Patent Publication Number: US-7907857-B2

Title: Hard imaging methods and hard imaging devices

Description:
FIELD OF THE DISCLOSURE 
     Aspects of the disclosure relate to hard imaging methods and hard imaging devices. 
     BACKGROUND OF THE DISCLOSURE 
     Imaging devices capable of printing images upon paper and other media are ubiquitous and used in many applications including monochrome and color applications. For example, laser printers, ink jet printers, and digital printing presses are but a few examples of imaging devices in wide use today for monochrome or color imaging. 
     Electrophotographic imaging processes utilize a photoconductor which may be electrically charged and then selectively discharged to form latent images. The latent images may be developed and transferred to output media to form hard images upon the media. Electrophotographic imaging processes are implemented in laser printer configurations and digital presses in illustrative examples. 
     Imaging devices of example embodiments of the present disclosure use a liquid marking agent to develop latent images. At least some embodiments of the disclosure are directed towards apparatus and methods for reducing a presence of bubbles in the liquid marking agent during hard imaging operations. Additional embodiments are described in the following disclosure. 
     SUMMARY 
     According to some aspects of the disclosure, hard imaging methods and hard imaging devices are described. 
     According to one embodiment, a hard imaging method comprises forming a plurality of latent images, using a development assembly, developing the latent images using a liquid marking agent, transporting the liquid marking agent relative to the development assembly during the developing, and performing a bubble reduction operation to reduce a presence of bubbles in the liquid marking agent during the developing and transporting compared with not performing the bubble reduction operation. 
     According to another embodiment, a hard imaging device comprises a development assembly configured to develop a plurality of latent images using a liquid marking agent, and a marking agent assembly configured to transport the liquid marking agent relative to the development assembly, wherein the marking agent assembly includes a bubble reduction apparatus configured to reduce a presence of bubbles in the liquid marking agent compared with a configuration of the marking agent assembly void of the bubble reduction apparatus. 
     Other embodiments are described as is apparent from the following discussion. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustrative representation of a hard imaging device according to one embodiment. 
         FIG. 2  is a functional block diagram of circuitry of the hard imaging device according to one embodiment. 
         FIG. 3  is an illustrative representation of development operations of the hard imaging device according to one embodiment. 
         FIG. 4  is an isometric view of a bubble reduction apparatus according to one embodiment. 
         FIG. 5  is a top view of the apparatus of  FIG. 4  according to one embodiment. 
         FIG. 6  is an illustrative representation of a development assembly comprising a bubble reduction apparatus according to one embodiment. 
         FIG. 7  is an illustrative representation of a bubble reduction apparatus according to one embodiment. 
         FIG. 7   a  is another illustrative representation of the bubble reduction apparatus of  FIG. 7  according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to some embodiments of the disclosure, hard imaging devices and hard imaging methods utilize a liquid marking agent to develop and form hard images upon media. One form of a liquid marking agent comprises ink particles suspended in a liquid carrier, such as oil. One suitable liquid marking agent is Electroink® available from the Hewlett-Packard Company. During example development operations using a liquid marking agent, the ink particle concentration of the liquid marking agent is increased by several times in a development assembly  18  and applied to a photoconductor to develop latent images formed thereon and at least a substantial portion of the remaining liquid carrier evaporates prior to transfer of the ink particles to media. 
     As described in further detail below, bubbles may be generated during hard imaging operations and entrained within the liquid marking agent. The presence of bubbles may cause defects in imaging and may cause erroneous results in monitoring of various characteristics of the marking agent, for example, during monitoring of the characteristics to implement calibration operations. Additional details regarding monitoring and calibration are discussed in a co-pending US patent application entitled Hard Imaging Methods, Liquid Marking Agent Monitoring Methods, And Hard Imaging Devices, naming Boaz Eden, William D. Holland, Omer Gila, and Moshe Peles as inventors, assigned to the assignee hereof, filed the same day as the present application, and the teachings of which are incorporated herein by reference. At least some embodiments of the disclosure provide apparatus and methods for reducing the presence of bubbles in the liquid marking agent. 
