Description

BACKGROUND

[001] When putting into operation a printer, e.g. a new printer or a printer which had been out of service for a certain time, or a new printhead is installed, it may be advis able to purge a printing fluid delivery system of the printer or a printhead chamber be fore use to ensure that the printing fluid delivery system and the printhead are filed with a continuous volume of printing fluid and that the printing fluid is in an appropri ate condition.

DESCRIPTION OF DRAWINGS

[002] The following detailed description will best be understood with reference to the drawings, wherein:

[003] Fig. 1 shows a schematic diagram of a printing fluid delivery system of a printer according to an example;

[004] Fig. 2 shows a schematic diagram of a printing fluid delivery system of a printer according to another example;

[005] Fig. 3 shows a schematic diagram of the printing fluid delivery system of Fig. 1 in combination with a printhead and a pressure regulator according to an example;

[006] Fig. 4 shows a schematic diagram of the printing fluid delivery system of Fig. 2 in combination with a purge unit according to an example; [007] Fig. 5 shows a schematic diagram of the printing fluid delivery system of Fig. 2 in combination with a purge unit according to an example;

[008] Fig. 6a and 6b show diagrams of a flow rate and a pressure differential over time during a purging process according to an example;

[009] Fig. 7 shows a flow diagram of a method of monitoring a fluid delivery system according to an example;

[0010]Fig. 8 shows a diagram of a pressure over time during a purging process ac cording to an example;

[0011 ]Fig. 9 shows a flow diagram of a method of monitoring a fluid delivery system according to an example;

[0012]Fig. 10 shows a diagram of a pressure over time during a purging pro cess according to an example;

[0013]Fig. 11 shows a flow diagram of a method of monitoring a fluid delivery system according to an example;

DESCRIPTION OF EXAMPLES

[0014] The following disclosure provides many different examples, for implementing different features of the disclosed subject matter. Specific examples of components, values, operations, materials, arrangements, or the like, are described below to simplify the present disclosure. These are merely examples and are not intended to be limiting. Other components, values, operations, materials, arrangements, or the like, are contemplated. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity, respective components designated by the same reference numerals may be implemented and may operate in an identical or similar way, without being bound to this.

[0015]Fig. 1 shows a schematic diagram of a printing fluid delivery system 100 of a printer architecture according to an example. In the example described, the printing fluid may be an ink, such as a color ink, including CMYK inks, and white ink. The ink may be a latex ink or another type of ink. In other examples, the printing fluid can be a type of conditioning fluid used in inkjet type printers, including 2D and 3D printers. The printing further may be any type of printable liquid. The printer may be, may in clude, or may be part of a desktop printer, a large format printer, a plotter or the like, for example. The printer may be a 2D printer or a 3D printer. Accordingly, in the fol lowing description, the printing fluid sometimes is referred to as ink, with the under standing that other types of printing fluids may be used.

[0016]ln the example of Fig.1 , the printing fluid delivery system 100 comprises a sup ply tank 110, an intermediate tank 120, a fluid interconnect 130, a controller 140, a fluid pump 150 and an air pressure pump or air pressure source 160. The supply tank 110 is shown as part of the printing fluid delivery system but also can be a compo nent external to the printing fluid delivery system 100 which is connected thereto but, as such, may not be part of the printing fluid delivery system. Whereas, the following description refers to the supply tank 110 as part of the printing fluid delivery system, the supply tank 110 may be a replaceable resource to be connected to the print fluid delivery system. The fluid interconnect 130 may provide a connection to a printhead inserted in a printer. A fluid channel portion 122 connected to an output of the inter mediate tank 120 merges with a fluid channel portion 112 connected to an output of the fluid pump 150 in a common fluid channel portion 132 connected to the fluid inter connect 130. The controller 140 may be programmed to control operation of the fluid pump 150, the air pressure source 160, the sensors and valves and to monitor opera tion thereof, as explained in further detail below.

[0017]A printhead may be supported in the printer by a printhead tray or a printhead carriage or the like which may include bays to receive and connect the printheads. Such bays may include power and signal ports to be connected to a printhead and fluid interconnects such as the fluid interconnect 130 shown in Fig. 1.

[0018]The fluid pump 150 may be associated with a drive motor 152 and a pressure relief valve 154. The drive motor 152 may e.g. be a DC motor controlled by a drive voltage, e.g. a PWM modulated drive voltage. The pressure relief valve 154 may be designed to prevent the pressure in the fluid delivery system from rising above a cut off pressure which could damage components of the fluid delivery system. Such cut off pressure may be in the range of 5 to 7 psi or about 35 to 50 kPa, for example. [0019] In this example, the intermediate tank 120 can include a variable fluid volume to contain a supply of printing fluid and a variable gas volume to receive pressurized gas, such as air, to pressurize the supply of printing fluid to feed the printing fluid from the intermediate tank 120. The variable fluid volume may be contained in a collapsi ble fluid reservoir, such as a collapsible ink bag, for example. The variable gas vol ume may be contained in a fluid tank container surrounding the collapsible ink bag and may be separated from the variable fluid volume by the bag material. In another example, the variable fluid volume and the variable gas volume may be contained in a common fluid tank container and be separated by a flexible membrane. The varia ble fluid volume and the variable gas volume are arranged relative to each other in such a way that they are separated but pressure applied to the gas volume can be transferred to the fluid volume and vice versa.

[0020]The supply tank 110 can include a fluid volume larger than that of the interme diate tank 120 to refill the intermediate tank during printing. For example, the supply tank 110 may have a fluid volume of several liters, such as about 2 L, 3 L or 5 L, to contain a printing fluid supply. The intermediate tank may have a smaller maximum fluid volume, e.g. a volume of less than a liter, such as a maximum fluid volume of about 500 ml_ or 700 mL or 750 ml_.

[0021]The fluid pump 150 may be designed to generate a flow rate of the printing fluid sufficient to refill the intermediate tank 120 during printing, i.e. a flow rate which is the same or larger than a maximum fluid flow rate from the intermediate tank 120 to the fluid interconnect 130. In an example, the flow rate generated by the fluid pump 150 may be in the order of 30 to 300 mL/minute or 40 to 200 mL/min, e.g. about 150 mL/min. The maximum flow rate from the intermediate tank 120 to the fluid intercon nect 130 may be in the order of 60 to 200 mL/min, depending on the rate at which printing fluid is ejected from the printhead. Depending on the type and size of the printer architecture, print head and the application, tank volumes and fluid flows may vary.

[0022]A fill state of the fluid tanks, such as the intermediate tank or the supply tank, can be measured using a fluid level sensor. The fluid level sensor can be a physical sensor provided in the tank or can be a differential pressure sensor, for example. In this example, the fluid level sensor of the intermediate tank 120 is implemented in a pressure differential senor 170 comprising an input connected to an air pressure sup ply line 162 in communication with the variable gas volume of the intermediate tank 120 and an input connected to the fluid channel portion 122 at the output of the inter mediate tank 120, as shown in Fig. 1. The differential pressure sensor 170 may oper ate based on a pressure difference between the air pressure supplied to the interme diate tank 120 and the fluid pressure in the fluid channel portion 122.

[0023]Printing fluid may be transferred from the intermediate tank 120 to the fluid in terconnect 130 using the air pressure pump or air pressure source 160 (in the follow ing, sometimes referred to as a pressure source), for example. Instead of air, another gas, e.g. an inert gas, can be used to generate a gas pressure on the intermediate tank 120. Accordingly, in the following description, reference to air should include also reference to another type of gas to be provided by the air pressure source 160. The intermediate tank 120 may act as a buffer of printing fluid, and during normal printing operation is pressurized with air or another gas using the pressure source 160 to supply printing fluid to the fluid interconnect 130 and eventually to the print- head, see Fig. 3, for example.

