Patent Publication Number: US-8122779-B2

Title: Electronic pipettor with improved accuracy

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of prior U.S. patent application Ser. No. 11/856,231, filed on Sep. 17, 2007, now issued as U.S. Pat. No. 7,540,205, entitled ELECTRONIC PIPETTOR ASSEMBLY. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to improvements in hand-held electronic pipettors. 
     BACKGROUND OF THE INVENTION 
     The use of hand-held electronic pipettors is widespread in clinical and research laboratory applications. Electronic pipettors are typically controlled by small microprocessors that are located within in the pipettor housing. The microprocessors are usually programmed through the use of user controls on the pipettor itself. Many electronic pipettors have a small screen display as well. Users can program the pipettor to aspirate a volume of liquid reagent or sample and dispense the aspirated volume or a series of aliquots in successive dispensing operations. Programmable electronic pipettors can also be configured to do more complex operations such as mixing in a pipettor tip, etc. 
     The electronics industry has seen many advances in recent years. For example, small-scaled LCD displays with improved clarity and enhanced color graphics capabilities, improved processing and memory capabilities, wireless communication devices, etc. are all prevalent. To date, however, it has been difficult to take advantage of many of these advancements in hand-held pipettors. Hand-held electronic pipettors, by their very nature, need to be compact and comfortable in the palm of the hand, yet provide ample room for the motor, the piston and cylinder assembly, and the ejection mechanism, as well as the programmable electronics and a power supply such as a rechargeable battery. An objective of the invention is to provide a configuration for a hand-held electronic pipettor that is able to physically accommodate recent electronic advancements yet meet the above described design requirements. 
     Another objective of the invention is to provide a hand-held electronic pipettor with improved pipetting accuracy. Pipetting accuracy is especially important when working with small volumes, such as 1 or 2 μl aliquots, however, accuracy can be difficult to attain when pipetting such small aliquots. Difficulties arise not only because of mechanical imprecision of the pipettor components, but also because liquid surface tension issues. Inherently, there are normally differences between forces acting on liquids being aspirated and forces acting on liquids being dispensed, and these differences can cause meaningful inaccuracies when pipetting small volumes. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention is a hand-held electronic pipettor that is designed particularly to be programmed and operated with one hand for the convenience of the user. More specifically, the pipettor has an elongated body adapted to be held in hand of the user with a finger hook on the rear side of the body. On the front side of the body there is a touch wheel control that is operated by the thumb of the user or with a finger from the hand not holding the pipettor. A user interface display is also located on the front side of the pipettor and is located preferably above the touch wheel control. The pipettor preferably comprises a microprocessor which is programmed with menu driven software for controlling information displayed on the user interface display and for programming the microprocessor to operate the pipettor. The user programs the pipettor using the touch wheel control. A run button is located on the front side of the pipettor body as well, and is located below the touch wheel control. The run button likewise is designed to be operated by the thumb of the user. The user activates the run button in order to run a procedure or the next step in the procedure that is programmed into the pipettor. The front side of the pipettor also preferably includes an ejector button to be operated by the thumb of the user. The ejector button is used to activate the ejection mechanism to remove pipette tips mounted to the tip mounting shaft on the pipettor. The ejector button is preferably located below the run button. In this manner, the touch wheel control, run button and ejector button can all be conveniently operated by the thumb of the user. 
     The preferred touch wheel control includes a circular touch pad that uses capacitance electronics to translate rotational movements of the thumb (or finger) into up and down cursor movements on the display, and an enter button located at the center of the circular touch pad. The circular touch pad also preferably includes four selector locations, namely a back button located at the top of the circular touch pad, right and left navigation buttons located on the right and left side of the circular touch pad, respectively, and a purge button located at the bottom of the circular touch pad. The back button allows the user to conveniently return to the previous screen or menu selection. The right and left navigation buttons allow the user to navigate via right or left menu prompts. The purge button allows the user to voluntarily stop the procedure and purge the pipettor, i.e., a full dispense and blow out, in order to purge the system to start another procedure. In accordance with the invention, each of these controls can be implemented conveniently using the thumb or finger of the user. 
