Patent Publication Number: US-8121504-B2

Title: Apparatus and method for measuring or controlling concentration of liquid developer

Description:
This application is based on Japanese Patent Application No. 2007-046907 filed on Feb. 27, 2007, in Japanese Patent Office, the entire content of which is hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to an apparatus and method for measuring or controlling the concentration of liquid developer used for image formation in an image forming apparatus. 
     BACKGROUND 
     What is commonly known in the prior art includes an image forming apparatus according to the electrophotographic method using a liquid developer. In such an image forming apparatus, an electrostatic latent image is formed on the photoreceptor, and this is developed by a liquid developer, whereby a toner image is formed. This toner image is transferred onto paper and is fixed thereon. 
     The liquid developer is formed by dispersing toner particles made up of resin and pigment into a carrier solution as an insulation solution exemplified by silicone oil in a high concentration. In order to develop an electrostatic latent image with this liquid developer, a thin layer of the developer of micron is formed on a developer carrier such as a development roller, and this thin-film developer is brought into contact with a photoreceptor. 
     As described above, when the liquid developer is used for development, formation of a uniform thin film of the developer of a predetermined concentration is crucial to obtain images with less variation of image density. To put it another way, it is important to maintain constant concentration of the liquid developer. 
     One of the techniques known in the conventional art to measure the concentration of the developer to maintain constant concentration of the liquid developer is the technique of calculating the developer concentration by detecting the light transmittance of the developer (Unexamined Japanese Patent Application Publication No. H09-281808 and Unexamined Japanese Patent Application Publication No. H11-73029). However, the method of calculating the developer concentration by light transmittance is accompanied by the problem of poor accuracy in measuring the concentration of highly concentrated developer. This is because, when the developer concentration is low, light transmittance is greatly changed by a change in concentration. However, when the concentration is increased, the light transmittance is reduced to the extent of reaching the point of saturation. 
     To solve this problem, the U.S. Pat. No. 6,131,001 discloses a method of using the viscosity of the developer to measure the concentration of a highly concentrated developer. The technique disclosed in this document uses, as the method of measuring the viscosity of developer, the method of obtaining the viscosity from the pressure difference of the developer in a pipe, the method of installing a viscometer in the tank accommodating the developer, and the method of obtaining the viscosity from the torque by the flowing developer. However, any of these methods has the problem of complicated structure and lack of sufficient precision. 
     Further, the Unexamined Japanese Patent Application Publication No. H06-277477 discloses the development of a technique of obtaining the viscosity of highly concentrated slurry (fuel mixture of coal and water) from the rotating torque in stirring operation by rotation, although this is not a liquid developer. However, despite comparatively simple measurement of the rotating torque, this technique has a defect of requiring the complicated procedure of calculating by detecting the amount of the liquid in a container. 
     SUMMARY 
     The object of the present invention is to solve the aforementioned problems and to provide a developer concentration measuring apparatus, developer concentration measuring method, developer concentration controlling apparatus and developer concentration controlling method wherein simple and highly sensitive measurement or control of the concentration of a highly concentrated developer can be ensured without the need for liquid amount control. 
     In view of foregoing, one embodiment according to one aspect of the present invention is an apparatus for measuring a concentration of a liquid developer: 
     a developer container having an opening, the developer container being adapted to contain the liquid developer for measurement; 
     a stirring mechanism which is adapted to stir the liquid developer contained in the developer container; 
     a load detector which is adapted to detect a load of stirring of the liquid developer when the liquid developer is stirred by the stirring mechanism; and 
     a controller which is adapted to calculate the concentration of the liquid developer based on the load detected by the load detector when an excessive amount of the liquid developer is overflowing or has overflowed through the opening of the developer container. 
     According to another aspect of the present invention, another embodiment is an apparatus for controlling a concentration of a liquid developer, comprising: 
     a developer container having an opening, the developer container being adapted to contain the liquid developer for measurement; 
     a stirring mechanism which is adapted to stir the liquid developer contained in the developer container; 
     a load detector which is adapted to detect a load of stirring of the liquid developer when the liquid developer is stirred by the stirring mechanism; 
     a supply developer container which is adapted to contain a supply developer; 
     a supply mechanism which is adapted to supply the developer contained in the supply developer container to the developer container; and 
     a controller which is adapted to control the supply of the supply developer by the supply mechanism based on the load detected by the load detector when an excessive amount of the liquid developer is overflowing or has overflowed through the opening of the developer container. 