[0024]The pressure source 160 operates by pressurizing the air volume inside the in termediate tank 120 and above or around the fluid volume, by applying an air pres sure which cycles between a lower_threshold_pressure and an upper_thresh- old_pressure. For example, the air pressure may be increased to the upper_thresh- old_pressure by activating the pressure source 160 to force part of the fluid volume out of the intermediate tank 160 and towards the fluid interconnect 130. The pressure source 160 is deactivated upon reaching the upper_threshold_pressure, and the air pressure will decrease due to part of the fluid being discharged from the intermediate tank 120. When the pressure reaches the lower_threshold_pressure, the air pressure source 160 may be again activated to increase the air pressure until it reaches the upper_threshold_pressure. This cycle may be repeated as long as the printer is oper ating to deliver printing fluid to the fluid interconnect 130. At the end of a print job or in a printing pause, the system can be depressurized.

[0025]Further, printing fluid may be transferred from the supply tank 110 to the inter mediate tank 120 using the fluid pump 150 to refill the intermediate tank 120 upon demand. A refill operation may be triggered by the differential pressure sensor 170, for example. The fluid pump 150 may be a different type of pump, such as a DC mo tor driven fluid pump. For example, a volumetric pump may be provided, the pump having specific flow rates at respective RPMs. As another example, a centrifugal pump may provide specific flow rates at respective RPMs.

[0026]For example, when the printing fluid is consumed from the intermediate tank 120, the intermediate tank 120 may be refilled from the supply tank 110 using the fluid pump 150, which pushes printing fluid into the intermediate tank 120. In one ex ample, the refill operation is continued until the differential pressure sensor 170, measuring fluid vs. air pressure, detects an end of refill process when the intermedi ate tank 120 is considered full. The end of refill process can be detected by correla tion of the differential pressure with a look up table of pressure values vs. fluid level, for example.

[0027]ln addition, as shown in Fig. 1 , an air pressure sensor 164 can be provided for monitoring the pressure source 160 and, further, an air relief valve 166 can be pro vided for depressurizing the fluid delivery system 100 and preventing the pressure in the fluid delivery system from rising above a level which could damage components of the fluid delivery system.

[0028]The pressure sensor 170 also can be configured to measure the absolute pressure in the fluid channel portion 312, e.g. an absolute pressure which is meas ured against ambient pressure. For this, the air input of the pressure sensor 170 can be connected to ambient atmosphere, instead of being connected to the air pressure supply line 162. For example, the air inlet of the pressure sensor 170 can be de signed to be switchable between different inputs, e.g. an input connected to air pres sure supply line 162 and an input connected to atmosphere.

[0029]The printing fluid delivery system 100 further can be configured to bypass the intermediate tank 120 and directly feed printing fluid from the supply tank 110 to the fluid interconnect 130. This can be achieved by pressurizing the intermediate tank 120 up to a pressure value above the highest pressure to be expected in the com mon fluid channel portion 132, under normal operating conditions. This will be ex plained in further detail with reference to Fig. 5 below. In this configuration, the fluid pressure sensor 170 may be decoupled from the air pressure in air pressure line 162 to operate as an absolute pressure sensor, i.e. a sensor measuring the pressure in the printing fluid channel against atmospheric pressure.

[0030]The fluid pump 150, the air pressure source 160, the air pressure sensor 164 and the air pressure sensor 164 and the differential pressure senor 170 as well as other components to be controlled or monitored in the fluid delivery system 100 are communicatively coupled to the controller 140 wherein communication can be wire less or wired to control operation of these components and to receive feedback sig nals from the sensors 164, 170 to thus control operation of the fluid delivery system 100. For ease of illustration, communicative coupling between the controller 140 and other components of the fluid delivery system 100 is not illustrated in the drawings and, in some drawings, the controller 140 may be omitted. Moreover, the fluid deliv ery system 100 and an associated printhead (not shown in Fig. 1) can be coupled to the same controller or different controllers to control a printing operation and any an cillary operations. The controller 140 can be a single control system, a distributed control system and can be implemented in hardware, firmware, software and combi nations thereof.

[0031 ]Fig. 2 shows a schematic diagram of another example of a fluid delivery sys tem 200 in a printer architecture according to an example. The same components as in Fig. 1 are designated by the same reference numbers. With regard to the type of printing fluid, volume of tanks, flow rates, pump types, controller operation and cou pling and other details of corresponding components and operation of the fluid deliv ery system, reference is made to the description of Fig. 1 above.

[0032]ln the example of Fig.2, the printing fluid delivery system 200 comprises a sup ply tank 110, a fluid interconnect 130, a controller 140, a fluid pump 150 and a pres sure sensor 210. The supply tank 110 also can be a component external to the print ing fluid delivery system 200 which is connected thereto but, as such, is not part of the printing fluid delivery system. The fluid interconnect 130 may provide a connec tion to a printhead inserted in the printer. The supply tank 110 and the fluid pump 150 may be configured as described above with reference to Fig. 1. The fluid delivery sys tem 200 of Fig. 2 is one which operates without intermediate tank, directly feeding printing fluid from the supply tank 110 to the fluid interconnect 130 through a printing fluid channel 212. The pressure sensor 210 can measure the pressure in the printing fluid channel 212, e.g. an absolute pressure which is measured against ambient pressure. The fluid pump 150 can be controlled to feed printing fluid to the fluid inter connect 130 by controlling a voltage applied to the fluid pump 150, e.g. using a pulse width control scheme, and the fluid flow can be monitored by the pressure sensor 210.

[0033]Whereas, Fig. 1 and 2 show printing fluid delivery systems 100, 200 including a single supply tank 110, intermediate tank 120 and fluid interconnect 130, a printing architecture may comprise a plurality of supply tanks, intermediate tanks and/or fluid interconnects, e.g. one for each color of Black, Cyan, Magenta and Yellow inks and possible additional inks and other fluid, e.g. a pre- or post-treatment fluid. The num ber of fluid interconnects may be different from the number of printheads connected thereto because a single printhead may be able to eject more than one color or type of ink and hence may be connected to more than one fluid interconnect.

[0034]ln a printer architecture designed for multiple types of ink, such as BCMY inks and conditioning fluids for example, a separate ink pump may be provided in respec tive separate fluid lines between respective supply tanks and respective intermediate tanks for each type of ink. Each fluid line may be connected to a respective fluid pres sure sensor. Further, a single pressure source or multiple pressure sources may be connected to each one of the respective intermediate tanks, with a single differential pressure sensor connected to the air pressure line downstream of the air pressure source or with multiple differential pressure sensor connected to air inlets of the re spective intermediate tanks.

[0035]Accordingly, the controller 140 may be programmed to separately control and monitor the fluid delivery system for each color or each type of printing fluid. For ex ample, the printer architecture may comprise a number of pairs of a print supply tank and an intermediate tank respectively in fluid communication via an associated pump, each pair and associated pump dedicated to a selected fluid type or color, wherein the controller is further programmed to independently control the fluid pumps dedi cated to different fluid types or colors. A diagnosis may be performed individually for each of different color inks or different fluid types.

[0036]Fig. 3 shows a schematic diagram of the printing fluid delivery system 100 of Fig. 1 in combination with a printhead 300 and a pressure regulator 350, according to an example. The pressure regulator 350 may be part of a service station (not shown) of a printer and may be part of a primer. A service station further may comprise a wiper, spittoon and the like, for priming and servicing printhead nozzles and spitting waste ink.

