DE102014000536A1 - Method and device for detecting an electrical conductivity in a water heater - Google Patents

Abstract

The invention relates to a measuring device for detecting an electrical conductivity of water in an electric instantaneous water heater (200) comprising at least two measuring electrodes (3) for spaced arrangement in a water-carrying channel (208) of the water heater (200), a measuring electronics for providing and evaluating an electrical signal influenced by the conductivity of the water, a transformer (5) having a primary side (51) and a secondary side (52), the measuring electronics being connected to the primary side (51) and the two measuring electrodes being connected to the (52) secondary side in such a way that the measuring electronics are galvanically decoupled via the transformer (5) with the measuring electrodes (3), the measuring electronics providing an electrical, primary-side alternating voltage signal on the primary side (51) of the transformer (5) as intended, so that on the secondary side (52) of the transformer (5) a secondary-side change adjusts the current across the measuring electrodes (3) and through the water as a function of the electrical conductivity of the water, the secondary-side current on the primary side (51) of the transformer (5) influences a primary-side current and the measuring electronics measures this primary-side current for determining the electrical conductivity , Furthermore, an electric instantaneous water heater (200) is proposed for heating water, wherein at least one electrical heating element, in particular a bare wire (202), for heating the water is provided in at least one channel (208) to be heated, a measuring device for measuring the electrical conductivity of the water to be heated or heated is provided with two electrodes (3) which are arranged in the at least one channel to be heated water (208) for detecting the conductivity of the water, and are connected to a measuring electronics for evaluation, and wherein the measuring device is interrupted when a predetermined Grenzleitwertes a voltage supply of the at least one heating element is exceeded.

Description

The present invention relates to a measuring device for detecting an electrical conductivity of water in an electric instantaneous water heater and, moreover, the present invention relates to an electric instantaneous water heater equipped therewith. Furthermore, the present invention relates to a method for detecting an electrical conductivity of water in an electric water heater.

Electric water heaters are known, they heat water with one or more electric heating coils, especially from a Blankheizdraht, or with a similar heating means, while the water flows through this water heater. Such water heaters are particularly found in the household and the water heated by them is then usually used immediately after heating, both temporally and locally. Such use may be, for example, showering or washing hands. A risk from the electric current of the heating element is avoided by the fact that the heated water, which produces an electrical connection in particular from Blankheizdraht to the user, due to its purity - and the calculated calculated discharge line - is only slightly electrically conductive. Therefore, such water heaters are harmless in this respect because of the defined discharge lines.

Exceptionally, however, a different situation may arise when contaminated, especially saline water enters the line. This can occur in areas with a dilapidated pipeline system, if, in addition, tide or a flood disaster or other extraordinary event causes contaminated or salty water to enter the pipeline system.

In order to address this problem, the use of continuous-wave water heaters with bare-wire may be dispensed with in such potentially endangered areas. There will be no risk. This also applies when using devices with tubular heaters, these in turn are more expensive and have worse control properties.

On the other hand, a water heater is a comparatively easy way to heat water. Such a water heater is compared to some other systems with expensive hot water tank comparatively inexpensive and space-saving and maintenance-prone. The heating by means of electric current is particularly cost-effective, if the use of a water heater, the holding warm water and thus the expenditure of the necessary energy is avoided.

Thus, if electrical instantaneous water heater with bare wire in such vulnerable areas to be used and the safety of users should also be guaranteed, a security system is necessary to preclude a hazard to users. However, an expensive security system contradicts the idea of offering a water heater as a cost-effective solution.

The present invention is therefore based on the object, at least one of o. G. To address problems. In particular, a solution is to be proposed, in a simple and cost-effective manner, the offering of a water heater even in such, o. G. vulnerable areas. At least, an alternative solution should be proposed compared to previous solutions.

