Patent Publication Number: US-9895898-B2

Title: Lid for an ink reservoir with mixing function

Description:
TECHNICAL FIELD AND PRIOR ART 
     The invention relates to the domain of continuous inkjet printers (CIJ). 
     It also relates to the architecture (the layout of the ink circuit) of CIJ printers, particularly in order to guarantee homogeneity of the ink. 
     Continuous inkjet (CIJ) printers are well known in the field of coding and industrial marking of various products, for example for high speed marking of barcodes, expiration dates on food products or references or distance marks on cables or pipes directly on the production line. This type of printer is also used in some decoration domains in which the technological possibilities of graphic printing are exploited. 
     These printers have several subassemblies of the type shown in  FIG. 1 . 
     Firstly, a print head  1  usually offset from the body of the printer  3 , is connected to the body through a flexible umbilical line  119  including hydraulic and electrical connections necessary for operation of the head by giving it flexibility that facilitates integration on the production line. 
     The body of the printer  3  (also called the cabinet) usually comprises three subassemblies: 
     an ink circuit in the bottom part of the cabinet (zone  4 ′), that firstly supplies a suitable quality ink to the head at a stable pressure, and secondly makes it possible to handle ink from jets that is not used for printing, 
     a controller located in the top of the cabinet (zone  5 ′), capable of managing action sequences and performing treatments for activation of different functions of the ink circuit and the head. 
     an interface  6  that provides the operator with the means of using the printer and being informed about its operation. 
     In other words, the cabinet comprises 2 subassemblies: the electronics and the electrical power supply and the operator interface at the top, and an ink circuit supplying nominal quality ink to the head at positive pressure and recovering ink not used by the head at negative pressure, at the bottom. 
       FIG. 2  diagrammatically shows a print head  1  of a CIJ printer. It comprises a drop generator  60  supplied with pressurised electrically conducting ink by the ink circuit  4 . 
     This generator is capable of emitting at least one continuous jet through a small dimension port called the nozzle. The jet is transformed into a regular sequence of identical size drops under the action of a periodic stimulation system (not shown) on the upstream side of the nozzle outlet. When the drops  7  are not to be used for printing, they are directed towards a gutter  62  that recovers them so as to recycle unused ink by returning the drops to the ink circuit  4 . Devices  61  placed along the jet (charge and deflection electrodes) electrically charge the drops on order and deflect them in an electrical field Ed. These drops are then diverted from their natural ejection trajectory from the drop generator. The drops  9  intended for printing are not directed to the gutter and are deposited on the support to be printed  8 . 
     This description can be applied to continuous inkjet (CIJ) printers called binary printers or continuous multi-deflected jet printers. Binary CIJ printers are fitted with a head in which the drop generator has a multitude of jets, each drop in a jet can only be oriented towards only two trajectories, namely print or recovery. In multi-deflected continuous jet printers, each drop in a single jet (or a few spaced jets) may be deflected on different trajectories corresponding to charge commands that are different from one drop to the next, thus scanning the zone to be printed along a direction that is the deflection direction, the other scanning direction of the zone to be printed is covered by a relative displacement of the print head and the support to be printed  8 . In general, the elements are arranged such that these 2 directions are approximately perpendicular. 
     An ink circuit of a continuous inkjet printer supplies firstly ink at regulated pressure, and possibly solvent, to the drop generator of the head  1  and creates a negative pressure to recover fluids not used for printing in return from the head. 
     It also manages consumables (ink and solvent distribution from a chamber) and controls and maintains the ink quality (viscosity/concentration). 
     Finally, other functions are related to user comfort and to the automatic handling of some maintenance operations in order to guarantee identical operation regardless of usage conditions. These functions include rinsing of the head with solvent (drop generator, nozzle, gutter), assistance with preventive maintenance such as replacement of limited life components (filters, pumps). 
     These various functions have very different purposes and technical requirements. They are activated and sequenced by the printer controller that will be more complex if there is a large number of sophisticated functions. 
     Inks containing pigments such as titanium oxide (rutile TiO 2  or anatase) in the form of sub-micronic size particles are particularly attractive for their whiteness and their opacity. They are used for marking and identification of black or dark supports. 
     Dense pigment particles naturally tend to sediment when ink is at rest. 
     The consequences of this inevitable sedimentation can be blocking of pipes or loss of opaqueness of markings. Therefore the ink circuit must be able to stir ink in one way or another such that the ink can maintain its homogeneity, or restore it after a fairly long rest time. 
     On the other hand, the viscosity, upon using a printer, will change from one value to another. In other words, the viscosity will not be a stable parameter during the operation of the printer. This viscosity variation is mainly due to three factors: 
     solvent evaporation, 
     solvent addition in the ink reservoir, which results from washing operations of all or part of the fluid circuit; these operations are performed using a solvent which is, following such operations, sent to the main reservoir, 
     temperature variations. 
     The ink quality (measured by the viscosity) is thus maintained by additions of solvent into the ink. Consequently, a problem arises of the optimum mixing of the ink and added solvent. 
     Another difficulty related to the ink quality is the presence of foam in the ink reservoir into which unprinted ink recovered by the print head gutter is returned. This foam is created by the inevitable intake of air with ink recovered through the gutter. In particular, water-based inks foam more than solvent-based inks. This air is evacuated through a vent. Preferably the ink circuit can defoam the ink sufficiently quickly to avoid creating an ink overflow through the vent. The question of recycling air mixed with ink to the head also arises. 
     In the specific field of ink jet printers, solutions have been suggested to meet the requirements related to the presence of dense pigments in the inks and/or to recover the ink not printed via the gutter of the printing head and/or to mix the solvent added to the ink, regardless of whether it is a solvent added to compensate for a solvent variation or for washing operations. 
     A 1 st  solution is set forth in  FIG. 3A  where reference  11  designates a reservoir which contains ink  13 , which can be drawn off via a duct  111  disposed in the bottom of the reservoir, for sending it to the printing head  1 . 
     According to this solution, a liquid (solvent and/or ink) which is fed to the circuit is introduced inside the ink volume  13  already present in the reservoir, under the free surface of this ink  13 . The ink which is fed can be ink which comes from an external cartridge, or ink which comes back from the printing head. The solvent which is fed can be solvent which comes from an external cartridge. But this solution causes high pressure variations at the outlet of the duct  111 , because of viscosity variations upon introducing the solvent. 
     A 2 nd  solution is set forth in  FIG. 3B  where identical references to those of  FIG. 3A  designate the same elements. 
     According to this solution, a liquid (solvent and/or ink) which is fed to the circuit is introduced above the ink volume  13  already present in the reservoir, above the free surface of this ink  13 . Once again, the ink which is fed can be ink which comes from an external cartridge, or ink which comes back from the printing head. The solvent which is fed can be solvent which comes from an external cartridge. But this solution causes the formation of two phases, the ink  13  on the one hand and, on the other hand, a phase  13   1  at the surface of the ink. This phase  13   1  substantially consists of solvent, which is not properly mixed with the ink. 
     A 3 rd  solution is set forth in  FIG. 3C  where identical references to those of  FIGS. 3A and 3B  designate the same elements. 
     According to this solution, ink E and solvent S are mixed by means  112  (for example a “T”) before they are injected in the reservoir  11 . The solvent can have the same origin as mentioned above. But this solution disturbs the hydraulic line on which the means  112  are located. 
     The problem arises of finding a new device and a new method for injecting ink and/or solvent in an ink reservoir of a CIJ type printer. 
     In general, the ink circuit of known inkjet printers capable of projecting dense pigment inks remains a costly element due to the large number of hydraulic components to be installed. 
     Therefore the problem arises of making some or all of the functions of an ink circuit in a CIJ type printer at low cost with a reduced number of components while guaranteeing minimum reliability, or in any case reliability expected by users, particularly related to homogeneity of pigment inks throughout consumption. Therefore a search is made to use the simplest possible components, particularly for functions such as controlling and maintaining the ink quality. This ink quality may be defined in terms of viscosity and/or concentration of the ink. 
     One particular problem is to reduce or to limit the variation in the opaqueness of the ink as a function of the ink consumption. The opaqueness of marking is related essentially (but not only) to the pigment concentration. If some of the pigments settle to the bottom of the reservoir, the pigment concentration in the liquid ink will be reduced and the opaqueness will be reduced. 
     Another problem is to reduce or to minimise the time necessary for homogenisation of the ink before printing is restarted, after a possibly long shutdown of the machine. 
     According to another aspect, the ink circuit comprises a large number of hydraulic, hydro-electric components, sensors, etc. Modern printers have many sophisticated and precise functions. Hydraulic components (pumps, solenoid valves, self-closing connections, filters, miscellaneous sensors) are present or are designed to satisfy a level of quality, reliability, performance and service for the user. And maintenance functions consume components because they are often automated. 
     Therefore there is also a need for an ink circuit architecture that minimises the number of components while guaranteeing a good level of performance and reliability and ease of maintenance allowing fast actions, minimising risks of dirt and that can be done by operators without any special training. 
     PRESENTATION OF THE INVENTION 
     The invention first relates to a reservoir lid for a continuous ink jet printer, including a so-called upper surface, a so-called lower surface, between which are included an upper part and a lower part of the lid, at least the latter being delimited sideways by a peripheral surface (S e ), or by an edge or a periphery defined by this peripheral surface, and: 
     at least one 1 st  duct, which passes through at least one part of the lid, for leading a 1 st  fluid from said upper part to said lower part and direct it, at least partly, sideways, to said peripheral surface (S e ), 
     at least one 1 st  chamber delimited by an internal surface into which said duct opens and by said peripheral surface (S e ), and means for flowing a liquid contained in this chamber along a direction parallel to said peripheral surface or along this peripheral surface. 
     The invention thus enables a recover function of a fluid to be integrated, for example ink from a printing head, in a lid of a reservoir. The structure of this lid enables, when installed on a reservoir, the internal wall of the reservoir to be used to guide the fluid introduced into the reservoir to the fluid pool already contained in the reservoir. The chamber is thus disposed against the peripheral wall while facing it. 
     An internal surface of the 1 st  chamber faces, at least partly, the peripheral surface Se. 
     The peripheral surface is preferably straight and is an extension of the edge or the periphery of the lower part of the lid. 
     End or front faces of said internal surface can be aligned with, or be positioned in, the peripheral surface Se. 
     The internal surface of the chamber can include one or more walls. 
     An upper portion or wall of the 1 st  chamber may be formed or closed by the lower part or the lower surface of the lid. 
     An upper wall of the chamber can be formed in a portion of the lower part of the lid and/or an upper part of the chamber can be closed by the lower part of the lid. Preferably, this upper wall or upper part faces the means for flowing a liquid contained in the chamber along a direction parallel to said peripheral surface. 
     The chamber can be open onto the peripheral surface Se, the internal wall of the reservoir coming to close it sideways when the lid closes the reservoir. The internal wall of the reservoir however leaves free the means for flowing a liquid contained in the chamber along a direction parallel to the peripheral surface, preferably along the same. 
     Alternatively, the chamber includes a wall which can be wholly formed with the rest of the chamber and which in turn bears against the internal wall of the reservoir. The latter however leaves free the means for flowing a liquid contained in the chamber along a direction parallel to the peripheral surface, preferably along the same. This wall advantageously has a curvature which corresponds to the internal surface of the reservoir. This wall is nearly the same as the surface Se. 
     According to a particular embodiment, the upper surface and the lower surface of the lid can be at least partly parallel to each other and to a plane (XY). Alternatively, they can be at least partly bent or have a curvature. 
     The 1 st  duct can pass through at least one part of the lid along at least one direction parallel to the peripheral surface (S e ), or perpendicular to the plane (XY). 
     The 1 st  duct can pass through the lid, from the upper surface to the lower surface. 
     The 1 st  duct can form a bend, thus causing the flow of a fluid flowing therethrough to change direction, to be finally directed to the surface Se. 
     The 1 st  duct can open into the internal surface of the chamber, for example in a wall of the chamber, for example through at least one port made in said surface or said wall. Preferably, said surface or said wall faces at least partly said peripheral surface Se. 
     The peripheral surface is straight, it enables an extension direction to be defined, or an axis Z. In the following, several indications are given, in particular of an angle with respect to this peripheral surface (or a plane tangent thereto). But they can also be given with respect to the axis Z or a plane XY, perpendicular thereto. 
     Preferably, at least one part of the duct is directed to said peripheral surface (S e ) along a direction forming, with this surface, an angle between 30° and 60°. 
     Regardless of the embodiment contemplated, the peripheral surface (S e ) can be cylindrical. 
     According to one embodiment, the lower part of the lid includes at least one peripheral part which projects from said lower surface, at least one part of said 1 st  chamber being made in said peripheral part. 
     Further, means can be provided to receive at least one measuring stick. 
     A lid according to the invention can further include fluid connection means, on the upper surface, to lead at least the 1 st  fluid to an inlet of the 1 st  duct. 
     Preferably, these fluid connection means include an inlet enabling the 1 st  fluid to be led along a direction perpendicular to the peripheral surface (S e ) or, optionally, parallel to the plane (XY). 
     A lid according to the invention can further include side means, for example a throat to receive a seal, for a sealing with the wall of a reservoir, these side means being disposed between said 1 st  chamber and the upper surface. 
     At least one further duct can advantageously pass through the upper part of the lid and open into a cavity delimited by the lower part. Such a duct enables the atmosphere present above an ink contained in a reservoir on which the lid is positioned to be put to atmospheric pressure. 
     Generally, the 1 st  duct can open into said 1 st  chamber alone. 
     According to one alternative, a lid according to the invention includes at least one 2 nd  duct, which passes through at least one part of the lid, to lead a 2 nd  fluid from said upper part to said lower part and direct it at least partly, sideways, to said peripheral surface (S e ), this 2 nd  duct opening into the 1 st  chamber. The 1 st  duct and the 2 nd  duct are advantageously at least partly parallel to each other. 
     According to this alternative, the 1 st  chamber can accommodate 2 fluids, which will be able to be mixed to each other, and then the mixture can be directed to the fluid pool already contained in the reservoir. Both fluids can be, on the one hand, ink that comes from a flow from the reservoir itself, and on the other hand, ink that comes from an external supply, for example an ink or solvent cartridge which comes from an external supply, for example a solvent cartridge or an intermediate solvent reservoir. 
     The 2 nd  duct can pass through at least one part of the lid along at least one direction parallel to the peripheral surface (S e ), or even perpendicular to the plane (XY). 
     The 2 nd  duct can pass through the lid, from the upper surface to the lower surface. The 2 nd  duct can, as the 1 st  duct, form a bend, thus leading the flow of a fluid flowing therethrough to change direction, to be finally directed to the surface Se. 
     The 2 nd  duct can open into the internal surface of the chamber, for example in a wall of the chamber, for example through at least one port made in said surface or said wall. Preferably, said surface or said wall faces at least partly said peripheral surface Se. 
     When the 1 st  duct, respectively the 2 nd  duct, opens into the 1 st  chamber through a 1 st  port, respectively a 2 nd  port, the sum of the cross-section area of the 1 st  port and the 2 nd  port, through which the 1 st  fluid and the 2 nd  fluid pass, is preferably lower than or equal to the cross-section area of the means for discharging the liquid from the chamber. 
     The lid can further include: 
     at least one 3 rd  duct, which passes at least one part of the lid, for leading a fluid, namely a liquid from said upper part to said lower part and direct it, at least partly, sideways, to said peripheral surface (S e ), 
     at least one 2 nd  chamber delimited by an internal surface into which opens said 3 rd  duct and by said peripheral surface (S e ), and means for flowing the fluid from this 2 nd  chamber along a direction parallel to said peripheral surface, or along this peripheral surface. 
     An upper portion or wall of the 2 nd  chamber may be formed or closed by the lower part or the lower surface of the lid. 
     In this alternative, the lid includes, on the one hand, a chamber in which a mixture can be made and, on the other hand, a chamber which enables another fluid to be collected, for example ink from a printing head of a printer. Both chambers are separated from each other. 
     The 2 fluids to form the mixture can be, on the one hand, the ink that comes from a flow from the reservoir itself, and on the other, the ink that comes from an external supply, for example an ink cartridge, or solvent that comes from an external supply, for example a solvent cartridge or an intermediate solvent reservoir. 
     The 3 rd  duct can open alone into said chamber. 
     An internal surface of the 2 nd  chamber faces, at least partly, the peripheral surface Se. When the lid is in a position for closing the reservoir, the 2 nd  chamber is disposed against the peripheral wall of the reservoir while facing it. 
     End or front faces of the internal surface of the 2 nd  chamber can be aligned with, or be positioned in, the peripheral surface Se. 
     The internal surface of the 2 nd  chamber can include one or more walls. 
     An upper wall of the chamber can be formed in a portion of the lower part of the lid and/or an upper part of the chamber can be closed by the lower part of the lid. Preferably, this upper part or upper wall faces means for flowing a liquid contained in the chamber along a direction parallel to said peripheral surface. 
     The 2 nd  chamber can be open onto the peripheral surface Se, the internal wall of the reservoir closing it sideways when the lid closes the reservoir. The internal wall of the reservoir however leaves free the means for flowing a liquid contained in the chamber along a direction parallel to the peripheral surface, preferably along the same. 
     Alternatively, the 2 nd  chamber includes a wall which can be integrally formed with the rest of the chamber and which in turn bears against the internal wall of the reservoir. The latter however leaves free the means for flowing a liquid contained in the 2 nd  chamber along a direction parallel to the peripheral surface, preferably along the same. This wall advantageously has a curvature which corresponds to the internal surface of the reservoir. This wall is substantially the same as the surface Se. 
     The 3 rd  duct can pass through at least one part of the lid along at least one direction parallel to the peripheral surface (S e ), or even perpendicular to the plane (XY). 
     The 3 rd  duct can pass through the lid, from the upper surface to the lower surface. The 3 rd  duct can form a bend, thus leading the flow of the fluid flowing therethrough to change direction, to be finally directed to the surface Se. 
     The 3 rd  duct can open into the lower surface of the 2 nd  chamber, for example in a wall of the chamber, for example through at least one orifice made in said surface or said wall. Preferably, said surface or said wall faces at least partly said peripheral surface Se. Regardless of the number of chamber(s) and, in each chamber, the number of ducts, either ducts, or each duct, enables, preferably, each fluid to be led at least partly under a level defined by a lower surface of the lid and/or under the side or peripheral sealing means, when present. 
     The invention also relates to a reservoir including a body and a lid as described in the present application, at least the 1 st  chamber being closed sideways by the internal surface of the wall of the reservoir body. This wall separates the inside of the reservoir from the external atmosphere. 
     Such a reservoir can further include means for transferring ink, contained in the reservoir, to the 1 st  duct of the lid. 
     The invention also relates to a continuous ink jet printer, including: 
     an ink circuit comprising a reservoir as described in the present application, for example as described above, 
     a printing head, 
     hydraulic connection means, for leading, from the ink reservoir on which the lid is deposited, an ink to be printed to the printing head. 
     The invention also relates to a continuous ink jet printer, including: 
     an ink circuit including a reservoir including a body and a lid as described in the present application, for example as described above, at least the 1 st  chamber being closed, sideways, by the internal wall of the reservoir body, 
     a printing head, 
     hydraulic connection means, for leading, from the ink reservoir, an ink to be printed to the printing head, 
     means for leading an ink to be recovered from the printing head to, depending on the embodiment, the 1 st  duct or the 3 rd  duct. 
     The invention also relates to a continuous ink jet printer, including: 
     an ink circuit including a reservoir including a body and a lid as described in the present application, of the type including a 1 st  and a 2 nd  duct, for example as described above, at least the 1 st  chamber being closed, sideways, by the internal wall of the reservoir body, 
     a printing head, 
     hydraulic connection means, for leading, from the ink reservoir, an ink to be printed to the printing head, 
     means for leading to the 1 st  duct an ink recovered at the bottom of the reservoir and, to the 2 nd  duct, an ink from an ink or solvent supply circuit from a solvent supply circuit. 
     The invention also relates to a continuous ink jet printer, including: 
     an ink circuit including a reservoir including a body and a lid as described in the present application, of the type including 3 ducts, for example as described above, at least the 1 st  chamber being closed, sideways, by the internal wall of the reservoir body, 
     a printing head, 
     hydraulic connection means, for leading, from the ink reservoir, an ink to be printed to the printing head, 
     means for leading:
         to the 1 st  duct, an ink recovered at the bottom of the reservoir,   to the 2 nd  duct, an ink or a solvent from an ink or solvent supply circuit,   to the 3 rd  duct, an ink to be recovered from the printing head.       

     The invention also relates to a method for operating a continuous ink jet printer, of the type described in the present application, for example as described above, in which ink is recovered from the printing head and sent to the 1 st  duct, and then in the 1 st  chamber, this ink then flowing along the internal wall of the reservoir. 
     The invention also relates to a method for operating a continuous ink jet printer, of the type described in the present application, for example as described above, in which: 
     ink is recovered at the bottom of the reservoir and led into the 1 st  duct, forming a 1 st  ink flow in the 1 st  chamber, 
     ink or solvent is sent, by the ink supply circuit, into the 2 nd  duct, forming a 2 nd  fluid flow in the 1 st  chamber, 
     both flows being mixed with each other in said 1 st  chamber, forming a mixture which flows along the internal wall of the reservoir. 
     The invention also relates to a method for operating a continuous ink jet printer, of the type described in the present application, for example as described above, in which: 
     ink is recovered at the bottom of the reservoir and led into the 1 st  duct, forming a 1 st  ink flow in the 1 st  chamber, 
     ink or solvent is sent, by the ink and/or solvent supply circuit, into the 2 nd  duct, forming a 2 nd  fluid flow in the 1 st  chamber, 
     both flows being mixed with each other in said 1 st  chamber, forming a mixture which flows along the internal wall of the reservoir, 
     ink being recovered from the printing head and sent to the 3 rd  duct, and then in the 2 nd  chamber, this ink then flowing along the internal wall of the reservoir. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a known printer structure, 
         FIG. 2  shows a known structure of a print head of a CIJ type printer, 
         FIGS. 3A-3C  illustrate solutions for supplying a reservoir, 
         FIGS. 4A-4E  represent an exemplary embodiment of a lid according to the invention, 
         FIGS. 5A-5E  represent another exemplary embodiment of a lid according to the invention, 
         FIG. 6  represents a top view of another exemplary embodiment of a lid according to the invention, 
         FIG. 7  illustrates the operation of measuring sticks in an ink reservoir of a CIJ type printer, 
         FIGS. 8A and 8B  represent exemplary embodiments of a reservoir according to the invention, with a flow of the ink from the lower part of the reservoir to its upper part, 
         FIGS. 9 to 13  represent aspects of a circuit of a CIJ type printer to which the invention can be applied. 
     
    
    
     DETAILED PRESENTATION OF EMBODIMENTS 
       FIG. 4A  represents a side view of an exemplary embodiment of a lid  33  for a reservoir of a printer, for example of the type schematically represented in  FIG. 7 or 8A-8B or 13 . 
     This lid extends between an upper surface  33   1  and a lower surface  33   2 . In the embodiment represented, both these surfaces are substantially parallel to each other and along a plane XY. By definition, the direction Z is the direction perpendicular to this plane. 
     A first part  33   a , called an upper part, bears against the top of the side wall(s)  19  of the reservoir (which separate inside the reservoir from the external atmosphere), as schematically represented in  FIGS. 4B and 4C . This first part  33   a  has, in the plane XY, for example a substantially square or rectangular shape. 
     A second part  33   b , called a lower part, has an external shape adapted to the internal shape of the reservoir that the lid will close. For example, this external shape is rectangular; alternatively it can be circular. In the latter case, this second part  33   b  has for example a circular ring shape, well seen in  FIG. 6  (bottom view). It has external dimensions adapted to those of the internal shape of the reservoir; in the case of a circular shape, it has an external diameter D substantially equal to the internal diameter of the reservoir on which the lid is intended to be positioned to close it. Its side edge defines a part of a straight cylindrical surface S e  or is part of a straight cylindrical surface S e , which corresponds to the internal wall of the reservoir, when the latter is closed by the lid  33 . A particular case is that of the cylindrical revolution surface, but, outside this particular case, are also included herein the straight cylinders, with a cross-section other than a circular one, for example a rectangular one. The cylindrical surface extends parallel to an axis Z, which will be the vertical axis when the lid is positioned on the reservoir. This second part is intended to be introduced into the upper part of the reservoir. It can advantageously include means for a sealing with the internal wall of the reservoir, for example a peripheral throat  335   e  which will allow to receive a seal. 
     A duct  331  passes through at least one part of the lid, it is preferably positioned in a part close to the external edge thereof. This duct enables a fluid to be led from the upper part  33   a  of the lid to the surface S e , in fact to the internal wall of the reservoir when the lid  33  is in a position on the same. The fluid is flown under the action of the pump of the circuit in which it flows, but also under the action of gravity. According to the embodiment illustrated, it includes a first part  331   a , which extends along a direction substantially perpendicular to the plane XY of the lid (or parallel to the surface S e  or to the axis Z). This first part is extended by a second part  331   b , which forms a bend with the first part. The duct  331  opens into a chamber, or cavity,  333 , through an aperture  341 . This chamber  333  can be made in a portion  33   3 . In the embodiment illustrated, the latter partly projects from the lower part  33   b  of the lid. This portion is an extension of the circular ring  33   b , under the lower surface  33   2 , on a part of its periphery. Further, this cavity, which faces the surface S e , is intended to face the internal surface of the wall  19 , when the lid  33  is installed on top of the reservoir. This arrangement is represented in  FIGS. 4B and 4C . An alternative is explained below in connection with  FIG. 4E . 
     The chamber  333  is delimited by an internal surface, which includes, in the embodiment illustrated, side walls  336 ,  337 , the front face  336   a ,  337   a  of which is substantially in the surface S e  and comes against, or bears against, the internal surface of the wall of the reservoir when the same is closed by the lid  33 ; these front faces  336   a ,  337   a  advantageously have a curvature which corresponds to the inner surface of the reservoir. The chamber is thus open into, or onto, the surface S e  or the internal wall of the reservoir which will close it sideways. A flat seal (not represented in the figures) can possibly be disposed between these front faces  336   a ,  337   a  and the inner surface of the reservoir. A wall  349 , in which the aperture  341  is made, delimits the bottom of the cavity by facing the surface S e  (and the internal wall of the reservoir when the same is closed by the lid  33 ). 
     The chamber  333  also includes, in its lower part, flowing means  338 , for example by means of a slot or at least an outlet port. According to one embodiment, these means face an upper wall  339  of the chamber. These flowing means will enable the fluid, which has penetrated the chamber  333 , to flow along the inner surface of the wall  19 . Preferably, these flowing means offer the fluid a surface area higher than or equal to the surface area of the port  341 . This condition makes it possible to ensure that the chamber  333  does not enable liquid to be retained, which would result in slowing flowing of this liquid to the reservoir. 
     The duct  331  enables the liquid to be directed to the surface S e  and to the internal or inner surface of the reservoir when the same is closed by the lid  33 , preferably under the sealing means  335   e  when the same are present and/or at least partly under a level defined by the lower surface of the lid (the plane XY for example). The chamber  333  enables the liquid that will be directed against the internal wall to be contained and then, through its outlet means, to be guided to the bottom of the reservoir (or along a direction opposite to the upper surface  33   1 ). 
     The cavity  333  has advantageously a sufficiently high volume not to be saturated and for the fluid not to overflow sideways. In practice, a volume between 50 mm 3  and 1000 mm 3  can be suitable. By way of indicating purposes, the reservoir volume is for example between 0.5 l and 10 l. 
     In the zone in which the duct  331  is made, the second part  33   b  of the lid  33  can have a local extra thickness, which extends perpendicular to the surface S e . In  FIGS. 4C and 6 , is seen the thickness e 1  of this part  33   b , which is lower than the thickness e 2  of the part in which the duct  331  is made. 
     Preferably, the duct  331  leads the fluid to the cavity  333  along a direction tilted to the means  338 . This tilt is a compromise between not spattering (or splashing) the wall and the requirement to limit the length of the duct (and thus the material necessary to be worked to make it). 
     For example this tilt is about 45° with respect to the plane XY (or with respect to the surface Se or to the axis Z) or, more generally, of an angle between 30° and 60° (with respect to the plane XY or with respect to the surface Se or to the axis Z). Thus, in the embodiment illustrated, the second part  331   b  of the channels which lead the fluid to the cavity  333  is tilted, for example by an angle of about 45°. 
     An alternative, also covered by the invention, is represented in  FIG. 4D , in which the first part  331   a  of the duct forms, with respect to the plane XY, an angle different from 90°, (or is not parallel to the surface Se or to the axis Z) for example an angle of 45°. If the second part  331   b  is itself tilted by about 45° with respect to the plane XY (or with respect to the surface Se or to the axis Z), then both these parts form, between them, an angle of about 90°. The accessibility is promoted in the configuration of  FIG. 4B , with the use of a connector  350  which affords a horizontal access. 
     After it has outflown through the means  338 , the fluid flows along the inner surface of the wall  19 : thereby, it can neither create spatters (which could happen if fluid drops were released on the surface of the liquid contained in the reservoir) nor disturb possible measurements, for example level measurements, which would be made inside the reservoir. This is in particular the case when the level measuring sticks  516 - 522  are provided, as illustrated in  FIG. 4C . When the fluid is ink which comes from the printing head of a CIJ printer, the wall allow to spread the ink on the internal surface of the side wall of the reservoir, which will result in spreading the ink on the one hand, and the air bubbles on the other hand, which bubbles can be contained in this liquid, because of the mixing undergone by the ink, with air, upon sucking in the recovery gutter. 
     An interface or connection element  350  can be disposed on the upper part  33   1  of the lid, to connect an external duct to the duct  331 . In  FIG. 4C , is represented a cross-section view of such an element  350  and it can be seen in a front view in  FIG. 4A . This element enables a fluid inlet  351  (as, for example, a  fir tree  connector) to be brought in communication with the duct  331 . It has therethrough a duct  353  which includes two parts, which form, between them, an angle of about 90°. This makes it possible to have an inlet  351  arranged in parallel to the plane XY (or perpendicular to Se), which is favourable from the overall space point of view, along a vertical direction (or along the axis Z) of the entire device. In the case of the embodiment of  FIG. 4D , the connection element  350 ′ is represented in the same figure. It provides an access at an angle of about 45° with respect to the plane XY or to the surface Se. 
     As illustrated in  FIG. 4A , a second duct  331 ′ can be disposed parallel to the duct  331 , this duct  331 ′ passing through the upper part  33   a  of the lid and opening directly into inside the reservoir through a port  331 ″ (see  FIG. 6 ): it does not require to be extended, inside the reservoir, by another duct, because it simply affords putting the atmosphere located above the liquid present in the reservoir to atmospheric pressure. The interface element  350  can be adapted to connect the inlets of both ducts  331 ,  331 ′ to inlets  351 ,  351 ′, as is illustrated in  FIG. 4A . 
     In  FIG. 4E  (in a bottom view) is represented the case where the cavity  333  is closed by a wall  347  (which faces the wall into which the port  341  opens) which itself bears against the internal surface of the side wall of the reservoir. This wall  347  advantageously has a curvature which corresponds to the internal surface of the reservoir. Its thickness is in the order of several tenths of mm, for example between 0.2 and 1 mm. A surface of this wall can thus be substantially the same as the surface Se. The fluid, which outflows from the duct  331 , is thus directed, in a 1 st  time to the internal surface of this wall and to the surface Se (and then to the internal surface of the wall of the reservoir when the lid is mounted thereto). And then, it flows against the internal surface of the internal wall of the reservoir, with the same effects as described above. 
     The means that have been described above in connection with  FIGS. 4A-4E  enable for example ink that comes back from the printing head to be brought into the reservoir, via the gutter  62  (see  FIG. 2 ). 
       FIG. 5A  represents another exemplary embodiment of a lid  33 . Common references to those of the preceding figures designate the same elements. 
     Ducts  431 ,  432  pass through the lid, preferably in a part close to the edge thereof. 
     These ducts enable fluids, in particular liquids, to be led from the upper part  33   a  of the lid to the surface S e , in fact to the internal surface of the wall of the reservoir when the lid  33  is in a position on the same. The fluids flow under the action of the pump of the circuit in which they circulate, but also under the action of gravity. These ducts are substantially parallel to each other and, according to the embodiment illustrated, include a first part  431   a ,  432   a , which extends along a direction substantially perpendicular to the plane XY of the lid (or parallel to the surface S e  or to the axis Z). Each of these first parts is extended by a second part  431   b ,  432   b , which forms a bend with the first part. Each of these ducts  431 ,  432  opens into a chamber, or cavity,  433 , called a mixing chamber or cavity, through an open aperture  441 ,  442 . This chamber can be made in a portion  43   3 . In the embodiment illustrated, this latter projects from the lower part  33   b  of the lid. This portion is an extension of the circular ring  33   b , under the lower surface  33   2 , on a part of its periphery. Further, this cavity is intended to face the wall  19 , when the lid  33  is installed on top of the reservoir. This arrangement is represented in  FIGS. 5B and 5C . An alternative is explained below in connection with  FIG. 5E . 
     This chamber  433  is delimited by an internal surface which includes, in the embodiment illustrated, side walls  436 ,  437 , the front face  436   a ,  437   a  of which is substantially in the surface S e  and bears against the internal surface of the wall of the reservoir when the same is closed by the lid  33 ; these front faces  436   a ,  437   a  advantageously have a curvature which corresponds to the internal surface of the reservoir. 
     The chamber is thus open into, or onto, the surface S e  or the internal wall of the reservoir which will close it sideways. A flat seal (not represented in the figures) can possibly be disposed between the front faces  436   a ,  437   a  and the internal surface of the reservoir. 
     A wall  449 , in which each aperture  441 ,  442  is made, delimits the bottom of the cavity by facing the surface S e  (and the inner wall of the reservoir when the same is closed by the lid  33 ). 
     The chamber  433  also includes in its lower part, flowing means  438 , for example at least one slot or at least one outlet port. According to one embodiment, these means face an upper wall  439  of the chamber, which can be substantially parallel to the plane XY or perpendicular to the surface Se or to the axis Z. These flowing means will enable the liquid, which has undergone mixing in the cavity  433 , to flow along the internal surface of the wall  19 . Preferably, these flowing means offer the fluid a surface area higher than or equal to the sum of the surface areas of each of the ports  441 ,  442 . This condition makes it possible to ensure that the cavity  433 , if it provides a mixing function, does not yet enable liquid to be retained, which would result in slowing flowing the mixture intended to the reservoir. These means  438  can be limited, sideways, by portions  436   i ,  437   i  which are extensions of the side walls  436 ,  437  substantially in parallel to the plane of XY. 
     These portions  436   i ,  437   i  indeed form a lower wall of the cavity, this lower wall being provided with the means  438 . 
     The ducts  431 ,  432  enable the liquids to be directed to the surface S e  and to the internal surface of the wall of the reservoir when the same is closed by the lid  33 , preferably under the sealing means  335   e  when the same are present and/or at least partly under a level defined by the lower surface of the lid (the plane XY for example). The chamber  433  enables these liquids directed to the internal surface of the wall to be mixed, to be temporarily contained therein and then, by its outlet means, to guide the mixture to the bottom of the reservoir (or along a direction opposite to the upper surface  33   1 ). 
     The cavity  433  has advantageously, on the one hand, a sufficiently low volume for the fluid coming through the ports  441 ,  442  to be efficiently mixed therein. But it has also, on the other hand, a sufficiently high volume not to create too much head loss on the path of the mixture in question. Indeed, it is preferred that a liquid, for example an ink mixture added from a cartridge and ink that comes from a recirculation from the reservoir bottom, arrives as quickly as possible in the liquid contained in the reservoir, or in any case without the fluid circuit it will have to travel slowing it down too much. In practice, a volume between 70 mm 3  and 2000 mm 3  can be suitable. 
     In the cavity  433  of  FIG. 5A , curved arrows are represented which correspond to turbulences and/or turbulent motions that the fluids undergo when they are in the cavity. These turbulent motions make it possible to ensure mixing of both liquids which arrive from the 2 ducts or channels  431 ,  432 . 
     In the zone in which the ducts  431 ,  432  are made, the second part  33   b  of the lid  33  can have a local extra thickness, which extends perpendicular to the surface S e . It is seen, in  FIGS. 5C and 6 , the thickness e 1  of this part  33   b , which is lower than the thickness e′ 2  of the part in which the ducts  431 ,  432  are made. 
     Preferably, the ducts  431 ,  432  lead the liquids in the cavity  433  with a tilt, to the means  438 . This tilt is a compromise between not spattering the wall and the requirement to limit the length of the duct (and thus the material necessary to be worked to make it). For example, this tilt is about 45° with respect to the plane XY (or with respect to the surface Se or to the axis Z) or, more generally, of an angle between 30° and 60° (with respect to the plane XY or with respect to the surface Se or to the axis Z). Thus, in the embodiment illustrated, the second part  431   b ,  432   b  of the channels which lead the fluids to the cavity  433  is tilted, for example by an angle of about 45°. 
     An alternative, also covered by the invention, is represented in  FIG. 5D , in which the first part  432   a  of the duct forms, with respect to the plane XY, an angle different from 90°, for example an angle of 45°. If the second part  432   b  is itself tilted by about 45° with respect to the plane XY (or with respect to the surface Se), then both these parts form, between them, an angle of about 90°. 
     After it outflows through the means  438 , the fluid mixture flows along the internal surface of the wall  19 : therefore, it can create neither spatters (which could happen if liquid drops were released on the surface of the liquid contained in the reservoir) nor disturb possible measurements, for example level measurements, which would be made inside the reservoir. This is in particular the case when the level measuring sticks  516 - 522  are provided, as illustrated in  FIG. 5C . 
     An interface or connection element  450  can be disposed on the upper part  33   1  of the lid, to connect 2 external ducts to the ducts  431 ,  432 . In  FIGS. 5C and 5A , are respectively represented, a cross-section and a front view of such an element  450 . This element enables fluid inlets  454 , respectively  455  (each as, for example, a “hose barb” connector) to be brought in communication with the duct  432 , respectively  431 . There are 2 ducts  452 ,  455  therethrough, each including two parts, which form, between them, an angle of about 90°. This makes it possible to have inlets  450 ,  455  disposed in parallel to the plane XY (or perpendicularly to Se), which is favourable from the accessibility point of view, along a vertical direction (or along the axis Z) of the entire device. In the representation of the  FIG. 5C , the inlet  454  is extended by a second connection  456 , directed downwardly from the reservoir, which enables an ink flow, which is pumped in the bottom of the reservoir, to be led to this inlet  454  in order to make up a recirculation flow. 
     In the case of the embodiment of  FIG. 5D , the connection element  450 ′ is represented on this same figure. It provides an access along an angle of about 45° with respect to the plane XY or to the surface Se. 
     In  FIG. 5E  (in a bottom view) is represented the case where the cavity  433  is closed by a wall  447  (which faces the wall into which the port  341  opens) which itself bears against the internal wall of the reservoir. This wall  447  advantageously has a curvature which corresponds to the internal surface of the reservoir. Its thickness is for example between 0.2 and 1 mm. This wall is substantially the same as the surface Se. The fluids, which outflow from the ducts  431 ,  432  are thus directed, in a 1 st  time to this wall and to the surface Se (and to the wall of the reservoir when the lid is mounted thereon). Then, their mixture can flow against the internal wall of the reservoir. 
     The means that have been described above in connection with  FIGS. 5A-5E  enable for example an ink mixture to be led to the reservoir, which comes from the bottom of the reservoir, by recirculation, and ink, or respectively solvent, which comes from an ink cartridge, respectively from a solvent cartridge. The chamber  433 , as has been described above, enables mixing of both these liquids to be made before the mixture is able to flow against the internal surface of the wall of the reservoir, to the liquid pool already contained therein. 
     Both aspects that have been presented above can be combined in a same reservoir lid. Thus, in  FIG. 6 , is represented a bottom view of a lid which includes a chamber  333  as described above in connection with  FIGS. 4A-4E  and a chamber  433  as described above in connection with  FIGS. 5A-5E . Identical references to those of the preceding figures designate the same elements. 
     In this figure, it is seen that the duct  331 ′ opens, through an aperture or port  331 ″, into a cavity formed or delimited by the lower part  33   b.    
     It is also seen in this figure that, in each of the 4 angles of the upper part  33   a , a port  35   i  (i=1-4) can be provided; these 4 ports will enable the entire lid to be attached to the upper part of the reservoir. 
     The lid can be provided with means for receiving one or more measuring sticks or rods for measuring the liquid level in the reservoir. 
     Thus, in  FIGS. 4C and 5C , are seen 4 measuring sticks or rods  516 ,  518 ,  520 ,  522  which plunge into the reservoir and which pass through the upper part  33   a  of the lid  33 . There are held by means  517 ,  519 ,  521  (the means for holding the stick  522  are not represented, because they are behind the means  517 ), for example nuts, each of which is screwed to a threaded portion which is an extension of each of the sticks. To enhance holding, insulating plates  530 , 531  can be provided on either side of the upper part  33   a  of the lid. These plates, as well as the upper part  33   a , have ports which enable each of the sticks  516 - 522  to be inserted. The same elements are found in  FIG. 5C . 
     The operation of all these measuring sticks will now be explained, in connection with  FIG. 7 , in which reference  19  designates again the reservoir wall. 
     In this figure, are represented 2 measuring sticks, or electrodes,  516 ,  518  and 2 reference sticks,  520 ,  522 . 
     Each of the reference rods comprises an electrodes covered, on most of its length, with a coating, or a sleeve  524 ,  526 , of a dielectric or electrically insulating material, which only allows an end portion of the corresponding electrode, having a length l R , to project. Thus, it enables a liquid level to be measured with a depth l R +p, p being the distance between the free end of the reference electrode and the bottom of the tank. 
     Each of the measuring rods  516 ,  518  includes an electrode which is, in turn, not covered with a sleeve, at least on the part which is included between the free end of the electrode, intended to be the closest to the bottom of the tank, and the maximum level h max  desired to be measured. The different electrodes are of conducting material, for example of stainless steel. 
     The pairs of electrodes are fed with a current by generator means  530 . The electrodes of each pair are electrically arranged in series with an electrical circuit connecting said electrodes in series. Preferably, the current supplied is an AC electric current, having a null mean to avoid any electrolysis. 
     Means  532 , for example a voltage detector, enable a voltage V M  between both measuring rods to be measured. For example, these means  532  include a resistor which enables both the intensity to be measured by the voltage measurement and the current to be limited in the circuit. 
     Preferably, these means enabling a measurement to be made perform a sampling on peak values, and then an amplification. 
     Means  534 , such as multiplexing means, typically a multiplexer as used in electronics, can be provided to perform, alternatively, a measurement at the terminals of both measuring rods and a measurement at the terminals of both reference rods. Thus, the pair at the terminals of which no measurement is made is completely disconnected and has no influence on the measurement made at the terminals of the other pair, and any coupling effect of the electrode pairs is avoided. In this configuration, the same voltage measuring means  532  can be used for measuring a voltage V R  between both measuring rods and for measuring a voltage V R  between both reference rods. 
     For example, a measurement is performed during 100 ms with both measuring rods, and then during 100 ms with both reference rods. The measurement durations with both measuring rods and then both reference rods can be equal, or different: for example, the ratio of the measurement period with both measuring rods to the measurement period with both reference rods can be between 5 and 10. 
     From the voltage measurement V M  and V R , an impedance, a measurement impedance (or its resistive component) R M  and a reference impedance R R  (or its resistive component) can be deduced respectively. 
     Then, R R /R M  is calculated to deduce therefrom the level h M  of the liquid height by the following formula:
 
 h   M   =K· ( R   R   /R   M )− K   0  
 
     Thus, R R /R M  (the ratio of the resistive components) is preferably calculated to deduce the ink level therefrom. This formula is independent from the liquid conductivity which, as will be seen herein below, is confirmed by experimental measurements. Surprisingly, it has been noticed that the reference resistance, per mm of ink, is different from the measurement resistance, per mm of ink. (The difference probably comes from the non-direct field lines (that is those which are not perpendicular to the electrodes), which are not identical for the reference resistance and the measuring resistance, because of structure differences in the bottom of the electrodes). 
     A correct measurement can be carried out as soon as the reference electrodes are fully dipped (this is in the case in  FIG. 7 ) and/or the measurement electrodes of a distance p′, with respect to the bottom of the tank, equal to p increased by a length corresponding to the active part of the reference electrodes (this is also the case in  FIG. 7 ). This previous condition results in p′≧p+l R ; p′ is preferably equal or close to p+l R . Alternatively the ends of the measurement electrodes can be protected by an insulating coating or insulating sleeves, having a length equal to or higher than the active part l R  of the reference electrodes. 
     In the opposite case, the formula h M =K·(R R /R M )−K 0  is not valid in the bottom of the tank as long as the ends (over the distance l R ) of the reference electrodes are not fully dipped (in this case, the measurement and reference impedance are equal, which gives a constant hm value). But, once the reference electrodes are fully dipped, the above formula can then be applied, coefficients K 0  and K 1  being experimentally determined. 
     Electronic means can be programmed, for example in the printer controller, to calculate h M  as a function of the R R  and R M  values. Measurement data are transmitted from the ink tank to the controller, which then carries out data processing and calculating the ink or liquid level. 
     If the ink level thus calculated is lower than a predetermined threshold level, the controller can trigger a tank filling operation. 
     If the ink level thus calculated is lower than a predetermined threshold level, the controller can trigger a reservoir filling operation. 
     It is understood that the implementation of the level measuring means by sticks or rods, directly in the reservoir, is a sensitive operation. In particular, such a measurement should not be disturbed by inopportune liquid flows, which would come from introducing, ink or solvent into the reservoir. A lid according to the invention enables, as explained above, the different liquids to be led to the reservoir wall, thus as far as possible from the level measuring sticks. Any disturbance of the same is thus avoided. 
     Even in the absence of the measuring sticks, holding an undisturbed ink level in the reservoir is to be favoured. In particular, this ink level should not be disturbed by inopportune liquid flows, which would come from introducing ink or solvent into the reservoir. A lid according to the invention enables, as explained above, the different liquids to be led to the reservoir wall, thus as far as possible from the level measuring sticks. Any disturbance of the ink level is thus avoided. 
     A lid according to the invention closes an ink reservoir. The 1 st  and, possibly, the 2 nd  chamber is/are closed sideways by the internal wall of the reservoir. 
       FIG. 8A  shows an example embodiment of an ink reservoir  10  to which the invention can be applied, for an ink circuit of a continuous inkjet printer. 
     This reservoir  10  is delimited by one or more sidewall(s)  19  and covered with the cap or lid or cover  33 , preferably according to one of the embodiments described above. The bottom  622  is preferably conical and has no horizontal surface or it has an extremely small horizontal surface, so as to accumulate the minimum amount of material. The tip of the cone is oriented towards the bottom of the device along the direction of liquid flow when the reservoir is placed vertically. To satisfy the condition for slip on an inclined surface, the cone angle from the horizontal is chosen such that it is greater than about 30° (and less than 60° or 80°), or less than about 60° (but more than 10° or 30°) from the vertical or the sedimentation direction of pigments. 
     The example of a part of the reservoir for which the wall is cone-shaped is given herein, but other forms are possible, for example a pyramid shaped wall or more generally a wall tapered or converging towards a portion that comprises an ink flow orifice. The section of the part thus delimited reduces towards this flow orifice. 
     Such a flow orifice or ink outlet  621  is made in an end part of the reservoir, particularly through the bottom end of the reservoir, in this case formed by the cone tip. 
     Starting from this outlet, a first pipe or duct  623  connects a pump  625  to said bottom end. 
     Preferably the pump  625  is located under a level that passes through the lower part of the reservoir or under this bottom part. This makes sure that it is always pressurised and primed. More generally, any device for displacement of ink from the bottom to the top of the reservoir can be used. 
     A second hose or duct  627  connects the output of the pump  625  to the inlet  454  of the interface means  450  disposed on the lid  33 , as has already been described above (see for example  FIG. 5A ). The ink thus enters again the reservoir, at a point above the maximum ink level in the reservoir, thus above the surface  635  of the ink present in the reservoir, which is found, for example, 10 mm or 50 mm from the top of the reservoir (defined by the lower face  33   2  of the lid  33 ). 
     Preferably the pump  625  provides permanent ink circulation with a flow greater than or equal to the ink sedimentation velocity. The pump flow is for example between 10 l/hour and 20 l/hour, it is for example of about 14 l/hour. This circulation takes place along a single direction from the bottom of the reservoir to the upper part. 
     The pump may be a membrane pump type or a peristaltic pump or a geared pump or a centrifugal pump or any other type of pump. 
     Preferably, it is capable of reaching a flow greater than the pigment sedimentation velocity over the entire surface of the cylindrical part of the reservoir. For example, a flow of more than 0.5 cm 3 /hour is sufficient for a reservoir for which the largest cross-sectional area is 50 cm 2 . 
     Pumping is preferably done permanently, regardless of whether or not the printer is in operation. This possibility is available if the pump  625  is dedicated to the circulation of ink, and is not governed by the operating rate of another function. 
     The reservoir  10  is provided with means  630  and/or  631  to draw off ink in order to pressurise it and to send it to the print head. Each of these means may be composed of a duct connected to a pump  637 ,  639  respectively, so that ink can be sent under pressure to the print head. 
     This drawing off may be made at a minimum distance d from the bottom of the reservoir and the surface of the liquid in the reservoir, that may for example be calculated using Stokes&#39; law as a function of the size grading of the largest ink pigment particles, the pigment density and the density of the dispersing medium: 
     
       
         
           
             v 
             = 
             
               
                 
                   
                     2 
                     · 
                     
                       r 
                       2 
                     
                   
                   ⁢ 
                   g 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   Δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   ρ 
                 
                 
                   9 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   η 
                 
               
               = 
               
                 
                   
                     d 
                     
                       
                           
                       
                       ⁢ 
                       2 
                     
                   
                   ⁢ 
                   g 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   Δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   ρ 
                 
                 
                   18 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   η 
                 
               
             
           
         
       
     
     where v is the sedimentation velocity in m/s, 
     r is the radius, D is the diameter of particles in m, 
     g is the gravitation constant 9.81 m/s 2 , 
     Δρ is the difference in density between the pigment and the liquid medium in kg/m 3 , 
     η is the dynamic viscosity in Pa·s, 
     and t is the time, where d=v·t, d is the distance from the lowest point of the reservoir. 
     A median zone  115  of the reservoir can be defined, for example located between: 
     a first level A, defined by the ink flow orifice or by a level located at not less than 1/20 th  or 1/10 th  or ¼ or ⅓ of the reservoir height, measured from its lowest point, as a proportion of the height H of the reservoir (itself measured between the lowest point in the reservoir and the highest point in the reservoir when the reservoir is in operation), 
     and a second level B defined by the upper third or quarter (once again measured as a proportion of the height H of the reservoir as explained above). In this zone  115 , the concentration of pigment in the ink remains approximately constant and equal to the initial nominal concentration. 
     One interesting point for the ink sampling point is approximately in the median zone  115  between the ink surface and the outlet orifice  621  located in the bottom of the reservoir. The distance D, measured along the vertical or the pigment sedimentation direction when the reservoir is in use, between the ink drawing off point and the orifice  621 , is for example not less than 10 mm, or 20 mm, or 50 mm. The position of this drawing off point  19   1  is preferably vertically in line with the orifice  621 . It can be determined as a function of physical parameters of the ink (particularly pigment size grading, pigment density, density of the dispersing medium), as explained above. The drawing off location is the location at which pigment concentration will remain nominal or approximately constant, preferably for as long as possible when recycling is not present. 
     Therefore, we chose a fixed drawing off point in order to maximise the recycling stop time as a function of the machine usage. 
     With a drawing-off point  19   1  positioned such as described above, drawing off may be made at any time without waiting for the recirculation between the bottom of the reservoir and the surface to homogenize the ink over the entire height of the liquid, after the printer is restarted after a rest period. In this way, the printer may be put into operation without delay, at least with a much shorter time than in previous embodiments. 
     Furthermore or as a variant, ink may be drawn off from the recirculation duct  623  at the bottom of the reservoir to supply the head under pressure. To achieve this, means  631  are used to draw off liquid from this duct. Drawing off from the duct  623  can feed the print head even when the ink level in the reservoir is located below means  19   1 , if there are any. 
     A device according to the invention can include either and/or both of the drawing off means  630 ,  631 , each with the respective advantages indicated above. 
       FIG. 8B  is another exemplary embodiment of an ink reservoir to which the invention can be applied, for an ink circuit of a continuous ink jet printer, the lid  33  having also available means, described above in connection with  FIGS. 4A-4E , to recover the ink that comes from the printing head. The reservoir illustrated in this figure includes the means that have already been described above in connection with  FIG. 7A , but it also includes interface means  350  disposed on the cover  33 , as has already been described above (see for example  FIG. 4A ). A hose or duct  357  leads the ink at the outlet of the gutter  62  to the inlet  351 . This recovered ink thus enters again the reservoir, also at a point above the maximum ink level in the reservoir, thus above the surface  35  of the ink present in the reservoir. 
     The lids  33  of  FIGS. 8A, 8B  can be provided with measuring sticks or rods, as explained above in connection with  FIG. 7 . 
     A continuous ink jet printer according to the invention can include: 
     an ink circuit including at least one reservoir according to the invention, 
     a printing head  1  ( FIGS. 1 and 2 ), 
     hydraulic connection means  119 , for leading, from the ink reservoir, an ink to be printed to the printing head. 
     Electrical connection means are on the other hand provided to electrically supply the printer. 
     Drawing off the ink in the bottom of the reservoir and moving it or bringing it back to the top of the reservoir, using the lid according to the invention, even with an extremely low flow rate, is sufficient to keep homogeneity of the ink in the entire reservoir. This represents a particularly interesting economy as regard means accordingly. 
     On the other hand, solid particles settle more quickly by sliding on a tilted surface as in a liquid, if the angle of the tilted surface with respect to the horizontal is higher than the sliding angle of the particles. 
     These considerations are applied to the embodiments described below. 
     An example of an architecture of a fluid circuit of a printer to which the invention can be applied is shown in  FIG. 9 . Reference identical to those used previously denote identical or corresponding elements. In particular, the flexible umbilical  119  is shown that contains hydraulic and electrical connections and the print head  1 , to which the printer architecture disclosed below can be connected. 
     This  FIG. 9  shows that the fluid circuit  4  of the printer comprises a plurality of means  10 ,  50 ,  100 ,  200 ,  300 , each means being associated with a specific function. 
     A removable ink cartridge  30  and a solvent cartridge  40  that is also removable are associated with this circuit  4 . Although the presence of cartridges can be recommended, including when the ink circuit is stopped (for example to enable active monitoring), the ink circuit may be without the cartridges  30 ,  40  when stopped or at rest. 
     Reference  10  refers to the main reservoir that contains a mix of solvent and ink. 
     Reference  100  (or solvent supply circuit) refers to all means that are used to draw off and possibly store solvent from a solvent cartridge  40  and to supply solvent thus drawn off to other parts of the printer, either to supply the main reservoir  10  with solvent, or to clean or maintain one or several of the other parts of the machine. 
     Reference  300  (or ink supply circuit) refers to all means of drawing off ink from an ink cartridge  30  and supplying the ink thus drawn off to supply the main reservoir  10 . As can be seen on this figure, according to the embodiment disclosed herein, these means  300  can be used to send solvent from means  100  to the main reservoir  10 . 
     Reference  200  (or ink pressurization circuit) denotes all means used to draw off ink from the main reservoir  10  and for sending it to print head  1 . According to one embodiment illustrated here by arrow  25 , it is also possible that these means  200  can be used to send ink to the means  300 , and then once again to the reservoir  10  which enables ink flow recirculation inside the circuit. This circuit  200  may also allow draining the reservoir in the cartridge  30  as well as cleaning of the connections of the cartridge  30  (in the case of the embodiment in  FIG. 12 , by changing the position of the valve  37 ). 
     The system shown on this figure also comprises means  50  of recovering fluids (ink and/or solvent) that returns from the print head, more precisely from the gutter  62  of the print head or from the head rinsing circuit. Therefore these means  50  are arranged on the downstream side of the umbilical  119  (relative to the flow direction of fluids returning from the print head). 
     As can be seen on  FIG. 9 , the means  100  may also allow sending solvent directly to these means  50  without passing through the umbilical  119  or the print head  1  or the recovery gutter  62 . 
     The means  100  may comprise at least three parallel solvent supplies, one to the head  1 , the 2 nd  to means  50  and the 3 rd  to means  300 . 
     Each of the means described above can be provided with means such as valves, preferably solenoid valves, for guiding the fluid concerned to the chosen destination. Thus, means  100  can be used to send solvent exclusively to head  1 , or exclusively to means  50  or exclusively to means  300 . 
     In a variant, the same means may enable sending solvent toward all means forming part of the ink circuit, for example for a complete rinsing of the entire circuit. 
     Each of the means  50 ,  100 ,  200 ,  300  described above can be provided with a pump that is used to process the fluid concerned (the 1 st  pump, 2 nd  pump, 3 rd  pump, 4 th  pump respectively). These various pumps perform different functions (the functions of their corresponding means) and are therefore different from each other, although these different pumps may be of the same type or a similar type (in other words, none of these pumps performs 2 of these functions). 
     The means  20  make it possible to ensure recirculation (they play the role of the pump  625  of  FIGS. 8A, 8B ). 
     In particular, the means  50  comprise a pump (1 st  pump) that pumps recovered fluid as described above, from the print head, and sends it to the main reservoir  10 . This pump is dedicated to recovery of this fluid from the print head and is physically different from the 4 th  pump of means  300  dedicated to transfer of the ink and/or from the 3 rd  pump of means  200  dedicated to pressurisation of the ink at the outlet from reservoir  10 . 
     The means  100  comprise a pump (2 nd  pump) that pumps solvent and sends it to means  50  and/or to means  300  and/or to the print head  1 . 
       FIG. 10  shows an even more detailed representation of means  100  that draw off solvent from a cartridge  40  and send it to the different parts of the device, for example to perform cleaning or unblocking operations. 
     These means comprise a pump  41  (the 2 nd  pump) and various fluid connection means, each comprising one or several conduits or one or several valves  39 ,  42 . One of these valves, the valve  42 , guides solvent to 2 possible channels, namely the print head  1  or the ink supply circuit  300 . In the latter case, when the means that enable solvent to enter means  300  are themselves closed, solvent is guided to means  50 . An anti-pulsation device  411  and a filter  412  may also be arranged in series with the pump. 
     An intermediate reservoir  14  may also be provided that may be provided with level measurement means  14 ′ and that may be supplied from a cartridge  40 , when the cartridge is connected to the circuit. Preferably, these means  14 ′ comprise an ultrasound sensor that provides good precision for detection of the solvent level. 
     This reservoir  14  may send solvent to the various means  50 ,  300  and/or to the print head  1 , and/or to the main reservoir  10  to clean them or to unblock their hydraulic components; it may also supply solvent to the main reservoir  10 . Solvent can also be drawn off from the cartridge  40  and sent directly to the various elements of the circuit, to perform the same operations (cleaning or unblocking or supply of the main reservoir  10 ). The source of the solvent is selected by a valve  39 . The  normally open  (NO) and  normally closed  (NC) positions of each valve are shown on this figure, as on the others. In this case, if the valve  39  is in the  NC  position ( FIG. 4 ), solvent is pumped from the cartridge  40 , and if it is in the  NO  position, solvent is pumped from the reservoir  14 . 
     The reservoir  14  may be supplied from the cartridge  40 , for example through a calibrated leak or restriction  45  located at its inlet. This leak also participates in generating pressure. The reservoir  14  may be filled as follows; the valve  39  is in the  NC  position (see  FIG. 13 ), so that solvent can be pumped from cartridge  40  through the pump  41 . The valve  42  is in the closed (NC) position, while inlets to means  50  and  300  are prohibited to solvent. 
     Solvent can be sent to these various means  50  (via duct  335 ),  300 , then possibly to the main reservoir  10  (via duct  337 ), using valve  42  and means located at the inlet to means  50 ,  300 , for example one inlet valve for each of these means. Therefore, 3 parallel channels are defined at the outlet from means  100  that, depending on the needs, will be used to send solvent to one and/or the other of these elements. 
     Means  100  may also comprise means  47  forming the pressure sensor, to measure the solvent pressure at the outlet from pump  41  and means  411 ,  412 . This information can be used to detect a pressure increase in the solvent, which can be the result of a blockage in one of the conduits in which solvent flows. 
       FIG. 11A  shows a more detailed representation of one embodiment of means  50  that allow recovery of fluids (ink and/or solvent) that returns from the print head. Therefore, two types of fluid can be brought together at the inlet to these means  50 ; ink from the recovery gutter  62  (see  FIG. 2 ) and solvent that was used to clean or rinse the print head  1  and/or the umbilical  119 . A duct  511  guides these fluids to the inlet to means  50 . 
     These means comprise a pump  53  (the 1 st  pump), possibly a filter  52  arranged in series with this pump, for example upstream from the pump, and means  51  forming the inlet valve. These means  51  comprise one or several valves, preferably a three-way valve. They exclusively send fluid either from head  1  (NO position of the valve in  FIG. 5 ) through the duct  511 , or solvent from means  100  (NC position of the valve in  FIG. 5 ) through the duct  335 , to the pump  53 . 
     Fluid pumped by the pump  53  can then be sent to the main reservoir  10  preferably through a set of means such as disclosed in connection with  FIGS. 4A-4E . 
       FIG. 11B  shows a variant of  FIG. 11A . On  FIG. 11B , 2 valves  51 - 1  and  51 - 2  are implemented, instead of a three-way valve. Valve  51 - 1  is on duct  511 , and makes it possible to interrupt a flow of fluid returning from the print head  1 ; valve  51 - 2  is on a duct through which clean solvent flows, and makes it possible to interrupt or block any flow of said clean solvent towards the pump  53 . The other reference on  FIG. 11B  are the same as on  FIG. 11A  and designate the same technical elements. Through the control of valves  51 - 1  and  51 - 2  (one of said valves being closed while the other one is open), this embodiment achieves the same result as with the one of  FIG. 11A : fluid is exclusively sent either from head  1  (open position of valve  51 - 1  in  FIG. 11B  and closed position of valve  51 - 2 ) through the duct  511 , or solvent from means  100  (open position of the valve  51 - 2  in  FIG. 11B  and closed position of valve  51 - 1 ) through the duct  335 , to the pump  53 . 
       FIG. 12  shows a more detailed representation of means  300 , in cooperation with the main reservoir  10  and the means  200 . 
     The main reservoir  10  is preferably provided with means  15  for detecting the level of ink contained in it (in fact the ink in it is mixed with the solvent). In the above examples, these means may comprise one or several rods for measuring the ink level. A cap  33  according to the invention may be adapted, as already explained, for implementing these rods. 
     Reference  301  refers to the cannula (or any equivalent means), that will provide fluid connection between the cartridge  30  and the rest of the circuit. 
     When the cartridge  30  is in position and contains ink, ink may be pumped by pumping means  31  (4 th  pump) towards the main reservoir  10  through fluid connection means, comprising conduits  346 ,  343 ,  344 ,  347  and one or more valve(s) (or solenoid valves)  133 ,  135  that may be 3-way type valves. Thus, the ink transfer pump  31  pumps ink from the cartridge  30  and the ink passes in sequence through valves  135  and  133  (in positions    12   , or “NC”, and    23   , or “NO” respectively in  FIG. 15 ), and through conduits  343 ,  344 ,  347  to reach the main reservoir  10 . The NO (respectively NC) state of the valve  135  corresponds to the position    23    (respectively    12   ) creating connections between conduits  345  and  343  (respectively  346  and  343 ). 
     Means  345 ,  35 , for example a duct and a valve respectively (when the valve is in position    32    (NO) in  FIG. 12 ) at the inlet to means  300 , can be used to receive solvent from means  100 . The means  300  will then increase the pressure of this solvent to a relative pressure ( gauge pressure  ) equal for example to between 0 and 5 bars or between 0 and 10 bars, in fluid connection means. 
     This solvent may be directed through the conduits  343 ,  344  depending on the open or closed state of the valves  135  and  133 : 
     to reservoir  10  (through the duct  347 , valve  135  in position    32    (NO), valve  133  in position    23    (NO)), to add solvent, for example for cleaning, into the reservoir  10 ; 
     to conduits  320  (through the duct  348 , valve  135  in position    32    (NO), valve  133  in position    21    (NC)). Since the valve  37  is in the NO position, solvent can then be directed to the cartridge  30  through conduits  344 ,  348  and  320 . 
     The means  200  at the outlet from the main reservoir  10 , comprise a pump  20  (3 rd  pump, or ink pressurization pump) that pump ink from the main reservoir  10 , or the fluid contained in the reservoir, and this ink or this fluid can be directed either towards the main reservoir itself (through the return duct  318  and a cover according to the invention, duct  318  joining duct  431 ) or towards the cartridge  30  itself (and into this cartridge) through one or several conduits  319 ,  320 . The ink path at the outlet from the pump  20  may be controlled by means of one or several valves  37 , preferably a 3-way valve. In  FIG. 12 , the position    21    ( NC ) of valve  37  directs the ink flow towards the duct  319 , and position    23    ( NO ) directs the ink flow towards the duct  318 . Fluid can be transferred to the print head  1  through a duct  21  that collects ink downstream from the pump  20 , preferably from a location between the outlet from the pump  20  and the valve  37 . The print head  1  comprises a valve that allows, or not, forming an ink jet and, possibly, printing. 
     Generally, the instructions to activate pumps and valves are sent and controlled by the control means  3  (also called “controller”). In particular, these instructions will control flow of solvent under pressure, from means  100  to various other means  1 , and/or  50 , and/or  300  of the circuit (and possibly through these latter means  300  to the main reservoir  10 ). 
     The control means  3  control the opening and the closing of each valve, as well as the activation of the pumping means, in order to circulate ink and/or solvent as disclosed in this application. In one or more memory or memory means, it also memorises data, for example ink and/or solvent level measurement data, and may also possibly process these data. The controller is also programmed to manage other operations, particularly printing operations. 
     The control means  3  may comprise a processor or microprocessor, programmed to implement a cleaning method according to the invention or one or several steps according to the invention. 
       FIG. 13  shows an in ink circuit in which the circuit and the elements described above, particularly with reference to  FIGS. 4B-12 , can be used. The different means  10 ,  50 ,  100 ,  200 ,  300  described above are combined. In this figure, numeric reference identical to those in the previous figures refer to identical or corresponding elements. 
     At the outlet on the main reservoir  10 , is disposed a filter  22 , and then the pump  20  and an anti-pulsation device  23 . A pressure sensor  24 , and, possibly, a temperature sensor can be provided, as illustrated in the figure: the data it provides are used by the controller to control the ink pressure to a set point, generally when the ink jet speed in the head is not available (for example when the jet ejection is stopped, or when the jet speed is not measurable). As already indicated above, the ink is sent to the printing head  1 , via the duct  21 , connected downstream of the anti-pulsation device  23 , between the pump  20  and the valve  37 . The printing head contains itself a valve which enables to allow, or not, an ink jet and possibly a printing to be produced. 
     The ink is filtered by the main filter  27  downstream of the sensor  24  before it is sent to the head  1 . 
     The intermediate reservoir  14  has been described above. A duct  141  can be used to bring the free volume located above each of the liquids contained in the reservoirs  10  and  14  to the same atmospheric pressure. This duct can be connected to the duct  331 ′, described above in connection with  FIG. 4A . 
     It should be noted that when the valve  42  is in the  NC  position while valve  35  is in the  NC  position, solvent flow is blocked both towards the cartridge  30  and towards the duct  343 ; therefore, solvent is thus directed to valve  51  or to restriction  45  (and then enters the intermediate reservoir  14 ). 
     It is understood that solvent and/or ink, from the cartridges  30 ,  40  can be sent to the reservoir. These additions are made occasionally over time, preferably upon instructions by the control means. The ink circulation, from the reservoir bottom to the lid, as explained in connection with  FIGS. 8A and 8B , is in turn continuous over time. Recovering ink from the head depends on the jet being started. 
     The invention is particularly useful for ink containing dense particle dispersions such as metals or metal oxide pigments, for example titanium, zinc, chromium, cobalt or Iron (such as TiO 2 , ZnO, Fe 2 O 3 , Fe 3 O 4 , etc.) in the form of micronic or sub-micronic particles. Such a pigment ink can for example be based on TiO 2 , and can be used for marking and identification of black or dark supports. 
     But it is also useful in the case of a non-pigment ink that can dry and form deposits of dry material in the conduits and connections of the ink circuit, as described above.