a pump 3 pumps the water 10 from the water reservoir 1 into a cup 9 . the water 10 enters the pump 3 via a channel or conduit 2 and is supplied by the pump to the channel 4 . the pump 3 pumps the water through the channel 4 via a consumable pad 8 into the cup 9 . alternatively , instead of the pump 3 a valve may be used if the lowest level of the water 10 in the water reservoir 1 is higher than the highest fill level in the cup 9 , such that the water 10 can fall from the reservoir 1 into the cup 9 without the need for a pump 3 . for example , the consumable pad 8 may contain coffee or thee . instead of the consumable pad 8 a user refillable holder for receiving grinded coffee or tea leaves may be present . alternatively , the setup shown may be used to brew filter coffee . although the pad 8 is shown to be placed in an open system such that the hot water has to fall through the pad by gravity , the system may be closed and the hot water may be applied under pressure to the pad 8 such as is usual in philips senseo machines or in espresso brewing machines . an electrical heater 5 has heater elements 50 which are arranged along the channel 4 to heat the channel 4 and the water 10 in the channel 4 when present . the portion of the channel 4 which is heated by the heater elements 50 may extend substantially vertical to improve the convection . the heater elements may comprise resistive wires which are heated by a current flowing there through . although a single heater element 50 is shown , alternatively several heater elements may be arranged in parallel or in series . the controllable electrical power can be supplied to all the heater elements or only to a subset of the heater elements . a sensor 6 is arranged near the channel 4 to sense the wall temperature of the channel 4 downstream the heater 5 . alternatively , the sensor 6 may be arranged inside the channel 4 to sense the water temperature of the water 10 leaving the heater 5 , or the sensor 6 may sense the wall temperature of a wall of the heater 5 . for example , this wall of the heater 5 may be a wall of the heater element 50 . optionally , a further temperature sensor 60 may be present which for example senses the temperature of the water 10 upstream of the heater 5 . the controller 7 has an input to receive the sensed temperature st 1 sensed by the temperature sensor 6 and optionally a further input to receive the sensed temperature st 2 sensed by the temperature sensor 60 . the controller 7 may use the different sensed temperatures st 1 and st 2 to obtain an optimal temperature profile of the water by controlling different issues with different temperatures , as will be elucidated later . alternatively , the controller 7 may use the temperature difference between the temperatures sensed by the two temperature sensors 6 and 60 . the controller 7 has outputs to supply control signals to the heater 5 and the pump 3 . the heater 5 can be controlled by controlling a level of a voltage applied to , or a level of a current flowing through , the heater elements 50 . the control may be continuously or time discrete . usually , although not essential , the heater elements are connected to the mains voltage ( not shown ) via an electronic switching device ( not shown ). the control signal supplied by the controller 7 may control the on - off duty cycle of the electronic switching device to control the average electrical power supplied to the heater elements 50 . consequently , also the heating power hp supplied by the heater elements 50 is controlled . the pump 3 can be switched on and off . alternatively , also the water flow through the pump 3 can be controlled by the controller 7 to even further decrease the temperature fluctuations of the heated water . if instead of the pump 3 , a valve can be used , the valve is switched on or off to pass the water 10 or to block the water 10 , respectively . the system shown in fig1 is used to elucidate with respect to the waveforms shown in fig2 a to 2c the known operation of the brewing machine , and to elucidate with respect to the waveforms shown in fig3 a to 3c an embodiment in accordance with the present invention . the waveforms shown in fig2 and 3 occur in a system in which the temperature sensor 6 senses the water temperature . similar waveforms occur if the temperature sensor 6 senses the wall temperature of the channel 4 inside or downstream outside the heater 5 . the waveforms may deviate more if the temperature of a wall of the heater 5 is sensed . alternatively , in an embodiment , when the two temperature sensors are present . one of the temperature sensors senses the wall temperature , while the other one senses the water temperature . the temperature sensor which senses the wall temperature is used to switch on the pump and to activate the closed loop , while the temperature sensor which senses the water temperature is used to control the temperature of the water during the closed loop phase . fig2 a to 2c show schematically waveforms for elucidating the known operation of a prior art water flow heater . fig2 a shows the heating power hp in watts supplied by the heater 5 . fig2 b shows both the wall temperature tw in degrees celsius of the channel 4 within the heater 5 , and the water temperature wt in degrees celsius of the water leaving the channel 4 at the position of the temperature sensor 6 . fig2 c shows the flow rate of the water 10 through the channel 4 in ml per second . all time periods , powers , temperatures and flow rates are examples only . at the instant t 0 , the preheating phase ph 1 starts and the controller 7 controls the heater 5 to supply the maximum heating power hpm . both the wall temperature indicated by the graph tw and the sensed water temperature indicated by the graph wt start increasing . it the instant t 1 the water temperature wt has reached the set point temperature or desired steady state level tlw and the preheating phase ph 1 ends . at this instant t 1 , the wall temperature tw is equal to tlt . if the sensor 6 is present it is possible to sense the wall temperature and no flow of liquid is required to sense the temperature at or near the heater position . alternatively , for example if only the sensor 60 is present , during the first phase a relatively small rate of flow of the liquid is applied to be able to sense the temperature of the liquid . at the instant t 1 , the controller 7 activates the pump 3 and the water 10 starts flowing through the channel 4 , see fig2 c . further , at the instant t 1 , the control loop is closed and the controller 7 starts controlling the heater 5 to supply a heating power hp dependent on the sensed temperature st . the start value of the closed loop is the steady state heating power hps . as is clear from fig2 b , the controller 7 starts operating in the closed loop mode when the water temperature wt is above the set point temperature tlw . consequently , in reaction the controller 7 decreases the heating power hp . however , due to inherent time delays caused by time constants in the system and the integrating action of the closed loop , it takes some time until the temperature wt reaches the set point temperature tlw again . now the heating power hp increases again to counteract