According to the invention, a measuring device for detecting an electrical conductivity of water in an electric instantaneous water heater according to claim 1 is thus proposed. This measuring device thus comprises at least two measuring electrodes, a measuring electronics and may include a transformer. The measuring electrodes are spaced apart in a channel of the water heater and connected via the transformer to the measuring electronics. The measuring electronics provide an AC electrical signal on the primary side of the transformer, which is then transmitted to its secondary side to which the measuring electrodes are connected. On the secondary side, an alternating current flows through the measuring electrodes and through the water into the channel of the instantaneous water heater. The current depends on the conductivity of the water and thus also acts on the transformer back to the primary side, at which also sets a conductivity-dependent current. The measuring electronics then measures this primary-side current and can thus determine the electrical conductivity or the specific resistance of the water.

In principle, it is known to determine an electrical conductivity via an electrical measurement. However, this solution proposed here is characterized, inter alia, by the fact that it is comparatively simple and therefore cost-effective. A special feature is that this measuring device is specifically aimed at the task of determining a conductivity of the water in the value range in which a use of a water heater can be problematic. This simply designed measuring device uses an AC signal, which has a Transformer is fed to a measuring section and its conductivity-dependent reaction also acts back on this transformer and only then evaluated by the measuring electronics. Especially with clean water with low conductivity, ie with high specific resistance, only a small amount of current is produced and the described couplings via the transformer, which require an alternating current, become correspondingly very inaccurate and possibly can no longer be utilized.

According to the invention, however, it has been recognized that this measuring device can in any case provide sufficient accuracy for the described intended use. Basically, the measuring device can be used in all household appliances that heat drinking or service water in the flow process. These include, in addition to water heaters for heating tap water for providing at a tapping point such as a tap or shower head, and coffee machines, washing machines and dishwashers, to name just a few examples.

Preferably, the measuring device is prepared to measure a resistivity of 0 kΩcm to 10 kΩcm. This depends, among other things, on the design or dimensioning of the probes. In these areas, the measurement can be made in accordance with an embodiment with a tolerance of less than 10%. The measuring device is therefore specially tailored to this area. This is particularly true of the described structural design. However, it may also relate to the design of some elements, such as the measuring electrodes used and the distance in which they are placed. Also, the voltage level that is used, in particular that provided as the primary-side AC voltage, may be oriented thereto. Preferably, a peak voltage of the AC signal of about 2.5 V is proposed here, that is, a voltage which is approximately 5 V from its minimum value to its maximum value.

An embodiment proposes that the measuring electronics is galvanically separated from the measuring electrodes by the transformer. It is thus proposed a transformer which has no galvanic contact between the primary and secondary side. As a result, the measuring electronics of voltages in the measuring section, so the water to be heated can be protected in a simple manner. Thus, the measuring device can be used without problems for a so-called. Blankheizdrahtdurchlauferhitzer in which a Blankheizdraht, which is designed in particular as a heating coil, is disposed in the water channel and the water there directly heated, in which a correspondingly high electric current flows through it. Such Blankheizdrähte can be controlled directly with a mains voltage, for example, based on a neutral conductor and thus regularly based on a ground potential 230 V may be. Such a high voltage can easily destroy a measuring electronics. The galvanic isolation across the transformer avoids such potential danger for the measuring electronics.

According to one embodiment, it is proposed that the measuring electronics have a voltage divider or another means for raising an electrical voltage in order to raise the primary-side AC voltage signal so that its voltage means rises above a value of 0 V, in particular above a value 2 V. In particular, the primary-side AC voltage signal should raise so that it is completely positive. An alternating voltage signal, which oscillates by ± 2.5 V around a zero point of the transformer, should thus be raised, for example, by 2.5 V, so that it no longer between +2.5 V and -2.5 V, but between 0 and 5 V oscillates. By raising the zero point of the transformer by 2.5 V, the switching voltage of the driver, which oscillates between 0 and 5 V, is perceived or effective for the transformer as an alternating voltage with a stroke of +/- 2.5 V. In this way, in particular the generation of the primary-side AC voltage signal can be simplified and the proposed measuring device allows such a voltage shift due to its construction with the transformer and this has virtually no effect on the self-adjusting current.

