Patent Publication Number: US-10760939-B2

Title: Liquid level sensing apparatus and related methods

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent arises from a continuation application of U.S. patent application Ser. No. 15/629,495 filed on Jun. 21, 2017, (now U.S. Pat. No. 10,401,209), which claims the benefit of U.S. Provisional Patent Application No. 62/353,461, filed Jun. 22, 2016. U.S. patent application Ser. No. 15/629,495 and U.S. Provisional Patent Application No. 62/353,461 are hereby incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure relates generally to liquid level sensing systems, and, more particularly, to liquid level sensing apparatus and related methods. 
     BACKGROUND 
     Clinical chemistry and immunoassay diagnostics provide automated analyzers for processing biological fluids. Automated clinical analyzers provide rapid results with relatively high accuracy. To automate clinical chemistry and immunoassay diagnostics, analyzer systems often employ liquid sensing methods to aspirate a sample fluid from a container or test tube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example automated analytical apparatus that may be implemented with an example liquid level sensing apparatus and related methods in accordance with the teachings of this disclosure. 
         FIG. 2  illustrates a top plan view of a portion of the automated analytical apparatus illustrated example pipette system implemented with the example liquid level sensing apparatus and related methods disclosed herein. 
         FIG. 3  is a block diagram of an example liquid level sensing apparatus in accordance with the teachings of this disclosure. 
         FIG. 4  is a schematic illustration of the example liquid level sensing apparatus of  FIG. 3 . 
         FIGS. 5-8  illustrate schematic illustrations of the example liquid level sensing apparatus of  FIGS. 2-4  at various stages of operation. 
         FIG. 9  is a flowchart representative of an example method of implementing the example liquid level sensing apparatus of  FIGS. 3-8 . 
         FIG. 10  is a block diagram of an example processor platform capable of executing the instructions of  FIG. 9  implementation of the liquid level sensing apparatus of  FIG. 3 . 
     
    
    
     The figures are not to scale. Instead, to clarify multiple layers and regions, the thickness of the layers may be enlarged in the drawings. Wherever possible, the same reference numbers will be used throughout the drawing(s) and accompanying written description to refer to the same or like parts. 
     DETAILED DESCRIPTION 
     Automated analytical apparatus (e.g., clinical chemistry and/or immunoassay diagnostic analyzers) rely on liquid level sensing to automate processing or testing of fluids (e.g., a biological fluid or sample), reagent liquids and/or buffer liquids contained or stored in containers or tubes. For example, detecting the presence of the liquid and/or locating a surface of the liquid permits controlled emersion of a pipetting apparatus into the liquid to enable a consistent amount of liquid volume to be aspirated or removed from a container. For example, known clinical analyzers typically employ liquid level sensing for use with open, uncapped containers. When an open, uncapped container is employed, a pipetting probe of the clinical analyzer may be positioned within the open-end container and a volume of the sample fluid may be aspirated or removed from the container. 
     Example liquid level sensing systems disclosed herein provide a pipetting system employing automated liquid level sensing capability. Specifically, example liquid level sensing systems disclosed herein may be used with automated analytical apparatus employing either open-end containers and/or closed or capped containers. Example liquid level sensing systems disclosed herein may include a pipetting system employing a pipetting probe to aspirate a liquid from a container and a cannula to pierce a septum or cap (e.g., a cover or top) of a container. In some examples, example pipetting systems disclosed herein enable a consistent amount of liquid volume to be aspirated or removed from a container. To detect the presence of the liquid and/or an amount or distance of emersion of the pipetting probe into the liquid, example liquid level sensing apparatus disclosed herein may be configured to emit or transmit a signal (e.g., a radio frequency signal) toward an antenna positioned adjacent the container. 
     In particular, to detect a liquid level of a container, example liquid level sensing apparatus disclosed herein may cause the pipetting probe to transmit a first signal. Additionally, to prevent the piercing cannula from degrading and/or otherwise interfering with a signal transmitted by the pipetting probe during level sense operation (e.g., when the pipetting probe is moving toward a liquid in a container), an example cannula disclosed herein may be configured to transmit a second signal. In some examples, the first signal is substantially the same (e.g., identical) to the second signal. In some examples, the first signal is different than, or phase-shift controlled relative to, the second signal. In some examples, the first signal and/or the second signal may have a frequency between approximately 0.3 Hz and 300 GHz. In some examples, the first signal and/or the second signal may be a digital signal and/or any other type of signal. For example, the first signal and/or the second signal may be digital pulse signal and/or any other type of signal modulation. In some examples, the first signal and/or the second signal may have a frequency between approximately a DC potential and 300 GHz. 
     In some examples, example liquid level sensing systems disclosed herein may include a phase-lock loop (e.g., an analog phase-lock loop, a digital phase-lock loop, etc.) for frequency generation, improved frequency stability and alignment (e.g., to maintain a relationship such as a phase-shift relationship, an equivalent relationship) between the first signal and the second signal. Thus, a phase-lock loop control system may be employed for signal phase integrity and/or noise and jitter control. 