[0037]The printhead 300 of this example includes two printing fluid chambers or res ervoirs 310, 320, for ease of description also referred to as printhead chambers, each provided for a different type of printing fluid, e.g. for different color inks, wherein re spective fluid levels of printing fluids are shown at 312, 322. The two printhead cham bers 310, 320, in the drawing, also are designated as Even and Odd, referring to odd and even print nozzle rows (not shown) of the printhead. The print head 300 com prises a first fluid port 314 and a second fluid port 324. In this example, the first fluid port 314 and hence the first printhead chamber 310, designated as Even chamber, are in fluid communication with the fluid distribution system 100 and connected to the fluid interconnect 130, for example. The second fluid port 324 and hence the second printhead chamber 320, designated as Odd chamber, can be used in combination with a further fluid distribution system (not shown). The printhead 300 further may comprise a nozzle plate (located at the downward facing surface of the printhead 300, not shown) to eject printing fluid from the printhead chambers 310, 320. The printhead 300 further may be received in a carriage or other printhead support in the printer.

[0038]ln the following, the printhead 300 is described with reference to the first print- head chamber 310 and associated components wherein the second printhead cham ber 320 may be configured, operated and monitored in a corresponding way.

[0039]The first fluid port 314 is in fluid communication with the interior of the print- head chamber 310 via a fluid pipe 316 and a regulator valve 318 which may be a switchable regulator valve. The regulator valve 318 comprises a stopper that is to block the outlet of the fluid pipe 316 into the printhead chamber 310 wherein the reg ulator valve 318 can be opened and closed, as described below.

[0040] In some examples, the regulator valve 318 may be coupled to the pressure regulator 350 of the service station such that the regulator valve can be actuated or switched using the pressure regulator 350. The regulator valve 318 may be actuated via an inflatable bag 352 in the printhead chamber 310, which may be inflated or de flated to adjust a pressure inside the printhead chamber 310. To this end, the inflata ble bag 352 may be connected selectively to ambient pressure and to an air pressure source 354 of the pressure regulator 350. The inflatable bag 352 further may be as sociated with pressure plates and a spring mechanism (not shown) to bias the pres sure plates against the inflatable bag 352 to control the regulator valve as a function of the internal pressure of the printhead chamber 310. Accordingly, the inflatable bag 352 may be inflated by (i) applying an external pressure thereto, via the pressure reg ulator 350, or by (ii) a corresponding pressure drop between ambient pressure ap plied to the inflatable bag 352 and the internal pressure of the printhead chamber 310 when the fluid level in the printhead chamber is falling. The first scenario (ii) may cor respond to a servicing task, such as priming or purging of the printhead, and the lat ter scenario (ii) may correspond to an ongoing printing operation.

[0041] When the inflatable bag 352 is inflated, the pressure inside the camber 310 will increase and the regulator valve 318 will be pressed away from the mouth of the fluid pipe 316, allowing printing fluid to flow into the printhead chamber 310. When the inflatable bag 352 is deflated, pressure inside the printhead chamber 310 de crease and the regulator valve 318 can be closed again. Additionally, the pressure regulator 350 may comprise a relief valve 356 which opens at a defined maximum pressure to avoid components of the printhead 300 being damaged due to an over pressure being applied to the inflatable bag 552 inside the printhead 300.

[0042] Between the mouth of the fluid pipe 316 and the nozzle plate (not shown) of the printhead 300, there further may be a filter element 330 which keeps any debris and particles in the printing fluid from reaching the nozzle plates. The filter element 330 may for example comprise or consist of a porous material or membrane.

[0043]New printheads may be delivered with a printhead chamber which is prefilled from a manufacturer with a shipping fluid. The shipping fluid may be designed for long-term storage and transport of printheads. Before printing, the shipping fluid is purged from the printhead chamber and the printhead chamber is filled with printing fluid. This can be done, for example, during a first printhead insertion process. The shipping fluid may be purged from the printhead chamber by moving the printhead to a service station which may include the pressure regulator 350, shown in Fig. 3, and further servicing components, such as a wiper and a spittoon, not shown in the draw ings. Pressure can be applied from the pressure regulator 350 to the inflatable bag 352 to pressurize the shipping fluid in the printhead chamber 310 and to open the regulator valve 318 so that printing fluid can flow from the fluid interconnect 130 through the fluid pipe 316 into the printhead chamber 310. The printing fluid will be fed from the initially full intermediate tank 120 by pressurizing the air in the intermedi ate tank 120 using the air pressure source 160, as explained above. Printing fluid continues to be fed from the intermediate tank 120 to the printhead 300 until all ship ping fluid in the printhead chamber 310 has been replaced by printing fluid. The purg ing process may be performed continuously or in cycles of activation and deactivation of the pressure regulator 350, as described in further detail below.

[0044]Several factors may affect the time and number of cycles to complete the purging process, i.e. to complete replacing all of the shipping fluid by printing fluid. These factors may include properties of the shipping fluid and the printing fluid, such as viscosity and density, temperature, hardware variability of the printing fluid delivery system and the like. Measuring the time or the number of purging cycles hence may not yield reliable information about the status of the purging process. Depending on circumstances, it may happen that not all of the shipping fluid is replaced by printing fluid so that a user may start printing with shipping fluid still being present in the print- head which may cause image quality and printhead reliability issues if shipping fluid is fired instead of printing fluid. It also may happen that, to be on the safe side, the purging process continues after all shipping fluid has been replaced which would cause waste of printing fluid and create additional costs.

[0045]The status and, in particular, be end of the purging process could be moni tored by visual inspection, asking a user to perform a diagnostic plot and, depending on the result, e.g. if there were image quality issues which could signal that there still is shipping fluid in the printhead, ask the user to run an additional purging process to complete removing any shipping fluid remaining in the printhead chamber. This, how ever, could lead to additional waste of printing fluid and worsens the overall customer experience. In the printing fluid delivery system of Fig. 1 and 3, the differential pres sure sensor 170 can be used to determine a status of the purging process. [0046]As explained above, in a printing fluid delivery system 100 having an interme diate tank 120, the fluid pressure to feed the printing fluid from the intermediate tank 120 to the fluid interconnect 130 and eventually to the printhead 300 is generated by the air pressure source 160 to pressurize the air inside the intermediate tank 120 above or around the ink volume therein. The intermediate tank 120 is refilled from the supply tank 110 when it starts emptying wherein the differential pressure sensor 170 is used to determine the fill level of the intermediate tank 120. As in a printing opera tion, when shipping fluid is to be purged from the printhead chamber 310, pressurized air is supplied from the air pressure source 160 to the intermediate tank 120 also in a purging process. In this case, as explained above, the pressure regulator 350 is acti vated to inflate the inflatable bag 352 of the printhead chamber 310 be purged. Once the inflatable bag 352 is inflated, the regulator valve 318 will open and there will be continuous flow of printing fluid from the intermediate tank 120 to the fluid intercon nect 130, the printhead chamber 352 and out of the printhead 300 through the asso ciated nozzle plate. This will move the shipping fluid out of the printhead chamber 310 and replace it with printing fluid.