According to a further embodiment, it is proposed that, in addition to the measurement of the primary-side current, a temperature of the water whose conductivity is to be determined is taken into account and used to determine the conductivity. Thus, the accuracy of the measurement can be further increased by the fact that the temperature, which may have an influence on the conductivity of water, is incorporated as an additional variable.

According to one embodiment, the measuring electronics generates the electrical, primary-side AC voltage signal via a pulse width modulation with a frequency in the range of 1 kHz to 10 kHz. As a result, the generation of the primary-side AC voltage signal can be achieved in a simple manner and with a low apparatus design. For example. Only a microprocessor needs to provide a corresponding switching signal to a driver, which generates the pulsed signal with the aid of an input DC voltage and generates an analogue voltage characteristic via a low-pass filter. Incidentally, the transformer can simultaneously perform the task of throttling the Apply low-pass filter for this pulsed signal.

According to the invention, an electric water heater is also proposed for heating water. Such a water heater has at least one water-carrying channel, in which the water is heated with an electric heating element, in particular with a bare wire. It is also possible for a plurality of such heating elements to be provided in a plurality of channel sections, in particular fluidically arranged one behind the other. In this channel or in one of these channels, if several are present, a measuring device is used for measuring the electrical conductivity. It has at least two electrodes, which are arranged in the at least one channel leading to the water to be heated for detecting the conductivity of the water, and are connected to a measuring electronics for evaluation. If the measuring device detects an exceeding of a predetermined limiting conductance, a voltage supply of the heating elements as a whole, if necessary, except for an electrical supply of a measuring and control electronics, is interrupted. Thus, a water heater with bare wire can be used in areas at risk, because it is assumed that an actual high conductivity occurs there only exceptionally, usually only for a short time.

Preferably, the interruption is active only for the duration of the exceeding of the limit conductance and falls below the predetermined limit conductance, the power supply for the at least one heating element is released again. An interruption in the supply of warm water can thus be limited to a minimum of time.

A favorable variant is when the shutdown takes place via a relay. In this way, a shutdown can be performed in a simple and fast manner, which also achieves galvanic isolation, so that a risk of electric shock over a waterway can be avoided.

As an advantageous variant, it is proposed here that a measuring device according to one of the embodiments described above is used. Accordingly, at least two electrodes may be provided in this channel or one of the channels. This can be done in the immediate vicinity or at a certain distance from the heating elements. Preferably, a temperature sensor is also arranged in the channel, if possible in the vicinity of the measuring electrodes, at least so that no active heating element is arranged between the measuring electrodes and the temperature sensor.

Preferably, the instantaneous water heater is prepared to switch off the at least one electrical heating element as soon as the detected conductance has exceeded a predetermined limiting conductivity or the detected specific resistance has fallen below a predetermined limiting resistance. The measuring device is thus connected to a safety shutdown device. The measuring device can give a value to a control unit, which then prevents the heating. Preferably, the switching off of the electric heating element or the electrical heating elements is carried out so that the entire heater, so the entire power electronics of the water heater is turned off. Once the conductivity is acceptable again can be turned on again.

Preferably, as elements of the water heater can be used, which are present anyway. In particular, an inlet pipe made of copper or an outlet pipe is also proposed from copper. Furthermore, as the electrode, in particular as a cooperating electrode, a temperature sensor housing or a temperature sensor receptacle can be used, which receives a temperature sensor in the water channel and is itself made of metal, preferably copper. The water channel, which is also the preferred embodiment, be made of non-conductive material, in particular plastic, but the temperature sensor receptacle can be made of metal and serve as an electrode, in particular in connection with an inlet or outlet pipe. Preferably, a heat sink, in particular a heat sink tube of a power semiconductor cooling is used for cooling power semiconductors for driving the at least one heating element. Such power semiconductors, in particular power semiconductor switches, heat up when driving the blank heating wire, can be cooled by the water to be heated and use metal heat sinks, at least according to one embodiment, which are arranged correspondingly for cooling in the service water or utility water. Preferably, such a power semiconductor circuit, in particular its heat sink, which is arranged on the power semiconductor or the power semiconductors for cooling, is not grounded and can thus represent an electrode to the ground and thus to the copper inlet or copper outlet.