     According to another aspect of the present invention, another embodiment is a method for measuring a concentration of a liquid developer, comprising the steps of: 
     containing the developer for measurement in a developer container having an opening; 
     stirring the liquid developer contained in the developer container; 
     detecting a load of the stirring of the liquid developer when an excessive amount of the liquid developer is overflowing or has overflowed through the opening of the developer container; and 
     calculating the concentration of the liquid developer contained in the developer container based on the detected load. 
     According to another aspect of the present invention, another embodiment is a method for controlling a concentration of a liquid developer, comprising the steps of: 
     containing the developer for measurement in a developer container having an opening; 
     stirring the liquid developer contained in the developer container; 
     detecting a load of the stirring of the liquid developer when an excessive amount of the liquid developer is overflowing or has overflowed through the opening of the developer container; and 
     controlling a supply of a supply developer to the liquid developer in the developer container based on the detected load. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view of the schematic structure of the image forming section  10  of an image forming apparatus as an embodiment of the present invention; 
         FIG. 2  is a layout drawing representing the schematic structure of the liquid development apparatus  4  of  FIG. 1 ; 
         FIG. 3  is a layout drawing representing the schematic structure of the developer concentration controlling apparatus  60  and developer concentration measuring apparatus  50  of  FIG. 2 ; 
         FIG. 4  is a configuration diagram of the apparatus for describing the operation of the developer concentration measuring apparatus  50 ; 
         FIG. 5  is a chart representing the relationship between the concentration and viscosity of the liquid developer; 
         FIG. 6   a  is a diagram showing the liquid surface when stirring the low-viscosity developer; 
         FIG. 6   b  is a diagram showing the liquid surface when stirring the high-viscosity developer; 
         FIG. 7   a  is a diagram showing the relationship between the developer viscosity and the amount of liquid regulated by the opening at the time of stirring; 
         FIG. 7   b  is a diagram showing the relationship between the amount of developer liquid and the load of stirring; 
         FIG. 8  is a flow chart showing the procedure of measuring the concentration in the developer concentration measuring apparatus; 
         FIG. 9  is a diagram showing the developer concentration measuring apparatus wherein the overall upper end of the wall surface forms an opening; 
         FIG. 10  is a diagram representing the developer concentration measuring apparatus using an ammeter as a load detector; 
         FIG. 11  is a flow chart representing the procedure of controlling the concentration in the developer concentration controlling apparatus; and 
         FIG. 12  is a chart representing an example of the change in stirring load T in the concentration controlling process. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following describes the embodiment of the present invention with reference to drawings: 
     (Structure and Functional Operation of Image Forming Section) 
       FIG. 1  is a cross sectional view showing an example of the schematic structure of a wet type image forming apparatus as an embodiment of the present invention. 
     In  FIG. 1 , numeral  1  denotes a photoreceptor drum that serves as an image carrier. The image forming section  10  includes a charging apparatus  2  for uniform charging of the surface of the aforementioned photoreceptor drum  1  installed around the photoreceptor drum  1 ; an exposure apparatus  3  for forming an electrostatic latent image by applying LED or laser beam to the charged photoreceptor drum  1 ; a liquid development apparatus  4  for developing the electrostatic latent image with a liquid developer; a transfer apparatus  5  for transferring the developed toner image onto a transfer media  7 ; and a cleaning apparatus  6  for removing the liquid developer remaining on the surface of the photoreceptor drum after transfer. 
     Further, an apparatus for coating part of the liquid developer to the photoreceptor drum  1  in advance or an apparatus for recovering part of the extra liquid developer on the photoreceptor drum  1  may be installed before and after the liquid development apparatus  4 . A recording material such as recording paper can be used as a transfer media  7 . Alternatively, an intermediate transfer belt can be used as a transfer media  7  so that the image is transferred again onto a final recording material. 
     The liquid development apparatus  4  includes a development roller  41  that carries a thin layer of the liquid developer on its surface so as to develop the latent image on the photoreceptor drum  1  as an image carrier; a conveyance roller  42  which is in contact with the development roller  41  to transfer onto its surface the liquid developer whose amount is controlled; and a supply roller  43  which is in contact with the conveyance roller  42  for supplying its surface with the liquid developer  8  inside the developer container  44 . 
     In  FIG. 1 , only one liquid development apparatus  4  is installed, but more than one liquid development apparatus  4  can be installed for color image formation. A desired color development method may be used, and the use or non-use of intermediate transfer may be determined as desired. A desired structure can be arranged depending of such options. 
     The photoreceptor drum  1  rotates in the arrow direction A in  FIG. 1 . The charging apparatus  2  charges the surface of the rotating photoreceptor drum  1  up to several hundred volts by corona discharge. On the downstream side of the charging apparatus  2  in the rotating direction of the photoreceptor drum, an electrostatic latent image whose surface potential is reduced to the level of a hundred volts is formed by the laser beam applied by the exposure apparatus  3 . 