     Example liquid level sensing systems disclosed herein electrically energize a cannula and the pipetting probe with electrically phase controlled signals. For example, a signal source (e.g., an oscillator) may provide electrically in-phase signals (e.g., the same signals or frequency, or a signal or frequency having a predetermined phase shift) to the cannula and the pipetting probe so that the pipetting probe and the cannula emit signals that are substantially the same or controlled (e.g. via phase shift). 
     In this manner, the pipetting probe and/or the piercing cannula may be composed of an electrically conductive material (e.g., a metallic material) to enable repeated piercing of containers without causing damage the cannula and without the cannula interfering or degrading the signal emitted by the pipetting probe when the pipetting probe is disposed in a container to detect a level of a liquid. As a result, the energized piercing cannula disclosed herein may puncture (e.g., vertically puncture) a top or cap of a closed container and the energized pipetting probe may be positioned through a channel or opening of the energized piercing cannula at similar or equal electrical potential (e.g., electrically phase controlled) without electrical impediment of the signal transmitted by the pipetting probe as the pipetting probe makes coordinated contact with liquid in the container. 
     An antenna (e.g. a transmitting or receiving antenna) may be positioned near or adjacent the container to receive or transmit the detected signal transmitted by the pipetting probe and/or the cannula, which may be analyzed for indication of contact with liquid in the container. In some examples, if the signal emitted by the cannula is phase controlled (e.g., radiates a frequency or signal having a phase shift) compared to the signal emitted by the probe, the antenna and/or a signal analyzer may be configured to differentiate (e.g., filter) the signal emitted by the probe and the signal emitted by the cannula. In addition, example liquid sensing systems disclosed herein maintain integrity of liquid level sensed during a descent of an electrically conductive pipetting probe through an electrically conductive piercing cannula that has pierced a closed or capped container. In some examples, an example piercing cannula disclosed herein may be energized using a wire to carry the signal (e.g., a radio frequency signal) from, for example, a signal source (e.g., a signal generator, a printed circuit board, etc.) to the piercing cannula. In some examples, the piercing cannula is energized using capacitive coupling to provide a connection between the signal source, the piercing cannula and/or the probe. 
       FIG. 1  is an example automated analytical apparatus (e.g., an immunoassay analytical system) in accordance with the teachings of this disclosure. The analytical system  100  of the illustrated example provides an automated continuous and/or random access analytical system, capable of simultaneously effecting multiple assays of a plurality of liquid samples. The analytical system  100  of the illustrated example includes a user input or interface to enable a user and/or other control system to input information or commands to the analytical system  100 . In some examples, the example liquid level sensing apparatus disclosed herein may be retrofitted with existing analytical systems employed in the field. For example, the liquid level sensing apparatus disclosed herein may retrofit a system disclosed in U.S. Pat. No. 5,627,522, which is incorporated herein by reference in its entirety. 
       FIG. 2  is plan view of the analytical system  100  of  FIG. 1 . The example analytical system  100  of the illustrated example employs a pipetting system  200  (e.g., a robotic arm pipetting system) employing an example liquid level system in accordance with the teachings of this disclosure. The pipetting system  200  obtains fluid from containers  202  held by a rotating carousel  204 . The containers  202  may include sample containers, reagent containers, and/or any other container. The containers  202  of the illustrated example may include a (e.g., pierceable) septum or cover  206  that covers an opening of the containers  202  (e.g., caps, lids, etc.). The example analytical system  100  of  FIGS. 1 and 2  includes a first carousel  208  that is serviced by a first pipetting system  210  and a second carousel  212  that is serviced by a second pipetting system  214 . The first pipetting system  210  and/or the second pipetting system  214  may employ the example liquid sensing system disclosed herein. To move (e.g., aspirate or deliver) fluids relative to the containers  202 , the first pipetting system  210  and the second pipetting system  214  include a drive system  216 . The drive system  216  of the illustrated example includes a translatable arm or guide  218  to move a probe or cannula relative to the containers  202  positioned in the first carousel  208  or the second carousel  212  in a first direction  220  (e.g., a horizontal direction) and a second direction  222  (e.g. a vertical direction, which would be into and out of the paper or screen in the orientation shown in  FIG. 2 ) that is different than the first direction  220  in the orientation of  FIG. 2 . For example, the translatable arm or guide  218  may include a first telescopically extending arm to move the cannula and/or the probe in the first direction  220  relative to the containers  202 , a second telescopically extending arm to move the cannula in the second direction  222 , and a third telescopically extending arm (e.g., slidable within the second telescopically extending arm) to move the probe in the second direction  222 . The third telescopically extending arm may be configured to move the probe independently of the second telescopically extending arm of the cannula. 
       FIG. 3  is a block diagram of an example pipetting system  301  implemented with an example liquid level sensing system  300  in accordance with the teachings of this disclosure. For example, the example pipetting system  301  and/or the liquid level sensing system  300  of the illustrated example may be used to implement the first pipetting system  104  and/or the second pipetting system  106  of the example analytical system  100  of  FIGS. 1 and 2 . It is to be understood that the liquid level sensing system  300  of the present invention can be utilized in any automated instrument where liquid level sensing is desired. 
     The pipetting system  301  of the illustrated example includes a pipetting probe  302  and a piercing cannula  304  that are movable relative to a container  306  (e.g., the container  202  of  FIG. 2 ) via a drive system  308  (e.g., the drive system  216  of  FIG. 2 ). For example, the pipetting probe  302  of the illustrated example transfers fluid to and/or from the container  306  (generically representing a container in a reaction vessel, a reagent pack, or a test container). In some examples, the container  306  may be supported by a holder  310 . The container  306  may be, for example, a test tube or other containers having a closed end provided by, for example, a cap (e.g. having a septum). The piercing cannula  304  pierces or provides an access opening to a closed end of the container  306  to enable the pipetting probe  302  to be inserted in a cavity of the container  306 . More specifically, the pipetting probe  302  passes through a passageway of the piercing cannula  304  to access the container  306 . 
     The drive system  308  of the illustrated example includes a probe positioner  312 , a first motor  314 , a cannula positioner  316  and a second motor  318 . The probe positioner  312  moves the pipetting probe  302  relative to the cannula  304  and/or the container  306  via the first motor  314 . For example, the probe positioner  312  moves the pipetting probe  302  relative to the cannula  304  and/or the container  306  in the first direction (e.g., the vertical or up and down direction relative to an upper surface or cap of the container  306 ) and the second direction (e.g., the sideways or horizontal direction relative to an upper surface or cap of the container  306 ) different than the first direction. The cannula positioner  316  moves the piercing cannula  304  relative to the container  306  via the second motor  318 . For example, the cannula positioner  316  moves the cannula  304  relative to the container  306  in a first direction (e.g., a vertical or up and down direction relative to an upper surface or cap of the container  306 ) and a second direction (e.g., a sideways or horizontal direction relative to an upper surface or cap of the container  306 ). The probe positioner  312  and/or the cannula positioner  316  may include a telescopically movable robotic arm (e.g., the guide  218  of  FIG. 2 ) to move the pipetting probe  302  and/or the cannula  304  in the first direction and a telescopically extending arm to move the pipetting probe  302  and/or the cannula  304  in the second direction. In some examples, the probe positioner  312  is a dedicated telescoping arm that moves independently from the cannula positioner  316 . In this manner, the probe positioner  312  can move the pipetting probe  302  in the first direction and/or the second direction relative to the cannula  304 , and the cannula positioner  316  can move the cannula  304  in the first direction and/or the second direction relative to the pipetting probe  302 . 
     To determine a position of the cannula  304  relative to the container  306  (e.g., an upper surface or cap of the container  306 ), the example pipetting system  301  employs one or more sensors  320 . In some examples, the sensors  320  (e.g., optical sensors, infrared sensors, etc.) determine an amount or distance the cannula  304  has moved relative to an upper surface or end of the container  306 . In some examples, the sensors  320  detect or measure a distance between an end of the cannula and the upper end of the container  306  (e.g., a closed container). In some examples, the sensors  320  detect the presence of the container  306  and/or an upper surface of the container  306 . In some such examples, the sensors  320  sense when the cannula  304  engages and/or pierces the upper surface (e.g., a septum or cap) of the container  306 . In some examples, the sensors  320  determine when the cannula  304  has penetrated the container  306  and is positioned in the cavity at a predetermined distance relative to the upper surface of the container  306 . 
     To control the operation of the probe positioner  312  via the first motor  314  and the cannula positioner  316  via the second motor  318 , the example pipetting system  301  of the illustrated example employs a controller  322 . For example, the controller  322  commands the second motor  318  to move the cannula  304  in the first direction and/or the second direction via the cannula positioner  316 , and the controller  322  commands the first motor  314  to move the pipetting probe  302  in the first direction and/or the second direction via the probe positioner  312 . To provide information regarding a position of the pipetting probe  302  and/or a position of the cannula  304  relative to the container  306  (e.g., an upper surface of the container  306 ), the pipetting system  301  of the illustrated example employs a position determiner  324 . The position determiner  324  may receive one or more signals from sensors  320  providing feedback information regarding a position of the pipetting probe  302  and/or a position of the cannula  304  relative to the container  306 . For example, the position determiner  324  determines a position of the pipetting probe  302 , a position of the cannula  304 , and/or a position of the container  306  and communicates this information to the controller  322 . The controller  322 , in turn, controls the movement of the pipetting probe  302  and/or the cannula  304  based on the position information provided by the position determiner  324 . For example, the controller  322  may prevent or stop operation of the first motor  314  until the position determiner  324  determines that the cannula  304  has pierced the container  306  and/or is positioned inside the container  306 . After the position determiner  324  determines that the cannula  304  is inside the container  306 , the controller  322  may cause or command the first motor  314  to operate the probe positioner  312  and cause the probe  302  to move inside the container  306  (e.g., via an access opening of the cannula  304 ). In some examples, the controller  322  commands the second motor  318  to stop movement of the cannula  304  when the position determiner  324  determines that the cannula  304  is inside a cavity of the container  306 . 
     To determine when the pipetting probe  302  engages a fluid (e.g., a biological sample or a reagent) in the container  306 , the pipetting system  301  of the illustrated example employs the liquid level sensing system  300 . The liquid level sensing system  300  of the illustrated example includes a processor  326 , a signal generator  328 , a signal analyzer  330 , and an input/output interface  332  that are communicatively coupled via a field bus  334 . The signal analyzer  330  of the illustrated example includes an amplitude detector  336  and a rate of change of amplitude detector  338 . In some examples, however, the signal analyzer  330  of the illustrated example includes the amplitude detector  336 . The liquid level sensing system  300  is communicatively coupled to the controller  322  via a wired and/or wireless communication channel or communication link  340 . 
     In the illustrated example, the signal generator  328  electrically energizes the pipetting probe  302  and the cannula  304  with an electrically phase controlled signal. In some examples, the signal generator  328  of the illustrated example electrically energizes the pipetting probe  302  and the cannula  304  upon movement of the pipetting probe  302  via the probe positioner  312  and/or the cannula  304  via the cannula positioner  316 . For example, the controller  322  may communicate or command the signal generator  328  via the input/output interface  332  and the communication link  340  to electrically energize the pipetting probe  302  and/or the cannula  304 . In some examples, the controller  322  communicates or commands the signal generator  328  to electrically energize the pipetting probe  302  and/or the cannula  304  after at least a portion of the cannula  304  is positioned or disposed inside (e.g., has pierced) the container  306  and prior to movement of the pipetting probe  302  toward the container  306 . 
     The signal generator  328  of the illustrated example electrically energizes the pipetting probe  302  to provide a first signal  342  to the pipetting probe  302  and electrically energizes the cannula  304  to provide a second signal  344  to the cannula  304 . For example, each of the pipetting probe  302  and the cannula  304  emits or transmits a radio frequency signal when the first signal  342  is provided to the pipetting probe  302  and the second signal  344  is provided to the cannula  304 . In some examples, the first signal  342  provided to the pipetting probe  302  is electrically in-phase with the second signal  344  provided to the cannula  304 . In some examples, the first signal  342  may be identical to the second signal  344 . For example, the first signal  342  of the pipetting probe  302  and the second signal  344  of the cannula  304  may each emit a signal having a frequency of approximately 125 kHz (e.g., plus or minus 10%). In some examples, the first signal  342  and/or the second signal  344  may have a frequency between approximately 0.3 Hz (or alternatively a DC potential) and 300 GHz. In some examples, the first signal  342  may be shifted out of phase (e.g., 90 degree phase shift, a 45 degree phase shift, or any other phase shift value) relative to the second signal  344 . For example, the first signal  342  may lag the second signal  344  by a predetermined phase shift. In some examples, the first signal  342  and/or the second signal  344  may have a waveform including a sine wave, a square wave, a triangular wave, a sawtooth wave, etc. In some examples, the signal generator  328  may include a first signal generator to generate the first signal  342  to the pipetting probe  302  and a second signal generator to generate the second signal  344  to the cannula  304 . The signal generator  328  of the illustrated example is a low impedance driver signal source. 
     In some examples, the signal generator  328  and, more generally, the example liquid level sensing system  300  may include a phase-lock loop control system for frequency generation, improved frequency stability and alignment between the first signal  342  and the second signal  344 . For example, the phase-lock loop circuit maintains a relationship between the signal of the cannula  304  and the signal of the pipetting probe  302 . In some examples, the phase-lock loop circuit maintains the electrically phase controlled electrical potential between the signal of the cannula  304  and the signal of the pipetting probe  302 . In some examples, a phase-lock loop circuit may include a voltage-controlled oscillator, a phase detector, a low-pass filter, a variable-frequency oscillator and a feedback that may include a frequency divider. 
     The liquid level sensing system  300  of the illustrated example detects when the pipetting probe  302  contacts a fluid (e.g., a liquid) inside a cavity of the container  306 . To determine when the pipetting probe  302  contacts the fluid in the container  306 , the example liquid level sensing system  300  employs the signal analyzer  330 . More specifically, as the pipetting probe  302  moves through the cannula  304  and/or inside the cavity of the container  306  and relative to the liquid in the container  306 , the liquid level sensing system  300  detects changes in the signal (e.g., the near-radio frequency (RF) signal) that is radiated by the pipetting probe  302  and received by an antenna  346  positioned adjacent the container  306  (e.g., an antenna or a plurality of antennas positioned adjacent the rotating carousel of  FIG. 2  or adjacent the container  306 ). The antenna  346  of the illustrated example is communicatively coupled to the signal analyzer  330  via the input/output interface  332  to transmit the received signal to the signal analyzer  330 . For example, the signal analyzer  330  continually monitors the signal of the pipetting probe  302  as the pipetting probe  302  moves through an air-filled portion (e.g., a non-liquid filled portion) of the container  306  above the liquid and detects a change in signal and a rate of change in signal to detect the signal of the pipetting probe  302  when the pipetting probe  302  contacts the liquid in the container  306 . The signal analyzer  330  (e.g., via a comparator) detects a change in the signal in response to the pipetting probe  302  contacting the liquid in the container  306  compared to the signal emitted by the pipetting probe  302  when the pipetting probe  302  is moving through the air-filled portion of the container  306  above the liquid (e.g., not in contact with the liquid  400 ). 
     More specifically, the amplitude detector  336  detects a change in the amplitude of the signal received by the antenna  346  as the pipetting probe  302  moves relative to the cannula  304  and/or the container  306 . The rate of change of amplitude detector  338  detects a rate of signal change received by the antenna  346  as the pipetting probe  302  moves through the air-filled portion towards the liquid filled portion. For example, both the amplitude detector  336  and the rate of change of amplitude detector  338  detect a change in the amplitude of the signal received by the antenna  346  as the pipetting probe  302  moves through the air (e.g., not in engagement with liquid in the container  306 ) and when the pipetting probe  302  comes into direct engagement with liquid in the container  306 . The example signal analyzer  330  determines the presence of liquid when both the amplitude detector  336  and the rate of amplitude detector  338  receive a signal indicative of the pipetting probe  302  coming into contact with the liquid. Basing liquid detections on both signal amplitude and rate of change of signal amplitude reduces the number of false or failed liquid detections. However, as the pipetting probe  302  contacts liquid in the container  306 , the amplitude of the signal generated by the pipetting probe  302  increases or, in other words, has a positive slope. Thus, for a given frequency of, for example, 125 kHz, the amplitude of the signal received by the antenna  346  may be greater than an amplitude of a system noise envelope when the pipetting probe  302  engages the liquid. The amplitude detector  336  detects a change in the amplitude of the detected signal as the pipetting probe  302  moves relative to the cannula  304  and/or the container  306 . To prevent false detections, the rate of change of amplitude detector  338  detects a change in the slope between the amplitude of the signal received by the antenna  346  and the slope of the system noise envelope. In this manner, if the slope of the detected change of amplitude is not greater than a threshold, the rate of change of amplitude detector  338  determines that the pipetting probe  302  has not contacted the liquid. If the slope is greater than a threshold, the rate of change of amplitude detector  338  determines that the pipetting probe  302  contacted the liquid. In some examples, the signal analyzer does not include the rate of change of amplitude detector  338  and instead only detects liquid when the amplitude detector  336  detects a change in the amplitude of a detected signal. 
     To remove or aspirate a volume of fluid from the container  306  when the liquid level sensing system  300  determines that the pipetting probe  302  is in contact with the liquid in the container  306 , the pipetting system  301  of the illustrated example employs a pump  348  (e.g., a syringe). For example, the liquid level sensing system  300  may be configured to send a signal to the controller  322  to activate the pump  348 . The controller  322  may command the first motor  314  and the second motor  318  to operate the probe positioner  312  and the cannula positioner  316  to remove the pipetting probe  302  and the cannula  304  from the container  306  after an amount of fluid is aspirated by the pipetting probe  302 . 
     The liquid level sensing system  300  enables the energized pipetting probe  302  to be positioned through a channel or opening of the energized piercing cannula  304  at an electrically phase controlled electrical potential (e.g., similar or equal frequency, controlled out of phase shift frequency, etc.) without electrical impediment of the signal emitted or transmitted by the pipetting probe  302  as the pipetting probe  302  makes coordinated contact with liquid in the container  306  while being adjacent (e.g., passing through) the cannula  304 . As noted above, a phase-lock loop control system may be employed for signal phase integrity and/or jitter control. 
     While an example manner of implementing the pipetting system  301  is illustrated in  FIG. 3 , one or more of the elements, processes and/or devices illustrated in  FIG. 3  may be combined, divided, re-arranged, omitted, eliminated and/or implemented in any other way. Further, the example liquid level sensing apparatus  300 , the example probe positioner  312 , the example cannula positioner  316 , the example controller  322 , the example positioner determiner  324 , the example processor  326 , the example signal generator  328 , the example signal analyzer  330 , the example amplitude detector  336 , the example rate of change of amplitude detector  338 , the example input/output interface  332  and/or, more generally, the example pipetting system  301  of  FIG. 3  may be implemented by hardware, software, firmware and/or any combination of hardware, software and/or firmware. Thus, for example, any of the example liquid level sensing apparatus  300 , the example probe positioner  312 , the example cannula positioner  316 , the example controller  322 , the example positioner determiner  324 , the example processor  326 , the example signal generator  328 , the example signal analyzer  330 , the example amplitude detector  336 , the example rate of change of amplitude detector  338 , the example input/output interface  332  and/or, more generally, the example pipetting system  301  of  FIG. 3  could be implemented by one or more analog or digital circuit(s), logic circuits, programmable processor(s), application specific integrated circuit(s) (ASIC(s)), programmable logic device(s) (PLD(s)) and/or field programmable logic device(s) (FPLD(s)). When reading any of the apparatus or system claims of this patent to cover a purely software and/or firmware implementation, at least one of the example liquid level sensing apparatus  300 , the example probe positioner  312 , the example cannula positioner  316 , the example controller  322 , the example positioner determiner  324 , the example processor  326 , the example signal generator  328 , the example signal analyzer  330 , the example amplitude detector  336 , the example rate of change of amplitude detector  338 , the example input/output interface  332  and/or, more generally, the example pipetting system  301  of  FIG. 3  is/are hereby expressly defined to include a non-transitory computer readable storage device or storage disk such as a memory, a digital versatile disk (DVD), a compact disk (CD), a Blu-ray disk, etc. including the software and/or firmware. Further still, the example pipetting system  301  of  FIG. 3  may include one or more elements, processes and/or devices in addition to, or instead of, those illustrated in  FIG. 3 , and/or may include more than one of any or all of the illustrated elements, processes and devices. 
       FIG. 4  is a schematic illustration of the example pipetting system  301  and the example liquid level sensing system  300  of  FIG. 3  to detect or sense a level of a liquid  400  in the container  306  via the pipetting probe  302  when using the cannula  304  to pierce the container  306 . The liquid level sensing system  300  includes a circuit board  402  (e.g., a printed circuit board) that includes the signal generator  328  to provide or generate the first signal  342  to the pipetting probe  302  and the second signal  344  to the cannula  304 . In some examples, the circuit board  402  is configured to receive approximately +5 Volts. 
     The first signal  342  generated by the signal generator  328  is transmitted to the pipetting probe  302  via a connector or first cable  408  (e.g., a coax cable). Similarly, the second signal  344  generated by the signal generator  328  is transmitted to the cannula  304  via a second connector or second cable  410  (e.g., a coax cable). In some examples, a phase-locked loop control system may be employed for frequency generation, improved frequency stability and alignment between the first signal  342  and the second signal  344 . 
     In some examples, neither the pipetting probe  302  nor the cannula  304  are grounded. In some examples, both the pipetting probe  302  and the cannula  304  are grounded. The antenna  346  of the illustrated example is communicatively coupled to the liquid level sensing system  300  via a connector or cable  412  (e.g., a coaxial cable). The antenna  346  of the illustrated example is grounded via a ground  414 . The antenna  346  is mounted or positioned in a stationary position beneath an area where liquid sensing is desired (e.g., beneath the container  306 ). In some examples, the antenna  346  includes a plurality of antennas positioned below the rotating carousel  204  of the analytical system  100  of  FIG. 1 . The antenna  346  transmits the received signal from the pipetting probe  302  to the liquid level sensing system  300  via the cable  412 . The example liquid level sensing system  300  of the illustrated example may employ a shield circuit  416  as described in greater detail in connection with  FIG. 10 . 
       FIG. 5  illustrates the pipetting system  301  in a first position  500  (e.g., a stationary or initial position). Referring to  FIG. 5 , the cannula  304  of the illustrated example includes a body  502  (e.g., a cylindrical body) having a first end  504  and a second end  506  opposite the first end  504 . The cannula  304  includes a channel or opening  508  to form an access or passageway  510  between the first end  504  and second end  506  (e.g., between an exterior of the container  306  and an interior of the container  306 ). The first end  504  of the cannula  304  of the illustrated example includes a cannula tip  512  (e.g., an angled tip) to facilitate piercing or puncturing a seal  514  (e.g., a cap) positioned at a first end  516  of the container  306  to provide access to a cavity  518  of the container  306  (e.g., a sealed container). The seal  514  of the illustrated example may include a septum that may be pierced (e.g., repeatedly pierced) by the cannula  304 . The pipetting probe  302  of the illustrated example is a hollow tube that passes through the passageway  510  of the cannula  304 . The cannula  304  and the pipetting probe  302  of the illustrated example are composed of electrically conductive material or combination of materials. For example, the cannula  304  and/or the pipetting probe  302  may be composed of aluminum, stainless steel, an alloy and/or any other electrically conductive material(s). The antenna  346  is positioned adjacent (e.g., underneath) a second end  520  of the container  306  opposite the first end  516 . In the first position  500  of  FIG. 5 , the pipetting probe  302  and the cannula  304  are spaced away (e.g., vertically spaced) from the container  306  such that neither the cannula  304  nor the pipetting probe  302  are in direct contact with the seal  514  and/or the container  306 . Further, in the example first position  500  of  FIG. 5 , the first signal  342  and the second signal  344  is not provided to the pipetting probe  302  and cannula  304 . 
       FIG. 6  illustrates the pipetting system  301  in a second position  600 . For example, in the second position  600  of  FIG. 6 , the liquid level sensing system  300  and/or the pipetting system  301  received a command to obtain a sample of liquid  400  from the cavity  518  of the container  306 . In response to receiving the command, the signal generator  328  provides the first signal  342  to the pipetting probe  302  and the second signal  344  to the cannula  304 . The pipetting probe  302  of the illustrated example is composed of an electrically conductive material to enable the pipetting probe  302  to emit the first signal  342  and/or the cannula  304  of the illustrated example is composed of an electrically conductive material to enable the cannula  304  to emit the second signal  344 . When electrically charged or energized with the first signal  342 , the pipetting probe  302  emits a first electrical field  602  relative to a longitudinal axis  604  of the pipetting probe  302 . When the cannula  304  is electrically charged or energized with the second signal  344 , the cannula  304  emits a second electrical field  606  relative to a longitudinal axis  608  of the cannula  304 . 
     The first signal  342  radiated by the probe traverses or radiates across the air space between the pipetting probe  302  and the antenna  346 . The first signal  342  is coupled from the pipetting probe  302  to the antenna  346  primarily by the first electrical field  602 . Because the first electrical field  602  is actually part of an electromagnetic field radiating from the pipetting probe  302 , the liquid level sensing system  300  may also be referred to as an “RF” (radio frequency) sensing system, although in some examples an actual frequency employed by the first signal  342  (e.g., 125 kHz) may be several octaves below standard radio frequencies. When the pipetting probe  302  and the cannula  304  are positioned in the air, the antenna  346  receives a relatively weak or small signal (e.g., a null) from the first electrical field  602  along the extension of the longitudinal axis  604  of the pipetting probe  302  and/or the second electrical field  606  along the extension of the longitudinal axis  608  of the cannula  304 . 
     Further, because the cannula  304  of the illustrated example radiates the second electrical field  606  provided by the second signal  344  (e.g., which may be identical or substantially similar to the first signal  342 ), the cannula  304  when composed of an electrically conductive material does not interfere or degrade the first electrical field  602  and/or the first signal  342  emitted or transmitted by the pipetting probe  302  when the pipetting probe  302  passes through the passageway  510  or is otherwise positioned adjacent the cannula  304 . Absent the second signal  344 , the cannula  304 , when composed of an electrically conductive material, may degrade or otherwise interfere with the first signal  342  radiated by the pipetting probe  302  when the pipetting probe  302  passes through the passageway  510  or is positioned adjacent the cannula  304 . Electrically grounding only the cannula  304  would not help prevent degradation of the first signal  342  because an electrically grounded cannula  304  attracts or shields the first signal  342  radiated by the pipetting probe  302  instead of enabling the first signal  342  to radiate toward the antenna  346  along the longitudinal axis  604  of the pipetting probe  302  if the pipetting probe  302  is not also grounded. Thus, in some examples, neither the pipetting probe  302  nor the cannula  304  are grounded. In some examples, both the pipetting probe  302  and the cannula  304  is grounded or at a DC potential. 
     When the liquid level sensing system  300  receives a command to obtain a sample of the liquid  400  in the container  306 , the controller  322  commands the second motor  318  to operate the cannula positioner  316 . To position the cannula  304  in the cavity  518  of the container  306 , the position determiner  324  receives positional information representative of a position of the cannula  304  relative to the container  306  from the sensors  320 . For example, the position determiner  324  determines the position of the container  306  relative to the cannula  304  and commands the cannula positioner  316  to move in a first direction (e.g., horizontally) via the second motor  318  to align with the container  306 . The sensors  320  detects the position of the seal  514 , and the controller  322  commands the second motor  318  to move in a second direction (e.g., vertically) toward the seal  514  via the cannula positioner  316 . The sensors  320  detect when the cannula  304  is immediately adjacent (e.g., in direct contact with) an upper surface  614  of the seal  514  and communicates this information to the controller  322 . The controller  322  may command or cause the second motor  318  to stop after a certain period of time lapses from the time of detection of the cannula  304  being adjacent the upper surface  614  of the seal  514 . For example, the position determiner  324  may instruct the controller  322  to command the second motor  318  to stop after 3 milliseconds from the time the position determiner  324  detects the cannula tip  512  being in direct contact with the upper surface  614  of the seal  514  to ensure that the cannula tip  512  pierces the seal  514  and at least partially enters the cavity  518  of the container  306 . Thus, the cannula tip  512  of the cannula  304  is at least partially positioned in the cavity  518  or the container  306  when the cannula  304  pierces the container  306 . In some examples, the sensors  320  may detect when the cannula  304  has pierced through the seal  514  and is positioned in the cavity  518 . After the cannula  304  has pierced the seal  514 , the controller  322  commands or causes the second motor  318  to stop (e.g., remove power to the second motor  318 ). 