[0047]Assuming that the intermediate tank 120 is full and the printing fluid delivery system 100 is feeding printing fluid of one type and the same type of printing fluid is also present in the printhead 300, the purging process will create a constant flow of printing fluid and the printing fluid pressure present in the ink channel portions 122, 132 and at the input of the differential pressure sensor 170 will correspond to the air pressure generated by the air pressure source 160. The differential pressure sensor 170 therefore will generate an output signal of zero (0). This, however, is different when the printhead chamber 310 of the printhead 300 is fully or partially filled with shipping fluid having a different characteristic from the printing fluid, e.g. a different viscosity and/or a different density. In this case, the flow rate in the printing fluid deliv ery system 100 will depend on the ratio between printing fluid and shipping fluid when feeding the printing fluid to the fluid interconnect 130 and into the printhead 300. Based on the same air pressure, the printing fluid delivery system will create a change in flow rate and hence a change in pressure drop as the ratio between print ing fluid and shipping fluid changes. By monitoring this pressure drop, using the dif ferential pressure sensor 170, a status of the purging process can be detected. [0048]ln some examples, ink used as printing fluid may have a density in the order of 1 to 1.05 and a viscosity in the order of 3 to 5, depending on the color, at room tem perature, whereas a shipping fluid for a corresponding printhead may have a density in the order of 1.1 g/cm ³ and a viscosity in the order of 2 cP (centipoise, 1 P = 1 crrr ¹ -g-s ¹ ) or a density in the order of 1.2 g/cm ³ and a viscosity in the order of 8 cP.

In an example, where the viscosity of the shipping fluid is lower than that of printing fluid, a flow rate of the shipping fluid in the fluid channel portion 122 would be largest when the printhead chamber 310 is completely filled with shipping fluid and the flow rate will decrease as shipping fluid is replaced by higher viscosity printing fluid. Once all shipping fluid is replaced by printing fluid and pure printing fluid is fed through the fluid channels, the flow rate will become constant. On the other hand, where the vis cosity of the shipping fluid is higher than that of printing fluid, a flow rate of the ship ping fluid in the fluid channel portion 122 would be lowest when the printhead cham ber 310 is completely filled with shipping fluid and the flow rate will increase as ship ping fluid is replaced by lower viscosity printing fluid until all shipping fluid has been replaced. At this point, the flow rate will become constant.

[0049]The flow rate Q in e.g. the fluid channel portion 122 is a function of geometry G of the printing fluid delivery system 100, fluid property IP, pressure at inlet Pi of the printing fluid delivery system, which would be the inlet of the fluid channel portion 122 or the intermediate tank, and pressure at outlet Po of the printing fluid delivery sys tem, which would be the outlet or nozzle plate of the printhead:

Q = f(G, IP, Pi, Po) wherein, in this example,

G = constant because the geometry of the printing fluid delivery system 100 is con stant,

Po = atmospheric pressure at the printhead nozzles which is about constant,

Pi = pressure in the intermediate tank which is about constant because a constant air pressure is supplied to the intermediate tank 120 from the air pressure source 160, and

IP which will vary as shipping fluid is increasingly displaced by printing fluid because the printing fluid and the shipping fluid have different viscosities and/or densities, for example. Accordingly, the flow rate varies with the ratio of printing fluid-to-shipping fluid. If the printing fluid flow varies with the ratio of printing fluid-to-shipping fluid, the differential sensor 170 will detect a corresponding change in pressure difference which can be attributed to the changing ratio.

[0050]As the differential pressure measured by the differential pressure sensor 170 depends on the pressure drop on the channel portion 122 which, in turn, depends on the flow rate, the measurement by the differential pressure sensor 170 can be used to monitor status of the purging process. Accordingly, the status of the purging pro cess may be monitored by monitoring the differential pressure sensor 170.

[0051 ]This can be explained further with reference to Fig. 6a, 6b and Fig. 7. Fig. 6a schematically shows a curve of a flow rate in the fluid channel portion 122, as seen at the pressure sensor 170, and Fig. 6b schematically shows an output signal of the fluid pressure sensor 170. Fig. 7 shows a flow diagram of an example of a purging process which may be performed in the system of Fig. 3 under control of the control ler 140.

[0052]The purging process starts from a pre-purging state, at 710, in which a new printhead 300 is installed in the printer and connected to the fluid interconnect 130. At this stage, the intermediate tank 120 is full and pressurized at a pressure Pi by the gas pressure source 160. The intermediate tank 120 being full may correspond to a state where the printing fluid volume in the intermediate tank 120 is at least as large as the volume of the printhead chamber 310. In one example, the pressure Pi applied by the gas pressure source 160 may be in the range of about 2 to 8 psi (psi = pounds per square inch) or about 15 to 60 kPa relative to atmospheric pressure. The interme diate tank 120 can be filled from the supply tank 100 using the fluid pump 150 prior to starting the purging process. The fill level of the intermediate tank 120 can be deter mined using the differential pressure sensor 170, as explained above. The intermedi ate tank 120 may hold a printing fluid volume which is larger than the fluid volume of the printhead chamber 310. For example, the printing fluid volume of the intermediate tank 120 may be in the range of about 500 to 700 ccm, and a volume of the printhead chamber 310 may be in the range of about 300 ccm.

[0053]ln the diagrams of Fig. 6a and 6b, the time period before the start of the purg ing process is designated as A. The flow rate during the time period A is zero (0) and the output of the differential pressure sensor 170 is zero (0) because the pressure of printing fluid in the fluid channel portion 122 corresponds to the pressure Pi applied by the gas pressure source 160. At this time, the printhead chamber 310 is filled with shipping fluid, and the ink delivery system 100 is filled with printing fluid.

[0054]The purging process is started, at 712, by the controller 140 activating the pressure regulator 350 at a time t1 , to inflate the inflatable bag 352 and open the reg ulator valve 318 to let printing fluid flow through channel portions 122, 132, fluid inter connect 130, fluid pipe 316 and into the printhead chamber 310. At this time, the con troller 140 also monitors the differential pressure sensor 170. Assuming that the ship ping fluid in the printhead chamber 310 has a viscosity smaller than the viscosity of the printing fluid, the flow rate of the printing fluid in channel portion 122 and seen at differential pressure sensor 170 will be largest at the onset of the purging process be cause it will be determined to a larger extent by the lower viscosity of the shipping fluid. Accordingly, at the onset of the purging process, the flow rate will climb up to a maximum value, as shown at t1 in Fig. 6a. As soon as the printing fluid starts flowing through the ink channel portion 122, a pressure drop will occur between the outlet of the intermediate tank 120 and the location of the pressure sensor 170, and the differ ential pressure sensor 170 will measure a corresponding pressure difference. Be cause, during the purging process, the ratio between the shipping fluid and the print ing fluid in the printing fluid delivery system 100 and the printhead 300 will keep changing, also the flow rate will keep changing and this can be detected by the differ ential pressure sensor 170.

[0055]At the onset of the purging process, when the printhead chamber 310 still is filled with shipping fluid, the pressure difference between the pressure in the interme diate tank 120 and the pressure at the differential pressure sensor 170 is largest, as shown at t1 in Fig. 6b. As shipping fluid starts being flushed out of the printhead chamber 310 by the printing fluid streaming into the printhead chamber 310, the flow rate will decrease, as shown in time period B of Fig. 6a. Further, the pressure drop over channel portion 122 will decrease and also the pressure difference between the pressure in the intermediate tank 120 and the pressure at the differential pressure sensor 170 will decrease, as shown in time period B of fig. 6b. During this time period B, the flow rate will keep changing because the ratio between the shipping fluid in the printhead chamber 310 and the printing fluid in the printing fluid delivery system 100 and the printhead chamber 310 will keep changing. The controller 140 will continue monitoring the differential pressure sensor 170.

[0056]0nce all of the shipping fluid has been flushed from the printhead chamber 310 and is replaced by printing fluid, at t2 in Fig. 6a, the flow rate will become con stant, as shown in time period C in Fig. 6a, because there is no more shipping fluid in the system. The pressure difference at the differential pressure sensor 170 will be come zero (0), as shown in time period C in Fig. 6b.

