ITTO980482A1 - DEVICE FOR LOADING AND DOSING LIQUID INTO THE TANK OF A HOUSEHOLD WASHING MACHINE, IN PARTICULAR A LAVAST - Google Patents

Description

Description of the industrial invention entitled:

"DEVICE FOR LOADING AND DOSING LIQUID INTO THE TANK OF A WASHING MACHINE FOR DOMESTIC USE, IN PARTICULAR A DISHWASHER"

SUMMARY

A device for loading and dosing liquid into the tub (7) of a washing machine for domestic use, in particular a dishwasher, of the type in which the dosage is carried out by detecting the level of the liquid inside the tub is described. (7), said device (6) comprising detection means (16A, 16B, 17A, 17B, 18A, 18B, 19A, 19B) located outside the tank (7) and operating according to the quantity of liquid loaded into it , a duct (C2,13, T) being provided to put said detection means (16A16B, 17A, 17B, 18A, 18B, 19A, 19B) in communication with said tank (7). According to the invention, closing means (V) are provided which are suitable for causing the occlusion of said duct (C2,13, T) at the end of an operation for loading liquid into the tank (7), in order to prevent substantial return of liquid from said tank (7) towards said detection means (16A, 16B, 17A, 17B, 18A, 18B, 19A, 19B).

DESCRIPTION

The present invention relates to a device for loading and dosing liquid in the tub of a washing machine for domestic use, in particular a dishwasher.

It is known that dishwashers comprise a washing tank, on the bottom of which the water taken from the water mains and necessary for washing the dishes is collected; for this purpose, the machine is equipped with a washing or recirculation pump which feeds one or more sprinkler members with the aforementioned water collected on the bottom of the tank.

The dosage of the water necessary for washing can be done in various ways.

The simplest solution is to provide for the timed opening of a solenoid valve for loading the water into the washing tank; this system, now practically abandoned, can however be very imprecise, since it does not take into account the always possible variations in pressure in the water supply network and the consequent variations in the flow rate of the solenoid valve.

The currently most widespread solution, substantially for reasons of economy, is to perform the washing water dosage by means of an electro-pneumatic pressure switch which, by means of an air trap, is able to detect the water level directly at the inside the washing tank, in order to provide a control criterion for the filling solenoid valve from the water mains.

This system, although economical, nevertheless requires a very precise calibration (and therefore expensive in the production phase) of the pressure switch: in fact, taking into account the fact that the washing tank has a rather large section, even a variation of a few millimeters in the level of water in the tank can result in an error of several liters in the dosage. This contrasts with the need to have machines with strictly controlled consumption.

The electromechanical pressure switch, over time, also tends to lose precision and this results in a water filling greater than that strictly necessary; on the other hand, the recalibration of the pressure switch, on washing machines already installed, is very critical to carry out, to the point that its replacement is preferable; in most cases, therefore, the Terror of water loading and dosing is passively accepted.

The fact that the air trap necessary for the operation of the pressure switch is practically located inside the washing tank, can also cause system malfunctions, due to the deposit of dirt particles right in correspondence with the air trap.

There are also known some solutions, currently abandoned, in which the detection of the water level is carried out directly inside the washing tank, by means of a mushroom-shaped float, the stem of which is able to produce the switching of an electrical contact; the operation of this float is however negatively influenced by the dirt particles which may be present in the washing tank, which could even cause the float itself to jam.

In other known solutions, the level that the water must reach within the washing tank is predetermined through the appropriate height positioning of a siphon, in such a way that, as soon as a minimum quantity of water exceeds the curvature of the siphon itself , a detection device causes the immediate interruption of the power supply to the fill solenoid valve.

However, these loading and dosing devices also have drawbacks, due to their positioning within the washing tank or adjacent to it, which can cause malfunctions, due to the heat that develops inside the tank, or to the deposit of particles of dirt inside the siphon or in correspondence with the detection device.