If such elements are used as electrodes, in particular if they have a significantly greater distance from each other than a mean diameter of the channel, then the geometric dimensions of the channel in the region between these two electrodes can be used as geometric dimensions of the measuring path. to determine the conductivity from the measuring current.

Preferably, the electrodes are arranged with a minimum distance from one another, which is greater than a distance of a few millimeters of two measuring electrodes, as they could be arranged especially in a laboratory device for measuring a conductivity. Preferably, the distance is greater than 1 cm, greater than 5 cm, in particular greater than 10 cm. As the material can be used advantageously metal. The use of copper would be problematic at a very close distance of the two electrodes, because even the smallest deposits can greatly reduce the distance of the electrodes in a relative sense. Using the proposed large spacing, this problem is less and the benefits of especially the high conductivity of copper can then be exploited.

Preferably, the measuring device, in particular the measuring electronics or measuring circuit electronics, a separate retrofittable assembly, which is connected to the electrodes and via a data interface, preferably via an I ² C bus, connected to a control electronics of the water heater and powered by the control electronics with voltage becomes. It can be retrofitted. For example, it is arranged in a carrier for attachment. The electrodes may be connected via flexible lines and only need to be arranged in the corresponding channel in a predetermined position, in particular with a predetermined distance from each other. For the interruption, which is triggered in the case of a too high conductivity, a relay can be used, or it can be used an existing interrupting the power supply relay, which is also controlled by the measuring device. According to one embodiment, the interruption takes place by means of Leistungshableiterschalter, in particular TRIACs.

It is advantageous if the measurement circuit electronics have a μcontroller and the μcontroller of the measurement circuit electronics determines an electrical resistance between the electrodes via a measurement circuit. The μController can also be referred to as a microprocessor or microcontroller.

To calculate the conductance, a temperature of the medium can be transmitted via a data interface from a higher-level control of the flow heater, so that the current conductivity of the medium is calculated from the temperature and the resistance and this is compared with a reference value. Via this data interface, a release signal can also be transmitted to the higher-level controller of the water heater in order to cancel an interruption when the current conductance is again smaller than the limit conductance.

Preferably, the limit conductance, which can also be referred to as reference conductance, adjustable. An adjustability can be realized in particular via a potentiometer, or it is realized via a parameter that can be set on an operating unit and implemented or adjusted via the measuring device, in particular via the microprocessor.

According to one embodiment, the electrodes are not directly adjacent to a few millimeters, as known from conventional measurement technology, but have a greater distance from one another. This is intended to obtain a better ratio of the resistance of the medium formed by the spacing of the electrodes, which forms the actual measured variable, and the surface resistance of the electrodes, which changes due to aging and / or calcification, in order to achieve a better long-term stability of the measuring signal, and cheaper to use electrode material that is subject to greater aging. As a result, z. As copper can be used. This is also advantageous for the use, for example, of the inlet or outlet pipe, the heat sink or the inclusion of the temperature sensor as an electrode.

The measurement circuit electronics can advantageously be accommodated on the control electronics board and form a unit and / or the μcontroller of the measurement circuit electronics and the μController of the control electronics can form a unit. It is favorable, after release of the heating power when exceeding the conductance, so the interruption of the supply voltage for the heating elements, in particular the bare hot wires, on a control panel, z. B. by a flashing display or a symbol in a display.

According to the invention, a method is also proposed for detecting an electrical conductivity, which operates as explained above in connection with the explanations on the measuring device and / or works as explained above in connection with explanations of embodiments of the proposed instantaneous water heater.

The invention will now be explained in more detail by way of example with reference to exemplary embodiments with reference to the accompanying figures.

1 schematically shows the structure of a measuring device according to an embodiment.

2 shows a flow heater according to an embodiment in a schematic circuit diagram.

3 shows a simplified flow diagram for a method for monitoring the conductivity of service water or fresh water in a device for electrically heating this water.