[0057]During the purging period B, the controller 140 monitors the differential pres sure sensor 170 to detect whether its output is zero (0), DR = 07, at 714. If no, the purging process continues, at 712. If yes, the controller 140 detects that the purging process is complete, at 716, and all of the shipping fluid has been purged from the printhead chamber 310 and is replaced by printing fluid. The pressure regulator 350 is deactivated by the controller 140 and feeding printing fluid from the intermediate tank 120 is stopped. Continuing the purging process at this point would create unnec essary waste of printing fluid and is avoided. Rather, the controller 140 can signal the end of the purging process and the printer can go to a standby or operation mode, during time period C. Additionally, detecting an output signal of the differential pres sure sensor 170 being zero (0) is a reliable sign that all shipping fluid has been purged and removes the risk of leaving shipping fluid in the printhead chamber 310. This allows the controller 140 to provide automatic feedback from the purging pro cess and improves quality and reliability of subsequent printing operation.

[0058]With reference to Fig. 6a and 6b, Fig. 3 and 7, the purging process has been described as a continuous fluid feeding process. In another example, purging can be performed in cycles, with the intermediate tank 120 continuously pressurized by the pressure source 160 and with the pressure regulator 350 operating in cycles. For ex ample, the pressure regulator 350 can be activated in cycles of short periods of times, e.g. for 10 to 20 seconds per cycle, with a waiting time of e.g. 5 to 10 seconds between cycles to cyclically feed the printing fluid from the intermediate tank 120 to the printhead chamber 310. In this example, the output signal of the pressure differ ential sensor 170 may be monitored during the activation periods of the pressure reg ulator 350, as described with reference to Fig. 6a, 6b and 7. [0059]0perating the pressure regulator 350 in cycles may avoid over stressing com ponents of the pressure regulator 350 and printhead 300, it also may allow printing fluid and shipping fluid in the printhead chamber 310 to settle and to create a steady, turbulent free flow of printing/shipping fluid through the printhead chamber 310 and out of the printhead 300. If the shipping fluid has a higher density than the printing fluid, cyclic operation may also help to keep shipping fluid and printing fluid apart as shipping fluid may settle below printing fluid near the nozzle plate during the waiting periods.

[0060]With reference to Fig. 6a and 6b, the purging process has been described for an example in which the viscosity of the shipping fluid is lower than that of the print ing fluid. In such a case, the flow rate is largest with a maximum amount of shipping fluid being present in the printhead chamber 310 and will decrease as the ratio of shipping fluid to printing fluid decreases. The pressure difference at the differential pressure sensor 170, at the onset of the purging process, then will be negative. In an other example, where the viscosity of the shipping fluid is larger than that of the print ing fluid, the flow rate would be smallest at the onset of the purging process, when a maximum amount of shipping fluid is present, and would increase as the ratio ship ping fluid to printing fluid decreases. Accordingly, in time period B of Fig. 6a, the flow rate would increase from t1 to t2. The pressure difference at the differential pressure sensor 170, at the onset of the purging process, would be positive. However, also in this second example, once all shipping fluid has been purged from the printhead chamber 310, the flow rate would be constant and the output signal of the differential pressure sensor 170 would be zero (0).

[0061 ]The method of monitoring the purging process using the differential pressure sensor 170 is insensitive to the type of shipping fluid and the type of printing fluid as long as there is a difference in one parameter in which both fluids differ from each other and which has an impact on the flow rate. The method further is insensitive to changes in the quality of the printing fluid and/or the shipping fluid, e.g. due to tem perature or aging due to storage time. The method of monitoring the purging process is also insensitive to changes in the hardware of the printing fluid delivery system 100 and/or the printhead 300. The method does not depend on any defined flow rate cre ated by the printing fluid delivery system 100 and the pressure regulator 350, for ex- ample. [0062]The method of monitoring the purging process may also allow detecting failure of components of the printhead 300, the printing fluid delivery system 100 or the pressure regulator 350 because, if e.g. the gas pressure source 160 or inflatable bag 352 or the regulator valve 318 do not operate properly, the controller 140 may detect that there is no printing fluid flow or a printing fluid flow deviating from an expected one by monitoring the output of the differential pressure sensor 170. Accordingly, the method may detect any malfunction in the printhead 300 or the printing fluid delivery system 100 before they are put into operation in a printing process.

[0063]The status of the purging process can be detected using no extra hardware in a printer architecture having a differential pressure sensor for monitoring the printing fluid level in the intermediate tank. It will be sufficient to provide a respective update of the software or firmware of the controller 140 to upgrade existing printers to per form the method of monitoring the printing fluid delivery system as described.

[0064]Further, whereas the method of purging the shipping fluid has been described for one printhead chamber 310, a corresponding method can be applied to the other printhead chamber 320 and further printheads, by providing associated printing fluid delivery systems supplying the printing fluid desired for the respective printhead chambers. For example, the printhead chambers 310, 320 may be connected to re spective printing fluid delivery systems 100, of which one is shown in Fig. 3, and the purging process may be performed for both printhead chambers simultaneously or subsequently. The same pressure regulator 350 may be used for both printhead chambers 310, 320 or different pressure regulators may be provided.

[0065]A further example is described with reference to Fig. 4. Fig. 4 shows a sche matic diagram of the printing fluid delivery system 200 of Fig. 2 in combination with a purge unit 400 according to an example. The same components as in Fig. 2 are des ignated by the same reference number. Reference is made to the description of Fig.

1 to 3 above. The purge unit 400 may include a purge chamber 410, an inlet 412 cou pled to the purge chamber 410 and a gas valve 414 coupled to the purge chamber 410, the gas valve 414 including a sealing material which, when dry, is air permeable and, when wetted, is air impermeable. An example of a sealing material is foam poly ethylene PE, e.g. one having a pore size of 20 to 30 mp. The purge unit 400 may also include multiple chambers, and corresponding multiple inlets and multiple gas valves coupled to the multiple chambers. Whereas Fig. 4 shows two inlets and two gas valves, for ease of illustration, the following description refers to one chamber 410, one inlet 412 and one gas valve 414. Similar to a printhead having more than one printhead chamber, a purge unit having more than one chamber, with associated inlets and gas valves, may be used in combination with more than one fluid delivery system for different types of printing fluids.

[0066]A new printer or a printer which had been out of operation for some time, may include an empty printing fluid delivery system 200 or one which is at least partially filled with air rather than printing fluid. Before the printer can be used for printing, the air is removed from the printing fluid delivery system 200. The purge unit 400 may be used in combination with the printing fluid delivery system 200 to remove the air from the printing fluid delivery system 200.

[0067]To this end, the air may be purged from the printing fluid delivery system 200 by connecting the fluid interconnect 130 with the purge unit 400 which may be in serted in a printhead support (not shown in the drawings), for example. The fluid pump 150 may be activated and printing fluid may be fed from the supply tank 110 to the inlet 412 of the purge unit. The fluid flow may push any air in the printing fluid de livery system 200 from the fluid channel 212 into the purge unit 400 wherein the fluid level in the purge chamber 410 rises and air or other gas is forced out through the gas valve 414.

[0068]At the onset of this purging process, the sealing material in the gas valve 2014 will be dry and will let air and other gas pass therethrough. With the printing fluid level rising in the purge chamber 410, it will eventually reach the sealing material of the gas valve 414 which is wetted by the printing fluid and will become air-impermeable. At this time, all air or other gas will have been purged from the printing fluid delivery system 200 and the purging process is complete.