     Referring to  FIG. 1 , an example of a hard imagine device  10  is shown according to one illustrative embodiment. The depicted arrangement of the hard imaging device  10  is configured to implement electrophotographic imaging wherein latent images are developed to form developed images which are subsequently transferred to output media. Examples of hard imaging devices  10  include digital presses (e.g., Indigo® presses available from the Hewlett-Packard Company) although other configurations may be used. 
     The hard imaging device  10  depicted in  FIG. 1  includes a photoconductor  12 , charging assembly  14 , writing assembly  16 , development assembly  18 , and a transfer assembly  20 . Hard imaging device  10  is configured to form hard images upon media  22 , such as paper or other suitable imaging substrates. Other hard imaging devices  10  may include more, less or alternative components or other arrangements in other embodiments. 
     In one operational embodiment, charging assembly  14  is configured to deposit a blanket electrical charge upon substantially an entirety of an outer surface of photoconductor  12 . Writing assembly  16  is configured to discharge selected portions of the outer surface of the photoconductor  12  to form latent images. Development assembly  18  is configured to provide a marking agent to the outer surface of photoconductor  12  to develop the latent images formed thereon. In one embodiment, the marking agent is a liquid marking agent. Ink particles of the liquid marking agent may be electrically charged to the same electrical polarity as the blanket charge provided to the outer surface of the photoconductor  12  and attracted to the discharged portions of the outer surface of the photoconductor  12  corresponding to the latent images to develop the latent images. The developed images are transferred by transfer assembly  20  to media  22 . 
     Referring to  FIG. 2 , an example of electrical components of hard imaging device  10  is illustrated according to one embodiment. The electrical components include a communications interface  52 , processing circuitry  54 , storage circuitry  56  and device components  58  in one embodiment of hard imaging device  10 . More, less or alternative components are provided in other embodiments of hard imaging device  10 . 
     Communications interface  52  is arranged to implement communications of hard imaging device  10  with respect to external devices (not shown). For example, communications interface  52  may be arranged to communicate information bi-directionally with respect to device  10 . Communications interface  12  may be implemented as a network interface card (NIC), serial or parallel connection, USB port, Firewire interface, flash memory interface, floppy disk drive, or any other suitable arrangement for communicating with respect to device  10 . In one example, image data of hard images to be formed may be received by communications interface  52 . 
     In one embodiment, processing circuitry  54  is arranged to process data, control data access and storage, issue commands, and control imaging operations of device  10 . Processing circuitry  54  may comprise circuitry configured to implement desired programming provided by appropriate media in at least one embodiment. For example, the processing circuitry  54  may be implemented as one or more of a processor and/or other structure configured to execute executable instructions including, for example, software and/or firmware instructions, and/or hardware circuitry. Exemplary embodiments of processing circuitry  54  include hardware logic, PGA, FPGA, ASIC, state machines, and/or other structures alone or in combination with a processor. These examples of processing circuitry  54  are for illustration and other configurations are possible. 
     Processing circuitry  54  is configured to control imaging operations of device  10 , such as the formation and development of latent images upon photoconductor  12 . Processing circuitry  54  may also operate as a control system in some embodiments described below to monitor levels of marking agent within development assembly  18  and to control flow of marking agent from development assembly  18  responsive to the monitoring of the level of the marking agent in the development assembly  18 . As described below, the monitoring and flow control is implemented in one embodiment to reduce the presence of bubbles in the liquid marking agent. 