     In accordance with another aspect of the invention, the pipettor is designed so that the center of gravity of the pipettor is located within the palm of the hand of the user holding the pipettor with their index finger wrapped around the housing underneath the finger hook located on the rear housing. This provides the user with a comfortable feel, and promotes accuracy in the placement of the physical location of the pipette tip by the user. In order to accommodate the relatively large number of electrical components on the pipettor, it is desirable to use a battery that has relatively significant weight, size and electrical storage capacity. Thus, it is preferred to use a battery having an elongated cylindrical shape as is common, but not typically used in connection with electronic pipettors. The housing is designed to provide structural support for the internal components of the pipettor, including the motor, and the elongated rechargeable battery. In accordance with this aspect of the invention, it has been found that mounting the battery in the housing so that the top of the battery is above the height of the finger hook, and the motor is mounted at a height substantially below the battery will locate the center of gravity of the pipettor in the palm of the hand of the user, and will also otherwise allow for the appropriate placement of internal pipettor components within the pipettor housing in a compact manner. In addition, the pipettor housing is designed with an internal vertical structure or wall which provides a compartment for the rechargeable battery, as well as preferably another compartment for an optional wireless communication chip. These compartments are accessible to the user if the user removes the rear housing, but the vertical structure isolates other electrical components from the user thereby protecting those other components, such as the color screen display, the capacitance circular touch pad, and a circuit board operating the pipettor. 
     In accordance with another aspect of the invention, it is preferred that the pipettor be equipped with at least one megabyte of flash memory, i.e., electrically erasable programmable read-only memory. The flash memory is helpful for many reasons, including software storage. 
     In accordance with another aspect of the invention, the invention involves the use of an aspiration look-up table, preferably located in flash memory, to determine the number of motor steps (or half steps) necessary to aspirate a selected aspiration volume. The motor step values in the aspiration table are preferably determined empirically to account for any aspiration accuracies in the pipettor, as is known in the art. To aspirate a volume of liquid, the user selects the aspiration volume, then a translator in flash memory determines an index value to which that aspiration volume corresponds. The index value is then correlated to the number of motor steps empirically determined to aspirate the appropriate amount of liquid. From the home position the motor retracts the piston the determined number of motor steps (or half steps) in order to aspirate the selected aspiration volume into the disposable pipette tip mounted on the pipettor. 
     In accordance with another aspect of the invention, when a user desires to dispense multiple aliquots, the pipettor uses a separate dispensing look-up table to determine the appropriate number of motor steps necessary to dispense the selected (or calculated) aliquot volume. The motor step values in the dispensing look-up table are empirically determined to account for dispensing inaccuracies. It has been found that the number of steps corresponding to aspirating a certain value is typically somewhat different than the number of steps for dispensing the same value, and therefore using separately developed empirical tables for as dispensing and aspirating can lead to significant improvements in dispensing accuracy especially when dispensing multiple aliquots of small volumes. 
     Preferably, the user can separately calibrate the aspiration look-up table and/or the dispensing look-up table in the field by shifting the values in the tables independently after using conventional calibration procedures. 
     Other features of the invention may be apparent to those skilled in the art upon reviewing the following drawings and description thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  a perspective view of a hand-held electronic pipettor constructed in accordance with a preferred embodiment of the invention. 
         FIG. 2  is a front elevation view of the pipettor shown in  FIG. 1 . 
         FIG. 3  is a side elevation view of the pipettor shown in  FIGS. 1 and 2  with an upper handle portion of the housing broken away to display internal components. 
         FIG. 4  is a rear perspective view of the pipettor shown in  FIGS. 1-3  with a rear housing member removed in order to show components accessible from the rear of the pipettor when the rear housing member is removed. 
         FIG. 5  is a view similar to  FIG. 4  which also schematically illustrates the insertion of a wireless communications chip into an insolated chamber accessible from the rear of the pipettor upon removing the rear housing member. 
         FIG. 6  is a view similar to  FIG. 5  showing the wireless communication chip installed. 
         FIG. 7  illustrates an aspiration look-up table and a dispensing look-up table in accordance with the preferred embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1-6  show a single channel pipettor  10  constructed in accordance with a preferred embodiment of the invention. While the embodiment shown is a single channel pipettor  10 , it should be understood that many aspects of the invention may apply to multi-channel pipettors as well. The single channel pipettor  10  shown in the drawings is a hand-held electronic pipettor which is programmable by the user to aspirate and dispense liquid samples or reagents. The hand-held pipettor  10  includes a mounting shaft  12  onto which a disposable pipettor tip  14  is mounted as is known in the art. Although not shown in the drawings, the pipettor  10  includes an aspiration cylinder and a piston that is driven by motor  26  (see  FIGS. 3-6 ) as will be explained in more detail below. The pipettor  10  has an elongated body consisting of an upper portion  16  and a lower portion  17 . The aspiration cylinder and piston resides in the lower portion  17 . A finger hook  28  is located on the rear side of the upper portion  16  of the pipettor  10 . The upper portion  16  of the pipettor  10  is adapted to be held in the hand of a user, in the user&#39;s palm, with the user&#39;s index finger residing against the bottom surface of the finger hook  28  and the user&#39;s thumb available to operate controls on the front side of the pipettor, as shown in  FIG. 3 . 
     The front side of the pipettor  10  includes a touch wheel control  30 , a run button  18  located below the touch wheel control  30 , an ejector button  20  located below the run button  18 , and a user interface display  32  located above the touch wheel control  30 . As can be seen in  FIG. 1  and in particular in  FIG. 3 , each of the touch wheel control  30 , run button  18 , and ejector button  20  can be operated conveniently by the thumb of a user, thereby enabling convenient one-handed programming and operation. If desirable, the user can also conveniently use the touch wheel control to program the pipette with a finger on the opposite hand. The touch wheel control  30  is used to control the pipettor  10 , preferably in accordance with menu driven software controlling the information displayed on the user interface display  32  and providing an interface for programming the microprocessor to operate the pipettor  10 . While not described in detail herein, the preferred features of the menu driven software are disclosed in co-pending patent application filed on even date herewith, now U.S. Pat. No. 8,033,188 and entitled “Pipettor Software Interface,” which is assigned to the assignee of the present application and incorporated by reference herein. The run button  18  located below the touch wheel control  30  is actuated by the user after the pipettor  10  has been programmed in order to run aspiration and dispensing procedures. The ejector button  20  located below the run button  18  is actuated by the thumb of a user in the direction of arrow  22  to manually eject the disposable pipettor tip  14  from the pipettor  10 . More specifically, downward movement of the ejector button  10  activates an ejector mechanism which pushes the ejector sleeve  24  downward against a spring bias to engage the top of the pipettor tip  14  and eject the tip  14  from the tip mounting shaft  12  on the pipettor  10 . The preferred ejector mechanism is disclosed in co-pending U.S. patent application Ser. No. 11/856,193, entitled “Pipettor Tip Ejection Mechanism”, Publication No. 2009/0071267A, filed on even date herewith, assigned to the assignee of the present application and incorporated herein by reference, now abandoned. The preferred configuration for the mounting shaft  12  and the disposable pipettor tip  14  are disclosed in co-pending U.S. patent application Ser. No. 11/552,384, now U.S. Pat. No. 7,662,343, entitled “Locking Pipette Tip and Mounting Shaft” which is assigned to the assignee of the present application and incorporated herein by reference. It should be understood, however, that the features of the present invention need not be limited to the preferred ejection mechanism or the preferred configuration for the mounting shaft and pipettor tips disclosed in the above-referenced, co-pending patent applications. In the preferred embodiment, the ejection mechanism is driven manually by the user actuating the ejector button  20 . The run button  18  and the touch wheel control  30 , on the other hand, provide electrical control or programming inputs. 
     Referring in particular to  FIG. 3 , a circuit board  34  contains circuitry for the user interface display  32 , the touch wheel control  30 , and the run button  18 , as well as a microprocessor  38 , and flash memory  36 , and circuitry for driving the stepper motor and for battery charging and power management. 
     Preferably, the pipettor  10  includes at least one megabyte of electrically erasable programmable read-only memory, i.e., flash memory  36 . Suitable flash memory  36  is available, e.g., from Atmel Corporation. The microprocessor  38  can be any suitable microprocessor, e.g., from Texas Instruments Inc. The microprocessor  38  is used for programming functions and data storage, in combination with the flash memory  36 . The microprocessor  38  and the flash memory  36  allow ample memory storage for programs, calibration information, screen saver images, etc. The flash memory  36  stores information even if power is lost. Preferably, the pipettor includes a reset button which will reset the RAM on the pipettor  10 , i.e., the microprocessor  38 , but programs stored in the flash memory  36  are maintained. The use of flash memory  36  allows storage of larger programs and data, as well as the ability to reprogram or re-flash new software for future enhancements. Flash memory  36  normally has good kinetic shock resistance whether powered or not. Moreover, as discussed below, use of the flash memory  36  allows the use of separate aspiration and dispensing lookup tables that can be programmed into flash memory to more accurately correlate aspirating and dispensing volumes to motor movement for specific pipettor models. The microprocessor  38 , as mentioned, contains menu driven software which controls the operation of the pipettor  10  in accordance with either canned or custom procedures and user input. 
     The preferred user interface display  32  is a color 128 by 128 pixel LCD display, such as is available from Truly Semiconductors Limited, headquartered in Hong Kong. The size of the screen for the preferred LCD display is 1.5 inches across the diagonal. It has been found that such a display provides clarity and ease of programming to the user. Further, using the color display allows the software to be color coded, as described in the above-incorporated co-pending patent application, now U.S. Pat. No. 8,033,188 entitled “Pipettor Software Interface”. The improved clarity of the display  32  allows the pipettor  10  to provide more complete information on the display  32 , such as eliminating the need for abbreviations on the display and/or providing a meaningful help function on board the pipettor. The display  32  provides interactive user interface output for all pipettor programming actions, indicators and help screens. Navigation of the menus displayed on a user interface display  32  is accomplished using the touch wheel control  30 . The user interface display  32  is mounted to the upper end of the circuit board  34  and is attached to the upper portion  16  of the pipettor  10 , such that the screen of the display  32  can be seen through an opening  40  in the pipettor housing. 
     The upper portion  16  of the pipettor includes a front housing member  42  and a rear housing member  44 . The front housing member  42  and the rear housing member  44  are attached together to enclose the internal components of the pipettor  10 .  FIG. 3  illustrates in cross-section the front housing member  42  and the rear housing member  44  with the enclosed internal components. In  FIGS. 4 ,  5  and  6 , the rear housing member has been removed. The circuit board  34  (containing the circuitry for the user interface display  32 , the touch wheel control  30 , the run button  18 , the microprocessor  38 , the flash memory  36 , and circuitry for driving the stepper motor and for battery charging and power management) is attached to the upper portion of the front housing member  42 , preferably using screws or the like. As mentioned, the front housing member  42  has an opening  40  through which the screen for the user interface display  32  can be viewed by the user. 
     As is known in the art, the preferred touch wheel control  30  includes a capacitance circular touch pad  46  and a central enter or “OK” button  48 , see  FIG. 2 . The circular touch pad  46  comprises a plastic molded cover with a flexible printed circuit board glued to its underside. The flexible printed circuit board is made of Kapton® polyimide film with printed copper etches on the backside. The printed copper etchings on the back side of the flexible material define regions around the circular touchpad  46  as is generally known in the art. The printed flexible circuit is electrically connected to a devoted microprocessor (Quantum 4k) which is designed to detect the capacitance of a finger or thumb in one or more regions of the circular touch pad  46 . As is also known in the art, the touch wheel control  30  is programmed to translate relative rotational movement into up and down scrolling movements on the display screen  32 , for example, clockwise motion of the thumb scrolls menu downward whereas counterclockwise motion of the thumb scrolls the menu upward. The plastic cover for the touchpad  46  also has four downwardly extending protrusions (not shown in the drawings) spaced equally around the circular touchpad  46 . There are six switches also mounted to the main circuit board  34 , four corresponding to the location of the downward protrusions on the circular touch pad  46 , one corresponding to the location of the enter button  48 , and one corresponding to the position of the run button  18 . Control signals are sent to the microprocessor  38  when the user presses any one of these switches. With respect to the switches activated by the circular touchpad  46 , the microprocessor  38  preferably includes software which uses information from the capacitance sensor to ensure that a proper control signal sent to the microprocessor  38  in the event that more than one switch is activated when the user depresses the touchpad  46 . In the software, pressing the enter button normally signals to select the value highlighted on the display  32  per the rotational movement detected on the circular touchpad  46 . The four selector locations on the circular touch pad  46  are preferably labeled with symbols on the circular touch pad  46 , as shown in  FIG. 2 . The top of the circular touch pad  46  shows a “go back” symbol  50  which serves as a back button for menu selections. When a user presses and holds the back button on the circular touch pad  46 , the menu on the display will go back to the previous menu or information displayed. The circular touch pad  46  also includes right and left navigation buttons  52 ,  54 , respectively. The user can navigate a right arrow menu prompt using the right navigation button  52 , and a left arrow menu prompt by pressing the left navigation button  54 . The circular touch pad  46  also includes a “purge” symbol  56 . When the user presses and holds the purge button  56 , the pipettor  10  will empty. In other words, the pipettor  10  is programmed to do a complete dispense and blowout when the user presses and holds the purge button  56 . More specifically, it is preferred that pressing the purge button  56  will display a prompt on the display  32  for the user to press the run button  18  to proceed with purging. Normally, after completing the final dispense in a typical dispense cycle, a blowout will be initiated automatically upon pressing of the run button  18 . As is known in the art, the pipettor  10  should be held with the tips out of the liquid to complete the blowout and avoid aspirating liquid back into the tip(s). Alternatively, the run button  18  can be held in the liquid to perform the blowout with the pipettor returning home upon releasing run button  18 . The “purge” button  56  allows a user to prematurely end the procedure, such as would be the case if the user wanted to start over. 
     As mentioned, there are four switches associated with the circular touch pad  46 , one switch associated with the enter or “OK” button  48 , and one switch associated with the run button  18 . Preferably, the switches associated with the circular touch pad control  46  and enter or “OK” button  48  are used for programming the pipettor, and the switch associated with the run button  18  is used to initiate pipetting protocols or pipetting steps. For a more complete description of the preferred embodiment, refer to the incorporated co-pending patent application, now U.S. Pat. No. 8,033,188, entitled “Pipettor Software Interface,” filed on even date herewith. 
     As mentioned, the motor  26  is preferably a stepper motor controlled by the microprocessor  38 . The preferred stepper motor  26  is DC powered, and capable of being controlled in full steps or half steps. A suitable stepper motor can be purchased from Haydon Switch and Instrument, Waterbury Conn. The preferred motor drives at 0.002 inches per each full step and 0.001 inches per each half step. Preferably, the microprocessor  38  is capable of supplying DC power the stepper motor  26  via pulse width modulation in order to control the average current provided to the motor. Pulse width modulation provides several advantages. First, pulse width modulation can be used to control the rate of acceleration, which in turn smoothes the acceleration and deceleration per each motor step. This prevents the motor from over shooting the desired position and also minimizes mechanical oscillation of the motor. In addition, slower acceleration at start up helps to overcome binding forces by rubber seals in a smooth fashion so there is no need to home after the pipette has been sitting idle, at least in single channel pipettors where the sealing binding forces are relatively small compared to multi-channeled pipettors. In addition, pulse width modulation can be used to reduce the current draw when desirable, which not only reduces energy consumption but also reduces heat generation. Further, it is preferred that the power management circuitry be designed to scavenge inductive energy in the motor back to the battery as is known in the art, to extend battery life. The characteristics of the preferred battery are discussed below. 
     As is known in the art, the stepper motor  26  provides rotational stepping or half stepping motion which is converted into axial motion of the piston within the aspiration and dispensing chamber. The full stroke movement of the preferred stepper motor is approximately 1.1 inches per second at top pipetting speed. However, it may be desirable in some circumstances to use a half stepping routine in order to improve pipetting accuracy. If higher pipetting speeds are desirable, it may be desirable to use half stepping at motor start up while the piston is released from binding its forces that have developed in seal areas in the pipettor  10 , and then convert to a full stepping routine for faster speed once the motor  26  is moving, and finally convert back to a half-stepping routine as the motor  26  approaches the end of the movement in order for increased accuracy. On the other hand, in pipettors  10  using the preferred motor, it may be desirable to operate in half stepping mode completely. Preferably as mentioned, it may be desirable to limit acceleration to the top pipetting speed through the combination of pulse width modulation to limit current supply to the motor and using half step control perhaps in combination with full step control to ensure stable acceleration to top speed. Moreover, it may desirable to adjust the top pipetting speed, for example the user may have the capability of selecting between 10 top pipetting speeds as disclosed in the incorporated co-pending patent application, now U.S. Pat. No. 8,033,188, entitled “Pipettor Software Interface” filed on even date herewith. In that case, it is preferred that acceleration and deceleration tables be loaded into flash memory  36  for each speed. 
     Depending on the application, the pipettor  10  and the tips  14  are designed to operate over a preferred volume range, as is generally known in the art. For example, in a single channel pipettor  10  as shown in the Figures, it may be preferred to design the pipettor  10  to operate over a volume range of 0.5 to 12.5 μl, or a range of 5 to 125 μl, or a range of 10 to 300 μl, or a range of 50 to 1,250 μl, or a range of 100 to 5,000 μl. Multi channel pipettors would likely not be designed for the higher volumes, especially if the pipettor included a large number of channels, such as 16 channels.  FIG. 2  shows a label  60  on the pipettor  10  for indicating the size of the pipetting volumes for which the pipettor is designed. 
     It is typical in the art to use a lookup table to correlate motor steps to pipetting volumes. In accordance with one aspect of the preferred embodiment of the invention, the pipettor  10  includes an aspiration lookup table that determines the number of motor steps necessary to aspirate a selected aspiration volume, and a separate dispensing lookup table that determines the number of motor steps necessary to dispense a selected or programmed volume. The values in the aspiration lookup table are determined empirically to account for aspiration inaccuracies in the pipettor  10 , whereas the values in the dispensing lookup table are determined to empirically account for dispensing inaccuracies in the pipettor  10 . This is particularly helpful because, as mentioned above, the forces acting on liquids being aspirated and forces acting on liquids being dispensed are quite different and this can have a substantial effect, especially when dealing with small aliquots. In addition, mechanical imprecision during aspiration is typically different than during the dispense cycle. Preferably, the aspiration and dispensing lookup tables reside in the flash memory  36 .  FIG. 7  illustrates portions of a data table containing data correlations for the translator, the aspiration look-up table  36 B and the dispensing look-up table  36 C for a 300 μl pipettor. The entire data table in  FIG. 7  preferably resides in flash memory  36 . 