[0069]0nce the sealing material of the gas valve 414 has become air-impermeable, gas and liquid cannot exit the purge unit 410 through the gas valve 414 and, with the fluid pump 150 continuing to be activated, the pressure in the printing fluid delivery system 200 will rise. If the end of the purging process is not detected or not detected in time, the pressure increase may lead to damaging components of the printing fluid delivery system 200 and/or to leakage of printing fluid. If the purging process is fin ished before it is complete, remaining air in the printing fluid delivery system 200 may compromise image quality and printhead life. The status of the purging process may be controlled and monitored by the controller 140, controlling the fluid pump 150 and monitoring the pressure sensor 210.

[0070]This can be explained further with reference to Fig. 8 and 9. Fig. 8 schemati cally shows a curve of a pressure as measured by pressure sensor 210, and Fig. 9 shows a flow diagram of an example of a purging process which may be performed in the system of Fig. 4 under control of the controller 140.

[0071]The purging process starts from a pre-purging state, at 910, in which a purge unit 400 is installed in the printer and connected to the fluid interconnect 130. At this stage, the supply tank 120 is full and connected in the printing fluid delivery system 200. The supply tank 110 being full may correspond to a state where the printing fluid volume in the supply tank 110 is at least as large as the volume of the purge cham ber 410 and any fluid channels of the printing fluid delivery system 200 leading up to the fluid interconnect 130.

[0072]The supply tank 110 may hold a printing fluid volume fluid volume of several liters, such as about 2 L, 3 L or 5 L, or in the range of about 500 to 5000 com, and a volume of the purge chamber 410 may be in the range of about 100 to 200 com. In the diagram of Fig. 8, the time period before the start of the purging process is desig nated as A. During the time period A, the fluid pump 150 is deactivated and there is no flow of printing fluid in the printing fluid delivery system 200. The output of the pressure sensor 210 is zero (0) because the fluid channel portion 212 will be empty, i.e. filled with air, and no pressure is applied to the fluid channel portion 212. At this time, the purge chamber 410 is empty, i.e. it is filled with air.

[0073]The purging process is started, at 912, by the controller 140 activating the mo tor 152 to drive the fluid pump 150, at a time t1 , to start feeding printing fluid from the supply tank 100 through the fluid channel portion 212 to the fluid interconnect 130 and into the purge chamber 410. At this time, the controller 140 also monitors the pressure sensor 210. At the onset of the purging process, as printing fluid starts to flow through the fluid channel portion 212 to the fluid interconnect 130 and into the purge chamber 410, the pressure measured by the pressure sensor 210 will start to rise smoothly, as shown in section B1 of time period B in Fig. 8. The pressure de tected by the pressure sensor 210 is the sum of the static pressure of a fluid column between the supply tank 110 and the purge unit 400 as the printing fluid advances to the purge unit 400 and a dynamic fluid channel loss caused by the flow rate delivered by the fluid pump 150: Psensor = Pcolumn + Ploss.

[0074]As air is displaced from the fluid channel portion 212 and into the purge cham ber 410, the rate of the printing fluid fed by the fluid pump 150 will increase and the pressure at pressure sensor 210 will rise accordingly, as shown in time period B1 in Fig. 6a. When the printing fluid reaches the purge chamber 410, the level of the print ing fluid will rise in the purge chamber 410 and the pressure at the pressure sensor 210 will rise accordingly until the printing fluid reaches the air vent 414.

[0075]The printing fluid, in particular, may be a printing liquid, such as ink, and con tact of the printing fluid with the air vent 414 will wet the sealing material located therein. Accordingly, as soon as the printing fluid level reaches the air vent 414 it will be sealed and will be impermeable against air, liquid and the printing fluid, in particu lar. This is illustrated at time t2 in the diagram of Fig. 8.

[0076]0nce the air vent 414 is sealed and, as long as the fluid pump 150 continues to operate, the printing fluid, which is still delivered by the pump 150, cannot advance any further and the pressure in the printing fluid delivery system will rise substantially wherein the pressure rise is detected by the pressure sensor 210, as illustrated by the pressure curve in time period B2 in Fig. 8. If the fluid pump 150 continues to oper ate, the pressure will increase up to the cut-off pressure of the relief valve 154 and may rise beyond the reading range of the pressure sensor 210.

[0077]At a defined pressure threshold (TH), the controller 140 may detect that the pressure sensor 210 is exceeding this defined pressure threshold e.g. at t3. Further, at the same defined pressure threshold (TH) or at a higher pressure, the pressure sensor 210 may go into saturation, which is illustrated at the time t4 in the diagram of Fig. 8. The saturation state corresponds to time period C in Fig. 8.

[0078]During the entire purging process, the controller 140 continues monitoring the pressure sensor 210. The controller 140 may detect start of the pressurize rise, at t1 , a change in the inclination of the pressure curve, e.g. at t2, the pressure curve ex ceeding a defined threshold (TH), e.g. at t3, and the pressure sensor going into satu ration, at t4. In different examples, any of the events of the change in the inclination of the pressure curve, e.g. at t2, the pressure curve exceeding a defined threshold (TH), e.g. at t3, and the pressure sensor going into saturation, at t4, may be used as an indication that the purging process is complete, i.e. that all air has been removed from the printing fluid delivery system 200 and has been pushed into the purge unit 400.

[0079]Accordingly, during the purging time period B, the controller 140 monitors the pressure sensor 210 to detect whether the pressure P measured by the pressure sensor 210 exceeds an absolute threshold (TH) or goes into saturation and/or whether the pressure change DR exceeds a DR threshold, at 914. Any of these events may be considered as “the pressure exceeding a threshold”. In a particular ex ample, the controller 140 may monitor whether the pressure sensor 210 goes into saturation and outputs a constant high-level signal, as shown in time period C in Fig. 8. This would be considered an indication of the pressure P at pressure sensor 210 having exceeded a threshold. As long as the pressure is below the threshold, the purging process continues, at 912. If the pressure or pressure change at the pressure sensor 210 has exceeded the threshold, the controller 140 detects that the purging process is complete, at 916, and that all of the air has been purged from the printing fluid delivery system 200.

[0080]The motor 152 and hence the fluid pump 150 are deactivated by the controller 140 and feeding of printing fluid from the supply tank 110 is stopped. Continuing the purging process at this point would create unnecessary waste of printing fluid and is avoided. Rather, the controller 140 can signal the end of the purging process and the printer can go to a standby mode, during time period C. The purge unit 400 may be removed and replaced by a printhead. This process allows the controller 140 to pro vide automatic feedback from the purging process and improves quality and reliability of subsequent printing operation.

[0081] Different examples of monitoring and detecting the status of the purging pro cess have been described with reference to Fig. 4, 8, and 9. Detecting the pressure sensor 210 going into saturation is a simple yet reliable method of detecting the end of the purging process. To this end, the fluid pump 150 may be adjusted to generate, while the air valve 414 is air-permeable, a printing fluid flow which creates a pressure on the ink channel portion 212 which is within the reading range of the pressure sen sor 210. In one example, this range e.g. may be from -0.5 psi to 2.5 of from -3.5 kPa to 17 kPa. The corresponding flow rate may be about 30 ccm/min, for example. When the absolute pressure in the printing fluid delivery system 200 increases due to the printing fluid rising in the purge unit 400 and being blocked by the air valve 414, the pressure sensor 210 goes into saturation. This situation can be used for detecting that the purging process is complete.

[0082]ln another example, using a pressure sensor 210 having a larger reading range, the status and, in particular, the end of the purging process also can be de tected based on a sudden increase of the pressure slope, such as at t2, and/or an in crease of the pressure beyond an absolute threshold value TH, such as at t3.