     The storage circuitry  56  is configured to store programming such as executable code or instructions (e.g., software and/or firmware), electronic data, databases, image data, or other digital information and may include processor-usable media. Processor-usable media may be embodied in any computer program product(s) or article of manufacture(s) which can contain, store, or maintain programming, data and/or digital information for use by or in connection with an instruction execution system including processing circuitry in the exemplary embodiment. For example, exemplary processor-usable media may include any one of physical media such as electronic, magnetic, optical, electromagnetic, infrared or semiconductor media. Some more specific examples of processor-usable media include, but are not limited to, a portable magnetic computer diskette, such as a floppy diskette, zip disk, hard drive, random access memory, read only memory, flash memory, cache memory, and/or other configurations capable of storing programming, data, or other digital information. 
     At least some embodiments or aspects described herein may be implemented using programming stored within appropriate storage circuitry  56  described above and/or communicated via a network or other transmission media and configured to control appropriate processing circuitry. For example, programming may be provided via appropriate media including, for example, embodied within articles of manufacture. In another example, programming may be embodied within a data signal (e.g., modulated carrier wave, data packets, digital representations, etc.) communicated via an appropriate transmission medium, such as a communication network (e.g., the Internet and/or a private network), wired electrical connection, optical connection and/or electromagnetic energy, for example, via a communications interface, or provided using other appropriate communication structure. Exemplary programming including processor-usable code may be communicated as a data signal embodied in a carrier wave in but one example. 
     Device components  58  include additional electrical components of the hard imaging device  10 . For example, device components  58  may include sensors, a pump, motors, a user interface, a level sensor for monitoring a level of marking agent in development assembly  18 , variable valves, and other additional electrical components which may be controlled or monitored by processing circuitry  54 . 
     Referring to  FIG. 3 , additional details of one embodiment of development assembly  18  are shown with respect to one embodiment of a marking agent assembly  30  of hard imaging device  10 . A single arrangement of assemblies  18 ,  30  of  FIG. 3  may be used for monochrome hard imaging devices  10 . In addition, a plurality of the arrangements of assemblies  18 ,  30  of  FIG. 3  may be used for individual ones of the colors of color hard imaging devices  10 . 
     Marking agent assembly  30  is configured to provide marking agent to development assembly  18  during imaging operations. Marking agent assembly  30  includes a reservoir  32  which contains a supply of the liquid marking agent in the presently described embodiment. A sensor  40  is configured to monitor properties (characteristics) such as density, temperature, and conductivity of the liquid marking agent in reservoir  32 . A sensor  42  may be used to calibrate sensor  40 . As discussed in the above-mentioned US patent application, it is desired in one embodiment to reduce the presence of bubbles in the liquid marking agent during calibration operations of sensor  40 . 
     A pump  34  is provided to transport the liquid marking agent from reservoir  32  via a supply hose  36  to a chamber  37  of development assembly  18 . Development assembly  18  may contain a roller  38  or other appropriate device for providing the liquid marking agent from the chamber  37  to the outer surface of photoconductor  12  to develop latent images. Unused marking agent is returned from chamber  37  to reservoir  32  via a return hose  39  in the depicted embodiment. Supply hose  36  may be referred to as a supply path and return hose  39  may be referred to as a return path in one embodiment. Other configurations of supply and return paths are possible. 
     As described in the example embodiments of the above-mentioned US patent application, bubbles may be caused by pumping operations of pump  34  during transporting of liquid marking agent from reservoir  32  to development assembly  18  and its return to the reservoir  32 . The US patent application mentioned above discloses methods and apparatus for reducing the presence of the bubbles in the liquid marking agent by cycling the pump  34  on and off and altering an operational frequency of the pump  34  during calibration operations. The disclosure of the present application provides additional apparatus and methods for reducing the presence of bubbles in the liquid marking agent during imaging operations of the hard imaging device  10  while hard images are being formed upon media. In one embodiment, at least some of the methods and apparatus of the above-mentioned US patent application and at least some of the methods and apparatus of the present disclosure may be combined and implemented in a single hard imaging device  10 . In other embodiments, only one of the methods and apparatus of the above-mentioned US patent application or the methods and apparatus of the present disclosure are implemented in a given hard imaging device  10 . 