     A liquid development apparatus  4  is installed on the downstream side of the exposure apparatus  3 , and the electrostatic latent image formed on the photoreceptor drum  1  is developed by the liquid developer  8 . 
     In the liquid development apparatus  4 , liquid developer  8  made of insulating solvent (hereinafter also referred to as “carrier solution”) and toner particles dispersed therein is stored in the developer container  44 , and the liquid developer  8  is supplied to the surface of the conveyance roller  42  by way of the supply roller  43 . 
     The conveyance roller  42  conveys a thin layer of the liquid developer  8  and transfers it onto the development roller  41 . A thin layer of liquid developer  8  is carried on the development roller  41 . Further, the toner particles in the thin layer of liquid developer  8  carried on the development roller  41  is moved to the electrostatic latent image on the photoreceptor drum  1  by the potential difference between electrostatic latent images on the development roller  41  and photoreceptor drum  1 , whereby an electrostatic latent image is developed. 
     The transfer apparatus  5  charges the transfer media  7  which is conveyed at the same speed as the peripheral speed of the photoreceptor drum  1 , or applies voltage thereto, whereby the toner image developed on the photoreceptor drum  1  is transferred onto the transfer media  7 . 
     A cleaning apparatus  6  for removing the liquid developer remaining on the surface of the photoreceptor drum  1  is installed on the downstream side of the transfer apparatus  5 . The liquid developer  8  remaining on the photoreceptor drum  1  is removed by the cleaning apparatus  6 . 
     The transfer media  7  with a toner image transferred thereon by the transfer apparatus  5 , if it is a recording material, is fed to the fixing apparatus (not illustrated) and is heated to fix the image. If the transfer media  7  is an intermediate transfer media such as an intermediate transfer belt, the toner image is transferred again onto the recording material. The recording material with the toner image transferred thereon is fed to the fixing apparatus wherein the image is fixed, and the material is ejected. 
     (Developer Composition) 
     The following describes the liquid developer  8  used for development: In the liquid developer  8 , colored toner particles are dispersed in carrier solution as a solvent at a high concentration. Additives such as a dispersant and charge regulating agent can also be added to the liquid developer  8 . 
     The insulating solvent which is nonvolatile at the normal temperature is used as the carrier solution. Toner particles are mainly made of a resin and pigment or dye for coloring purposes. The resin has a function of uniformly dispersing the pigment or dye in itself and a function as a binder when fixing the image on the recording material. 
     The volume-average particle diameter of the toner is preferably in the range of 0.1 μm or more without exceeding 5 μm. If the toner average particle diameter is below 0.1 μm, development performances are much deteriorated. If the average particle diameter is over 5 μm, the image quality is deteriorated. 
     The percentage of the mass of toner particles to the mass of the liquid developer is preferably in the range of about 10 through 40%. If it is below 10%, toner particles tend to settle out, and this raises the problem of stability with time when stored for a long time. Further, to get required image concentration, a large quantity of developer must be supplied, and this will increase the amount of carrier solution attached on paper. The paper must be dried at the time of fixing, and environmental problem occurs due to the evaporated vapor. If this percentage is over 40%, the viscosity of the liquid developer will be excessive, and handling difficulties will occur in the manufacturing process. 
     (Structure and Operation of Development Apparatus) 
       FIG. 2  shows the schematic structure of the, liquid development apparatus  4 . 
     The developer container  44  accommodates liquid developer  8 . 
     The supply roller  43  is arranged to be partially immersed in the liquid developer  8  in the developer container  44 . Rotating in the arrow-marked direction D, this roller draws up the liquid developer  8  from the developer container  44 . The high-viscosity liquid developer  8  is fed as it is attached to the surface of the supply roller  43  by its adhesibility. 
     The regulating member  45  is arranged so as to be in contact with the supply roller  43  in the counter direction to the rotating direction, as illustrated. It is intended to regulate the amount of the developer attached on the surface of the supply roller  43  to be fed. This arrangement peels down the excess amount of developer, and a thin layer of developer is formed on the surface of the supply roller  43 , whereby the developer is fed to the next conveyance roller  42 . 
     A rubber roller is generally used as a conveyance roller  42 . The conveyance roller  42  is arranged face to face with the supply roller  43  and is rotated in the arrow-marked direction C in contact therewith. At this nip portion, a thin layer of developer formed on the surface of the supply roller  43  is transferred onto the surface of the conveyance roller  42  and is fed toward the development roller  41 . 