The shape of the floats used in such solutions, as said cylindrical or parallelepiped, can also cause inaccuracies in the detection, due to the friction to which they are subject. These floats in fact have a tendency to adhere or possibly get stuck to the walls of the relative housing chamber, and to collect dirt particles.

Given that the loading of a quantity of water greater than that necessary contrasts with the needs of containing consumption (the water must also be heated for washing purposes), the pressure switch system is replaced, in the case of high-performance washing machines. range, from solutions in which the water dosing is carried out outside the washing tank.

In this perspective, solutions are known according to which the dishwashing machine is equipped with a measuring tank, connected by branch to the water supply duct to the washing tank, so that a part of the water coming from the water mains reaches the tank directly and a part reaches the aforementioned tank.

The measuring tank has a reduced water capacity compared to that necessary for washing and a float level sensor is placed in it; the sensor operates on a small quantity of water, proportional to that loaded in the tank, and closes the water loading solenoid valve when the preset level for the tank is reached. In other words, therefore, the level sensor operates on a fraction of the water fed to the tank, which fills a small section tank, thus reducing detection errors.

Even this system, however, is not immune from detection errors, due to possible differences in the distribution of the flow rate in the branch pipe that feeds the metering tank; moreover, the system proves to be decidedly expensive, compared to those previously mentioned.

Also known are loading and dosing systems which provide for the repeated supply of water to one or more tanks, having a capacity equal to a fraction of that required for washing; in this case at least one tank is equipped with a float level sensor, also in this case of cylindrical or parallelepiped shape, which controls the usual filling solenoid valve, and the water supply to the washing tank takes place by subsequent decanting of the contents of the tank in the tank itself.

Such systems may require long times to carry out the filling of the water necessary for washing, and are in any case complicated and expensive to manufacture.

The last two types of systems mentioned also have the disadvantage of requiring considerable precision in the molding phase of the tank body, in order to obtain the seats for the level sensors, and to fix them therein.

In essence, therefore, the known dosage solutions currently in use can be divided into two categories, depending on the detection system used:

- systems that provide detection means operating on the entire amount of liquid loaded into the tank, ie in which the dosage is carried out by detecting the level of the liquid present directly inside the washing tank; such systems are advantageous from the point of view of cost-effectiveness, but disadvantageous from the point of view of dosing precision, reliability and cleanliness;

- systems that provide detection means operating on a reduced portion of the amount of water required for washing, ie in which the dosage is carried out outside the washing tank; such systems are advantageous from the point of view of dosing accuracy, cleaning reliability, but disadvantageous from the point of view of production costs and operating times.

The present invention aims to solve the aforementioned drawbacks of known devices.

In this context, a first object of the present invention is to indicate a device for loading and dosing liquid into the tub of a washing machine for domestic use, in particular a dishwasher, of the type in which the dosage is carried out by detecting the level of the liquid present inside the washing tank, of simple and compact construction, and therefore economical, and which allows, compared to known solutions adopting a similar philosophy, to achieve greater dosing precision, greater reliability of use and an easier and cheaper realization.

A further object of the present invention is to indicate a device of the aforesaid type of increased reliability, by virtue of the presence of suitable means for preventing the backflow, towards the means for dosing the liquid, of any residual dirt present in the washing tank.

These and other purposes are achieved according to the present invention by a device for loading and dosing liquid into the tub of a washing machine for domestic use, in particular a dishwasher, incorporating the characteristics of the attached claims, which form an integral part of the present. Description.

Further objects, characteristics and advantages of the present invention will become clear from the following detailed description and from the annexed drawings, provided purely by way of non-limiting example, in which:

- Fig. 1 schematically represents a part of the hydraulic circuit of a washing machine, in particular a dishwasher, using a device for loading and dosing liquid made according to the dictates of the present invention;

- Fig. 2 represents a schematic section of a device according to the present invention, made according to a possible embodiment thereof, in a first operating condition;

- Fig. 3 shows a first detail of the device of Fig. 2 through a schematic front view;

- Fig. 4 shows through a front view and a schematic side view a second detail of the device of Fig. 2;

- Fig. 5 represents a schematic section of a device of Fig. 2, in a second operating condition.