The schematic representation of 1 shows two duct walls 1 a canal to carry water 2 , which thus represents the measuring medium whose conductivity or specific resistance is to be detected. Through one of the channel walls 1 are two electrodes 3 also called probes 3 may be referred to, liquid-sealing passed therethrough to detect the conductivity of the water 2 or measuring medium 2 to carry a measuring current. The electrodes 3 are on a transformer 5 on its secondary side 52 connected. At its primary side 51 becomes the transformer 5 acted upon by a primary-side AC voltage supplied by the AC generator 6 is produced. The alternator 6 This is done by a microcontroller 7 , which can also be referred to as μController for simplification, are driven with a corresponding switching signal. The alternator 6 thus generates an AC voltage via a pulse width modulation, which thus at this primary side 51 of the transformer 5 is entered.

It adjusts itself according to a measurement signal that in 1 illustratively as a measurement signal 8th a line is assigned, which - clearly speaking - from the transformer 5 to the microcontroller 7 leads. The microcontroller 7 thus also takes the evaluation of this measurement signal 8th in front. In fact, the measurement signal 8th also a signal that is on the primary side 51 of the transformer 5 established.

The microcontroller 7 thus controls the alternator 6 on, so that an AC signal, sinusoidal as possible, generated and on the primary side 51 of the transformer 5 enters. It then adjusts a secondary-side measuring current from the secondary side 52 over one of the electrodes 3 through the water 2 or measuring medium 2 to the other electrode 3 and back to the secondary side 52 of the transformer 5 flows. Accordingly, the measurement signal 8th one from the microcontroller 7 is suspended.

The channel wall 1 is also a temperature sensor 4 , which can also be referred to as a temperature sensor, performed liquid-sealing and can be a temperature of the water 2 or measuring medium 2 to capture. Accordingly, this temperature sensor 4 a temperature value 9 to the microcontroller 7 back. The microcontroller 7 thus evaluates to determine the conductivity of the water 2 the measuring current 8th as well as the temperature value 9 out.

2 shows in the simplified circuit representation of a water heater 200 with a blank heating wire 202 and a control and power electronics 204 , The blank heating wire 202 is shown schematically here and is also representative of other Blankheizdrähte. The water heater 200 works basically so that water to be heated through the inlet 206 flows in and in a duct or duct system 208 in the water heater 200 to be led. Therein it also becomes in a section of the bare wire or blank heating wire 202 heats and finally flows through the outlet 210 in the heated state.

The blank heating wire 202 is thereby by the control and power unit 204 driven. This has one or for each Blankheizdraht a Ansteuertriac 212 on, which is described schematically here. For cooling the drive triac 212 is a heat sink 214 provided with the not yet heated water in the channel or channel system 208 connected is. This still unheated water can thus first as cooling water for the Ansteuertriac 212 be used. The heat sink 214 can be provided as a cooling tube through which all the water flows, which is then to be heated.

For the power supply here is a three-phase power supply 216 intended. To control the Ansteuertriacs 212 and as a result, to drive each blank heating wire 202 is a microprocessor 218 intended. As soon as water is tapped, a flow meter detects 220 a volume flow v., so that the control and power unit 204 control the heating, in particular can start. Near the flowmeter 220 is also an inlet temperature sensor 222 disposed, which detects an inlet temperature θ _i , and it is an outlet temperature sensor 224 provided, which provides an outlet temperature θ _o . The microprocessor 218 can evaluate these values, thereby heating up and thus controlling the blank heating wire 202 to control as optimally as possible. In particular when a volume flow occurs, the heating can be switched on. If the volume flow is weak and / or the inlet temperature is already high or the outlet temperature high, a blank heating wire may only be activated reduced or if, for example, three blank heating wires are present, only one or only two of them will be activated.

In addition, as a function block is a conductivity meter 226 shown with the help of a conductivity sensor 228 the electrical conductivity of the water in the channel or channel system 208 detected. As a result, a conductivity value or its reciprocal, that is a resistance value to the control and power electronics 204 , in particular to the microprocessor 218 be given what is symbolized there as resistance Ω.