     As mentioned previously, liquid marking agent is provided by the supply path from reservoir  32  to development assembly  18  and unused liquid marking agent is returned by the return path to reservoir  32  during imaging operations. The solid ink particles of the liquid marking agent are concentrated by development assembly  18  to develop latent images in one embodiment. For example, the solid ink particles may be electrically charged in one embodiment and attracted to the latent images on the photoconductor  12 . Unused and re-diluted ink is returned to reservoir  32  where the solids and other constituents used to develop the images are reintroduced at proper concentrations. 
     Liquid marking agent transported from development assembly  18  to reservoir  32  may include air in the form of bubbles. As mentioned previously, the presence of bubbles in the liquid marking agent during imaging operations is problematic inasmuch as imaging problems may result when the liquid marking agent is pumped into the development assembly  18 . For example, imaging problems which may negatively affect print quality include voids in the concentrated ink layer applied to the photoconductor  12  due to the bubbles and which may result in voids in developed images. Without the presence of bubbles, the concentrated ink layer corresponding to the developed image should be relatively void free. 
     As mentioned above, movement of the liquid marking agent through the development assembly  18  and its return to reservoir  32  generates bubbles in the liquid marking agent. A major source of bubbles is collision of marking agent which was returned via hose  39  with the marking agent already present in reservoir  32  and by the flow of liquid marking agent around various components (e.g., rollers) inside the development assembly  18 . At least some embodiments of the present disclosure are directed towards reducing air entrained by liquid marking agent leaving chamber  37  and entering hose  39  to reduce the presence of bubbles in the liquid marking agent during imaging operations including development of latent images. 
     In one configuration of hard imaging device  10 , development assembly  18  is placed elevationally above reservoir  32  and a siphoning effect resulting from gravity is created during flow of liquid marking agent from chamber  37  to reservoir  32 . Observation of liquid marking agent within chamber  37  and hose  39  reveals that air is entrained in the liquid marking agent after flow of the liquid marking agent is established in hose  39  in one embodiment. This described example is related to a siphon effect where falling fluid within the confines of hose  39  is pulled downwardly faster than without the presence of the confinement (i.e., hose  39 ). 
     In one embodiment, when liquid marking agent first arrives at development assembly  18 , liquid marking agent fills chamber  37  until a steady state ink level is established in development assembly  18 . The rising level creates a pressure or “head” which causes a flow rate of marking agent outgoing via hose  39  and a flow rate of marking agent incoming via hose  36  to be the same. If the outgoing rate of liquid marking agent is slower than a rate of incoming marking agent, for example, the level of the marking agent rises in chamber  37  until the increased pressure is sufficient to equalize the rates. However, once siphoning action starts in hose  39  following the introduction of marking agent to hose  39 , the outgoing rate increases and the level of marking agent in chamber  37  decreases. Eventually, the level of marking agent present in chamber  37  drops below an outlet opening at an interface  41  of chamber  37  and hose  39  and air is sucked into the liquid marking agent in hose  39  leading to the formation of bubbles. 
     In example embodiments of the disclosure, apparatus and methods are described to reduce the formation and presence of bubbles in the liquid marking agent being returned to reservoir  32 . As described below in some illustrative embodiments, a bubble reduction apparatus is configured to perform bubble reduction operations to reduce the formation and presence of bubbles in the liquid marking agent compared with arrangements wherein the bubble reduction operations are not performed. 
     In one embodiment, the level of liquid marking agent may be stabilized in chamber  37  if flow rates of liquid marking agent in hose  39  may be adjusted (e.g., restricted). In one configuration, the bubble reduction apparatus is configured to perform a bubble reduction operation comprising selectively restricting flow of the liquid marking agent in the hose  39  to prevent the level of marking agent in the chamber  37  from dropping below interface  41  wherein air is entrained in the liquid marking agent entering hose  39 . In one embodiment, the flow of the liquid marking agent in hose  39  is restricted in response to an increase in flow of the liquid marking agent within hose  39 . In one embodiment, the size of an aperture of the return path is varied to selectively restrict the flow of the liquid marking agent. 
     As mentioned above, a siphon action is created during transport of the liquid marking agent in hose  39 . The siphon action is created at a moment in time following the initial introduction of liquid marking agent into chamber  37  and hose  39 . In one embodiment, methods and apparatus are disclosed for controlling the flow rate of liquid marking agent in hose  39  before and/or after a moment in time when the siphoning action starts. 
     In one embodiment, the steady state outgoing flow rate of liquid marking agent in hose  39  is roughly three times the flow rate before siphoning action starts. In one embodiment, it is desired to provide additional restriction of flow of the liquid marking agent in hose  39  after the siphon action has started to reduce or minimize suction of air into the liquid marking agent entering hose  39 . In some example embodiments, the level of liquid marking agent is stabilized in chamber  37  (e.g., the level is above interface  41  and air suction is avoided) if one or more opening in the return path for transporting liquid marking agent to reservoir  32  is reduced to approximately 30% of the opening(s) size prior to creation of the siphon action. However, in one embodiment, the restricted size of the opening is not implemented prior to the siphon action inasmuch as a level of liquid marking agent in the chamber  37  may rise rapidly and spill out of the development assembly  18 . Accordingly, in one embodiment, the bubble reduction apparatus is configured to implement variable opening(s) in the return path for transporting liquid marking agent from chamber  37  to reservoir  32  wherein additional restriction is provided after the siphon action has started compared with restriction prior to starting of the siphon action. 
     The state of the bubble reduction apparatus before the presence of the siphon action may be referred to as a non-restricted state (i.e., providing substantially no or comparably less flow of the marking agent in hose  39 ) while the state of the bubble reduction apparatus after the siphon action starts may be referred to as a restricted state (i.e., providing additional restriction to flow of the marking agent compared with the non-restricted state). 
     Referring to  FIGS. 4 and 5 , a bubble reduction apparatus  60  in the form of a siphon-induced flow restrictor  62  which may be provided in line with return hose  39  is shown. Referring to  FIG. 4 , apparatus  60  may be positioned at interface  41  ( FIG. 3 ) in one configuration where an end  65  is coupled with chamber  37  and an opposite end  66  is coupled with return hose  39  to provide flow of liquid marking agent in the illustrated direction  67 . Apparatus  60  may be positioned at other locations of the return path configured to return unused marking agent from development assembly  18  to reservoir  32  in other embodiments. 
     Still referring to  FIGS. 4 and 5 , end  65  includes a plurality of openings  64 ,  72 . One or more openings  64  are located radially outward from opening  72 . In the depicted embodiment, two openings  64  are located symmetrically about opening  72 . A stopper  70  is provided to at least partially plug opening  72  and to reduce flow of liquid marking agent through apparatus  60  following initiation of the siphon action. In the depicted embodiment, a stop  69  internal to the apparatus  60  is fixed in position relative to a housing of apparatus  60 . A spring  68  is connected with the stop  69  and stopper  70 . During operation and prior to the creation of the siphon action, spring  68  is configured to urge the stopper  69  upwardly in a non-blocking orientation as shown in  FIG. 4  permitting liquid marking agent to freely flow through opening  72 . Prior to the occurrence of the siphon action, liquid marking agent may flow in parallel through openings  64 ,  72 . 