     A low-hardness rubber roller is used as the development roller  41 . The development roller  41  is arranged face to face with the conveyance roller  42 , and is rotated in the arrow-marked direction B in contact therewith. At this nip portion, a thin layer of developer fed by the surface of the conveyance roller  42  is scraped off by the development roller  41 , and a thin layer of developer is carried and fed by the surface of the development roller  41 . Thus, the development roller  41  functions as the developer carrier. 
     In this case, the conveyance roller  42  forms a thin layer of developer which is conveyed to the developer carrier. The supply roller  43  may be designed to serve this function as well. To put it another way, it is possible to adopt the method wherein the developer is transferred from the supply roller  43  directly to the development roller  41 . 
     The development roller  41  is rotated in contact with the photoreceptor drum  1  as an image carrier, and the latent image on the photoreceptor  1  is developed by a thin layer of developer fed to the nip portion relative to the photoreceptor drum  1 , namely the development area. 
     However, after the latent image on the photoreceptor drum  1  has been developed, a thin layer of developer remains on the surface of the development roller  41 . When the remaining developer is fed to the development area again without being removed, the next development may be adversely affected. The removing member  46  is a cleaning blade to remove the remaining developer. 
     (Structure for Collecting and Recycling the Developer) 
       FIG. 2  also shows the schematic structure for collecting and recycling the removed remaining developer in the liquid development apparatus  4 . 
     As described above, the developer remaining on the development roller  41  is removed by the removing member  46 . The recovered developer is collected in a container to be discarded, or is reused. The present embodiment employs a configuration in which the developer is efficiently used without a container for discarding by reusing the recovered developer. 
     The developer having been scraped off from the surface of the development roller  41  by the removing member  46  is stored in the recovered developer container  53  as a recovered developer. 
     The recovered developer stored in the recovered developer container  53  is fed to the developer concentration controlling apparatus  60  so that it is adjusted to a desired level of concentration and is reused. The recovered developer container  53  is provided with a recovered developer supply mechanism  53   a . This recovered developer supply mechanism  53   a  is designed to feed the recovered developer to the developer concentration controlling apparatus  60 , and a normal pump capable of drive control can be used as this mechanism. 
     The recovered developer having been adjusted to a desired level of concentration by developer concentration controlling apparatus  60  is conveyed to the developer container  44  of the liquid development apparatus  4 , wherein it is reused. 
     &lt;Structure of the Developer Concentration Controlling Apparatus&gt; 
     The schematic structure of the developer concentration controlling apparatus  60  of  FIG. 2  is again shown in  FIG. 3 . 
     The developer concentration controlling apparatus  60  includes a developer concentration measuring apparatus  50 , first supply developer container  54 , first supply mechanism  54   a , second supply developer container  55  and second supply mechanism  55   a.    
     The first supply developer container  54  stores the liquid developer (including the case of carrier solution alone) having a concentration lower than a desired level, as the first supply developer. The developer is supplied to the developer concentration measuring apparatus  50  by the first supply mechanism  54   a . A normal pump capable of drive control can be used as the first supply mechanism  54   a.    
     The second supply developer container  55  stores the liquid developer of a concentration higher than a desired level as the second supply developer, for example, and this developer is sent to the developer concentration measuring apparatus  50  by the second supply mechanism  55   a . A normal pump capable of drive control can be used as the second supply mechanism  55   a.    
     In the developer concentration controlling apparatus  60 , the recovered developer having been fed for concentration control is sent to the developer concentration measuring apparatus  50  as a developer for concentration measurement. 
     The developer concentration measuring apparatus  50  measures the concentration of the developer for concentration measurement. The first supply mechanism  54   a  or the second supply mechanism  55   a  is driven in response to the result of comparison between the developer for concentration measurement and a desired level of concentration, whereby the first supply developer or the second supply developer is supplied. To put it another way, if the concentration is higher than a desired level, the developer (first supply developer) of lower concentration is sent. If the concentration is lower than the desired level, the developer (second supply developer) of higher concentration is sent. 
     In the developer concentration controlling apparatus  60 , measurement of the concentration by the aforementioned developer concentration measuring apparatus  50  and the supply of supply developer are continued until the concentration of the developer for concentration measurement reaches the desired level. 
     When the concentration of the concentration measurement developer has reached the desired level, concentration controlling procedure terminates. The developer for concentration measurement, the concentration of which has been controlled, from the developer concentration controlling apparatus  60  is supplied to the developer container  44  of the liquid development apparatus  4 . 