Fig. 1 schematically represents a part of the hydraulic circuit of a washing machine, made according to the dictates of the present invention; in the exemplified case, the invention is applied to a dishwasher.

In this figure, 1 indicates a connection to the water mains (such as a tap) to which a conduit 2 for feeding mains water to the dishwasher is connected; on this duct 2 there is a solenoid valve 3 for controlling the supply of mains water to the dishwasher; downstream of the solenoid valve 3, on the duct 2 there are placed in series a non-return device 4, generally known as an air break or air-break, and a decalcifying device 5. Note that the decalcifying device 5 may not be indispensable for a dishwasher, if the water has low hardness in the area where it is installed.

The devices 3, 4 and 5 are well known in their construction and operation, and therefore they will not be described in detail here; however, it is considered useful to remember that the presence of a jump in the air has the consequence that, in the hydraulic circuit of a washing machine, a certain amount of air is usually introduced, which is transported towards the washing tank; part of this air usually accumulates in the section of the duct closest to the tank, following the filling of the latter.

Downstream of the decalcifier, again on the duct 2, there is inserted a dosing device 6 for the washing liquid, made according to the dictates of the present invention; a tube T connects the device 6 to the washing tank of the machine, indicated with 7, which is equipped with a discharge duct 8, connected to a suitable discharge pump 9.

As can be seen, the device 6 is housed outside the washing tank 7, in a remote position with respect to this; in the exemplified case, the device 6 is also located at a height slightly higher than that of the bottom of the tank 7.

The tank 7 is also in communication with a suitable washing or recirculation pump, indicated with P, which supplies one or more sprinkler members I with the liquid collected from the bottom of the tank 7; also the aforementioned sprinkler members and the washing pump are of known construction and operation, and therefore will not be described in detail here.

In Fig. 2 a possible embodiment of the metering device 6 is shown in schematic form; it should be noted that in this figure, the portion of duct 2 coming from the decalcifying device 5 has a different course with respect to that illustrated in Fig. 1; the operation of the device 6 is in any case substantially similar in the two illustrated versions.

The device 6 comprises a body 11 in plastic material, for example made of two polypropylene half-shells welded with a hot blade, in which a connection fitting is defined, indicated as a whole with R.

The fitting R has a first connection 12, for connection to the duct 2, and a second connection 13, for connection to the pipe T; as can be seen, the connection 12 and the duct 2 have a smaller section than the connection 13 and the tube T. Three channels are defined between the connections 12 and 13 and the body 11, namely: - a first channeling Cl, aimed at putting the duct 2 in communication with the tube T, that is the outlet of the connection 12 with the inlet of the connection 13;

- a second channel C2, which puts in direct communication the area surrounding the channeling Cl, and therefore the attachment 13, with a first and a second chamber 16A and 16B, defined inside the body of the device 6;

- a third channel C3, which connects the upper part of the attachment 13, and therefore the tube T, with a third chamber 16C, defined inside the body of the device 6.

The two rooms 16A and 16B are both communicating in their lower part with the canalization C2 and are open upwards (and therefore in communication with the atmosphere); chamber 16C is also open upwards, but is isolated from chambers 16A and 16B. Fig. 3 shows a schematic front view of the fitting R, where the ducts Cl, C2 and C3 can be seen; in this figure, S indicates a seat for fixing a non-return valve, which is indicated in Fig. 2 with the reference V, which is fixed on the connection 13.

The valve V, shown in side and front view in Fig. 4, is essentially constituted by an element in rubber or similar material, having a peripheral ring VI, suitable for elastic coupling with the seat S of the fitting R, and a part central V2, which in its rest position is capable of at least closing the channels Cl and C2 with respect to the attachment 13; V3 indicates the spaced edges that connect the central part V2 to the peripheral ring VI, and with V4 the openings, present on the VI ring; the dimensions of the central part V2, and the presence of the spaced edges V3 and the openings V4 mean that the channeling C3 is never completely closed by the central part V2.