[0083] In different examples, an absolute pressure threshold TH at which the control ler detects an end of the purging process may be in the order of 2 psi or 15 kPa, for example. A saturation threshold of the pressure sensor 210 may be in the order of 2.5 psi or 17kPa, for example.

[0084]Additionally, as shown in Fig. 4, a pressure relief valve 154 may be provided to bypass the fluid pump 150. The pressure relief valve 154 may be designed to prevent the pressure in the fluid delivery system 200 from rising above a cut-off pressure which could damage components of the fluid delivery system. Such cut-off pressure may be higher than the pressure threshold and may be in the range of 5 to 7 psi or about 35 to 50 kPa, for example.

[0085] Detecting an output signal of the pressure sensor 210 is a reliable sign that all air has been purged from the printing fluid delivery system 200 and removes the risk of leaving air bubbles therein. This process further removes the risk of overfilling the purge chamber 410 and any associated risk of ink leakage, in particular, when the purge unit 400 is removed. Additionally, the process avoids that the fluid pump 150 keeps working although the fluid delivery system 200 is filled with printing fluid and hence avoids overstressing the components of the printing fluid delivery system 200. This improves quality and reliability of subsequent printing operation. [0086]The method of monitoring the purging process using the pressure sensor 210 is insensitive to the type of printing fluid. The method further is insensitive to changes in the quality of the printing fluid, e.g. due to temperature or aging due to storage time. The method of monitoring the purging process is also insensitive to changes in the hardware of the printing fluid delivery system 200 and/or the purge unit 400.

[0087]The method of monitoring the purging process may also allow detecting failure of components of the printing fluid delivery system 200 because, if e.g. the motor 152 or the fluid pump 150 do not operate properly, the controller 140 may detect that there is no printing fluid flow or a printing fluid pressure deviating from an expected one by monitoring the output of the pressure sensor 210. For example, providing an automatic feedback from the purging process enables to automatically diagnose a self-prime capability of the fluid pump 150 and trigger a recovery process automati cally to prevent failures during the purging process. Accordingly, the method may de tect malfunction in the printing fluid delivery system 200 before it is put into operation in a printing process.

[0088]The status of the purging process can be detected using no extra hardware in a printer architecture having a pressure sensor for monitoring the printing fluid supply in the printing fluid delivery system 200. It will be sufficient to provide a respective up date of the software or firmware of the controller 140 to upgrade existing printers to perform the method of monitoring the printing fluid delivery system as described.

[0089]Further, whereas the method of purging the printing fluid delivery system 200 from air has been described for one type of printing fluid, a corresponding method can be applied to additional printing fluid delivery systems, by connecting a further printing fluid delivery system to another purge chamber and supplying the printing fluid to the other purge chamber of the purge unit or to another purge unit.

[0090]Another example of a method of monitoring a printing fluid delivery system is described with reference to Fig. 5, 10, and 11. In the example of Fig. 5, a printing fluid delivery system 100, such as the one shown in Fig. 1 , is connected to a purge unit 400, such as the one described with reference to Fig. 4. The same components are designated by the same reference numbers as in previous figures. Full reference is made to the above description of Figs. 1 to 4, and 6 to 9 wherein corresponding components may be designed, controlled and operated in a corresponding way, with out all of the details being repeated hereinbelow. The same or similar functionality as described above with reference to Fig. 4, 8, and 9 may be achieved in the example described with reference to Fig. 5, 10, 11 , without all of the details being repeated hereinbelow.

[0091 ]The example described with reference to Figs. 5, 10 and 11 provides a pro cess in which the printing fluid delivery system 100 is purged from air and, subse quently, the intermediate tank 120 is filled or refilled with printing fluid from the supply tank 110.

[0092]As described with reference to Fig. 4, a new printer or a printer which had been out of operation for some time, may include an empty printing fluid delivery sys tem which is filled with air rather than printing fluid or a printing fluid delivery system including a considerable amount of air bubbles in the fluid channels. Before the printer can be used for printing, the air is removed from the printing fluid delivery sys tem 100. The purge unit 400 may be used in combination with the printing fluid deliv ery system 100 to remove the air from the printing fluid delivery system 100. To this end, the air may be purged from the printing fluid delivery system 100 by connecting the fluid interconnect 130 with the purge unit 400 which may be inserted in a print- head support (not shown in the drawings), for example.

[0093]ln the example of Fig. 5, if the fluid pump 150 is activated, the printing fluid would normally be fed to the intermediate tank 120 to fill or refill the intermediate tank 120. In this example, however, the printing fluid supplied by the supply tank 110 is first used to remove any air from the fluid channel portions 112, 132, before filling the intermediate tank 120 and eventually putting the printer in an operating or standby mode.

[0094]To enable the printing fluid to be fed from the supply tank 110 to the fluid inter connect 130 and the inlet 412 of the purge unit, the intermediate tank 120 is pressur ized by the air pressure source 160 at a pressure higher than that of the printing fluid flowing in the fluid channel portions 122, 132 when printing fluid is supplied from the supply tank 110. The printing fluid then will initially not flow into the intermediate tank 120 but will flow through fluid channel portions 112, 132 and push any air in the print ing fluid delivery system 100 from the fluid channel portions 112, 32 into the purge unit 400.

[0095]As described with reference to Fig. 4, the fluid level in the purge chamber 410 will rise and air or other gas will be forced out from the purge unit 400 through the gas valve 414. At the onset of this purging process, the sealing material in the gas valve 2014 will be dry and will let air and other gas pass therethrough. With the print ing fluid level rising in the purge chamber 410, it will eventually reach the sealing ma terial of the gas valve 414 which is wetted by the printing fluid and will become air-im permeable. At this time, all air or other gas will have been purged from the printing fluid delivery system 100 and the purging process is complete.

[0096]0nce the sealing material of the gas valve 414 has become air-impermeable, gas and liquid cannot exit the purge unit 410 through the gas valve 414 and, with the fluid pump 150 continuing to be activated, the pressure in the printing fluid delivery system 100 will rise. At this point in time, in the example of Fig. 5, the pressure in the fluid channel portion 112 may rise above the pressure in the intermediate tank 120 and, when the fluid pump 150 continues to feed printing fluid from the supply tank 110, the intermediate tank 120 may be filled with printing fluid, as described in further detail below.

[0097]The status of the purging process and the refill of the intermediate tank 120 may be controlled and monitored by the controller 140, controlling the fluid pump 150 and monitoring the pressure sensor 170.

[0098]This can be explained further with reference to Fig. 10 and 11 . Fig. 10 sche matically shows a curve of a pressure as measured by the differential pressure sen sor 170, and Fig. 9 shows a flow diagram of an example of a purging and refill pro cess which may be performed in the system of Fig. 5 under control of the controller 140.

[0099]The purging process starts from a pre-purging state, at 1110, in which a purge unit 400 is installed in the printer and connected to the fluid interconnect 130. At this stage, the supply tank 120 is full and connected in the printing fluid delivery system 100. The supply tank 110 being full may correspond to a state where the printing fluid volume in the supply tank 110 is at least as large as the volume of intermediate tank 120, the purge chamber 410 and any fluid channels of the printing fluid delivery sys tem 100. The intermediate tank 120 may be empty or partially filled and depressur ized. In the diagram of Fig. 10, the time period before the start of the purging process is designated as A. During the time period A, the fluid pump 150 is deactivated and there is no flow of printing fluid in the printing fluid delivery system 100. The output of the differential pressure sensor 170 is zero (0) because the fluid channel portions 112, 132 will be empty, i.e. filled with air, and no pressure will be applied to the fluid channel portions 112, 122, 132 or the intermediate tank 120. At this time, the purge chamber 410 is empty, i.e. it is filled with air.