     &lt;Structure of Developer Concentration Measuring Apparatus&gt; 
     The developer concentration measuring apparatus  50  includes a developer container for concentration measurement  51 , ejected developer container  52 , control section  61 , stirring member  62 , drive section  63 , and load detector  64 . 
     The developer container for concentration measurement  51  is used to store the developer for concentration measurement, and to measure the concentration. Driven by the drive section  63 , the stirring member  62  stirs the developer for concentration measurement in the developer container for concentration measurement  51  and the load is detected by the load detector  64 , whereby the concentration is measured. 
       FIG. 4  shows the schematic structure of the developer concentration measuring apparatus  50 . 
     The developer container for concentration measurement  51  is a cylindrical container, and an opening  51   a  is arranged on the side of the upper portion. When the liquid level has become higher than the opening due to an increase of the amount of the stored developer for concentration measurement, the excess amount of the developer flows out the opening, and thus, the opening  51   a  keeps the highest liquid level of the developer for concentration measurement at a constant level. The top end of the developer container for concentration measurement  51  defines an opening, as shown in  FIG. 9 . It is also possible to arrange such a configuration that the developer for concentration measurement overflows from the whole of the top end. The developer container for concentration measurement  51  is provided with a developer supply mechanism  51   b  ( FIG. 3 ). This developer supply mechanism  51   b  is designed to feed the concentration-controlled developer to the developer container  44 . A normal pump capable of drive control can be used as the developer supply mechanism  55   b.    
     The ejected developer container  52  receives the aforementioned overflowing developer for concentration measurement, and stores it. It is possible to make such arrangements that the liquid developer remaining in the ejected developer container  52  is discarded, or is used as a recovered developer to be subjected to concentration control. When the top end of the developer container for concentration measurement  51  defines an opening, the ejected developer container  52  is arranged immediately below the developer container for concentration measurement  51  so as to receive the developer for concentration measurement overflowing from the whole of the top end, as shown in  FIG. 9 . The structure of  FIG. 9  is adopted in the structure of  FIG. 3 . For ease of explanation of the function of the opening  51   a , the structure of  FIG. 4  will be used for the following description. 
     The stirring member  62  is a stirring blade, for example. It is installed inside the developer container for concentration measurement  51 , and is driven and rotated by the drive section  63 , whereby the stored developer for concentration measurement is stirred. The drive section  63  is a motor, for example, and is used to rotate the stirring blade as a stirring member  62  under predetermined conditions. 
     The load detector  64  is an apparatus to detect the load when the stirring member  62  is driven by the drive section  63 . Various apparatuses can be used as the load detector  64 . In the present embodiment, used is a load detector  64   a , which is arranged between the motor  63  and stirring blade  62  for detecting the torque resulting from rotation of the stirring blade. The dynamic torque meter  64   a  detects the torque required to rotate the stirring blade at a predetermined rotational speed and sends the output value corresponding to that torque to the control section  61 . As the load detector, an ammeter  64   b  can be used instead of this dynamic torque meter  64   a , as shown in  FIG. 10 . The ammeter  64   b  measures the current required to rotate the motor  63  at a predetermined rotational speed and outputs the measurement result to the control section  61 . 
     The control section  61  controls the operations of these components and calculates the concentration of the developer for concentration measurement or the value corresponding to the concentration. Further, based on the comparison with the desired level of concentration, the control section  61  controls the operation of supplying the first supply developer for concentration control or the second supply developer. The control section  61  can be made up of a microcomputer, memory, etc. 
     The aforementioned developer concentration measuring apparatus  50  is arranged in such a way as to calculate the concentration by detecting the load at the time of stirring the developer for concentration measurement. This is because the viscosity differs according to the concentration of the measurement developer, whereby the load for stirring is different. 
       FIG. 5  is a chart representing the relationship between the concentration and viscosity of the developer. 
     The liquid developer is made of a carrier solution with the toner dispersed therein, as described above. The concentration of the liquid developer is expressed by the concentration of toner in the developer. As shown in  FIG. 5 , viscosity is changed if there is a change in the developer concentration. There is a great change in viscosity especially in the highly concentrated area, and a sufficient sensitivity of measurement is provided. Further, a change in the developer viscosity is accompanied by a change in the load required to stir it. Viscosity, hence concentration, can be measured from the stirring load, using this relationship. 
     To get the viscosity from stirring load, conditions at the time of stirring should be kept constant. Apparatus conditions as well as rotational conditions (if the stirring blade is to be rotated) and the amount of liquid developer to be stirred are kept constant. However, the following problems are involved in keeping the amount of liquid constant: 
     The time and effort for measurement are required to keep constant the amount of liquid to be measured. In the case of repeated measurements, the amount of liquid must be made constant at every measurement. The amount of the liquid will be increased if the developer for concentration control is replenished. Accordingly, the amount of the liquid must be controlled at every replenishment. 
     The present embodiment does not use the method wherein the amount of liquid at the time of stirring is always kept constant at any viscosity. Instead, the present embodiment is arranged so that the amount of liquid at the time of stirring is kept at the same level for the developer of the same viscosity. To put it another way, when the developer has the same viscosity, liquid level is regulated to be constant by the opening  51   a , whereby the same amount of liquid can be obtained.  FIG. 6   a  and  FIG. 6   b  show the difference in the liquid level at the time of stirring due to the difference in the developer concentration, i.e., the difference in the viscosity.  FIG. 6   a  shows the situation of stirring the developer of low viscosity, and  FIG. 6   b  shows the situation of stirring the developer of high viscosity. In this case, the stirring conditions are assumed as the same. 
     As will be apparent from the  FIG. 6   a  and  FIG. 6   b , the liquid surfaces  81  are different between the high-viscosity developer and low-viscosity developer even under the same stirring conditions. In the case of a low viscosity developer ( FIG. 6   a ), the developer liquid is moved outward by centrifugal force caused by the rotation of the stirring blade, and the liquid surface  81  forms a deep V-shape. By contrast, in the case of a high viscosity developer ( FIG. 6   b ), the developer liquid does not move much outward by centrifugal force, and the liquid surface  81  forms a less deep V-shape. 
     Thus, if there is an opening  51   a , the upper limit in the height of the liquid surface at the outermost portion is regulated, and the amount of liquid is changed according to developer viscosity. When control is conducted so that the developer overflows from the opening  51   a , the amount of the developer of a predetermined viscosity can be kept to a predetermined level. 
     It goes without saying that the amount of liquid differs if there is a difference in developer viscosity. Since a predetermined liquid level, hence a predetermined amount of liquid, can be obtained in accordance with each viscosity, it is possible to conform the stirring load according to viscosity to the amount of liquid. 
     This arrangement eliminates the need of controlling the amount of liquid at every viscosity measurement. The required control is provided automatically by the opening  51   a . Even if the developer of different concentration should be replenished during the measurement, the amount of liquid is automatically controlled in response to a change in viscosity. 
     Referring to  FIG. 7   a  and  FIG. 7   b , the following describes that the developer viscosity, hence concentration, can be calculated from stirring load even if the amount of liquid is changed in response to the developer viscosity: 
       FIG. 7   a  is a diagram showing the relationship between the developer viscosity and the amount of liquid regulated by the opening at the time of stirring.  FIG. 7   b  is a diagram showing the relationship between the amount of developer and the load of stirring. When these diagrams are put together, the relationship between the developer viscosity and stirring load can be obtained. 
     In  FIG. 7   a , the R 1 , R 2  and R 3  indicate the cases wherein there is a difference in the rotational speed of the stirring blade as a stirring member  62 . The R 1  denotes the greater rotational speed, R 3  the smaller speed, and R 2  the speed intermediate between them. 
     Particularly, when the speed is set to a greater level (R 1 ), a change in developer viscosity appears as a difference in the amount of liquid developer regulated by the opening. When the developer viscosities are N 1  and N 2  (N 1 &gt;N 2 ), the amounts of liquid developer are L 1  and L 2  (L 1 &gt;L 2 ), respectively. 
     In  FIG. 7   b , N 1  and N 2  indicate the cases of different developer viscosities. N 1  and N 2  of  FIG. 7   a  have the relationship of N 1 &gt;N 2 . It is apparent that, as the viscosity is greater, the stirring load is greater; and as the amount of liquid developer is greater, the stirring load is greater. 
     In the present embodiment, the amount of liquid is regulated according to the developer viscosity. For example, the developers of viscosities N 1  and N 2  have the amounts of liquid of L 1  and L 2 , respectively. As illustrated, for L 1 , the stirring load is T 1  denoted by the crossing point of L 1  and the chart N 1 . For L 2 , the stirring load is T 2  denoted by the crossing point of L 2  and chart N 2 . To put it another way, the difference between viscosity N 1  and N 2  appears as the difference between T 1  and T 2  in the stirring load. 
     Because of such a correspondence, even if a difference in the amount of liquid is caused by the developer viscosity, the corresponding viscosity, hence concentration, can be calculated from the stirring load. 
     Assume, on the other hand, that measurement is made according to a constant amount of liquid at all times without the amount of liquid being regulated by an opening. Again assume that, similarly, developer viscosities are N 1  and N 2  (N 1 &gt;N 2 ) in  FIG. 7   b . Since the amount of liquid is constant independently of viscosity, the stirring loads are T 1  and T 3  for N 1  and N 2 , respectively, when the amount of liquid is assumed L 1 . 