The elasticity of the flaps V3 is such that the central part V2, under the thrust of the water passing through the fitting R, is susceptible to movement between a closed position (Fig.

5) and an open position (Fig. 2), as will be better clarified later on.

Returning to Fig. 2, with 17A and 17B two floats are indicated, made of polymer, respectively inserted in the chambers 16A and 16B and suitable for sliding in them; according to an important aspect of the invention, the floats 17A and 17B are made in the form of spheres, for example in blown and tumbled polypropylene or polystyrene.

With 18A and 18B two electric microswitches are indicated, of a per se known type and operation, fixed in the upper area of the body 11.

The microswitches 18A and 18B are equipped with respective control rods 19A and 19B, adapted to be moved by the balls 17A and 17B which, during the phases of loading the water into the washing tank 6, tend to rise in the chambers 16A and 16B, as will be clarified below.

It should be noted that, in the case exemplified here, the ball 17A and the microswitch 18A are provided for controlling the level of the liquid loaded into the tank 7, while the ball 17B and the microswitch 18B are provided for safety purposes, in the event that the first microswitch 18A does not intervene. For this purpose, as can be seen, the rod 19B of the microswitch 18B is arranged to switch at a higher height, and therefore at an intervention level, which is higher than the position and tripping threshold of the microswitch 18A and the relative rod 19A. .

The operation of the machine represented in Fig. 1 is as follows.

When the user of the dishwasher, with known methods, starts a washing cycle, a programmer or timer device (not shown for simplicity) commands the opening of the solenoid valve 3.

The water entering the water mains, after passing the air jump device 4, enters the softener 5; the water leaving the softener 5, then filtered and softened, is conveyed to the fitting R of the device 6, and precisely to the connection.After passing through the connection 12, the water continues along the canalization Cl, in the direction indicated by arrow F of Fig. 1 and determines the opening of the valve V, in the position visible in Fig. 2.

The water can then pass into the attachment 13, and then continue along the tube T and reach the washing tank 7, which is progressively filled.

The progressive increase of the water level in the tank 7 is transferred, by means of the same tube T and by virtue of the presence of the canalization C2, also to the chambers 16A and 16B, with the consequent progressive filling of the same; this filling also determines the progressive raising of the balls 17A and 17B within the chambers 16A and 16B.

The water supply to the tank 7 continues until the ball 17A moves the rod 19A by a predetermined angle, thus causing the switching of the microswitch 18 A. This switching constitutes a command signal to interrupt the power supply to the solenoid valve. 3, and then cause it to close. In this condition, therefore, the tank is filled with a predetermined level of water, which depends on the intervention threshold determined by the working position of the microswitch 18A.

The aforementioned command signal also has the function of restarting the electric motor of the programmer or timer of the machine, which during the water filling phase is typically stopped, in order to enable the execution of the subsequent phases foreseen by the washing cycle. .

Following the aforementioned interruption of the water flow, the valve V returns to the initial rest position, closing the fitting 13; this situation is illustrated in Fig. 5. It should be noted that the valve V is kept in the closed position, as well as by the elastic force of the flaps V3, also and above all by the pressure of the water which tends to rise in the tube T, in the direction indicated by arrow F.

In this condition, when the connection 13 is closed, therefore, any substantial backflow of dirty water towards the device 6 is prevented, as well as the contamination of the dosing chambers 16A and 16B.