     Referring to  FIG. 7 , in the preferred system, the pipettor  10  displays (or calculates from inputted data) a list of aspiration and dispensing volumes  36 D on the display screen  32 . For the 300 μl pipettor  10 , the volumes are displayed in 0.5 μl increments. The volume listings  36 D in  FIG. 7  show these increments without the decimal point. The displayed volumes correspond to a numerical index, for example ranging from 1 to 1,000, or 1 to 1,200, although a smaller range such as 1-600 can be used for smaller sizes such as the illustrated 300 μl pipettor. The full range of index values  36 A, for example 1-600 in  FIG. 7  corresponds to the full stroke of the piston. Each index value 1-600 also corresponds to a motor half motor step (i.e., 0.001 inches) value for aspiration  36 B and for dispensing  36 C. The translator in flash memory  36  correlates the displayed (or calculated) volume  36 D to a respective index value  36 A. For aspiration, the index value  36 A points to a position in an aspiration lookup table  36 B which also resides in the flash memory  36 . The aspiration lookup table  36 B correlates the index value  36 A to a corresponding retraction distance in terms of the number of motor steps (or half steps) necessary to aspirate the selected volume  36 D. It is this number of motor steps (or half steps) that is input into the microprocessor  38 , and which the microprocessor uses for operation. In a similar manner, the flash memory  36  also contains a dispensing lookup table  36 C that correlates dispensing volumes  36 D to an extension distance in terms of motor steps (or half steps) similar to that described above with respect to the aspiration lookup table, which the microprocessor  38  uses to control precise dispensing steps. The values in columns  36 B and  36 C for the 300 μl pipettor indicate the number of motor half steps. For neat pipetting, i.e., when the full amount of the aspirated liquid is to be dispensed, it is necessary only to use the aspiration look-up table  36 B. On the other hand, the dispensing look-up table  36 C is used, for example when dispensing multiple aliquots. 
     The translator index values  36 A in flash memory  36  and the aspiration and dispensing lookup tables in flash memory  36  are determined separately for each pipettor model. The translator normally correlates the full range of pipettor volume to the index scale, for example 1 to 1,200 or 1-600 in the case of the 300 μl pipettor, although sometimes it may be desirable to use less than the full piston stroke. The aspiration lookup table  36 B is preferably determined via empirical analysis for each pipettor model similar to the techniques used in the prior art such as Nishi U.S. Pat. No. 3,915,651 entitled “Direct Digital Control Pipettor,” incorporated herein by reference. The output of the aspiration lookup table  36 B, as mentioned, is the number of motor steps (or half steps) that is necessary to aspirate the displayed (or calculated) volume, based on empirical data that accounts for various inaccuracies in the pipettor  10 , including mechanical inaccuracies and inaccuracies due to air pressure and surface tension effects, as well as any other inaccuracies. While the aspiration lookup table  36 B is determined for each model, it can preferably be calibrated by the user from time to time using conventional laboratory calibration techniques and then by shifting the table  36 B either upward or downward. With respect to multiple dispensing, the pipettor  10  preferably retracts the piston an amount for the overall volume of the liquid, then retracts the piston to draw an additional volume for a fixed number of motor steps, and then reverses the motor to extend the piston the same number of motor steps, thus leaving in the tip  14  the selected volume of liquid. The purpose of the motor reversal is to accommodate the effect of mechanical backlash in the piston drive mechanism, as described in prior art Klein U.S. Pat. No. 4,399,711. The total selected volume can be entered in various modes into the pipettor. For example, the total volume can be selected, and then the number of aliquots can be entered, or the number of aliquots can be entered with a constant volume for each aliquot being entered, or custom or variable amounts can be entered for each aliquot. In any event, the translator in the flash memory  36  correlates the selected dispensing volume  36 D or volumes to an index scale  36 A which is then correlated to an empirically determined number of motor steps  36 C for piston extension in order to dispense the appropriate volume. 
