Abstract:
A humidity determination system for a hardcopy device, such as an inkjet printing mechanism, determines ambient humidity so printing routines may be adjusted to provide fast, high quality output in all environments. The system includes a thermal device having a viewable surface with a temperature which changes in response to a control signal. An optical sensor observes the viewable surface to detect an optical change in the sensor, such as when the surface passes through the dew point. A controller determines humidity from an ambient temperature reading and the temperature of the viewable surface at which the optical change occurs. With this information, the controller may change operating parameters of the hardcopy device. A hard copy device having such a humidity determination system is also provided, along with methods of determining humidity and operating a hardcopy device.

Description:
The present invention relates generally to inkjet printing mechanisms, and more particularly to an optical system for determining ambient humidity where an inkjet printing mechanism is operating, so printing routines may be adjusted to provide fast, high quality output in all environments. 
     Inkjet printing mechanisms use pens which shoot drops of liquid colorant, referred to generally herein as “ink,” onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are described and shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). 
     To clean and protect the printhead, typically a “service station” mechanism is mounted within the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. To facilitate priming, some printers have priming caps that are connected to a pumping unit to draw a vacuum on the printhead. During operation, partial occlusions or clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a clearing or purging process known as “spitting.” The waste ink is collected at a spitting reservoir portion of the service station, known as a “spittoon.” After spitting, uncapping, or occasionally during printing, most service stations have a flexible wiper, or a more rigid spring-loaded wiper, that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead. 
     To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast printing with darker blacks and more vivid colors, pigment based inks have been developed. These pigment based inks have a higher solids content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to use plain paper. 
     Various environmental factors affect inkjet printing routines, servicing routines, and other aspects of printer performance. Unfortunately in the past, there has been no way to economically provide an environmental factor input to a printer controller to allow the controller to modify these printing, servicing and other routines to provide optimum performance in light of the current environmental conditions. One environmental factor, temperature, may currently be monitored using temperature sensing resistors within the inkjet printheads; however, more important to printer performance than temperature is the environmental factor of humidity. Unfortunately, the currently available humidity sensors are far too expensive for the home and small business inkjet printing markets, with manufacturer&#39;s material costs for capacitive sensors ranging several dollars per sensor not including the cost of their support electronics, while voltage output humidity sensors currently cost about ten dollars each. Moreover, the currently available capacitive humidity sensors are inaccurate, so their inaccuracy coupled with their high cost renders their use unjustifiable in the home and small business inkjet printing market. 
     If humidity could be both economically and accurately measured for communication to a printer controller, a variety of performance enhancements could be made based upon knowledge of the ambient humidity. For example, presently to provide optimum performance in varying environmental conditions, inkjet printing, servicing, and other routines are based on a “worst case scenario” assumption of the environmental conditions, here meaning a high humidity environment for printing and a low humidity environment for printhead servicing, as well as for vapor transfer calculations which account for ink evaporation from the pens. The terms “dry” and “humid” are used herein to assist the reader in understanding which end of the scale refers to which condition. For instance, a “dry” condition normally is associated with a desert environment, whereas a “humid” condition is normally associated with a tropical environment, although it is apparent that during a cloud burst a desert may become a very humid environment for a short period of time. 
     In high humidity conditions, the media may already be moist and partially saturated before ever being loaded into a printer, and high humidity increases the drying time of aqueous-based inks. These high humidity conditions may lead to increased cockle of the media, a term referring to the swelling of the paper fibers when saturated with ink, causing a buckling which in extreme conditions may cause the media to buckle so high that the printhead crashes into the media, smearing the printed image and possibly damaging the printhead. Thus, a high humidity assumption increases the dry time delay for the media over that required in normal or low humidity conditions, which slows media throughput while a printer waits for one sheet to dry before depositing the next sheet on top of the previously printed sheet in the output tray. Furthermore, the low humidity assumptions for servicing increase the duration of servicing routines, which further slows media throughput. 
     Low humidity conditions contribute to hue shift problems, where various components of the ink evaporate over time, for instance by leaking at the printhead/cap sealing interface. In “off axis” printing systems, where the printheads carry only a small supply of ink across the printzone and are replenished with ink delivered from a stationary main ink reservoir through flexible tubing, some of the ink volatiles leach through the tubing walls to atmosphere. Any loss of one ink component changes the ink composition, resulting in changes in ink performance, often manifested as a hue shift in the resulting image. For instance, with fewer volatiles, the resulting ink dispensed by the printhead has a higher concentration of dyes or colorants, yielding a darker image than originally intended. To compensate for these ink composition changes, ambient humidity information may be used for vapor transfer rate calculations to allow for hue adjustment based on calculated dye load changes over time within the inkjet cartridges. 
     As another example of the impact of this high humidity assumption on printer performance, when performing duplex printing one typical duplexer unit typically holds a sheet after printing the first side for nearly seven seconds before reversing the sheet and beginning printing on the opposite surface. In low humidity conditions, such as in a desert setting, holding a sheet of paper for seven seconds as one would in a humid region unnecessarily delays duplex printing. These same delays are incurred to avoid cockle problems when printing single sided sheets. For pen servicing, it would be desirable to know the ambient humidity so the type of servicing routine performed on the printheads following uncapping and before a print job may be optimized. Additionally, by knowing a humidity history of the printer, vapor transfer rate calculations may be made to determine the amount of ink lost due to evaporation, which then may be used in conjunction with drop counting or other measures to predict when an inkjet cartridge is nearing an empty condition, allowing an operator to be warned before the cartridge runs dry. 
     Clearly, a variety of different printing, servicing and other performance operations may be adjusted and optimized if only the ambient humidity were input to the printing mechanism. Thus, one goal herein is to provide an ambient humidity input to an inkjet printing mechanism, which may use this input to optimize printer performance to provide fast high quality hard copy outputs. 
    
    
     DRAWING FIGURES 
     FIG. 1 is a fragmented, partially schematic, perspective view of one form of an inkjet printing mechanism including a peltier optical humidity sensing system for determining ambient humidity which affects inkjet printing. 
     FIG. 2 is an enlarged, perspective view of one form of a service station of FIG.  1 . 
     FIG. 3 is an enlarged, side elevational views of the service station of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates an embodiment of an inkjet printing mechanism, here shown as an inkjet printer  20 , constructed in accordance with the present invention, which may be used for printing for business reports, correspondence, desktop publishing, and the like, in an industrial, office, home or other environment. A variety of inkjet printing mechanisms are commercially available. For instance, some of the printing mechanisms that may embody the present invention include plotters, portable printing units, copiers, cameras, video printers, and facsimile machines, to name a few. For convenience the concepts of the present invention are illustrated in the environment of an inkjet printer  20 . 
     While it is apparent that the printer components may vary from model to model, the typical inkjet printer  20  includes a chassis  22  surrounded by a housing or casing enclosure  24 , typically of a plastic material. Sheets of print media are fed through a printzone  25  by a print media handling system  26 , constructed in accordance with the present invention. The print media may be any type of suitable sheet material, such as paper, card-stock, transparencies, fabric, mylar, and the like, but for convenience, the illustrated embodiment is described using paper as the print medium. The print media handling system  26  has a feed tray  28  for storing sheets of paper before printing. A series of conventional motor-driven paper drive rollers (not shown) may be used to move the print media from tray  28  into the printzone  25  for printing. After printing, the sheet then lands on output tray portion  30 . Alternatively, the sheet may be directed to pass through a duplexing mechanism, such as a modular duplexing mechanism  31 , which turns the sheet over for printing on the opposite surface from the surface first printed upon. One suitable duplexing mechanism is described in U.S. Pat. No. 6,167,231, currently assigned to the present assignee, the Hewlett-Packard Company. The media handling system  26  may include a series of adjustment mechanisms for accommodating different sizes of print media, including letter, legal, A-4, envelopes, etc., such as a sliding length and width adjustment levers  32  and  33  for the input tray, and a sliding length adjustment lever  34  for the output tray. 
     The printer  20  also has a printer controller, illustrated schematically as a microprocessor  35 , that receives instructions from a host device, typically a computer, such as a personal computer (not shown). Indeed, many of the printer controller functions may be performed by the host computer, by the electronics on board the printer, or by interactions therebetween. As used herein, the term “printer controller  35 ” encompasses these functions, whether performed by the host computer, the printer, an intermediary device therebetween, or by a combined interaction of such elements. The printer controller  35  may also operate in response to user inputs provided through a key pad (not shown) located on the exterior of the casing  24 . A monitor mounted on the casing  24  or coupled to the computer host may be used to display visual information to an operator, such as the printer status or a particular program being run on the host computer. Personal computers, their input devices, such as a keyboard and/or a mouse device, and monitors are all well known to those skilled in the art. 
     A carriage guide rod  36  is mounted to the chassis  22  to define a scanning axis  38 . The guide rod  36  slideably supports a reciprocating inkjet carriage  40 , which travels back and forth across the printzone  25  and into a servicing region  42 . One suitable type of carriage support system is shown in U.S. Pat. No. 5,366,305, assigned to Hewlett-Packard Company, the assignee of the present invention. A conventional carriage propulsion system may be used to drive carriage  40 , including a position feedback system, which communicates carriage position signals to the controller  35 . For instance, a carriage drive gear and DC motor assembly may be coupled to drive an endless belt secured in a conventional manner to the pen carriage  40 , with the motor operating in response to control signals received from the printer controller  35 . To provide carriage positional feedback information to printer controller  35 , an optical encoder reader may be mounted to carriage  40  to read an encoder strip extending along the path of carriage travel. 
     Housed within the servicing region  42  is a service station  44 . The service station  44  includes a translationally movable pallet  45 , which moves in a forward direction indicated by arrow  46 , and in a rearward direction indicated by arrow  47 , when driven by a motor  48  operating in response to instructions received from the controller  35 . While a variety of different mechanisms may be used to couple the drive motor  48  to the pallet  45 , preferably a conventional reduction gear assembly drives a pinion gear which engages a rack gear formed along the undersurface of the pallet  45 , for instance as shown in U.S. Pat. Nos. 5,980,018 and 6,132,026, both currently assigned to the present assignee, the Hewlett-Packard Company. 
     In the printzone  25 , the media sheet receives ink from an inkjet cartridge, such as a black ink cartridge  50  and/or a color ink cartridge  52 . The cartridges  50  and  52  are also often called “pens” by those in the art. The illustrated color pen  52  is a tri-color pen, although in some embodiments, a set of discrete monochrome pens may be used. While the color pen  52  may contain a pigment based ink, for the purposes of illustration, pen  52  is described as containing three dye based ink colors, such as cyan, yellow and magenta. The black ink pen  50  is illustrated herein as containing a pigment based ink. It is apparent that other types of inks may also be used in pens  50 ,  52 , such as thermoplastic, wax or paraffin based inks, as well as hybrid or composite inks having both dye and pigment characteristics. 
     The illustrated pens  50 ,  52  each include reservoirs for storing a supply of ink. The pens  50 ,  52  have printheads  54 ,  56  respectively, each of which have an orifice plate with a plurality of nozzles formed therethrough in a manner well known to those skilled in the art. The illustrated printheads  54 ,  56  are thermal inkjet printheads, although other types of printheads may be used, such as piezoelectric printheads. These printheads  54 ,  56  typically include a substrate layer having a plurality of resistors which are associated with the nozzles. Upon energizing a selected resistor, a bubble of gas is formed to eject a droplet of ink from the nozzle and onto media in the printzone  25 . The printhead resistors are selectively energized in response to enabling or firing command control signals, which may be delivered by a conventional multi-conductor strip (not shown) from the controller  35  to the printhead carriage  40 , and through conventional interconnects between the carriage and pens  50 ,  52  to the printheads  54 ,  56 . 
     Preferably, the outer surface of the orifice plates of printheads  54 ,  56  lie in a common printhead plane. This printhead plane may be used as a reference plane for establishing a desired media-to-printhead spacing, which is one important component of print quality. Furthermore, this printhead plane may also serve as a servicing reference plane, to which the various appliances of the service station  45  may be adjusted for optimum pen servicing. Proper pen servicing not only enhances print quality, but also prolongs pen life by maintaining the health of the printheads  54  and  56 . To hold the pens,  50 ,  52  in place securely against alignment datums formed within carriage  40 , preferably the carriage  40  includes black and color pen latches  57 ,  58  which clamp the pens  50 ,  52  in place as shown in FIG.  1 . 
     FIG. 2 shows one form of the service station  44 , constructed in accordance with the present invention. The pallet  45  may carry a variety of different servicing members for maintaining the health of the printheads  54 ,  56 , such as printhead wipers, primers, solvent applicators, caps and the like. These various servicing members are represented in the drawing figures as black and color caps  60 ,  62  for sealing the printheads  54 ,  56  of pens  50 ,  52 , respectively. Preferably, the pallet  45  is housed between a lower frame portion  64 , and an upper frame portion  66  of the service station  44 . As mentioned above, the motor  48  drives the pallet  45  in the forward and reverse directions of arrows  46  and  47  to bring the various servicing components into contact with the printheads  54 ,  56 . The frame lower portion  64  preferably defines a waste ink reservoir or spittoon  68 , which receives ink purged from the printheads  54 ,  56  in a spitting routine. 
     The service station  44  includes an ambient humidity determination system  70  constructed in accordance with the present invention, here shown as being mounted in-part along an outboard wall  72  of the lower frame  64 . As used herein, the term “inboard” refers to items facing toward the printzone  25 , and the term “outboard” refers to items facing away from printzone. First an explanation of the construction of the ambient humidity determination system  70  will be given, followed by a discussion of its operation. The outboard wall  72  supports a platform  74 , which projects outwardly from the wall. The peltier humidity determination system  70  includes a temperature controlling member or thermal device, shown in the illustrated embodiment as a peltier temperature controlling member or thermal device  75 , which changes temperature in response to a control signal from controller  35 . 
     The peltier thermal device  75  includes a base  76  supported by platform  74 , with the base operating as a heat sink. The peltier device  75  also has a target  78 , which may be of a metallic material. Sandwiched between the heat sink base  76  and the target plate  78  is a heating and cooling generator unit  80 , which may be made up of alternating negatively doped semiconductor sections  82  and positively doped semiconductor sections  84 . The generator unit  80  operates in response to a control signal from controller  35  to cool the target  78  or heat the target  78  in selected discreet temperature steps. Peltier heating/cooling devices are commercially available, and functionally equivalent, controllable heating and cooling devices may be substituted for the illustrated peltier device. Now the construction of the illustrated peltier thermal device  75  is understood, we can turn to a discussion of how it may be used to determine ambient humidity in system  70 . 
     The maximum atmospheric water vapor content is a strong function of the atmospheric temperature. Indeed, one earlier way to determine ambient humidity was through the use of a “sling thermometer” which had two thermometers mounted in a metal frame attached to a small length of chain or string. One thermometer is called a “dry bulb” thermometer, and the other is called a “wet bulb” thermometer. The wet bulb thermometer has a small piece of cloth tied around it, and the cloth is wet with water just prior to taking a measurement. To measure the ambient humidity, the metal frame is twirled around in a circle using the chain or string. When the temperatures have stabilized, both the wet bulb and dry bulb temperatures are recorded, then using this data in conjunction with a chart or graph, the ambient humidity is determined. Of course today, this data may be inputted into a computer or programmed calculator to arrive at the ambient humidity, rather than referring to a chart or graph. 
     These same basic principles are used here in the ambient humidity determination system  70 . First the peltier device receives a cooling control signal from the controller  35 , and in discreet temperature steps, the target  78  is cooled. Eventually, the target  78  reaches a temperature where the moisture from the air begins to condense and collect on the exposed target surface, just as moisture collects on a cold glass of lemonade on a warm summer day. The peltier device  75  may also operate in the opposite fashion. When starting from a cooled state where the target  78  is covered with condensation, the generator unit  80  may operate in response to a heating control signal from controller  35  to gradually heat the target  78  in discreet steps until the moisture returns to the atmosphere, similar to turning on the defrosters in your car to remove frost from the windshield. The temperature at which the moisture appears on the target  78  during cooling, and the temperature at which the moisture disappears on heating are the same temperature at a given ambient humidity, with this temperature being known as a “dew point.” 
     This accumulated moisture changes the light reflective properties of the target  78  from the dry state, with these moisture droplets scattering incoming light rays. Now if there were a way to monitor and determine at what temperature this condensation began, then one would know the equivalent of the “wet bulb” temperature. As mentioned in the Introduction section above, one or both of the inkjet printheads  54  and  56  are typically manufactured with on-board temperature sensing resistors, so the ambient temperature can be monitored using these printhead temperature sensing resistors to arrive at a “dry bulb” temperature. Thus, the problem becomes, how to monitor at what temperature the peltier target  78  begins to gather condensation. 
     Fortunately, some inkjet printers, such as the DeskJet® 990 model color inkjet printer produced by the Hewlett-Packard Company of Palo Alto, Calif., are equipped with an optical sensor which is used to detect ink droplets and incoming media. Such an optical sensor, or a monochromatic optical sensor as described in the Hewlett-Packard Company&#39;s U.S. Pat. No. 6,036,298, shown in FIG. 3 as an optical sensor  85 , may be used as another component of the illustrated ambient humidity determination system  70 . The illustrated optical sensor  85  includes a body  86 , which in the illustrated embodiment is supported by an outboard side wall of the printhead carriage  40 . The body  86  houses several components, including an illuminating element  88 , such as a blue or violet-blue light emitting diode (“LED”). The body  86  also houses a photo sensor  90 , along with optional electronics for the photo sensor, such as an amplifier  92 . 
     The photo sensor  90  receives light through a lens element  94 , with the field of view of light passing to lens  94  being limited by a window, or F-stop  95 . Optionally, an optical filter (not shown) may be placed in the F-stop window  95 . The sensor body  86  may also house additional illuminating elements of different colors, along with additional photo sensors and related lens elements, etc., such as one photo sensor for monitoring diffractive reflection from the target  78 , and another photo sensor for monitoring spectral reflection from the target  78 . FIG. 3 shows the LED element  88  illuminating the peltier target  78  with an illuminating beam  96 . The illuminating beam  96  impacts the peltier target  78 , and then reflects off the target to form a reflected beam  98 , which passes through any optical filter element, through the F-stop  95 , and through lens  94 , before being received by the photo sensor  90 . 
     As mentioned above, the reflective properties of the target  78  change from those at a dry state to those at a wet state, with the amount of reflected light in beam  98  dropping in the wet state as the moisture droplets scatter the incoming light beam  96 . In the dry state, the metallic target  78  is quite reflective, so a greater amount of light is reflected in beam  98  than in the wet state. The photo sensor  90  measures the difference in the reflected beam  98 , then supplies a high signal to the controller  35  in the dry state, and a low signal to the controller  35  in the dry state. When the optical sensor  85  is monitoring the peltier target  75 , the controller  35  then looks for a change from high-to-low when the generator unit  80  is cooling, and from low-to-high when the generator unit  80  is heating. Since the generator unit  80  operates in response to a control signal received from the controller  35  to increase or decrease the target temperature in discreet degree increments, the controller  35  knows at what temperature the condensation either formed or evaporated, with this temperature being the dew point, or “wet bulb” temperature. Now the controller  35  has information for both the wet bulb temperature, and the dry bulb temperature form monitoring the temperature sensing resistors on printheads  54 ,  56 , the controller  35  may determine the ambient humidity. Actually, the ambient humidity calculation may be omitted if the controller  35  is programmed to react in various ways to these wet-bulb and dry-bulb temperature inputs, so the claims appended below this detailed description are not to be construed as being limited to requiring this optional ambient humidity calculation. 
     In operation, one preferred ambient humidity detection scheme, which is useful for printer  20  when operating under normal room temperature conditions, first lowers the temperature of the target  78  in a series of relatively coarse discreet steps. After reaching the dew point where condensation begins to collect on target  78 , and perhaps having overshot the exact dew point because large temperature steps were being taken, the generator unit  80  then begins to raise the temperature in a series of finer or smaller temperature increase steps. The optical sensor  85  continues to monitor the target  78  during this temperature increasing routine, with the controller  35  noting a more precise temperature at which the collected condensation evaporates from the target  78 . The more precise nature of the dew point temperature is a result of the smaller temperature increments being taken on the “fine adjust” heating cycle, as opposed to the larger temperature increments taken on the “coarse adjust” cooling cycle. For example, on the coarse adjust cooling cycle, temperature steps may be made in 5° or 10° increments, whereas for the fine adjust heating cycle, the temperature steps may be on the order of 1° or 5° increments. Following the heating cycle, another cooling cycle may be initiated in even finer temperature steps, followed yet by another heating cycle. The number of cooling/heating cycles employed, along with the exact temperature steps taken during each cycle, may be optimized to consume the shortest time or to provide the most accurate results. 
     FIG. 1 shows an optional alternative embodiment of an ambient humidity determination system  70 ′, where a thermal device a  100 , which may be constructed as described above for the thermal device  75 , is mounted in the printzone  25  to a portion of the media support system, here shown as a platen  102 . In other implementations, it may be desirable to support the thermal device on other components and in other locations viewable by an optical sensor, such as on the printer chassis  22  or casing  24 . Furthermore, the optical sensor need not be mounted on the printhead carriage  40 , but instead may be mounted at a fixed stationary location on the printer  20 , as long as the optical sensor is located to monitor the thermal device target  78  for condensation accumulation and evaporation. In the illustrated embodiment, the peltier thermal device  100  is located toward the far left of the platen  102 , remote from the service station  44 , to avoid having the target  78  become contaminated with ink aerosol generated by printheads  54 ,  56  during spitting routines over the service station spittoon  68 . Preferably, the peltier device  100  is mounted along the platen  102  in a position where the optical sensor  85  passes over the target  78  when slewing or reciprocating back and forth across the printzone  25  in the direction of the scanning axis  38 . 
     Now the operation of the ambient humidity determination system  70 ,  70 ′ is understood, the various ways in which the controller  35  may modify the operation of printer  20  in response to the ambient humidity information will be described. First it should be understood that printer operating systems do not need to be modified in response to all ambient humidity changes. Indeed, some inkjet printing mechanisms may be more sensitive to certain variations in humidity, while other printing mechanisms appear to be relatively immune to those same changes but sensitive to other changes. The exact operating changes, as well as the exact ambient humidity levels at which such changes occur, need to be tailored to best meet the needs of different printing mechanism designs. For instance, for one printer only under very dry conditions on the order of 10-20% relative humidity, or under very humid conditions on the order of 80-90% relative humidity, the print routines may be affected, while conditions between these extremes on the order of 30-70% relative humidity, are considered to be in a normal operating range, where print modes are unaffected by humidity. Thus, only above 70% ambient humidity, and below 30% ambient humidity does the controller of this example adjust the operating routines. 
     One change in operating routines of printer  20  may occur at around ambient conditions of 80% relative humidity. At this higher (80%) humidity, printing routines may be slowed to allow more time for volatiles within the inks to dry. Additionally, a time delay may be inserted between printing sheets in a multiple sheet print job, allowing a previously printed sheet to dry before the next sheet is dropped upon it in the output tray  30  to avoid smearing the earlier printed sheet. This delay or dry time may be adjusted, such as by increasing the dry time delay in high humidity conditions and decreasing the dry time delay in low humidity conditions. In an inkjet printing mechanism having auxiliary drying capability, such as in printers having internal heaters, additional heat may be applied in high humidity conditions to speed drying of the ink and reduce the drying time to a shorter interval. 
     As another example, under these relatively dry conditions, for instance on the order of 20% ambient humidity, print speeds may be increased because dry conditions allow the volatiles within the inks to dry more quickly. For instance, during duplex printing operations, where there is normally a seven second delay time between printing a first side of a sheet and a second side, the delay time may be decreased from a nominal seven second delay time to three or four seconds. Thus, by allowing the printer controller  35  to understand through the use of the peltier humidity determination system  70 ,  70 ′ that the printer is in a humid environment, in this example above 80% humidity, print quality is increased by allowing additional dry time for the inks on multiple page print jobs. Similarly, by allowing the controller  35  to know the printer is in a relatively dry environment, here less than 20% relative humidity, throughput is increased by eliminating some of the additional dry time required during nominal conditions especially in duplex printing. 
     As mentioned in Introduction section above, the earlier capacitive humidity sensors are currently available at a cost of approximately several dollars each, not including the cost of their support electronics, while voltage output humidity sensors cost about ten dollars each. In contrast, the illustrated peltier device  75  costs under a dollar, which imposes little additional cost on the overall printer  20 , while at the same time greatly improving performance. Moreover, if the optical sensor  85  is already installed in the printing unit for monitoring the media and/or ink droplets printed on a page, there is no additional cost associated with adding the optical sensor as the peltier target reader. 
     The ambient humidity determination system  70  may collect environmental data over time, storing this data within a storage portion of controller  35 . This monitoring of the various environmental factors by the system  70  is advantageously accomplished without requiring the carriage  40  to move from the servicing region  42 . Specifically, by obtaining a humidity history using the stationary sensor  70 , the water vapor transfer rate may be calculated to accommodate for evaporation of the inks from within pens  50 ,  52  over time. This water vapor transfer rate, in addition to counting the number of droplets fired by each printhead  54 ,  56  may be used to predict the amount of ink remaining in each of the pens  50 ,  52 . Thus, a history of the ambient humidity while the pens have been capped may be gathered by controller  35 . For example, under higher humidity conditions, the printheads  54 ,  56  are less susceptible to clogging. Thus, under high humidity conditions fewer drops need to be expended during pre-printing spitting routines. 
     As mentioned in the Introduction section above, low humidity conditions also contribute to hue shift problems, where various components of the ink, such as water or volatiles, evaporate or dissipate over time, for instance by leaking at the printhead/cap sealing interface or through ink delivery tubing in off-axis printing systems. If the controller  35  has a record of the changes in the ambient humidity, and knows the rates of evaporation over time under these humidity conditions, the controller may estimate the change(s) in ink composition over the lifetime of an ink supply. Knowing these changes in the ink composition over time, the controller  35  may then compensate for these changes by conducting vapor transfer rate calculations, for instance, by printing fewer dots per unit area for an aged printhead having a higher concentration of dyes or colorants due to evaporated volatiles. Thus, the controller  35  may compensate for these ink composition changes to allow for hue adjustment based on calculated dye load changes over time within the ink supplies. Furthermore, this evaporation information may be used by the controller  35  to more accurately predict an upcoming out-of-ink condition when used in conjunction with a drop-counting or other system for anticipating when the pens  50 ,  52  may run dry. For instance, a simple drop-counting routine may indicate an abundant ink supply remains and fail to give an operator any warning, while in reality; the pen is nearly dry due to evaporation and a warning should be given to tell the operator to have a replacement cartridge on hand. 
     Additionally, use of the peltier humidity determination system  70 ,  70 ′ allows the various print modes to be adjusted based on environmental conditions. As mentioned above, during duplex printing jobs throughput may be adjusted to correspond to the various changes in ambient temperature and humidity, to increase throughput and/or improve print quality over results obtained using nominal or worst case assumptions about environmental conditions. Furthermore, using the service station mounted humidity determination system  70  allows for variations in the pre-print mode servicing routines, as well as other servicing routines performed during print jobs. For example, under dry conditions the nozzles of both of the printheads  54 ,  56  are more subject to clogging, so to accommodate for this, pre-print spitting routines may be more vigorous than required under nominal conditions. Moreover, knowing this ambient humidity information which influences printer  20  may allow for more accurate line feed calibration, which refers to the advancing of the media through the printzone  25 . Line feed calculations may be impacted by expansion and contraction of the media path encoder disk, which is used to track the movement of the media through the printzone  25 . In some embodiments, the encoder disk may absorb water so in a humid environment the disk expands, adding a nominal offset to the timing of the counts as an optical sensor reads equally-spaced radial lines appearing near the disk periphery. Additionally, other media movement path components, such as drive rollers, may change shape or enlarge due to high ambient moisture conditions, impacting line feed accuracy for longer media advances which are more sensitive to runout errors in both the drive rollers and in the encoder feedback system.