In case of failure of the microswitch 18A, for example due to any malfunction of the same, the water will continue to be loaded into the tank 7 and therefore to cause a further level increase in the rooms 16A and 16B. In this case, the water supply to the tank 7 continues until the ball 17B comes into contact with the rod 19B and moves it at a predetermined angle, thus causing the switching of the microswitch 18B. Also in this case, therefore, such switching generates a signal, that is a power cut, apt to cause the closing of the filling solenoid valve 3. This signal can possibly also be used to control the operation of the pump 9 of Fig. 1, in order to drain the excess water from the tank 7, or to trigger a second solenoid valve, possibly provided for safety purposes along the duct 2. This signal can finally also be used to activate a visual and / or acoustic indication alarm for the user.

The intervention of the microswitch 18B therefore has the function of preventing the water level in the tank 7 from exceeding a predetermined safety limit, defined by the intervention threshold of the microswitch 18B itself, thus avoiding possible flooding. In any case, the operation of the valve V, in the event of intervention of the microswitch 18B, is similar to that already described previously.

As you can guess, at the time of discharging the washing liquid from tank 7, the water level within the chambers 16A and 16B will gradually drop, until all the liquid has been discharged.

This will determine the progressive lowering of the balls 17A and 17B towards the initial position, visible in Fig. 2, with the consequent "rearming" of the microswitch 18A (and possibly of the microswitch 18B, if the aforementioned discharge follows a safety intervention of the device ).

Note that the aforementioned "resetting" of the microswitches, following a drain, is not necessarily such as to automatically enable a new opening of the solenoid valve 3, since this is however also subject to the control of the dishwasher programmer.

The chamber 16C and the relative channel C3 are of advantageous use in the case of washing machines, and in particular dishwashing machines, equipped with so-called "dynamic" water loading or dosing systems, i.e. systems in which the washing pump is activated even before the water filling from the water mains has been completed.

In this type of applications it happens that the predetermined level, detected on the bottom of the tank to determine the closure of the filling solenoid valve, corresponds to a volume of liquid which is actually lower than that present in the hydraulic circuit of the dishwasher, in fact, in this condition , part of the liquid is in circulation, through the washing pump and the relative sprinklers.

Therefore, if for any reason the washing cycle is interrupted (for example because the user opens the door of the machine to add dishes to be washed), the washing pump stops, the water present in the spraying circuit falls on the bottom of the tank, causing a consequent increase in level.

Note that in such cases, it is possible for air to accumulate in the liquid supply duct to the tank, following the progressive filling of the latter during normal filling (see what is mentioned at the opening of the description, by the way of the air jump device), and this air pocket can prevent the level increase caused by the interruption of the washing cycle from finding an outlet in the tube for feeding the liquid to the tank. Therefore, the aforementioned level increase could in certain circumstances cause an overflow of liquid from the tank.

In the case of the present invention, on the contrary, the fall of the liquid present in the spraying circuit certainly determines a return of water along the pipe T, which is allowed by the presence of the channel C3.

In fact, the air eventually accumulated in the tube T, is free to exit, behind the thrust of the liquid that rises in the tube T itself, from the canalization C3, towards the chamber 16C, but without the risk of contamination of the chambers 16A and 16B, which are insulated by means of valve V. As mentioned, in fact, this valve V is made in such a way as to occlude the ducts C1 and C2, but not the duct C3, not even in the condition illustrated in Fig. 5.

Optionally, the hydraulic circuit of the dishwasher can be designed so that, in the aforementioned case of interruption of the washing, a part of the liquid that goes up the tube T can reach the chamber 16C; it should be noted that there are no particular problems if this liquid contains residues of dirt, since the chamber 16C does not have dosing functions, and does not have suitable means, such as floats and microswitches.

In any case, when the washing cycle is subsequently restarted (in the case exemplified above, by closing the door), the washing pump returns to operation, sucking up the water present on the bottom of the tank, and therefore also recalling any excess water that may be present. in tube T and chamber 16C.

In the preferred form of the invention, the intervention point of the microswitches 18A and 18B, and the microswitches themselves (but not the end of the control rods), are located at a height greater than the water overflow level (N, Fig. 1), that is the maximum level above which the water could flow out of the tank 7, in case of failure of the loading and dosing system, or of the solenoid valve 3.