The conductivity sensor 228 may, for example, directly a conductivity in the channel or channel system 208 measure up. This can also be done at the location shown in the 2 in the channel 208 Electrodes may be provided to measure the resistance there. A signal, in particular current signal, can be present in the conductivity block 226 or conductivity measuring block 226 evaluated and as resistance Ω to the control and power electronics unit 204 especially the microprocessor 218 are given. Possibly. can the microprocessor 218 taking into account a temperature, in particular the inlet temperature θ _i , the value of the electrical conductivity or the electrical resistance Ω still improve.

Additionally or alternatively, existing elements may be used as electrodes to conduct the conductivity measurement. Such a variant is in the 2 represented, namely, the conductivity block 226 as an alternative or option to an inlet pipe 230 made of copper used as an electrode and on the other hand, the heat sink 214 , which is also metallic, in particular made of aluminum, used as a second electrode. The inlet pipe 230 made of copper and the heat sink 214 have a comparatively large distance from each other, in any case, the measuring path through the water in the channel or channel system 208 between inlet pipe 230 and heat sink or cooling tube 214 comparatively large, in any case very large compared to a conventional conductivity meter. The measuring section is also large in comparison to the average channel diameter. Due to the large distance can be achieved, inter alia, that distance fluctuations between the inlet pipe 230 and the cooling tube 214 in any case relatively small relative to their absolute distance and thus have little influence on the conductivity measurement. The inlet pipe 230 can in this case via the ground connection 232 be grounded. The cooling tube 214 must not be grounded accordingly, because otherwise a measuring current will not come from the inlet pipe 230 to the cooling tube 214 would pass through the water and by the way it goes without saying that the channel or the channel system 208 at least in this area between inlet pipe 230 and cooling tube 214 not conductive, so for example. Made of plastic, is formed.

Alternatively, an outlet tube 234 , which is also made of copper, act as an electrode for conductivity measurement. As a counter electrode, so that a measuring current can flow between these two electrodes, could be a temperature sensor recording 236 serve the the outlet temperature sensor 224 receives. The conductivity measurement would also evaluate water at approximately the same temperature for the entire measuring section. Also the outlet pipe 234 can in this case via the ground connection 232 be grounded, the temperature recording 236 In any case, this is not grounded in this case.

The comparatively long measuring distances for measuring the conductivity of the water over a measuring current between the respectively selected electrodes can be very inaccurate. But they can also reduce problems with bubbles in the water. Especially a very long measuring distance as between the inlet pipe 230 and the heat sink 214 If these are used as electrodes, very pure and thus very weakly conductive water can lead to a very small, if not undetectable, measuring current. However, if the water is heavily polluted, in particular very saline, a measuring current can flow and thus also the measuring accuracy increases significantly. For this case, only the high accuracy is needed.

2 This is also illustrated by the fact that in the case of a very high conductivity of the water, the control of the blank heating wire 202 is interrupted, in particular that even the three-phase power supply 216 is interrupted, so basically the entire water heater 200 is tension-free. Preferably, a functionality of the microprocessor 218 and the conductivity gauge. In any case, the shutdown of the power electronics can be done when the conductivity increases above a predetermined Grenzleitwert, using the Grenzwerteinstellvorrichtung 238 can be specified.

The manner of shutdown according to at least one embodiment is in 3 illustrated. The process is in the initialization block 350 After initialization, a query is made in the query block 352 , It is queried, ie compared, whether the detected conductivity is greater than a predetermined conductivity reference value or whether the specific resistance is smaller than a correspondingly predetermined specific reference resistance. This reference value for comparison may, according to one embodiment, be in the limit value setting device 238 , in the 2 is shown to be adjusted. This reference value may, for example, also depend on which regulations prevail in one country or how far the downstream water route behind the blank heating wire is to the consumer.

Anyway, a positive comparison in the query block 352 to a branch to the shutdown block 354 Turning off the water heater 200 but at least the blank heating wire 202 or all blank heating wires 202 triggers.

As a result, the instantaneous water heater can not be used at the moment, and the signal block is displayed to the user 356 a signal output, such as the generation of a flashing signal.

Following this, you can immediately re-query in the query block 352 respectively. Basically, by a correspondingly frequent clocking of this computer-implemented query almost a continuous query. If this leads to the result that the conductance does not exceed the reference conductance or the resistivity does not fall below the limiting resistance, the power supply, at least the control of the blanket heating wires, becomes 202 switched on or remains switched on, if it had already been switched on. This is illustrated by the power on block 358 , In particular, the shutdown block 354 on the one hand and the switch-on block 358 on the other hand, the same switch, in particular a power supply main switch, access, the Abschaltblock 354 a shutdown and the power on block 358 causes a switch. If through the power on block 358 switching on, this also causes the signal block 356 no longer send out the signal indicating the switched off device.

There are thus known blank wire heater, which are also characterized by the direct heating of the medium (drinking water) by very short Anheizzeiten. The usually underlying discharge lines in the inlet and outlet together with the necessary grounding of the metallic inlet and outlet points of the medium to be heated (drinking water) are used for safe operation.

The discharge lines in the inlet and outlet are dimensioned so that the Fehlerableitstrom is kept so low even with the worst service water quality with high conductance, that in this case can not be life-threatening currents at ungerdetem device.

Overall, this results in comparison with classic tubular heater water heaters a very high level of protection, because even with poor or no grounding due to design, the maximum Fehlerableitstrom, respectively the contact voltage is limited so that there is no danger to the user.

In some regions, due to special conditions of the water supply, the conductance values can rise temporarily to such an extent that the leakage currents reach a few hundredths of an ampere. If the PE conductor connection is carried out correctly, the danger potential for the user is low and comparable to that of conventional tubular heater systems.

The inventive solution can now be achieved in a simple manner that even at extremely high conductivities of water and no existing or poor protective conductor connection, no unacceptably high contact voltages can occur.

Often, instead of the conductance, the leakage current is used as a criterion. Again, too high, namely impermissibly high leakage currents can be reliably prevented. For this purpose, according to one embodiment, it is proposed to carry out a measurement of the resistance between the first or last radiator bolt lying in the water and in each case the inlet and outlet pipe. Thus, a measurement of the resistance between the first radiator bolt lying in the water and the inlet pipe is carried out or a measurement of the resistance between the last radiator bolt lying in the water and the outlet pipe is carried out. It is also possible to use elements other than measuring electrodes.

With known geometry of the flow channels, which thus simultaneously form discharge channels for the leakage currents, the resistance between the measuring electrodes can be extrapolated to the resistance of the discharge lines. If, for example, the outlet section is considered as a tube of 100 cm in length and the measuring section between the electrodes 1 cm = z. B. 1 kOhm, then the resistance of the discharge outlet, so the discharge line at the outlet, can be calculated to 100 kOhm, In a known mains voltage of, for example, 230 V would result in a leakage current of 2.3 mA. This calculation can be done before switching on the heating power. If the calculation results in a value which is above a set limit value, for example above 10 mA, switching on could then be prevented.

Claims (14)