     However, once the siphon action is initiated, the flow of liquid marking agent through apparatus  60  is restricted. In particular, spring  68  is configured such that the increased pressure resulting from an increased rate of flow of the marking agent through hose  39  caused by the siphon action overcomes the force of the spring  68  and causes the stopper  70  to obstruct opening  72  reducing or precluding flow of liquid marking agent through opening  72  while liquid marking agent continues to flow through openings  64  in the described embodiment. In one embodiment, opening  72  has a diameter of approximately ¾″ and two openings  64  are provided with individual dimensions of approximately ⅛″ by ⅜″ for use with an outlet orifice size at end  66  of approximately ⅝″ and wherein hose  39  has a length of approximately 1 meter. Openings  64 ,  72  of different sizes or numbers may be used in other embodiments. As mentioned above, the flow rate of liquid marking agent through apparatus  60  is reduced by approximately 70% following initiation of the siphoning action in one embodiment. 
     Following the ceasing of delivery of marking agent to chamber  37  by hose  36  and the draining of marking agent within chamber  37  and hose  39  to reservoir  32 , the spring  68  is configured to again urge the stopper  70  upwardly to a non-restricting position. 
     Referring to  FIG. 6 , another embodiment of a bubble reduction apparatus  60   a  is shown in the form of a siphon-induced flow restrictor  62   a  implemented within a housing  80  of development assembly  18  which defines chamber  37 . In the depicted embodiment, a lower portion of housing  80  which is configured to receive and collect unused liquid marking agent in chamber  37  is shown. Return hose  39  is coupled with a lower portion of housing  80  at interface  41  in the depicted embodiment. Return hose  39  receives unused marking agent via an exit opening  94  from chamber  37  and transports the unused marking agent to reservoir  32  (not shown in  FIG. 6 ) in one embodiment. 
     Marking agent is introduced from hose  36  to an upper portion  82  of development assembly  18  during imaging operations to form hard images including development of latent images. Some of the marking agent is applied by roller  38  to a surface of photoconductor  12  (roller  38  and photoconductor  12  are shown in one embodiment in  FIG. 3 ) to develop latent images. Unused marking agent flows downwardly from upper portion  82  as indicated by arrows  84 . 
     In the depicted embodiment, a divider  81  which includes an opening  92  is positioned above exit opening  94 . Divider  81  extends laterally between the left and right portions of housing  80  but a gap is provided between divider  81  and either a front or rear wall of the housing (the front or rear wall is not shown in  FIG. 6 ) which permits some marking agent to flow as indicated by arrows  88  around the divider  81  into chamber  37 . 
     In addition, marking agent may also flow through opening  92  into chamber  37  as indicated by arrow  86  prior to creation of a siphon action in return hose  39 . For example, in the depicted arrangement, a stopper  90  in the form of a floating ball is positioned above opening  92  and a spring  91  is coupled with an interior wall of housing  80  and is configured to provide stopper  90  in the spaced position relative to opening  92  prior to the creation of the siphon action. 
     As mentioned above, a siphon action is created in hose  39  during transporting of the marking agent within hose  39  to reservoir  32 . A flow rate of the marking agent in hose  39  is increased by the siphon action and which creates a suction force which overcomes the force of spring  91  and pulls stopper  90  into a blocking position  90   a  with respect to opening  92  and which produces increased restriction of flow of marking agent via hose  39  to reservoir  32  compared with flow of the marking agent with stopper  90  spaced from opening  92 . Marking agent may continue to flow as indicated by arrows  88  around divider  81  during the presence of the siphon action and while stopper  90  is located at position  90   a.    
     Following the ceasing of delivery of marking agent to chamber  37  by hose  36  and the draining of marking agent within chamber  37  and hose  39  to reservoir  32 , the spring  91  is configured to again move the stopper  90  upwardly to a non-restricting position. 
     Referring to  FIGS. 7 and 7   a , a bubble reduction apparatus  60   b  in the form of another embodiment of a siphon-induced flow restrictor  62   b  is shown. Restrictor  62   b  is positioned in line with return hose  39  and may be located at interface  41  or at other locations of the return path in illustrative examples. A direction of marking agent flow through apparatus  60   b  is indicated by arrow  106 . 
     Referring to  FIG. 7 , apparatus  60   b  includes a tubular housing  100  and a flap  102  in the depicted embodiment. Flap  102  is supported by a hinge which permits flap  102  to pivot. A spring  104  is coupled with flap  102  at coupling  105  and housing  100  at coupling  107  and is configured to provide the flap  102  at the “open” position as depicted in  FIG. 7  prior to the creation of the siphon action within return hose  39 . In the open position, the flap  102  is angled relative to the flow  106  so that a higher flow force is imparted on the upper portion of the flap  102 . 
     Referring to  FIG. 7   a , operation of apparatus  60   b  is described following the creation of the siphon action within return hose  39 . In the illustrated embodiment, the suction force overcomes the force of spring  105  and moves flap  102  to the “closed” position  102   a  as depicted. The closed position  102   a  of flap  102  provides a smaller aperture through apparatus  60   b  compared with the open position of flap  102  shown in  FIG. 7 . Accordingly, flow of marking agent through apparatus  60   b  has additional restriction in the configuration of apparatus  60   b  shown in  FIG. 7   a  compared with  FIG. 7 . 
     The above-described apparatus  60 ,  60   a ,  60   b  are illustrative passive embodiments which are configured to provide selective restricted flow of the liquid marking agent in the return path in the presence of the siphon action in the return path without monitoring circuitry or external control circuitry. In other embodiments of the disclosure, active and passive/active hybrid arrangements of bubble reduction apparatus are described. For example, in one passive/active hybrid configuration, the apparatus  60   b  of  FIGS. 7 and 7   a  may include a sensor (e.g., one of device components  58  of  FIG. 2 ) and control system (processing circuitry  54  of  FIG. 2 ) configured to sense a level of marking agent within chamber  37 . Following the provision of flap  102  at the closed position  102   a  of  FIG. 7   a  by a sufficient suction force, the control system may control the position of flap  102  and leakage of marking agent around flap  102  responsive to monitoring of the level of the liquid marking agent in chamber  37  in one embodiment. For example, in one embodiment, apparatus  60   b  may include an electronically controlled lead screw to control the position of flap  102  and to vary a partial opening defined by flap  102  and housing  100  while apparatus  60   b  is in a restricted state of operation. The control may be referred to as fine tune control to provide leakage about flap  102  to account for flow variations of the marking agent within the return hose  39 . The control system is configured to maintain a desired amount (e.g., a substantially constant height of marking agent) within chamber  37  which is sufficient to avoid the introduction of air into return hose  39 . 
     In another embodiment, the bubble restriction apparatus may be active wherein the control system monitors a height of the marking agent in chamber  37  before and after the creation of the siphon action and controls a variable valve (e.g., butterfly valve) in the return path to selectively restrict the flow of the marking agent in the return path to provide and maintain a substantially constant level of marking agent within chamber  37  responsive to the monitoring. Active or hybrid arrangements of the bubble restriction apparatus may have some advantages over some passive systems including accounting for flow variations of marking agent within return hose  39 . Hybrid systems similar to the apparatus  60   b  of  FIGS. 7 and 7   a  have advantages in some arrangements by providing a robust passive arrangement for quickly transitioning from a non-restricted state to a restricted state at the time of creation of the siphon action while also providing monitoring and control for flow variations occurring during the restricted state of operation. 
     Aspects herein have been presented for guidance in construction and/or operation of illustrative embodiments of the disclosure. Applicant(s) hereof consider these described illustrative embodiments to also include, disclose and describe further inventive aspects in addition to those explicitly disclosed. For example, the additional inventive aspects may include less, more and/or alternative features than those described in the illustrative embodiments. In more specific examples, Applicants consider the disclosure to include, disclose and describe methods which include less, more and/or alternative acts than those methods explicitly disclosed as well as apparatus which includes less, more and/or alternative structure than the explicitly disclosed structure. 
     The protection sought is not to be limited to the disclosed embodiments, which are given by way of example only, but instead is to be limited only by the scope of the appended claims.