Therefore, with the arrangement of the microswitches 18A and 18B in an elevated position, the use of balls 17A and 17B of significant dimensions (preferably in the order of 18-30 mm in diameter) and the choice of rods 19A and 19B having a certain length it is possible to distance the parts under electrical voltage (ie the microswitches themselves) from the water loaded in the device 6 and also to prevent any overflow water from the tank 7 from coming into contact with them.

For this purpose, it should be noted that, advantageously, the intervention point of the microswitches could be further raised by simply providing more than one sphere within one or each chamber 16A and / or 16B.

It should also be emphasized, according to an important aspect of the invention, that the spherical shape of the floats 17A and 17B causes friction, within the relative sliding chambers 16A and 16B, which is far less than the substantially cylindrical or parallelepiped shaped floats used up to now in the known metering devices, as explained at the beginning of the present description, thus reducing the risk of errors or jamming; for the same reasons, the spherical shape of the floats according to the invention guarantees a constant thrust on the actuation rods of the microswitches.

The spherical structure, thanks to its intrinsic characteristics, also avoids the possibility of alterations in the external shape of the float (the so-called "boats"), which in known devices can occur due to prolonged use.

A substantial advantage of the spherical shape of the floats is their ability to "self-clean"; in other words, these spherical floats, during their handling phases, tend to rotate naturally around their geometric center, and therefore to free themselves of any residues of dirt deposited on them.

As mentioned, the presence of channeling C3 and chamber 16C is particularly advantageous in the case of washing machines equipped with dynamic water loading systems.

The invention can in any case find advantageous application even in the case of machines equipped with static loading, ie the type in which the washing pump is started only at the end of the liquid dosing phase. For such applications, therefore, it is not necessarily necessary to provide the channeling C3 and the chamber 16C, since there is no risk of increases in the level of liquid in the tank, due to interruptions in the washing.

In this case, in fact, when the opening of the solenoid valve 3 is closed, under the command of the timer, the non-return valve V also closes, as explained above, which prevents any backflow of dirt inside the device. 6.

However, note that in such cases, any air present accumulates right between the water present in pipe T, at the time of interruption of the flow, and valve V; this air pocket therefore constitutes itself a good "barrier" against the backflow of liquid, and possibly dirt, within the device 6. The presence of the valve V guarantees in any case the risk of contamination of the chambers 16A and 16B, and of the floats contained in them.

From the above description the characteristics of the present invention are clear. In particular, a device has been described for loading and dosing liquid in the tub of a washing machine for domestic use, in particular a dishwasher, of the type in which the dosage is carried out by detecting the level of the liquid present inside the tub. ; the device comprising detection means placed outside the tank 7 and operating on the basis of the amount of liquid loaded into it, a duct being provided to put the detection means in communication with the tank. According to the invention, closing means are provided to produce the occlusion of said duct following the intervention of the detection means, or at the end of an operation for loading liquid into the tank, in order to prevent the substantial return of liquid from the tank towards the detection means.

The detection means may comprise floats designed to cause the switching of electrical contacts to interrupt the supply of liquid to the tank, the floats being housed in respective chambers defined in the body of the device; this body is hydraulically connected to the tank in such a way that the progressive rise in the level of the liquid in the latter determines a consequent raising of a float in the respective chamber, which occurs progressively until it causes the switching of a contact, the switching threshold of the contact corresponding to the attainment of a predetermined level of liquid within the tank. The float or floats used are preferably spherical in shape.

In the preferred form of the invention, the closing means comprise a non-return valve, in particular made of elastic material and / or operated by the same liquid.

According to an important aspect of the invention, vent means are provided which, even following the intervention of the detection means, are able to allow the return of liquid from the tank into said duct, particularly in the case of liquid loading systems of the type dynamic. These vent means comprise at least one channel that puts said duct in constant connection with the atmosphere, regardless of the operating condition of said closing means, and a chamber defined in the body of the device.