[00100] The purging process is started, at 1112, by the controller 140 activating the motor 152 to drive the fluid pump 150 at a time t1 , to start feeding printing fluid from the supply tank 100 through the fluid channel portions 112, 132 to the fluid inter connect 130 and into the purge chamber 410. Further, at t1 , air pressure may be ap plied to the intermediate tank 120 by the air pressure source 160, the air pressure be ing larger than a maximum expected pressure of the printing fluid which is supplied by the fluid pump 150 through the fluid channel portions 112, 132. The pressure gen erated by the fluid flow may be calculated as the sum of the static pressure of a fluid column between the supply tank 110 and the purge unit 400 as the printing fluid ad vances to the purge unit 400 and a dynamic fluid channel loss caused by the flow rate delivered by the fluid pump 150: Psensor = Pcolumn + Ploss.

[00101] The pressure provided by the pressure source 160 may be in a range or 3 to 5 psi or 5 to 7 psi, corresponding to about 20 to 35 Pa or about 35 to 50Pa, for example. Applying pressure to the intermediate tank 120 ensures that the printing fluid is fed from the supply tank 110 to the fluid interconnect 130, rather than into the intermediate tank 120.

[00102] At this time, the controller 140 monitors the differential pressure sensor 170. At the onset of the purging process, as printing fluid starts to flow through the fluid channel portion 212 to the fluid interconnect 130 and into the purge chamber 410, the pressure generated by the fluid flow will start rising. The differential pressure sensor 170 is measuring the pressure difference between the pressure in the fluid channel portions 122, 132 and the pressure applied to the intermediate tank 120 by the pressure source 160 which, at this time, is negative. In this example, the pressure amount may be larger than a saturation value of the differential pressure sensor 170. Accordingly, the differential pressure sensor 170 may go into negative saturation, as shown in time period B in Fig. 10. If a differential pressure sensor 170 having a larger reading range were to be used, the differential pressure sensor 170 could measure the relative negative pressure during the purging process which may not be constant but which would still stay negative during the purging process in time period B.

[00103] As air is displaced from the fluid channel portions 112, 132 and into the purge chamber 410, the rate of the printing fluid fed by the fluid pump 150 will in crease and the pressure at differential pressure sensor 170 will rise accordingly wherein, as indicated above, the differential pressure sensor 170 may still remain in negative saturation, as shown in time period B in Fig. 10. A pressure sensor having a larger reading range may detect an increasing pressure throughout the process, de pending on the reading range of sensor.

[00104] When the printing fluid reaches the purge chamber 410, the level of the printing fluid will rise in the purge chamber 410 and the pressure at the differential pressure sensor 170 will rise accordingly until the printing fluid reaches the air vent 414. The printing fluid may in particular be a printing liquid, such as ink, and contact of the printing fluid with the air vent 414 will wet the sealing material located therein. Accordingly, as soon as the printing fluid level reaches the air vent 414 it will be sealed and will be impermeable against air, liquid and the printing fluid, in particular. This is illustrated as a time t2 in the diagram of Fig. 10. Once the air vent 414 is sealed and, as long as the fluid pump 150 continues to operate, the printing fluid, which is still delivered by the pump 150, cannot advance any further and the pressure in the printing fluid delivery system will rise substantially wherein the pressure rise, as illustrated by the pressure curve at time t2 in Fig. 10, may be detected by the pres sure sensor 170.

[00105] During the purging time period B, the controller 140 monitors the differ ential pressure sensor 170 to detect whether the pressure differential, i.e. the differ ence between the air pressure applied to the intermediate tank 120 and the pressure at the differential pressure sensor 170, DR, exceeds a threshold (TH), e.g. is larger than zero (0), at 914. When the differential pressure DR exceeds the threshold TH, such as TH = 0, the controller 140 may detect that the purging process is complete and that all of the air has been purged from the printing fluid delivery system 100, at 1116.

[00106] In the example of Fig. 5, 10, and 11 , the fluid pump 150 may continue to operate to refill the intermediate tank 120, at 1118, shortly after the end of the purg ing process. The printing fluid supplied from the supply tank 110 by the fluid pump 150 will flow into the intermediate tank 120, once the pressure in fluid channel portion 112 is larger than that in the intermediate tank 120. At this stage, refilling of the inter mediate tank 120 may proceed as described above, with reference to Fig. 1 . The in termediate tank 120 may be refilled from the supply tank 110 using the fluid pump 150, which pushes printing fluid into the intermediate tank 120. During the refill pe riod, designated as time period C in Fig. 10, the controller continues to monitor the differential pressure sensor 170, at 1120. In one example, the refill operation is con tinued until the differential pressure sensor 170, measuring fluid vs. air pressure, de tects an end of refill process by detecting that the differential pressure DR has reached a further threshold, which may be an end-of-refill/end-of-purge_threshold, when the intermediate tank 120 is considered full. The end of refill process can be detected by correlation of the differential pressure DR with the further threshold stored in a look up table of pressure values vs. fluid level, for example. If the differen tial pressure DR is below the further threshold, the refill process continues, at 1122. If the differential pressure DR reaches or exceeds the further threshold, e.g. at t3 in Fig. 10, the process of purging the printing fluid delivery system 100 from air and refilling the intermediate tank 120 with printing fluid is considered complete, at 1122 in Fig.

10.

[00107] Once the intermediate tank 120 is detected full, e.g. at t3, the motor 152 and hence the fluid pump 150 may be deactivated, the printer can go in a standby mode. For example, the intermediate tank 120 may be depressurized and the purging unit 400 may be removed and replaced with a printhead, in time period D in the diagram of Fig. 10. This process further removes the risk of overfilling the purge chamber 410 and any associated risk of ink leakage, in particular, when the purge unit 400 is removed. Additionally, the process avoids that the fluid pump 150 keeps working although the fluid delivery system 100 and the intermediate tank 120 are filled with printing fluid and hence avoids overstressing the components of the printing fluid delivery system 100. This process allows the controller 140 to provide automatic feedback from the purging and refill processes and improves quality and reliability of subsequent printing operation.

[00108] In one example, a method of monitoring a purge process in a printing fluid delivery system is provided, the printing fluid delivery system including a printing fluid channel and a fluid interconnect, the method comprising feeding a printing fluid to the fluid interconnect via the printing fluid channel, monitoring a pressure of the printing fluid in the printing fluid channel, and analyzing the pressure to derive information on a status of the purge process in the printing fluid delivery system.

[00109] In another example, a printing fluid delivery system is provided, the system comprising a printing fluid channel and a fluid interconnect, a pump associated with the printing fluid channel, a pressure sensor associated with the printing fluid channel, and a controller, the controller programmed to activate the pump to feed printing fluid to the fluid interconnect; to query the pressure sensor to signal a pressure difference between a pressure reference value and a pressure in the printing fluid channel; to monitor the pressure difference over time; and to determine a status of feeding the printing fluid to the fluid interconnect based on the monitored pressure difference over time.

[00110] In a further example, a printer is provided, the printer including a con troller, and a printing fluid delivery system, the printing fluid delivery system compris ing a printing fluid channel connected to a fluid interconnect, an ink pump coupled to the printing fluid channel, and a pressure sensor coupled to the printing fluid channel, the controller programmed to activate the ink pump to feed printing fluid through the printing fluid channel; query the pressure sensor to signal a pressure on the printing fluid channel, to moni tor the pressure over time, and to determine a status of a purge process in the print ing fluid delivery system based on the monitored pressure over time.

[00111] The foregoing outlines features of several examples to better understand the aspects of the present disclosure. The present disclosure may be used as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein.