Patent Publication Number: US-8109613-B2

Title: Ink supplying method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Division of U.S. patent application Ser. No. 11/617,256 filed Dec. 28, 2006, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ink jet recording apparatus, an ink supplying mechanism, and an ink supplying method for ejecting an ink from an ink jet head while circulating the ink. 
     2. Description of the Related Art 
     A technique for ejecting an ink from a nozzle of an ink jet head while circulating the ink in an ink jet recording apparatus is disclosed in, for example, JP-T-2002-533247 (the term “JP-T” as used herein means a published Japanese translation of a PCT patent application) or US 2002/0118256A1. In such an ink jet recording apparatus, for example, an upstream side tank, an ink jet head, and a downstream side tank are connected by a conduit. A liquid surface of the upstream side tank and a liquid surface of the downstream side tank are kept constant. An ink in the upstream side tank circulates to flow into the ink jet head through an upstream side channel and flow into the downstream side tank through a downstream side channel. 
     In such an ink jet recording apparatus, to prevent deficiencies such as inclusion of air and ink leakage and secure a satisfactory printing characteristic, maintenance of a proper circulation flow rate is demanded. In the technique described above, a circulation flow rate depends on a channel resistance of a channel extending from the upstream side tank to the downstream side tank via the upstream side channel, the ink jet head, and the downstream side channel and a difference between the height of the upstream side tank and the height of the downstream side tank. Therefore, in order to adjust the flow rate, it is necessary to adjust the flow rate according to positions of the upstream side tank, the downstream side tank, the ink jet head, and the like. In other words, for example, in order to increase the flow rate, it is necessary to increase the difference between the height of the upstream side tank and the height of the downstream side tank. Thus, the upstream side tank has to be lifted and the downstream side tank has to be lowered. However, usually, since an arrangement of tanks is often physically limited, it is difficult to adjust the heights. Further, since the channel resistance changes according to the change of the difference between the heights, it is difficult to secure a desired flow rate. 
     On the other hand, in the ink jet head, in order to secure the satisfactory printing characteristic, an ink pressure near the nozzle is extremely important. It is necessary to keep the ink pressure near the nozzle in a proper range. However, in the technique described above, when there is no ejection of the ink or an ejection quantity of the ink is small, the ink pressure near the nozzle depends on a channel resistance of a channel extending from the upstream side tank to the nozzle in the ink jet head via the upstream side channel, a channel resistance of a channel extending from the nozzle in the ink jet head to the downstream side tank via the downstream side channel, and the heights of the liquid surfaces of the upstream side tank and the downstream side tank. Therefore, in order to obtain an ink pressure in an appropriate nozzle position, it is necessary to adjust the height of the upstream side tank and the height of the downstream side tank. Consequently, the physical limitation on the arrangement of tanks and the change of channel lengths make it difficult to adjust the heights. 
     BRIEF SUMMARY OF THE INVENTION 
     According to an aspect of the invention, there is provided an ink supplying mechanism including a circulating system that connects an ink jet head having a nozzle, a pressure chamber opposed to the nozzle, and an upstream port and a downstream port that communicate with the pressure chamber, an upstream side tank that communicates with the ink jet head via the upstream port and is capable of storing an ink, a downstream side tank that communicates with the ink jet head via the downstream port and is capable of storing the ink, and a circulating pump that feeds the ink from the downstream side tank back to the upstream side tank. The ink supplying mechanism has a relief valve that is capable of opening and closing at least a liquid surface of the downstream side tank with respect to the atmospheric pressure, closes the relief valve, drives the circulating pump, sets the liquid surface of the downstream side tank to a negative pressure, and feeds the ink from the downstream side tank back to the upstream side tank via a feedback channel to circulate the ink. 
     Objects and advantages of the invention will become apparent from the description which follows, or may be learned by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The accompanying drawings illustrate embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain the principles of the invention. 
         FIG. 1  is a diagram schematically showing an overall structure of an ink jet recording apparatus according to a first embodiment of the invention; 
         FIG. 2  is a partial sectional view showing a structure around a nozzle of an ink jet head according to the first embodiment; 
         FIG. 3  is a diagram schematically showing an overall structure of an ink jet recording apparatus according to a second embodiment of the invention; 
         FIG. 4  is a diagram showing an operation of the ink jet recording apparatus according to the second embodiment; 
         FIG. 5  is a diagram showing the operation of the ink jet recording apparatus according to the second embodiment; 
         FIG. 6  is a diagram showing the operation of the ink jet recording apparatus according to the second embodiment; 
         FIG. 7  is a diagram showing the operation of the ink jet recording apparatus according to the second embodiment; 
         FIG. 8A  is a sectional view showing an ink drop condition around a nozzle according to the second embodiment; 
         FIG. 8B  is a sectional view showing the ink drop condition around the nozzle according to the second embodiment; 
         FIG. 8C  is a sectional view showing the ink drop condition around the nozzle according to the second embodiment; 
         FIG. 9A  is a sectional view showing an ink drop condition around the nozzle according to the second embodiment; 
         FIG. 9B  is a sectional view showing the ink drop condition around the nozzle according to the second embodiment; 
         FIG. 10A  is a sectional view showing an ink drop condition around the nozzle according to the second embodiment; 
         FIG. 10B  is a sectional view showing the ink drop condition around the nozzle according to the second embodiment; 
         FIG. 11  is a diagram schematically showing an overall structure of an ink jet recording apparatus according to a third embodiment of the invention; 
         FIG. 12  is a graph showing a relation between a circulation flow rate and a nozzle pressure of the ink jet recording apparatus according to the third embodiment; 
         FIG. 13  is a graph showing a relation between a circulation flow rate and a nozzle pressure of the ink jet recording apparatus according to the third embodiment; 
         FIG. 14  is a graph showing a relation between a circulation flow rate and a nozzle pressure of the ink jet recording apparatus according to the third embodiment; 
         FIG. 15  is a diagram showing a relation between a circulation flow rate and a nozzle pressure of the ink jet recording apparatus according to the third embodiment; 
         FIG. 16  is a diagram showing a relation between a circulation flow rate and a nozzle pressure of the ink jet recording apparatus according to the third embodiment; and 
         FIG. 17  is a partial sectional view showing a structure of an ink jet head according to a modification of the first embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
     An ink jet recording apparatus and an ink supplying method according to an embodiment of the invention will be hereinafter explained with reference to  FIGS. 1 and 2 . In the figures, components are schematically shown by enlarging, reducing, or simplifying the components as appropriate. An ink jet recording apparatus  1  forms an image by ejecting an ink on a not-shown recording medium from a nozzle  17  of an ink jet head  11  while circulating the ink. The ink jet recording apparatus  1  includes an ink supplying mechanism  10 . The ink supplying mechanism  10  includes the ink jet head  11 , an upstream side tank  25  serving as an ink supply source, a downstream side tank  30  that stores the ink, a first conduit  41 , a second conduit  42 , and a third conduit  43  that connect the ink jet head  11 , the upstream side tank  25 , and the downstream side tank  30  and form a circulation path for the ink, a circulating pump  35  serving as an ink sending mechanism that circulates the ink, and a filter  36 . 
     The ink jet head  11  shown in  FIG. 2  includes an orifice plate  18  having the nozzle  17 . A pressure chamber  19  opposed to the nozzle  17  is formed on the rear side of the orifice plate  18 . An ink  20  circulates through the pressure chamber  19 . The pressure chamber  19  is formed narrower than a circulation path that communicates with the conduits. An actuator  22  is provided in the pressure chamber  19  formed on the opposite surface side of the nozzle  17  in  FIG. 2 . In the pressure chamber  19 , when the actuator  22  is driven, an ink droplet  20   a  is ejected from the nozzle  17 . As the actuator  22 , for example, an actuator that directly or indirectly deforms a pressure chamber using a piezoelectric element such as a PZT, an actuator that drives a diaphragm with static electricity, an actuator that directly moves an ink with static electricity, or an actuator that heats an ink with a heater to generate air bubbles and generate a pressure is used. However, the actuator  22  is not limited to these actuators. The ink jet head  11  has an upstream port  11   a  and a downstream port  11   b . The upstream port  11   a  of the ink jet head  11  is connected to the upstream side tank  25  via the first conduit  41 . The downstream port  11   b  is connected to the downstream side tank  30  via the second conduit  42 . In the ink jet head  11  constituted as described above, the ink  20  flows from the right to the left, for example, as indicated by an arrow in  FIG. 2 , through the pressure chamber  19 . 
     As shown in  FIG. 1 , the upstream side tank  25  is arranged above the ink jet head  11 . The upstream side tank  25  has an ink inlet  25   a  and an ink outlet  25   b  and has a function as an ink supply source for supplying an ink. The upstream side tank  25  includes an upper tank  26  and a lower tank  27 . A liquid surface of the lower tank  27  is opened to the atmosphere. The upstream side tank  25  is connected to the upstream port  11   a  of the ink jet head  11  via the first conduit  41 . The upper tank  26  is a replaceable bottle. When the ink in the upper tank  26  is exhausted, a user replaces the upper tank  26  with a new ink-filled bottle. The upper tank  26  and the lower tank  27  are connected via a ventilation pipe  28  and an ink supply pipe  29 . As the ink is consumed from the ink jet head  11 , the liquid surface of the lower tank  27  lowers and the bottom end of the ventilation pipe  28  separates from the liquid surface of the lower tank  27 . At this point, the air is led into the upper tank  26  through the ventilation pipe  28 , the bottom end of which is exposed. When the ink pushed out by this air in the upper tank  26  falls into the lower tank  27  through the ink supply pipe  29 , the liquid surface of the lower tank  27  rises. According to the rise of the liquid surface of the lower tank  27 , the liquid surface of the lower tank  27  reaches the bottom end of the ventilation pipe  28 . Then, since the ventilation pipe  28  is closed, the inflow of the air into the upper tank  26  stops and the supply of the ink is cut off. In this way, the ink is supplied and the liquid surface of the lower tank  27  is controlled. 
     When there is a margin in a setting range of a proper pressure near the nozzle  17 , i.e., the pressure chamber  19  in the ink chamber of the ink jet head  11  (an average excluding a high-frequency component generated by the actuator for an ink ejection operation), the height of the liquid surface does not have to be strict. In this case, it is possible to suppress a change in the height of the liquid surface with respect to a change in a volume by using a shallow container with a large cross section as the upstream side tank  25 . In that case, when the ink in the upstream side tank  25  decreases, the user may directly supply an ink to the upstream side tank  25 . The structure of the replaceable bottle does not have to be provided. 
     The ink in the upstream side tank  25  is supplied to the upstream port  11   a  of the ink jet head  11  via the first conduit  41 . A valve V 1  (an opening and closing mechanism) that is capable of opening and closing the circulation path is provided in the first conduit  41 . The valve V 1  is closed when the supply of the ink is stopped but is opened during the normal operation. 
     The downstream side tank  30  is an ink tank having the ink inlet  30   a  and an ink outlet  30   b . The downstream side tank  30  stores an ink and has a function as a pressure source. The downstream side tank  30  is arranged below the ink jet head  11 . The ink inlet  30   a  is connected to the downstream port  11   b  of the ink jet head  11  via the second conduit  42 . The ink outlet  30   b  is connected to the upstream side tank  25  via the third conduit  43  including the circulating pump  35  and the filter  36 . The circulating pump  35  has a function of circulating the ink  20  by pumping up the ink in the downstream side tank  30 , filtering the ink with the filter  36 , and pumping up the ink to the upstream side tank  25  via the third conduit  43 . For example, like a tube pump, the circulating pump  35  closes when circulation is stopped. The same function may be realized by connecting a diaphragm pump and a check valve in series. The circulating pump  35  is controlled by, for example, ON/OFF control or speed control. 
     The downstream side tank  30  has an air layer in an upper part thereof. An openable and closable valve V 2  (a pressure adjusting mechanism) is provided above this air layer. By opening and closing the valve V 2  with a control unit  37 , it is possible to selectively open to the atmosphere pressure, or close the liquid surface of the downstream side tank  30 . Two liquid surface sensors S 1  and S 2  (liquid surface detectors) are provided in the downstream side tank  30 . The liquid surface sensors S 1  and S 2  have a function of detecting whether the liquid surface of the ink in the tank has reached a first level and a second level set in advance, respectively. When the liquid surface is at the first level, a volume of the air layer of the downstream side tank  30  is V. When the liquid surface is at the second level, a volume of the air layer of the downstream side tank  30  is V+ΔV. 
     The insides of the upstream side tank  25 , the downstream side tank  30 , the first conduit  41 , the second conduit  42 , the third conduit  43 , and the pressure chamber  19  communicate with one another to form a circulation path  40 . Not-shown air filters for preventing inclusion of foreign matters are provided in atmosphere opening sections of these components. When the ink tends to evaporate, mechanisms such as mazes for preventing evaporation may be provided in the atmosphere opening sections of the respective components. 
     A flow rate of the circulating pump  35  is set to, for example, 120% of a maximum circulation flow rate planned. A difference of levels of the liquid surface of the upstream side tank  25  and the orifice plate surface  18  of the ink jet head  11  is Hu and a difference of levels of the liquid surface of the downstream side tank  30  and the orifice plate  18  surface of the ink jet head  11  is Hl. 
     A channel resistance from the tip of the first conduit  41  in the upstream side ink tank  25  to the neighborhood of the nozzle  17  in the ink chamber of the ink jet head  11 , i.e., a channel resistance in an upstream side channel is Ru. A channel resistance from the neighborhood of the nozzle  17  in the ink chamber of the ink jet head  11  to the tip of the second conduit  42  in the downstream side tank  30 , i.e., a channel resistance of a downstream side channel is Rl. For simplification of the following explanations, it is assumed that Ru and Rl include a channel resistance in the ink jet head  11 . 
     In this embodiment, since cross sections of the respective tanks  25  and  30  are sufficiently large, channel resistances from the liquid surfaces in the tanks to connection points of the conduits  41  and  42  are usually negligible. If the channel resistances are not negligible, the channel resistances only have to be added to Ru and Rl, respectively. 
     When the ink jet head  11  has a branch at a middle point of a circulation channel in the inside thereof and has the nozzle  17  at the end of the branch, Ru only has to be considered a channel resistance from the upstream side tank  25  to this branch point and Rl only has to be considered a channel resistance from the branch point to the downstream side tank  30 . 
     Values of Rl and Ru are products of a constant depending on a physical shape of a channel and a viscosity of an ink. It is assumed that the ink is a nonvolatile oil ink having a specific gravity ρ. A gravitational acceleration is g and the atmospheric pressure is Patm. 
     It is assumed that an ejection flow rate is sufficiently low compared with a circulation flow rate. In this case, pressure losses in the ink supplying mechanism  10  and the ink jet head  11  depend on the circulation flow rate more than the ejection flow rate. In general, a dynamic pressure due to a circulation flow near the nozzle  17  at the bottom end of the ink jet head  11  is sufficiently low and negligible. In such an ink supply mechanism  10 , usually, a Reynolds number is sufficiently small and an influence of a turbulent flow is negligible. 
     An operation of an initial supply of an ink in the ink jet apparatus  1  will be explained. 
     In an initial state, an ink is supplied to the upstream side tank  25 , and then the valve V 1  is opened and the circulating pump  35  is stopped. When the valve V 2  is opened in this state, the ink flows into the downstream side tank  30  from the upstream side tank  25  through the first conduit  41 , the ink jet head  11 , and the second conduit  42 . 
     In this case, by closing the tip of the nozzle  17  with a not-shown closing cap until the initial supply is finished, it is possible to prevent the ink from flowing out from the nozzle  17  of the ink jet head  11 . When all conditions that a pressure ρgHu is low, a diameter of the nozzle  17  is small, and the ink does not adhere to the surface of the orifice plate  18  are satisfied, the ink does not flow out from the nozzle  17  even if the closing cap is not used. Thus, in such a case, the closing cap does not have to be provided. 
     When the ink accumulates in the downstream side tank  30  and a liquid surface sensor S 1  detects that the liquid surface exceeds the first level, which is a low level reference, the circulating pump  35  operates according to the control by the control unit  37  corresponding to a result of the detection. The ink is fed from the downstream side tank  30  to the upstream side tank  25 . Thereafter, while the liquid surface exceeds the first level, the circulating pump  35  operates. The liquid surface of the upstream side tank  25  slightly rises according to the operation of the circulating pump  35 . However, this change is sufficiently small and negligible. 
     In this state, a circulating flow of the ink is generated. In the circulating flow, the ink flows from the upstream side tank  25  through an upstream side channel including the first conduit  41 , the ink jet head  11  and a downstream side channel including the second conduit  42  and returns to the upstream side tank  25  through a feedback channel including the circulating pump  35 , the filter  36 , and the third conduit  43 . The circulating pump  35  operates intermittently. A circulation flow rate in this case is determined by Hu, Hl, Ru, Rl, ρ, and g. When a value of the circulation flow rate is Q1, Q1=ρg(Hu+Hl)/(Ru+Rl). 
     A pressure near the nozzle  17  is determined by Hu, Hl, Ru, Rl, ρ, and g. When a value of the pressure is Pn1 (gage pressure), Pn1=ρgHu−(ρg(Hu+Hl)(Ru/(Ru+Rl)). 
     In this case, Pn1 is set to, for example, about −0.1 kPa to prevent the ink from overflowing the nozzle  17 . Q1 is set to a value smaller than a planned circulation flow rate. This state is a low-speed circulation state. Since Q1 is smaller than the planned circulation flow rate, a position of the downstream side tank  30  does not have to be lowered by a great degree. Therefore, even if there is a physical limitation, it is possible to easily constitute the ink jet recording apparatus  1 . 
     An operation for increasing a circulation flow rate and reducing a pressure near the nozzle  17  to a value suitable for ink ejection (increasing an absolute value) will be explained. 
     The valve V 2  that opens the air layer of the downstream side tank  30  to the atmospheric pressure is closed and the circulating pump  35  is caused to operate until the liquid surface in the downstream side tank  30  reaches the second level. When it is detected by the liquid surface sensor S 2  that the liquid surface of the downstream side tank  30  is lower than the second level, the circulating pump  35  is stopped. Thereafter, the circulating pump  35  is caused to operate only while the liquid surface of the circulating pump  35  exceeds the second level. In this case, the liquid surface of the downstream side tank  30  lowers by ΔHl and the liquid surface of the upstream side tank  25  rises by ΔHu. These values are sufficiently small compared with Hl and Hu. A change in potential heads for ΔHl and ΔHu is sufficiently small and negligible. 
     At a point when the valve V 2  is closed, the air layer of the downstream side tank  30  has a volume V. Since the liquid surface is lowered from this state, the volume of the air layer of the downstream side tank  30  increases to V+ΔV. Therefore, the air layer of the downstream side tank  30  is decompressed. When a gauge pressure in the air layer of the downstream side tank  30  in this state is PL (a negative value), PL=−(ΔV/(V+ΔV))Patm. Patm is the atmospheric pressure. 
     When a circulation flow rate in this case is Q2, Q2=(ρg(Hu+Hl)−PL)/(Ru+Rl)=Q1+(−PL/(Ru+Rl)). In other words, the circulation flow rate increases from Q1 by (−PL/(Ru+Rl)). 
     When a pressure near the nozzle  17  is Pn2 (a gage pressure), Pn2=ρgHu−(ρg(Hu+Hl)−PL)(Ru/Ru+Rl)=Pn1+PL(Ru/(Ru+Rl). In other words, the pressure near the nozzle  17  shifts to a negative pressure side from Pn1 by −PL(Ru/(Ru+Rl)). 
     Q2 should be set to a target proper circulation flow rate and Pn2 should be set to a proper pressure near the nozzle  17 . A proper value of the circulation flow rate is set in, for example, a range of one to twenty times as high as a maximum flow rate at the time of printing. A proper value of the pressure near the nozzle  17  is set in, for example, a range of pressures equal to or lower than 0 kPa and equal to or higher than −3 kPa. 
     When the circulation is stopped and the circulating pump  35  is put on standby while the pressure near the nozzle  17  is kept in the proper range, the circulating pump  35  is caused to operate as follows. First, the valve V 2  is opened and an operation condition of the circulation pump  35  is fed back to the liquid surface sensor S 1  to set a reference level to the first level. In this way, the ink circulation system shifts from a high-speed circulation state to a low-speed circulation state. The valve V 1  is closed slowly. As a result, the pressure near the nozzle  17  falls gradually. In this case, since the pressure near the nozzle  17  is a negative value, an absolute value thereof becomes large. When a convergent value of the pressure near the nozzle  17  in this case is Pn3 (a gage pressure), Pn3=−ρgHl. Pn3 is set to, for example, −3 kPa. 
     The ink jet recording apparatus  1  or the ink supplying mechanism  10  according to this embodiment has effects described below. It is possible to adjust a circulation flow rate and a pressure near the nozzle to proper values according to adjustment of the circulating pump  35  and internal pressures of the tanks. Therefore, even when there is a limitation on the arrangement of the ink jet head  11  and the tanks  25  and  30 , it is possible to secure a proper flow rate and a proper pressure. In other words, even if a position of the downstream side tank  30  changes and a potential head of the liquid surface of the downstream side tank  30  with respect to the surface of the orifice plate  18  of the ink jet head  11  changes, it is possible to obtain a desired circulation quantity and a desired nozzle pressure by, according to the change, changing a difference between the heights of the liquid surface sensors S 1  and S 2  and adjusting a pressure in the air layer of the downstream side tank  30  at the time when the valves are closed. Thus, it is easy to arrange the downstream side tank  30  in a position advantageous in terms of a structure. 
     Even if the downstream side tank  30  is located above the ink jet head  11 , if a difference between the heights of the liquid surface sensors S 1  and S 2  is set large and a negative pressure in the air layer of the downstream side tank  30  is set to a proper value, it is possible to obtain a desired circulation quantity and a desired nozzle pressure. 
     Moreover, it is possible to enjoy benefits of a circulation system by adjusting a circulation flow rate according to a situation, using the low-speed circulation state and the high-speed circulation state according to the situation, and maintaining a pressure near the nozzle at a proper pressure. In other words, the likelihood of stagnation and precipitation of the ink is reduced, and the temperature of the system is stabilized, and if filtering, degassing, and deforming are performed during circulation, it is possible to modify the ink to be more suitable for fly of ink jet according to circulation. Even if air bubble are generated somewhere in the system, it is possible to increase a circulation flow rate to a degree enough for pushing the air bubbles to the downstream side tank  30  and releasing the air bubbles. On the other hand, by setting the circulation flow rate not to be too high, it is possible to prevent inclusion of the air in a negative pressure section and foaming on a gas-liquid interface from being caused and prevent air bubbles, particles, and the like in the ink from being sent to near the nozzle of the head and prevent a shear stress from being applied to the ink to affect stability of the ink when the ink passes a narrow section of a channel. 
     Second Embodiment 
     An ink jet recording apparatus  2  according to a second embodiment of the invention will be explained with reference to  FIGS. 3 to 10 . In the figures, components are schematically shown by enlarging, reducing, or simplifying the components as appropriate. Explanations of components same as those in the first embodiment are omitted. 
     The ink jet recording apparatus  2  shown in  FIG. 3  includes plural ink jet heads  11  to  16 , the upstream side tank  25  serving as an ink supply source, the downstream side tank  30  that stores an ink, and a supply tank  45  that supplies the ink to the upstream side tank  25 . 
     The plural (six) ink jet heads  11  to  16  have the same structure as the ink jet head  11  according to the first embodiment. 
     The upstream side tank  25  and the supply tank  45  are connected via a fourth conduit  44  that has a valve V 3 , which is capable of opening and closing, in the middle. The supply tank  45  is located above the upstream side tank  25  and the fourth conduit  44  is arranged to be inclined downward from the supply tank  45  to the upstream side tank  25 . 
     The supply tank  45  may be a replaceable cartridge like the upper tank  26  in the first embodiment or may be a tank in which an ink is poured from above. An internal pressure of the supply tank  45  is opened to the atmospheric pressure. The ink in the supply tank  45  is poured into the upstream side tank  25  through the fourth conduit  44 . 
     The upstream side tank  25  has an air layer in the upper part thereof. An openable and closable valve V 4  is provided above this air layer. By opening and closing the valve V 4  with the control unit  37 , it is possible to selectively open or close the liquid surface of the upstream side tank  25  with respect to the atmosphere pressure. 
     A liquid surface sensor S 3  is provided in the upstream side tank  25 . The liquid surface sensor S 3  has a function of detecting whether the liquid surface of the ink in the tank has reached a third level set in advance. Since the valve V 3  is opened and closed according to the control by the control unit  37  corresponding to a result of the detection by the liquid surface sensor S 3 , it is possible to adjust a flow state of the ink. Consequently, the liquid surface of the lower tank of the upstream side tank  25  is maintained constant. 
     A valve V 5 , which is capable of opening and closing the circulation path, is provided in the first conduit  41  extending vertically to the bottom of the upstream side tank  25 . The first conduit  41  below the valve V 5  is formed as a columnar pipe having an internal diameter of 6 mm and length of 5 mm. The first conduit  41  of the columnar pipe shape is divided into six below the valve V 5  to form fifth conduits  45 . The six fifth conduits  45  are connected to upstream ports  11   a  to  16   a  of the six ink jet heads  11  to  16 , respectively. The fifth conduits  45  are formed to extend horizontal or slightly lower and not to rise from the dividing sections to the upstream ports  11   a  to  16   a  of the ink jet heads  11  to  16 . 
     The second conduit  42  that connects the upstream side tank  25  and the downstream side tank  30  is formed in a columnar pipe shape having an internal diameter of 6 mm like the first conduit. An openable and closable valve V 6  is provided in the second conduit  42 . The second conduit  42  is divided into six sixth conduits  46  below the valve V 6 . The six sixth conduits  46  are connected to downstream side ports  11   b  to  16   b  of the ink jet heads  11  to  16 , respectively. The sixth conduits  46  are formed to extend horizontal or slightly rise and not to lower from the downstream side ports  11   b  to  16   b  of the ink jet heads  11  to  16  to the dividing sections. 
     The six ink jet heads  11  to  16  have the width of 50 mm, respectively. Therefore, when all the six ink jet heads  11  to  16  are used, it is possible to perform printing with the width of 300 mm. Internal diameters and lengths of the six fifth conduits  45  are φ3×100 mm, φ3×155 mm, φ3×210 mm, φ3×265 mm, φ3×320 mm, and φ3×375 mm in order from the one connected to the ink jet head  11  closest to the columnar pipe to the one connected to the ink jet head  16  most distant from the columnar pipe. Internal diameters and lengths of the six sixth conduits  46  are φ3×106 mm, φ3×160 mm, φ3×214 mm, φ3×267 mm, φ3×321 mm and φ3×375 mm in order from the one connected to the ink jet head  11  closest to the columnar pipe to the one connected to the ink jet head  16  most distant from the columnar pipe. 
     In the second conduit  42 , a section above the valve V 6  extends upward vertically in the inside of the upstream side tank  25  and the tip thereof is opened to the air layer. In the second conduit  42 , a valve V 8 , which is capable of opening and closing the circulation path, is provided below the branch point. The tip portion of the second conduit  42  located further below the valve V 8  is opened to the inside of the downstream side tank  30 . The length of the columnar pipe from the branch point to the tip inside the downstream side tank  30  is 143 mm. 
     Two liquid surface sensors S 4  and S 5  are provided in the downstream side tank  30 . The liquid surface sensors S 4  and S 5  have a function of detecting whether the liquid surface of the ink in the tank has reached a fourth level and a fifth level set in advance, respectively. The liquid surface sensor S 4  is set in a position higher than the liquid surface sensor S 5 . The downstream side tank  30  has an air layer in the upper part thereof. An openable and closable valve V 7  is provided above this air layer. By opening and closing the valve V 7  with the control unit  37 , it is possible to selectively open or close the liquid surface of the downstream side tank  30  with respect to the atmosphere pressure. An internal pressure of the air layer of the downstream side tank  30  is measured by a pressure sensor  31 . 
     The downstream side tank  30  is formed in, for example, a cylindrical shape having a cross section of 50 mm 2  and height of 10 mm. When the liquid surface is at the fifth level, an air layer volume is 5 mL. The third conduit  43 , which connects the downstream side tank  30  and the upstream side tank  25 , includes the circulating pump  35  and the filter  36 . The ink in the downstream side tank  30  is fed back to the upstream side tank  25  via the circulating pump  35  and the filter  36 . 
     The ink is a nonvolatile oil ink having a specific gravity of 0.85 and a viscosity of 10 mPas. The respective ink jet heads  11  to  16  have 636 nozzles having a surface diameter of 27 μm subjected to ink repellent finishing. It is possible to eject ink droplets of 42 pL from the respective nozzles at a frequency of 6240 Hz. An ink flow rate at the time when all the 636 nozzles of one ink jet head continuously eject the ink is 10 mL/min. 
     A channel resistance between the upstream side ports  11   a  to  16   a  and the downstream side ports  11   b  to  16   b  of the respective ink jet heads is set to 3.85×10 9  Pa·s/m 3 . A ratio of a channel resistance on the upstream side and a channel resistance on the downstream side viewed from the surface of the orifice plate  18  is set to 1:0.96. 
     Channel resistances of the fifth conduits  45  on the upstream side are 5.03×10 8  Pa·s/m 3 , 7.80×10 8  Pa·s/m 3 , 1.06×10 9  Pa·s/m 3 , 1.33×10 9  Pa·s/m 3 , 1.61×10 9  Pa·s/m 3 , and 1.89×10 9  Pa·s/m 3 , in order from the one connected to the ink jet head  11  closest to the columnar pipe to the one connected to the ink jet head  16  most distant from the columnar pipe. 
     Channel resistances of the sixth conduits  46  on the downstream side are 5.33×10 8  Pa·s/m 3 , 8.05×10 8  Pa·s/m 3 , 1.08×10 9  Pa·s/m 3 , 1.34×10 9  Pa·s/m 3 , 1.61×10 9  Pa·s/m 3 , and 1.89×10 9  Pa·s/m 3 , in order from the one connected to the ink jet head  11  closest to the columnar pipe to the one connected to the ink jet head  16  most distant from the columnar pipe. 
     A channel resistance of the first conduit  41  on the upstream side including the valve V 5  is 3.77×10 6  Pa·s/m 3  and a channel resistance from the branch point of the second conduit  42  on the downstream side including the valve V 8  to the tip in the inside of the downstream side tank  30  is 4.72×10 7  Pa·s/m 3 . 
     The liquid surface of the upstream side tank  25  is located higher than the surface of the orifice plates  18  of the ink jet heads  11  to  16  by 12 mm. A head pressure obtained by locating the liquid surface higher is 100 Pa. The liquid surface of the downstream side tank  30  is located lower than the orifice surfaces of the ink jet heads by 120 mm. A head pressure obtained by locating the liquid surface lower is 1 kPa. 
     Operations from the initial state to filling of an ink in the ink jet recording apparatus will be explained. In  FIGS. 3 to 7 , portions in which the ink is filled are indicated by hatching. In the initial state shown in  FIG. 3 , the ink is stored in the supply tank  45 . When the valves V 4 , V 5 , V 6 , and V 8  are opened and then the valve V 3  is opened from this state, as shown in  FIG. 4 , the ink flows down from the upper tank to the lower tank. While the ink flows down, the valve V 7  is closed. As shown in  FIG. 5 , the ink flows down from the supply tank  45  to the downstream side tank  30  through the fourth conduit  44 , the upstream side tank  25 , the first conduit  41 , the ink jet heads  11  to  16 , and the second conduit  42 . While the liquid surface sensor S 3  detects that the liquid surface of the upstream side tank  25  exceeds the third level, the valve V 3  is closed to adjust the liquid surface. 
     When the ink in the second conduit  42  has reached the valve V 6 , the valve V 6  is closed and the valve V 7  is opened. It is possible to judge whether the ink has reached the valve V 6  according to time from the start of the supply. It is also possible to judge whether the ink has reached the valve V 6  according to a value of a pressure gauge  31  (a pressure detector) of the downstream side tank  30 . When a reading of the pressure gauge  31  coincides with a potential pressure of the ink at the height from the downstream side tank  30  to the valve V 6 , it is possible to judge that the ink has nearly reached the position of the valve V 6 . In this embodiment, even if the ink in the second conduit  42  overflows to the air layer of the upstream side tank  25  passing the valve V 6 , no problem is caused in particular. High accuracy is not required for timing. 
     The circulating pump  35  is set to operate when the liquid surface of the downstream side tank  30  exceeds the fourth level. As shown in  FIG. 6 , when the ink accumulates in the downstream side tank  30  and exceeds the fourth level, the conditions set are satisfied. Thus, the circulating pump  35  operates. The circulating pump  35  pumps up the ink in the downstream side tank  30  to the upstream side tank  25  via the filter  36  and the third conduit  43  forming the feedback channel. In this case, the ink in the upstream side tank  25  may be slightly higher than the third level. However, an influence on a pressure distribution of the circulating system is small and negligible. This state is a low-speed circulation state in which the ink circulates slowly. 
     During the operation, first, a positive pressure is given to the respective nozzles  17  of the ink jet heads  11  to  16 . A value of the positive pressure decreases as the ink is filled on the downstream side. A maximum value of the positive pressure given is about 100 Pa. To prevent the ink from dripping because of the positive pressure, the nozzles  17  of the ink jet heads  11  to  16  only have to be closed by not-shown caps during the operation. Besides, as explained later, by keeping a condition for maintaining a proper meniscus, it is possible to prevent the ink from dripping from the nozzles  17  of the ink jet heads  11  to  16  even if the caps are not provided. 
     A circulation flow rate in this case is calculated as 62 mL/min in total of the six ink jet heads  11  to  16 . Circulation flow rates of the respective ink jet heads  11  to  16  are 13 mL/min, 12 mL/min, 11 mL/min, 10 mL/min, 9 mL/min, and 8 mL/min in order from the ink jet head  11  closest to the columnar pipe. Pressures near the nozzles  17  are substantially equal at −434 Pa in all the ink jet heads  11  to  16 . Printing is also possible in this state. 
     A procedure for increasing circulation speed to 180 mL/min in total of the six ink jet heads  11  to  16  in order to enjoy the advantages of the ink circulating system will be explained. As shown in  FIG. 7 , the valve V 7  is closed, the downstream side tank  30  is closed, and the circulating pump  35  is caused to operate until the pressure gauge  31  indicates −2110 Pa. The circulating pump  35  is set to operate only while the pressure gauge  31  indicates a pressure below −2110 Pa. In this case, although the air layer in the downstream side tank  30  is expanded by decompression, the liquid surface slightly lowers by about 0.2 mm because of the decompression. A change in a potential pressure due to this change in the liquid surface is sufficiently low and negligible. It can be said that it is more desirable to manage the conditions in this embodiment by directly measuring a pressure than managing the conditions according to the liquid surface as in the first embodiment. It goes without saying that, when it is possible to accurately detect the liquid surface, the conditions may be managed according to the liquid surface as in the first embodiment. If a shape of the downstream side tank  30  is different and, for example, if a volume of the air layer is larger, the liquid surface management may be more advantageous than the pressure management. Thus, any one of the managements may be used. This state is a high-speed circulation state in which the ink circulates at 180 mL/min. 
     Circulation flow rates of the ink jet heads  11  to  16  are 38 mL/min, 34 mL/min, 31 mL/min, 28 mL/min, 26 mL/min, and 24 mL/min in order from the ink jet head  11  closest to the columnar pipe. Pressures near the nozzles  17  are substantially equal at −1.46 kPa in all the ink jet heads  11  to  16 . 
     In the above explanation, the ink jet heads  11  to  16  do not eject the ink or eject the ink only a little. However, when the ink is ejected, since a flow rate on the upstream side increase and a flow rate on the downstream side decreases, pressures near the nozzles  17  shift further to the negative pressure side. When the ink jet heads  11  to  16  eject a maximum quantity of ink, the pressures near the nozzles  17  (an average excluding a high-frequency component generated by the actuator for an ink ejection operation) shift to the negative pressure side most. Pressures near the nozzles  17  of the ink jet heads  11  to  16  in that case are calculated as −1.68 kPa, −1.7 kPa, −1.72 kPa, −1.73 kPa, −1.77 kPa, and −1.79 kPa in order from the ink jet head  11  closest to the columnar pipe. All the pressures in these nozzle positions are within a range of proper values. 
     The liquid surface sensor S 5  is not always necessary for the operations described above. However, it is possible to use the sensor for abnormality detection. The liquid surface sensor S 5  is set, for example, 1 mm below the position of the liquid surface sensor S 4 . In the normal operation, the liquid surface should not be lower than the liquid surface sensor  55  during circulation. Thus, if the liquid surface of the downstream side tank  30  becomes lower than the height of the liquid surface sensor S 5 , it is possible to detect, as abnormality, ink leakage somewhere in a passage of the ink extending from the upstream side tank  25  to the downstream side tank  30  through the ink jet heads. 
     Conditions for prevention of ink drop will be explained. In general, in a circulation supply system, energy per a unit volume of the ink supply source on the upstream side viewed from the height of the surface of the orifice plate  18  (a sum of a static pressure and a potential pressure on the liquid surface of the upstream side tank  25 ) is usually larger than a pressure P 1  suitable for ink ejection of an ink jet head by an upstream side channel resistance×a circulation flow rate. 
     Therefore, even if the meniscus is in a state of a concave shape shown in  FIG. 8A  during circulation, when the circulation stops because of some reason, a negative pressure of the meniscus decreases and changes to a positive pressure. The meniscus projects from the tip of the nozzle and swells as shown in  FIG. 8B . 
     Besides before the start of circulation at the beginning of warm-up, the meniscus is in such a state, for example, when electric power is saved in the standby state and circulation is stopped for emergency stop. A degree of the swell of the meniscus depends on an ink pressure near the nozzle. In the circulation supply system in this embodiment, the degree of the swell of the meniscus depends on a head difference between the liquid surface of the upstream side tank  25  and the surface of the orifice plate  18 . 
     When the pressure near the nozzle is high, the meniscus swells more and changes from the state in  FIG. 8B  to a state in  FIG. 8C . When the pressure near the nozzle reaches P 2 , it is impossible to keep an ink droplet on the tip surface of the nozzle  17 . The ink  20  drops or spreads to the orifice plate  18  passing the tip of the nozzle  17  and drops. 
     The drop of the ink at the time of standby or the like is not preferable because the ink is consumed excessively and a section around the nozzle is stained. Therefore, it is advisable to set the energy per a unit volume of the ink supply source on the upstream side viewed from the height of the surface of the orifice plate  18  (the sum of a static pressure and a potential pressure on the liquid surface of the upstream side tank  25 ) smaller than P 2 . For example, in the second embodiment, since the static pressure on the liquid surface of the upstream side tank  25  is 0 (the atmospheric pressure) and the potential pressure thereof is 100 Pa, the energy per a unit volume of the ink supply source on the upstream side viewed from the height of the surface of the orifice plate  18  is 100 Pa. On the other hand, P 2  is equal to or higher than about 2 kPa in actual measurement. Therefore, if the surface of the orifice plate  18  is cleaned as described later, the drop of the ink is prevented. 
     To lower a reduced pressure on the surface of the orifice plate  18  of the ink supply source on the upstream side while maintaining the meniscus pressure Pn at the time of circulation, the upstream side channel resistance should be reduced. For this purpose, the ink supply source on the upstream side should be set as close as the ink jet head  11 . A structure according to the second embodiment is set in this way. 
     When there is no adhesion of the ink near the nozzle  17  and the nozzle  17  is maintained clean, the ink  20  does not overflow the nozzle  17  in the state in  FIG. 8C  and drop. Therefore, the drop of the ink is prevented by maintaining the surface of the nozzle  17  clean or drying the ink jet head  11  prior to an ink filling operation or the like. Consequently, the ink is prevented from dropping from the nozzle  17  and a static pressure as high as P 2  is allowed. 
     On the other hand, even if an ink pressure near the nozzle  17  is lower than P 2 , if a meniscus  21  in  FIG. 8B  formed in a convex shape by wipe or the like is broken, the ink spreads over the orifice plate  18  as shown in  FIG. 9A  and drops at a pressure P 3  lower than P 2  as shown in  FIG. 9B . 
     As shown in  FIG. 10A , when a distance from the nozzle  17  to the surface of the orifice plate  18  is relatively small, the ink  20  invades the side of a nozzle plate at a pressure P 3 ′. As shown in  FIG. 10B , when the orifice plate  18  has a concave section larger than the hole of the nozzle  17  in the surface thereof, the ink flows out to the uppermost step of the orifice plate  18 , on which the ink should not usually adhere, at a pressure P 3 ″ or more. The flow-out of the ink is not preferable because the section around the nozzle is stained. Therefore, it is more desirable to keep a reduced pressure on the nozzle surface of the ink supply source on the upstream side at a pressure equal to or lower than P 3 , P 3 ′, or P 3 ″. Magnitudes of P 1 , P 2 , and P 3  depend on a shape of the section around the nozzle, an angle of contact between a nozzle material and the ink, and a surface tension of the ink and obtained by a calculation or an experiment. A relation among the pressures is P 2 &gt;P 3 ″&gt;P 3  and P 3 ′&gt;0&gt;P 1 . 
     In this embodiment, effects same as those of the ink jet recording apparatus  1  according to the first embodiment are obtained. In the ink jet recording apparatus  2  according to this embodiment, it is also possible to cope with plural ink jet heads. 
     Third Embodiment 
     An ink jet recording apparatus according to a third embodiment of the invention will be explained with reference to  FIGS. 11 to 16 . Explanations of components same as those in the first embodiment or the second embodiment are omitted. In the figures, components are schematically shown by enlarging, reducing, or simplifying the components as appropriate. 
     The ink jet recording apparatus  3  shown in  FIG. 11  includes the ink jet head  11 , the upstream side tank  25  that stores an ink supplied to the ink jet head  11 , the downstream side tank  30  that stores the ink, the supply tank  45  that supplies the ink to the downstream side tank  30 , the conduits  41  to  44  that form a circulation path for the ink, and the circulating pump  35  serving as an ink sending mechanism that circulates the ink. 
     The ink jet head  11  has the same structure as the ink jet head  11  according to the first embodiment. 
     Both the upstream side tank  25  and the downstream side tank  30  are arranged lower than the ink jet head  11 . The upstream side tank  25  is connected to the upstream port  11   a  of the ink jet head  11  via the first conduit  41 . The downstream side tank  30  is connected to the downstream side port  11   b  of the ink jet head  11  via the second conduit  42 . The upstream side tank  25  and the downstream side tank  30  are connected via the third conduit  43 . The third conduit  43  includes the circulating pump  35  having an ink sending function and the filter  36 . The inside of the downstream side tank  30  is connected to the supply tank  45 , which stores the ink supplied to the downstream side tank  30 , via the fourth conduit  44 . The supply pump  38  having an ink sending function is provided in the middle of the fourth conduit  44 . 
     The supply tank  45  may be a replaceable cartridge or may be a tank in which the ink is poured from above. An internal pressure of the supply tank  45  is opened to the atmosphere. The ink in the supply tank  45  is poured into the downstream side tank  30  through the fourth conduit  44  via the supply pump  38 . 
     The upstream side tank  25  is formed in a columnar shape without a change in a cross section. Two liquid surface sensors S 6  and S 7  are provided in the upstream side tank  25 . The liquid surface sensors S 6  and S 7  have a function of detecting whether the liquid surface of the ink in the tank has reached a sixth level and a seventh level set in advance, respectively. The height of the air layer above the seventh level is set as hau. The air layer of the upstream side tank  25  is connected to the atmosphere via an openable and closable valve V 9 . By opening and closing the valve V 9  with the control unit  37 , it is possible to selectively open or close the liquid surface of the upstream side tank  25  with respect to the atmosphere pressure. Moreover, a pressure gauge  32  that is capable of measuring a pressure in the air layer inside the upstream side tank  25  is provided in the upstream side tank  25 . 
     The downstream side tank  30  is formed in a columnar shape without a change in a cross section. Two liquid surface sensors S 8  and S 9  are provided in the downstream side tank  30 . The liquid surface sensors S 8  and S 9  have a function of detecting whether the liquid surface of the ink in the tank has reached an eighth level and a ninth level set in advance, respectively. The height of the air layer above the liquid surface sensor S 8  is set as hal. The air layer of the downstream side tank  30  is connected to the atmosphere via an openable and closable valve V 10 . By opening and closing the valve V 10  with the control unit  37 , it is possible to selectively open or close the liquid surface of the downstream side tank  30  with respect to the atmosphere pressure. Moreover, the pressure gauge  31  that is capable of measuring a pressure in the air layer inside the downstream side tank  30  is provided in the downstream side tank  30 . 
     The plural tanks  25 ,  30 , and  45 , the head  11 , and the conduits  41  to  44  constitute a circulation system that can circulate the ink. 
     The seventh level and the eighth level are at the same height and set below the nozzle by height h. The ninth level is set blow the eighth level by −Δhl (Δhl is a negative value). The sixth level is set above the seventh level by Δhu. 
     Internal volumes of a section connected to the valve V 9  and the pressure gauge  32  and a section connected to the valve V 10  and the pressure gauge  31  are sufficiently small. If there is a change in a cross section in the upper parts of the upstream side tank  25  and the downstream side tank  30  or the internal volumes of the section connected to the valve V 9  and the pressure gauge  32  and the section connected to the valve V 10  and the pressure gauge  31  are ineligible, hau and hal only have to be corrected by replacing the tanks with tanks of a columnar shape having the same volume and without a change in a cross section. 
     For example, like a tube pump, both the circulating pump  35  and the supply pump  38  close when stopped. The same function may be realized by connecting a diaphragm pump and a check valve in series. The circulating pump  35  and the supply pump  38  are controlled by, for example, ON/OFF control or speed control. 
     A specific gravity of the ink in this embodiment is 0.85 and h=120 mm. A channel resistance Ru from the upstream side tank  25  to the surface of the orifice plate  18  is Ru=4×10 9  Pa·s/m 3  and a channel resistance R 1  from the surface of the orifice plate  18  to the downstream side tank  30  is Rl=4×10 9  Pa·s/m 3 . hau=51 mm, hal=49 mm, Δhu=1 mm, and Δhl=−1 mm. A cross section of the upstream side tank  25  and a cross section of the downstream tank  30  are the same. The atmospheric pressure is 101 kPa and a gravitational acceleration is 9.8 m/s 2 . 
     Operations from the initial state to filling of an ink in the ink jet recording apparatus  3  will be explained. In the initial state, the ink is stored in the supply tank  45 . When the valve V 10  is opened and the supply pump  38  is caused to operate, the ink is fed to the downstream side tank  30  and stored therein. When the valve V 9  is opened and the circulating pump  35  is caused to operate, the ink in the downstream side tank  30  is flows into the upstream side tank  25  via the filter  36 . In this case, it is possible to adjust a level of the ink by driving the circulating pump  35  and the supply pump  38  as appropriate while monitoring the liquid surface sensors S 6 ,  57 ,  58 , and S 9 . The liquid surface of the upstream side tank  25  is adjusted to the seventh level and the liquid surface of the downstream side tank  30  is adjusted to the eighth level. In this state, the height of the liquid surface of the upstream side tank  25  and the height of the liquid surface of the downstream side tank  30  coincide with each other. 
     The valve V 9  and the valve V 10  are closed to slowly drive the circulating pump  35 . According to the driving of the circulating pump  35 , the ink flows through the first conduit  41 , the ink jet head  11 , and the second conduit  42  in this order to be filled in the circulating system. 
     The circulating pump  35  is stopped in this state. When a circulating flow stops, the valve V 9  and the valve V 10  are opened. Since a total quantity of the ink is reduced by a quantity filled in the circulating system including the first conduit  41 , the ink jet head  11 , and the second conduit  42 , the supply pump  38  and the circulating pump  35  are driven as appropriate again while monitoring the liquid surface sensors S 6 , S 7 , S 8 , and S 9  to adjust the respective liquid surfaces to the seventh level and the eighth level. 
     In this state, the circulation is stopped and the liquid surface of the ink jet head  11  is located above the surface opened to the atmosphere by h=120 mm. Therefore, a negative pressure of −ρgh=−1 kPa is applied to the neighborhood of the nozzle of the ink jet head  11 . This negative pressure is an appropriate value as an ink pressure at the time when the ink is not ejected. 
     An operation for circulating the ink will be explained. In a state in which the ink is filled, the valves V 9  and V 10  are closed and the circulating pump  35  is driven until the liquid surface of the upstream side tank  25  reaches the position of the liquid surface sensor  56 . Thereafter, the circulating pump  35  is controlled to maintain the position of the liquid surface sensor S 6 . In this case, since the air in the upstream side tank  25  is compressed, the pressure therein rises. Since the air in the downstream side tank  30  expands, the pressure therein falls. Since the cross section of the upstream side tank  25  is uniform, a volume of the air layer is proportional to the height of the air layer. Therefore, a gauge pressure Pau in the air layer of the upstream side tank  25  is Pau=Δhu/(hau−Δhu)×101 kPa=1/(51−1)×101 kPa=2.02 kPa. In this case, a quantity of the ink in the upstream side tank  25  decreases by a volume obtained by multiplying Δhu by the cross section of the upstream side tank  25 . However, since a total quantity of the ink in the circulation path does not change if the pump  38  is stopped, a quantity of the ink in the downstream side tank  30  increases by the same volume. Since the cross sections of the upstream side tank  25  and the downstream side tank  30  are the same, Δhl=−Δhu=−1 mm. Since the cross section of the downstream side tank  30  is uniform, a volume of the air layer is proportional to the height of the air layer. Therefore, a gauge pressure Pal of the air layer of the downstream side tank  30  is Pal=Δhl/(hal−Δhl)×101 kPa=−1/(49+1)×101 kPa=2.02 kPa. 
     Since the liquid surface of the upstream side tank  25  rises 1 mm and the liquid surface of the downstream side tank  30  falls 1 mm, a potential pressure of 17 Pa acts in a circulation direction. Since a differential pressure between the upstream side tank  25  and the downstream side tank  30  is 4.04 kPa, a circulation flow rate is (4040+17 Pa)/8×10 9  Pa·s/m 3 ×100 3 ×60=30 4 mL/min. A pressure Pn near the nozzle  17  is obtained by dividing Pau−ρg(h−Δhu) and Pal−ρg(h−Δhl) by Ru and Rl. Since Ru=Rl and Δhu=−Δhl, Pn=−ρgh=−1 kPa. This is identical with that before the start of the circulation and is within a range of proper values. 
     When the ink jet head  11  ejects the ink, a flow rate on the upstream side increases and a flow rate on the downstream side decreases. Thus, Pn shakes further to a negative pressure side than −1 kPa. It is possible to consider that this pressure change is equivalent to a pressure loss at the time when an upstream side channel resistance and a downstream side channel resistance are arranged in parallel and the ink of an ejection flow rate is fed. When a maximum ejection quantity Qi of the ink jet head  11  is set to 10 mL/min as in the second embodiment, a pressure loss Ploss is Ploss=Ru*Rs/(Ru+Rs)*Qi=2×10 9  Pa·s/m 3 ×10 mL/min×1(100 3 ×60)=333 Pa. Thus, a pressure near the nozzle  17  (an average excluding a high-frequency component generated by an actuator for an ink ejection operation) fall to about −1.33 kPa when a maximum quantity of the ink is ejected. This value is within the range of proper values. 
     When a flow rate is higher and Pn at the time of ejection excessively shifts to the negative pressure side, Ru and Rl should be reduced. For example, it is possible to reduce Ru and Rl by increasing or decreasing diameters of the conduits. When the ink jet head  11  continues the ejection, since a total quantity of the ink in the circulating system decreases, the supply pump  38  is driven to fill the ink. For example, when the liquid surface of the downstream side tank  30  falls below the ninth level, it is advisable to drive the supply pump  38  to supply the ink. 
     In this embodiment, the liquid surface sensors S 6 , S 7 , S 8 , and  59  need to correctly detect a level difference of +/−1 mm. However, when it is desired to ease the requirement of accuracy of the liquid surface sensors S 6 , S 7 ,  58 , and S 9 , hau and hal only have to be set higher than those in this embodiment while maintaining a ratio of hau, hal, Δhu, and Δhl. 
     In the following explanation, in the ink jet recording apparatus  3 , the liquid surface sensor  56  is lifted and the liquid surface sensor S 9  is lowered to change a circulation flow rate to 0-100 mL/min. When the liquid surface sensor S 6  is lifted and the liquid surface sensor S 9  is lowered by the same degree, a pressure in the upstream side tank rises and a pressure in the downstream side tank falls. As a result, the circulation flow rate increases. While the height of the liquid surface sensor S 6  is changed, when the height of the liquid surface sensor  59  is shifted in the opposite direction by the same degree and the circulation flow rate is changed to 0-100 mL/min, a pressure near the nozzle  17  changes as shown in  FIG. 12  with respect to the circulation flow rate. In other words, when the circulation flow rate is higher than 30 mL, the pressure near the nozzle  17  shifts to the positive pressure side. When a target circulation flow rate is higher than 30 mL, a difference between hau and hal, i.e., a difference between the heights of the air layers of the upstream side ink tank and the downstream side ink tank before the start of the circulation should be increased. For example, when hau=52 mm and hal=48 mm, a relation between the circulation flow rate and the pressure near the nozzle  17  is flat in a wider area as shown in  FIG. 13 . 
     Moreover, instead of changing the heights of the air layers of the upstream side tank  25  and the downstream side tank  30 , the cross sections of the upstream side tank  25  and the downstream side tank  30  may be changed. For example, when hau=50 mm and hal=50 mm, if a ratio of the cross sections of the upstream side tank  25  and the downstream side tank  30  is 1:1, a relation between the circulation flow rate and the pressure near the nozzle  17  is as shown in  FIG. 14 . Thus, as the flow rate increases, the pressure near the nozzle  17  increases. Thus, if a ratio of the cross sections of the upstream side tank  25  and the downstream side tank  30  is set as 1:0.9, a relation between the circulation flow rate and the pressure near the nozzle  17  is as shown in  FIG. 15  and is flat in a wider area. 
     In the example explained above, the meniscus pressure near the nozzle  17  changes in a concave shape with respect to the circulation flow rate. However, if a potential head of the liquid surface of the downstream side tank  30  falls by a great degree when the circulation flow rate increases by, for example, forming the downstream side tank  30  in a conical shape having a smaller cross section in the lower part thereof, it is possible to set the pressure near the nozzle  17  not to change even when the circulation flow rate changes. 
     An adjusting method for not changing the pressure near the nozzle  17  before and after the operation of the circulation pump  35  will be explained. Here, a volume of the air layer in the initial state of the upstream side tank  25  is Vu, a volume of the air layer in the initial state of the downstream side tank  30  is V 1 , a volume of the ink moving from the downstream side tank  30  to the upstream side tank  25  is ΔV, the height of rise from the initial state of the liquid surface of the upstream side tank  25  is Δhu, the height of fall from the initial state of the liquid surface of the downstream side tank  30  is −Δhl, a channel resistance from the upstream side tank  25  to the surface of the orifice plate  18  is Ru, a channel resistance from the downstream side tank  30  to the surface of the orifice plate  18  is Rl, a specific gravity of the ink is ρ, a gravitational acceleration is g, the atmospheric pressure is Patm, an increased air pressure in the upstream side tank  25  is Pu (a gauge pressure), a decreased air pressure in the downstream side tank  30  is Pl (a gauge pressure), an initial liquid surface height of the downstream side tank  30  with respect to the height of the surface of the orifice plate  18  is h, and a pressure near the nozzle  17  is Pn. 
     In the initial state, Pn=ρgh. When the circulating pump  35  is caused to operate and the ink of Δv moves, Pu=Δv/(Vu−ΔV)Patm and PL=−ΔV/(Vl+ΔV)Patm. A potential pressure on the liquid surface of the upstream side tank  25  is ρg(h+Δhu) and a potential pressure on the liquid surface of the downstream side tank  30  is ρg(h+Δhl). 
     When it is assumed that Ru=Rl to simplify a calculation, Pn=(½){Pu+ρg(h+Δhu)+PL+ρg(h+Δh 1 )}=ρgh+(½)(Pu+Pl+ρpghΔhu+ρgΔhl)=ρgh+(½){ΔV(V 1 −Vu)+2ΔV 2 }/{(Vu−ΔV)(Vl+ΔV)Patm+(ρg/2)(Δhu+Δhl). 
     To prevent the pressure near the nozzle  17  from changing before and after the operation of the circulating pump  35 , {ΔV(Vl−Vu)+2ΔV 2 }/{(Vu−ΔV)(Vl+ΔV)}Patm=ρg(Δhl+Δhu)−Δhl=(Patm/ρg){ΔV(Vl−Vu)+2ΔV 2 }/{(Vu−ΔV)(Vl+ΔV)}+Δhu. 
     If the upstream side tank  25  has a columnar pipe shape having an area Su, ΔV=SuΔhu and −Δhu=ΔV/Su. Thus, −Δhl=Patm/ρg{ΔV(Vl−Vu)+2ΔV 2 }/{(Vu−ΔV)(Vl+ΔV)}+(ΔV/Su) (Equation 1). 
     When Vu=Vl=V, −Δhl=2(Patm/ρg)(ΔV 2 /V 2 −ΔV 2 )+ΔV/Su (Equation 2). Therefore, when the liquid surface of the downstream side tank  30  falls below Δhl, the cross section of the downstream side tank  30  only has to be adjusted such that Equation 1 or Equation 2 holds to have a volume change of ΔV 
     It is also possible to adjust a channel resistance ratio of the upstream side channel and the downstream side channel instead of the heights of the air layers or the cross section ratio of the upstream side tank  25  and the downstream side tank  30  to adjust a pressure change characteristic of the pressure near the nozzle  17  with respect to a flow rate. For example, hau and hal are set as hau=50 mm and hal=50 mm and channel resistances are set as Ru=4.4×10 9  Pa·s/m 3  and Rl=4.0×10 9  Pa·s/m 3  by extending the upstream side channel while keeping the cross section ratio of the upstream side tank  25  and the downstream side tank  30  at 1:1. Then, a relation between the circulation flow rate and the pressure near the nozzle  17  is as shown in  FIG. 16  and is flat in an area wider than that in  FIG. 14 . 
     In this embodiment, effects same as those of the ink jet recording apparatus  1  according to the first embodiment are obtained. Moreover, it is possible to lower the pressure in the downstream side tank not only by closing the downstream side tank and raise the pressure in the upstream side tank but also by making it possible to close the upstream side tank. This makes it possible to improve a degree of freedom of the arrangement of the tanks  25 ,  30 , and  50  and the ink jet head  11 . 
     The invention is not limited to the embodiments described above. It goes without saying that, in carrying out the invention, elements of the invention such as specific shapes of the components may be changed in various ways without departing from the spirit of the invention. For example, in the embodiments, the circulating pump  35  is controlled according to detection of the liquid surface sensors. However, the circulating pump  35  may be caused to operate at a constant flow rate. In the embodiments, the supply of the ink is controlled according to detection of the liquid surface sensor  3 . However, the supply of the ink may be controlled such that a weight of the downstream side tank  30  is fixed. 
     The supply of the ink from the supply tank  45  may be performed by the supply pump  38  or may be controlled by a valve using a natural supply flow rate determined by a liquid surface height of the supply tank  45 , a negative pressure in the downstream side tank  30 , and a channel resistance from the user tank to the downstream side tank  30 . 
     In the embodiments, the supply pump  38  is controlled according to detection by the liquid surface sensors. However, it is also possible that the supply pump  38  is made rotatable regularly and reversely, a value obtained dividing values of the pressure gauge  31  and the pressure gauge  32  by Ru and Rl is calculated, when the value is smaller than 0, the supply pump  38  is rotated regularly to supply the ink, and, when the value is larger than 0, the supply pump  38  is rotated reversely to feed the ink back to the supply tank  45 . Such a control may be performed to set the calculation value to 0. By performing the control, even when hau and hal change, since an influence on the pressure near the nozzle is only by a degree of a potential pressure difference. Thus, there is an advantage that it is unnecessary to too strictly adjust hau and hal. 
     In this way, when the supply pump  38  are capable of rotating regularly and reversely, the upstream side tank  25  and the downstream side tank  30  do not always have to be lower than the ink jet head. It is also possible that the upstream side tank  25  and the downstream side tank  30  are located above the ink jet head and the valves are closed to rotate the supply pump  38  reversely and generate a negative pressure. For example, the liquid surfaces of the upstream side tank  25  and the downstream side tank  30  are set in a position 30 mm above the nozzle and hau and hal are set as hau=hal=50 mm. In this case, since the valve  1  and the valve  2  are opened, it is likely that the ink drops from the nozzle. However, the drop of the ink is prevented by the method explained in the second embodiment. Subsequently, the valve  1  and the valve  2  are closed. According to a value obtained by dividing readings of the pressure gauge  1  and the pressure gauge  2  by Ru and R 1 , i.e., in this embodiment, an average Pave of the readings of the pressure gauge  31  and the pressure gauge  32  because Ru=Rl, when Pave is further on the positive pressure side than −1 kPa, the supply pump  38  is rotated reversely to feed the ink back to the supply tank  45  and, when Pave is further on the negative pressure side than −1 kPa, the supply pump  38  is rotated regularly to supply the ink. Then, a nozzle pressure is −1 kPa. In this case, the liquid surfaces of the upstream side tank  25  and the downstream side tank  30  are lower than those in the beginning. Subsequently, when the circulating pump is driven at 30.4 mL/min, the liquid surface of the upstream side tank  25  rises and the liquid surface of the downstream side tank  30  falls. The liquid surface of the upstream side tank  25  and the liquid surface of the downstream side tank  30  in this state are Δhu=0.38 mm and Δhl=−1.67 mm with a point when the valves are closed, i.e., the position 30 mm above the nozzle as a reference. This height change in the liquid surfaces is negligibly small as an influence on the pressure near the nozzle. Even in this period, it is possible to maintain the pressure near the nozzle substantially at −1 kPa from a period before the circulation start until a period during circulation if the supply pump  38  is controlled to rotate regularly and reversely as appropriate such that Pave=−1 kPa. 
     It is possible to remove redundant sensors not in use. However, the sensors may be used for abnormality detection without being removed. It is possible to learn abnormality from a relation between a liquid surface sensor and a pump flow rate. For example, when the circulating pump  35  is driven at a constant flow rate from a circulation stop state, time until a position of the liquid surface sensor of the upstream side tank  25  is detected may be measured. If the time is longer than a predetermined range, there is abnormality from the circulating pump  35  to the upstream side tank  25  or there is abnormality in the operation of the pump. It is possible to use the pressure gauges for abnormality detection as described below. For example, when the upstream port is not connected, a pressure detected by the pressure gauge  31  does not rise even if the circulating pump  35  is operating. Thus, it is possible to learn abnormality earlier than judging the abnormality with the liquid surface sensor. It is also possible to judge that there is abnormality somewhere when readings of the liquid surface sensor and the pressure sensor are different from predictions. It is possible to measure time until the liquid surface sensor reaches a predetermined position after the circulating pump  35  is started and, when the time is not in a predetermined range, judge that there is abnormality. For example, when the circulating pump  35  is started from the circulation stop state and the liquid surface of the upstream side tank  25  does not reach the liquid surface sensor within a predetermined time, the circulating pump has failed in feeding the ink or there is ink leakage ahead of the upstream side channel. Conversely, when the upstream side tank  25  reaches the liquid surface sensor in time shorter than the predetermined time, it is possible to judge that the upstream side tank  25  is not hermetically sealed. Presence or absence of abnormality may be detected according to whether fluctuation in a liquid surface height or fluctuation in a pressure during circulation is within a predetermined range. 
     In the example described in the embodiments, as shown in  FIG. 2 , the ink jet heads  11  to  16  eject the ink  20  while circulating the ink  20  via the pressure chamber  19 . However, a method of supplying the ink is not limited to this. For example, like an ink jet head  50  shown in  FIG. 17 , it is also possible to apply a method of circulating and supplying the ink to an ink storing unit  52 . The ink jet head  50  includes plural nozzles  51 , heat generating elements  51   a  formed in association with the nozzles  51 , the ink storing unit  52 , and channels  53  and  54  that communicate with an upstream side and a downstream side of the ink storing unit  52 . When the channels  53  and  54  are connected to the fourth conduit  40  and the fifth conduit  41  in the ink supplying mechanism  10  according to the embodiments, functions and effects same as those in the embodiments are obtained. In this form, pressure chambers  52   b  and the nozzles  51 , in which meniscuses are formed, are provided via slits  52   a  to be spaced apart from the ink storing unit  52 . It can be considered that the ink storing unit  52  is a branch point of the pressure chambers  52   b  and the nozzles  51  via an ink circulating section and the slits  52   a . When the ink is circulated to such a head, if the heights of the ink storing unit  52  and the surface of the nozzles  51  are hardly different, a meniscus pressure at the branch point and a meniscus pressure in the nozzle are substantially equal when the ink is not ejected. Therefore, it may be considered that an ink pressure in the ink storing unit  52  is the meniscus pressure in the nozzles. When the ink is ejected, it may be considered that the meniscus pressure in the nozzles falls by a pressure obtained by multiplying an ejection flow rate by a channel resistance from the branch point to the nozzles. 
     Moreover, an ink jet head used for this ink jet apparatus may be a type that branches to an actuator and nozzles from the middle of a circulation path via a filter. In this case, if the heights of the filter and the surface of the nozzles  51  are hardly different, it may be considered that, in a state in which the ink is not ejected, a pressure in the nozzles is identical with a pressure in a section where a primary side of the filter is in contact with the circulation path. It may be considered that, when the ink is ejected, the pressure in the nozzles falls by a pressure obtained by multiplying an ejection flow rate by a channel resistance from the primary side of the filter to the nozzles. As the actuator  21 , other than those described in the embodiments, for example, actuators of a piezoelectric type, a piezoelectric share mode type, a thermal ink jet type, and the like are also applicable. 
     When there are plural nozzle openings in the surface of an orifice plate and heights of the openings are different, it may be considered that an average of the heights of the nozzles is the height of the surface of the orifice plate as long as a difference in pressures near the nozzle due to the difference in heights does not exceed a range of proper pressures near the nozzle. In this case, when a direction of an ink circulation flow in a head is set in a direction from a section near a low nozzle to a section near a high nozzle, it is possible to reduce the difference in pressures near the nozzle due to the difference in heights. Thus, the direction of the ink circulation flow may be set in this way. 
     In the first embodiment, the circulating pump  35  is caused to operate according to a reading of the liquid surface sensor to obtain the gauge pressure PL of the air layer of the downstream side tank  30 . However, there is also a method of providing a pressure sensor for measuring a gauge pressure of the air layer of the downstream side tank  30  instead of providing the liquid surface sensor and causing the circulating pump to operate only while a result of the measurement is larger than PL (a negative value) (an absolute value is smaller) to directly maintain the pressure PL. 
     Further, instead of judging an output of the liquid surface sensor or the pressure sensor with respect to a threshold to control on and off of the pump, the output of the liquid surface sensor or the pressure sensor is changed to an analog output. The circulating pump performs control for changing a flow rate according to the analog output value instead of the on and off control such that a flow rate of the circulating pump coincides with a target flow rate when the output of the liquid surface sensor or the pressure sensor is a predetermined value. This makes it possible to realize smooth control with less pulsation. 
     The constitution of each of the embodiments may be combined with the constitutions of the other embodiments. Specifically, plural ink jet heads may be provided in the first embodiment and the third embodiment. The supply pump  38  may be used and the supply tank  50  may be arranged below the ink jet head in the first embodiment and the second embodiment. Besides, the directions, the materials, the numbers, the specific shapes, and the like of the components may be changed without departing from the spirit of the invention. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the invention as defined by the appended claims and equivalents thereof.