The advantages of the present invention are also clear from the above description; in particular, the following should be emphasized:

- the simplicity of operation, as the water dosage in the tank is based on the use of floats and microswitches;

- simplicity of construction, since the device comprises components with high repeatability in the production process and long mechanical life, ie spherical floats and microswitches, usually guaranteed for thousands of switching cycles; likewise, the body of the device according to the invention can be obtained with a simple thermoplastic molding or hot plate welding operation;

- the presence of means suitable for avoiding the backflow of dirty water into the device allows to increase the efficiency of the device and its reliability over time;

- compactness and reduced size;

- the presence of the vent consisting of the channeling C3 and the open chamber 16C avoid problems of water overflow from the tank, in the event of interruptions in the washing cycle, in the case of machines equipped with dynamic water loading systems; - Reliability in detection, as the spherical shape of the float used has friction, within the relative chambers, much less than the floats used up to now in known solutions, thus reducing the risk of errors or jamming; for the same reasons, the spherical shape of the float ensures a constant thrust on the actuation rod of the relative microswitch;

- the spherical shape avoids the possibility of alterations in the external shape of the float (the so-called "boats") due to its prolonged use, thanks to the characteristics of a spherical structure, and allows to obtain a "self-cleaning" effect of the floats, which during the handling phases tend to naturally rotate around their geometric center, and therefore to get rid of any residues of dirt deposited on them;

- for the aforementioned reasons, more than one float can be housed in the same chamber, which is not recommended in the case of cylindrical or parallelepiped floats according to the known art, for reasons of reliability;

- Spherical structures, suitable for use in the device according to the invention, are very common and easily available on the market, and therefore of limited cost.

It is clear that numerous other variants are possible for those skilled in the art to the device for loading and dosing liquid into the tub of a washing machine for domestic use, in particular a dishwasher, described as an example, without thereby departing from the areas of novelty inherent in the inventive idea.

Claims (22)

CLAIMS 1. Device for loading and dosing liquid into the tub (7) of a washing machine for domestic use, in particular a dishwasher, of the type in which the dosage is carried out by detecting the level of the liquid present inside the tub ( 7), said device (6) comprising detection means (16A, 16B, 17A, 17B, 18A, 18B, 19A, 19B) located outside the tank (7) and operating according to the quantity of liquid loaded therein, a duct (C2,13, T) being provided to put said detection means (16A, 16B, 17A, 17B, 18A, 18B, 19A, 19B) in communication with said tank (7), characterized in that means are provided closure (V) designed to produce the occlusion of said duct (C2,13, T) at the end of a liquid loading operation in the tank (7), in order to prevent the substantial return of liquid from said tank ( 7) towards said detection means (16A, 16B, 17A, 17B, 18A, 18B, 19A, 19B). 2. Device, according to claim 1, characterized in that said detection means (16A, 16B, 17A, 17B, 18A, 18B, 19A, 19B) comprise at least a first float (17A) adapted to cause the switching of a first electrical contact (18A) to interrupt the supply of liquid to the tank (7). 3. Device, according to claim 2, characterized in that said first float (17A) is housed in a first chamber (16A) defined in a body (11) of said device (6). 4. Device, according to claim 3, characterized in that said body (11) is hydraulically connected to the tank (7) in such a way that the progressive raising of the liquid level in the tank (7) determines a consequent raising of the first float ( 17A) in the first chamber (16A), which occurs progressively until said switching of said first contact (18A), the switching threshold of said first contact (18A) corresponding to the attainment of a first predetermined level of liquid within the tank ( 7). 5. Device according to claim 1, characterized in that said closing means (V) comprise a non-return valve (V). 6. Device according to claim 1 or 5, characterized in that said closing means (V) are made of elastic material, such as rubber or similar material. 7. Device according to claim 1 or 5, characterized in that said closing means (V) are operated by means of liquid present in said conduit (C2,13, T). 8. Device, according to claim 2, characterized in that said first float (17A) is spherical in shape. 9. Device, according to claim 1, characterized in that vent means (C3,16C) are provided which, even at the end of an operation for loading liquid into the tank (7), are able to allow the return of liquid from the tank (7) in said duct (C2,13, T), particularly in the case of liquid loading systems of the dynamic type. 10. Device, according to claim 9, characterized in that said vent means (C3,16C) comprise at least one duct (C3) which constantly connects said duct (C2, I3, T) with the atmosphere, regardless of the operating condition of said closing means (V). 11. Device, according to claim 10, characterized in that said means (C3,16C) comprise a second chamber (16C) defined in the body (11) of said device (6) and connected to said channel (C3). 12. Device, according to claim 1, characterized in that hydraulic connection means (R) are provided between said body (11) and said tank (7), which comprise a fitting (R) having an inlet (12) and a outlet (13), the inlet being connected to a liquid supply line (2) and the outlet being connected to said conduit (C2,13, T). 13. Device, according to the preceding claim, characterized in that a third channel (C2) is defined between said fittings (12,13), adapted to put said outlet fitting (13) in direct communication with said first chamber (16A) . 14. Device according to the preceding claim, characterized in that said second channel (C3) departs from said fitting (R). 15. Device, according to at least one of the preceding claims, characterized in that said detection means (16A, 16B, 17A, 17B, 18A, 18B, 19A, 19B) comprise at least a third chamber (16B) defined in said body (11 ), for housing at least a second float (17B) adapted to cause the switching of a second electrical contact (18B), said third chamber (16B) being hydraulically connected to the tank (7) so that the progressive raising of the liquid level in the tank (7) determines a consequent raising of the second float (17B) in the third chamber (16B), which occurs progressively until it causes said switching of the second contact (18B), the switching threshold of said second contact (18B) ) corresponding to the attainment of a second predetermined level of liquid within the tank (7). 16. Device according to the preceding claim, characterized in that also said second float (17B) is spherical in shape. 17. Device according to claim 15 or 16, characterized in that said third chamber (16B), said second float (17B) and said second contact (18B) are provided for safety purposes, in the case in which said first contact ( 18A) does not intervene, the switching threshold of said second contact (18B) being at a higher height than the switching threshold of said first contact (18A). 18. Device according to one or more of the preceding claims, characterized in that at least one of said contacts is in the form of a microswitch (18A, 18B), equipped with a control rod (19A, 19B) adapted to be moved by said float (17A, 17B). 19. Device according to one or more of the preceding claims, characterized in that at least two spherical floats are provided within the same chamber. 20. Method for loading and dosing liquid into the tank (7) of a washing machine for domestic use, in particular a dishwasher, of the type in which the dosage is carried out by detecting the level of the liquid present inside the tank ( 7) by means of a device (6) comprising detection means (16A, 16B, 17A, 17B, 18A, 18B, 19A, 19B) located outside the tank (7) and operating on the basis of the quantity of liquid loaded in this, a duct (C2,13, T) being provided to put said detection means (16A, 16B, 17A, 17B, 18A, 18B, 19A, 19B) in communication with said tank (7), characterized in that, following the intervention of said detection means (16A, 16B, 17A, 17B, 18A, 18B, 19A, 19B), the occlusion of said duct (C2, 13, T) is produced, in order to prevent the substantial return of liquid from said tank (7) towards said detection means (16A, 16B, 17A, 17B, 18A, 18B, 19A, 19B). 21. Method, according to claim 20, characterized by the fact that, also following the intervention of said detection means (16A, 16B, 17A, 17B, 18A, 18B, 19A, 19B), the return of liquid from the tank (7) in said duct (C2,13, T), particularly in the case of liquid loading systems of the dynamic type. 22. Device for loading and dosing the washing liquid into the tub of a washing machine for domestic use, in particular a dishwasher, according to the teachings of the present description and of the annexed drawings.