for the too low temperature wt . but as shown in fig2 b , the water temperature wt lies below the set point temperature tlw during quite a long period of time . in the end the water temperature stabilizes at the set point temperature tlw . the closed loop phase ph 3 lasts from the instant t 1 to the instant t 2 . it has to be noted that the water temperature may show an overshoot because at the instant t 1 when the pump starts , the water at the entrance portion of the flow through heater has already the same high temperature as the rest of the water in the flow through heater but will be additionally heated when flowing through the flow through heater towards its outlet . at the instant t 2 , the controller 7 switches off the heater 5 and the pump 3 and the water flow stops . the wall temperature tw of the heater 5 starts decreasing and the water temperature wt starts increasing because of the still high wall temperature tw . fig3 a to 3c show schematically waveforms occurring in an embodiment of the beverage brewing machine in accordance with the present invention . fig3 a shows the heating power hp supplied by the heater 5 in watts . fig3 b shows both the wall temperature tw of the channel 4 at the position where the temperature sensor 6 is arranged in degrees celsius , and the water temperature wt of the water leaving the channel 4 in degrees celsius . fig3 c shows the flow rate of the water 10 through the channel 4 in ml per second . all time periods , powers , temperatures and flow rates are examples only . at the instant t 10 , the known preheating phase ph 1 starts and the controller 7 controls the heater 5 to supply the maximum heating power hpm . both the wall temperature indicated by the graph tw and the sensed water temperature indicated by the graph wt start increasing . it the instant t 11 the water temperature wt has reached the set point temperature or desired steady state level tlw and the preheating phase ph 1 ends . at the instant t 11 , the wall temperature tw is equal to tlt . at the instant t 11 , at which the open loop phase ph 2 starts , the controller 7 activates the pump 3 and the water 10 starts flowing through the channel 4 , see fig2 c . further , at the instant t 11 the controller 7 controls the heater 5 to supply the maximum heating power hpm . alternatively , during the open loop phase ph 2 , the controller 7 may control the heater 5 to supply the steady state heating power hps , or any other suitable power level , sequence of power levels , or a continuously changing heating power hp . the open loop phase ph 2 ends at the instant t 12 at which the known closed loop phase ph 3 starts . the instant t 12 is determined by the water temperature wt dropping below the set point temperature tlw . at the instant t 12 , the known closed loop phase ph 3 starts . the controller 7 keeps the pump 3 activated and the water 10 keeps flowing through the channel 4 . further , at the instant t 12 , the control loop is closed and the controller 7 starts controlling the heater 5 to supply a heating power hp dependent on the sensed temperature st . the start value of the closed loop is preferably the steady state heating power hps . as is clear from fig3 b , immediately after the start of the closed loop mode the water temperature wt is below the set point temperature tlw . consequently , the controller 7 increases the heating power hp . however , due to inherent time delays caused by time constants in the system and an integrating action of the closed loop , it takes some time until the temperature wt crosses the set point temperature tlw . now the heating power hp decreases to counteract for the too high water temperature wt . as shown in fig3 b , the water temperature wt now lies below the set point temperature tlw during a relatively short period of time only . thus , the comparison of the water temperature curve wt shown in fig3 b with that of fig2 b shows that the water temperature wt at the start of the brewing operation has become more constant . in the end the water temperature stabilizes at the set point temperature tlw . the closed loop phase ph 3 lasts from the instant t 12 to the instant t 13 . optionally , at the instant t 13 , the controller 7 switches off the heater 5 but keeps the pump 3 active . in this manner the heater 5 and the channel 4 are cooled down rapidly to prevent generation of steam . this cooling phase is well defined such that it is possible to compensate during the heating phase such that the correct average temperature of the liquid is obtained . it should be noted that the above - mentioned embodiments illustrate rather than limit the invention , and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims . for example , a filter may be arranged between the pump 3 and the flow through heater 5 . an optional temperature sensor may be arranged to sense the temperature of the liquid 10 leaving the liquid reservoir 1 or of the liquid entering the heater 5 . such an extra temperature sensor enables a feed - forward control compensating for a varying temperature of the liquid 10 . the temperature sensor st 2 upstream the flow through heater may be arranged near to the outlet , for example to check whether the liquid temperature is not higher than the desired temperature . it has to be noted that the liquid may be water and that a powder may be mixed with the heated water to obtain a beverage such as hot milk or hot chocolate . in the claims , any reference signs placed between parentheses shall not be construed as limiting the claim . use of the verb “ comprise ” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim . the article “ a ” or “ an ” preceding an element does not exclude the presence of a plurality of such elements . the invention may be implemented by means of hardware comprising several distinct elements , and by means of a suitably programmed computer . in the device claim enumerating several means , several of these means may be embodied by one and the same item of hardware . the mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage .
a liquid flow through heater for heating a liquid comprises a channel and an electric heater element for heating at least a portion of the channel . a temperature sense unit senses a temperature indicative of the temperature of the liquid . a flow control means controls a flow of the liquid through the channel . a controller controls in a first phase , the electric heater element to pre - heat at least the portion of the channel , and the flow control means to obtain a rate of flow of the liquid through the channel which is zero or relatively small with respect to a rate of flow during a second and / or third phase . the controller controls in the second phase succeeding the first phase , the electric heater element to supply a predetermined heating power independent on the sensed temperature , and the flow control means to obtain a flow of the liquid through the channel , and in the third phase succeeding the second phase , the electric heater element to supply a heating power in dependence on the sensed temperature to substantially stabilize the sensed temperature on a desired target value , and the flow control means to obtain a flow of the liquid through the channel .