     By way of example, referring to  FIG. 7 , a displayed aspiration volume of 300 μl corresponds to an index value of 600, and an index value of 600 corresponds to 968 motor half steps as shown in column  36 B. Therefore, the microprocessor would instruct the motor to retract 968 half steps to aspirate 300 μl into the pipette tip  14  (apart from any kind of motor reversal). Further, a selected or calculated dispense volume of 12.5 μl corresponds to an index value of 25 in  FIG. 7 , and an index value of 25 corresponds to 40 motor half steps in column  36 C. Therefore, the microprocessor would instruct the motor to extend  40  half steps to dispense a 12.5 μl aliquot. To dispense another 12.5 μl aliquot, the volume in column  36 D would be 25 (i.e., 12.5+12.5), and the corresponding values (not shown) in columns  36 A and  36 C would be used to determine the number of additional motor half steps that the microprocessor would instruct the motor to extend. 
     The battery  58  is preferably a rechargeable, long life, lithium ion battery, having a voltage of 3.6 volts and a capacity of 1050 milliamp hours. The battery preferably has an elongated cylindrical shape, and has a 2-pin connector at its top end which is plugged into the main circuit board  34 . The pipettor  10  has an electrical power supply port  62  for charging the battery  58 , and also preferably is adapted for stand charging, e.g., using spring connectors on its backside. As mentioned, the main circuit board  34  contains electronic components for battery charging and power management. The battery should provide for at least two to four hours of continuous pipettor use. While many types of rechargeable batteries can be used in accordance with the invention, a suitable battery can be purchased from Great Power Battery Company, Hong Kong. Preferably, the amount of remaining battery life should be indicated on the display of the pipettor  10 . 
     The front housing member  42  is made of molded plastic polycarbonate and includes structural support and framing elements to assist in the mounting of the internal components of the pipettor  10 . For example, the front housing  42  includes framing platform  64  as well as vertical support structure  66 . The vertical support structure  66  is molded in a shape providing two compartments on its rear side, namely a battery compartment  68  and a wireless communications chip compartment  70 . As mentioned, the battery  58  has an elongated cylindrical shape as is common. The upper end of the rechargeable battery  58  resides in the battery compartment  68  whereas its lower end is supported on the frame platform  64 . Suitable electrical connection terminals are provided at the upper end of the compartment  68  as is known in the art. The motor  26  is mounted to the lower end of the platform  64 . Referring in particular to  FIG. 3 , the upper end of the battery  58  is mounted so that it is at a height above the height of the finger hook  28 , whereas the motor  26  is mounted at a height substantially below the battery  58 . The battery  58  and the motor  26  provide most of the weight in the upper portion  16  of the pipettor  10 . With the preferred configuration having the battery  58  mounted so that a top portion of the battery  58  is above the height of the finger hook  28 , the center of gravity of the pipettor  10  resides within the palm of the user. 
     While the pipettor  10  is designed for on board programming, it may be desirable to establish communication between the pipettor  10  and a personal computer (PC) for various reasons. In the preferred embodiment of the invention, the pipettor  10  can be adapted to accommodate a wireless communication chip  72  such as a Bluetooth wireless communication chip. The wireless chip  72  allows communication between the pipettor  10  and a PC without cluttering lab space with wires. Preferably, a wireless component can be connected to the PC either internally or using a USB connector, but several pipettors could wirelessly communicate with a single PC. For example, typical wireless communication systems have an open field connectivity of about 300 meters. However, because wireless communication is not necessary in many pipetting applications, many users may not wish to have this feature. Therefore, in accordance with one aspect of the invention, the pipettor  10  is designed to simplify the addition of a wireless communication chip  72  to the pipettor  10 , either at the factory or in the field. More specifically, the vertical support structure  66  on the front housing member  42  has compartment  70  for accommodating the wireless chip  72 . A user in the field can remove the rear housing member  44  (by removing screws) and can obtain access to the battery  58  as well as the wireless chip compartment  70 . At the same time, however, the vertical support structure  66  isolates the other electronic components the pipettor  10  from the user, thereby protecting the integrity of the pipettor  10 . Note that the compartment  70  includes an electrical connection  74 . To establish communication between the wireless communication chip  72  and the pipettor microprocessor  38 .