     When the difference between T 1  and T 2  in this embodiment is compared with that of aforementioned T 1  and T 3 , the difference between T 1  and T 3  is smaller. To put it another way, the measurement sensitivity according to the structure in the present embodiment is improved over the case wherein the amount of liquid is kept constant at the time of measurement. 
     (Operation of Measuring the Concentration in Developer Concentration Measuring Apparatus) 
       FIG. 8  is a flow chart showing the procedure of measuring the concentration in the developer concentration measuring apparatus. 
     When the measurement of developer concentration has started, the control section  61  drives the recovered developer supply mechanism  53   a  in Steps S 11  so that the developer for concentration measurement, i.e., the recovered developer collected in the recovered developer container  53  will be stored in the developer container for concentration measurement  51 . 
     In this case, what is important is the amount of the developer to be stored. The developer must be made to overflow through the opening  51   a  at the time of measurement, i.e., at the time of stirring. The amount of developer to be stored must be set taking this into account. The amount of developer conforming to the maximum storage capacity has only to be stored for ease of operation. 
     In the next Step S 12 , the control section  61  makes the drive section  63  drive the stirring member  62 . To put it more specifically, the stirring blade rotates at a predetermined rotational speed to stir the developer for concentration measurement. A commonly used method can be employed to control the rotating speed of the drive section  63 . The liquid surface of the developer for concentration measurement is formed in a V-shape and, the excess developer flows to the ejected developer container  52  through the opening  51   a . When the developer liquid level is placed under control, the stirring load is detected by the load detector  64 , and the result of detection is inputted to the control section  61 . 
     In this case, the stirring load must be detected after stirring has been started, and the amount of liquid developer has been changed and stabilized. Further, it is important at the time of stirring that the stirring blade should be immersed completely in the developer, and should be located sufficiently below the V-shaped liquid surface. This is essential to maintain high precision measurement. 
     In the next Step S 13 , the control section  61  calculates the developer concentration based on the stirring load having been detected. 
     When the correspondence between the stirring load and developer concentration is measured in advance, and the result is formulated in a Table stored in the control section  61 , the developer concentration can be calculated merely by referencing the Table. Further, it is also possible to make such arrangements that the aforementioned correspondences shown in  FIG. 7   a  and  FIG. 7   b  are stored as relational expressions, and the viscosity and concentration are calculated whenever required. 
     When developer concentration is measured as part of the operation of controlling the developer concentration, calculation of the concentration is not always essential. It is sufficient to store the data of the stirring load corresponding to a desired level of concentration and to determine whether the level is greater or smaller than this data. 
     The process of calculating the developer concentration and measuring the developer concentration is now completed. 
     (Operation of Controlling the Concentration in Developer Concentration Controlling Apparatus) 
       FIG. 11  is a flow chart representing the procedure of controlling the concentration in the developer concentration controlling apparatus. 
     When the control of developer concentration has started, in Step S 21 , the control section  61  provides control in such a way that the developer for which the concentration is to be controlled, i.e., the recovered developer in the recovered developer container  53  is stored in the developer container for concentration measurement  51  as the developer for concentration measurement. This is the same as the Step S 11  in the flow of measuring the developer concentration in  FIG. 8 . 
     This is followed by the Step S 22  which serves as a step for detecting the stirring load, similarly to the Step S 12 . 
     The next Step S 23  is basically the same as the Step S 13  in the flow of measuring the developer concentration in  FIG. 8 . It serves as a step for calculating the concentration. 
     Here the purpose is not to calculate the concentration but to control the concentration to a desired level. It is sufficient only if it is possible to determine whether or not the concentration is higher or lower than the desired level and what the difference is. Thus, management for controlling the concentration can be directly made using the value of stirring load corresponding to concentration. In this case, the “concentration” in the following description should be read as the “stirring load value” corresponding thereto. 
     The concentration controlling process ranges from the next Step S 24  to the End. While the concentration is controlled and the supply developer is replenished in the following process, the aforementioned stirring load detection and concentration calculation are continued. Based on these operations, the following concentration controlling process is also repeated until the concentration is reached the desired level. 
     In Step S 24 , a decision is made to see whether or not the concentration calculated in the aforementioned process has reached the desired level. A range of appropriate concentration is set in advance as the desired level of concentration by taking the concentration range into account, the concentration range which can be used in a liquid development apparatus. The control section  61  determines whether or not the calculated concentration lies within the range of concentration. 
     When the concentration reaches the desired level of concentration (Step S 24 : YES), the concentration controlling operation terminates. When the concentration does not reach the desired level of concentration yet (Step S 24 : NO), the procedure in Step S 25  is executed. 
     In Step S 25 , a decision is made to see whether the calculated concentration is greater or smaller than the desired level. The range of concentration in Step S 24  should be also used as a criterion for this step. In the case that the amount to be replenished is to be controlled in the subsequent Steps S 26  and S 27 , how much greater of smaller may be calculated in this step. 
     When the calculated concentration is greater than the desired level of concentration (Step S 25 : YES), the procedure in Step S 26  is executed. When the calculated concentration is smaller than the desired level of concentration (Step S 25 : NO), the procedure in Step S 27  is executed. 
     In Step S 26 , the control section  61  causes the first supply mechanism  54   a  to replenish the first supply developer (low-concentrated developer) to the developer container for concentration measurement  51  from the first replenishing developer container  54 . Since detection of the stirring load is continued during this time, replenishment and measurement are performed concurrently. This is enabled by the automatic control of the amount of liquid by the opening  51   a.    
     If the replenishment speed of the first supply mechanism  54   a  can be changed, replenishment may be controlled based on the difference in concentration obtained in Step S 25 . 
     In Step S 27 , the control section  61  ensures the second supply mechanism  55   a  to replenish the second supply developer (high-concentration developer) to the developer container for concentration measurement  51  from the second replenishing developer container  55 . Similarly to the case of Step S 26 , both replenishment and measurement are performed concurrently by the automatic control of the amount of liquid by the opening  51   a.    
     If the replenishment speed of the second supply mechanism  55   a  can be changed, replenishment can be controlled according to the difference in concentration obtained in Step S 25 , similarly to the case of step S 26 . 
     If the concentration of the recovered developer is in the direction of getting higher, the second supply developer (high-concentration developer) has not to be replenished. Further, the second supply developer container  55  has not to be installed. If the concentration of the recovered developer is in the direction of getting lower, the first supply developer (low-concentration developer) has not to be replenished. 
     After the procedure in Step S 26  or S 27  has been executed, the system goes to Step S 22 , and stirring load detection and concentration controlling processes are continued concurrently until the desired level of concentration is obtained, as described above. 
     When a desired level of concentration is obtained (Step S 24 : YES), the control section  61  supplies the concentration-controlled developer of the developer container for concentration measurement  51  to the developer container  44  of the liquid development apparatus  4 , and the developer concentration controlling terminates. Alternatively, control of the concentration of the next recovered developer can be started. 
       FIG. 12  is a chart representing an example of the change in stirring load T in the concentration controlling process. 
     A change in the stirring load T with respect to the lapse of time t is represented by the curve T (t). The TS denotes a reference load, which indicates the stirring load corresponding to a desired developer concentration level. Thus, if the T (t) agrees with the TS, the desired level of concentration has been obtained. 
     In the chart of  FIG. 12 , t 0  indicates the stirring load detection start-up time point. Detection of the stirring load T (t) starts at this time. In this example, the T (t) is greater than reference load TS. To be more specific, the concentration of the developer being measured is higher than the desired level of concentration. 
     The t 1  denotes the concentration controlling startup time point. The supply of the first supply developer (low-concentration developer) starts at this time. Both the developer replenishment and stirring load detection are continued concurrently, and T (t) starts to reduce with the lapse of time t. 
     The t 2  denotes the concentration controlling termination time point. During the period from t 1  to t 2 , while the stirring load is detected, the supply of the supply developer continues if the detected T (t) is greater than the reference load TS. The T (t) reaches the reference load TS at the time point t 2 , and therefore, the replenishment of the developer terminates. 
     After that, the stirring load T (t) is equal to the reference load TS, and the concentration controlling terminates because the desired level of concentration has been obtained. 
     As described above, in the present embodiment, concentration can be easily calculated by only keeping the liquid level constant using the opening, without having to control the amount of liquid constant at the time of measuring the concentration. Further, both liquid replenishment and measurement can be performed concurrently. Thus, while the concentration is controlled by the supply developer, concentration can be measured. This arrangement realize a substantial reduction in the time and effort required to measure the developer. Further, the measurement sensitivity can be enhanced using a change in the liquid amount caused by concentration, i.e., viscosity. By controlling the concentration in this manner, the image forming apparatus can efficiently use the recovered developer whose concentration is accurately controlled. 
     It is to be expressly understood, however, that the present invention is not restricted to the aforementioned embodiments. The present invention includes the embodiment which is modified without departing from the spirit and scope of the invention claimed.