Measuring device for detecting an electrical conductivity of water in an electric instantaneous water heater ( 200 ) comprising: - at least two measuring electrodes ( 3 ) for spacing in a channel carrying water ( 208 ) of the water heater ( 200 ), - measuring electronics for providing and evaluating an electrical signal influenced by the conductivity of the water, - a transformer ( 5 ) with a primary page ( 51 ) and a secondary side ( 52 ), where - the measuring electronics on the primary side ( 51 ) and - the two measuring electrodes on the ( 52 ) Secondary side are connected, so that the measuring electronics via the transformer ( 5 ) with the measuring electrodes ( 3 ), wherein, - the measuring electronics intended, an electrical, primary-side AC voltage signal on the primary side ( 51 ) of the transformer ( 5 ), so that - on the secondary side ( 52 ) of the transformer ( 5 ) a secondary-side alternating current across the measuring electrodes ( 3 ) and adjusted by the water as a function of the electrical conductivity of the water, - the secondary-side current at the primary side ( 51 ) of the transformer ( 5 ) influences a primary-side current and - the measuring electronics measures this primary-side current for determining the electrical conductivity. Measuring device according to claim 1, characterized in that the measuring electronics by the transformer ( 5 ) from the measuring electrodes ( 3 ) is galvanically isolated. Measuring device according to claim 1 or 2, characterized in that the measuring electronics has a voltage divider or other means for raising an electrical voltage in order to raise the primary-side AC signal so that its voltage center is above a value of 0 V, in particular above a value of 2 V. raises and / or raises the primary-side AC signal so that it is completely positive. Measuring device according to one of the preceding claims, characterized in that it is prepared for measuring a resistivity of 0 kΩcm to 10 kΩcm, preferably with a tolerance of less than 10%. Measuring device according to one of the preceding claims, characterized in that in addition to the measurement of the conductivity, the temperature of the water in the region of the measuring electrodes ( 3 ) and taken into account for determining the conductivity. Measuring device according to one of the preceding claims, characterized in that the measuring electronics generates the electrical, primary-side AC voltage signal via a pulse width modulation with a frequency in the range of 1 to 10 kHz. Electric instantaneous water heater ( 200 ) for heating water, wherein - in at least one channel leading to the water to be heated ( 208 ) at least one electrical heating element, in particular a bare wire ( 202 ), is provided for heating the water, - a measuring device for measuring the electrical conductivity of the heated or heated water is provided with two electrodes ( 3 ), which in the at least one channel leading to the water to be heated ( 208 ) are arranged for detecting the conductivity of the water, and are connected to the evaluation with a measuring electronics, and wherein - the measuring device is interrupted when a predetermined Grenzleitwertes a voltage supply of at least one heating element is exceeded. Electric instantaneous water heater ( 200 ) according to claim 7, characterized in that after an interruption of the power supply when exceeding the predetermined Grenzleitwertes the interruption is active only for the duration of the overshoot and falls below the predetermined Grenzleitwertes the power supply for the at least one heating element is released again. Electric instantaneous water heater ( 200 ) according to claim 7 or 8, characterized in that for interrupting the power supply, a relay is provided. Electric instantaneous water heater ( 200 ) according to one of claims 7 to 9, characterized in that a measuring device according to one of claims 1 to 6 is used. Electric instantaneous water heater ( 200 ) according to one of claims 7 to 10, characterized in that as electrodes ( 3 ) electrically conductive, preferably made of copper, water-contacting elements are used, in particular that at least one, in particular two of the following elements are each used as an electrode: - an inlet pipe ( 230 ) through which the water in the water heater ( 200 ), - a heat sink ( 214 ) a power semiconductor cooling for cooling power semiconductors for driving the at least one heating element ( 202 ), - a temperature sensor housing ( 236 ) for receiving a temperature sensor ( 224 ) for measuring a temperature of the water and - an outlet pipe ( 234 ) through which the water leaves the water heater. Electric instantaneous water heater ( 200 ) according to one of claims 7 to 11, characterized in that the electrodes ( 3 ) - Are arranged with a minimum distance to each other, which is greater than 1 cm, preferably greater than 5 cm, in particular greater than 10 cm and / or - are made of copper. Method for detecting an electrical conductivity, comprising the steps of - generating an electrical, primary-side AC voltage signal, - transmitting the primary-side AC voltage signal by means of a transformer ( 5 ) from its primary page ( 51 ) to its secondary side ( 52 ), - guiding the to the secondary side ( 52 ) of the transformer ( 5 ) transmitted AC signal via an electrode ( 3 ) by a water to be heated to a second electrode ( 3 ) and from this back to the transformer ( 5 ), - detecting a secondary side of the transformer ( 5 ) adjusting AC signal and - determining an electrical conductivity from the primary side set and detected AC signal. A method according to claim 13, characterized in that for carrying out the method, a measuring device according to one of claims 1 to 6 and / or an electric instantaneous water heater ( 200 ) according to one of claims 7 to 12 is used.

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