       FIG. 7  illustrates the pipetting system  301  in a third position  700 . In the third position  700  of  FIG. 7 , the pipetting probe  302  is positioned in the passageway  510  of the cannula  304 . For example, the controller  322  may command or cause the first motor  314  to operate to move the pipetting probe  302  (e.g., vertically) toward the container  306  via the probe positioner  312 . As shown in  FIG. 7 , although the cannula  304  is composed of a metallic or electrically conductive material, the first electrical field  602  generated by the first signal  342  through the pipetting probe  302  is not affected by the cannula  304  because the second signal  344  is radiating the second electrical field  606  through the cannula  304 . In some instances, the second signal  344  of the cannula  304  may prevent degradation of the first signal  342  emitted by the pipetting probe  302 . 
       FIG. 8  illustrates the pipetting system  301  in a fourth position  800 . In the example fourth position  800  of  FIG. 8 , the pipetting probe  302  contacts the liquid  400  in the cavity  518  of the container  306  via the passageway  510  provided by the cannula  304 . In particular, the liquid level sensing system  300  determines when the pipetting probe  302  contacts the liquid  400  and provides a signal to the controller  322  to command or cause the first motor  314  to stop (e.g., remove power to the first motor  314 ). In operation, when the pipetting probe  302  is lowered in the cavity  518  of the container  306  and contacts the liquid  400 , the first signal  342  from the pipetting probe  302  to the antenna  346  increases or is greater (e.g., a greater intensity) than a signal received by the antenna  346  from the pipetting probe  302  when the pipetting probe  302  is in air (e.g., not in contact with the liquid  400  in the cavity  518 ). The signal increases because the liquid  400  in the container  306 , in effect, propagates the signal transmitted by the pipetting probe  302  by directing the electromagnetic field of the pipetting probe  302  toward the receiving antenna  346 . In other words, the amplitude of the received first signal  342  is greater when the pipetting probe  302  contacts the liquid  400  than when the pipetting probe  302  is not in contact with the liquid  400 . The antenna  346  receives a stronger signal (e.g., a signal with a greater amplitude) transmitted by the pipetting probe  302  when the pipetting probe  302  is in contact with the liquid  400  and communicates the signal to the signal analyzer  330 . 
     In turn, signal analyzer  330 , via the amplitude detector  336  and the rate of change of amplitude detector  338 , determines if the pipetting probe  302  contacts the liquid  400 . As noted above, the amplitude detector  338  detects a spike or change in amplitude provided by the first signal  342  when compared to, for example, a system noise envelope, and amplitude of the first signal  342  when the pipetting probe  302  is not in direct contact with the liquid  400 , etc. The rate of change of amplitude detector  338  prevents false positives by analyzing a slope of a curve of the amplitude detected by the amplitude detector  338  to determine if the detected slope is greater than a threshold. The rate of change of amplitude detector  338  detects whether the slope of the amplitude detected by the amplitude detector  336  is greater than a threshold and communicates this result to the signal analyzer  330 . The signal analyzer  330  determines that the pipetting probe  302  has contacted the liquid  400  based on the signal provided by the antenna  346  when an amplitude is detected and a rate of change of the amplitude detected is greater than a threshold. As a result, the example liquid level sensing system  300  detects contact with the liquid  400  upon a tip  802  of the pipetting probe  302  contacting the liquid  400 . For example, when the pipetting probe  302  contacts the liquid  400  in the container  306 , a signal propagates through the liquid  400 . The signal to the cannula  304  helps prevent degradation of the signal that enters or broadcasted by the pipetting probe  302  when, for example, the cannula  304  is composed of an electrically conductive material. Thus, the second signal  344  to the cannula  304  helps promote the first signal  312  through the pipetting probe  302  to reach the antenna  346 . 
     When the signal analyzer  330  determines that the pipetting probe  302  is in contact with the liquid  400  (e.g., using the amplitude detector  336  and/or the rate of change of amplitude detector  338 ), the signal analyzer  330  instructs the controller  322  to stop operation of the first motor  314  (e.g., remove power to the first motor  314 ). The controller  322  instructs the pump  348  to activate to enable a sample of the liquid  400  in the cavity  518  of the container  306  to be aspirated in the hollow body of the pipetting probe  302 . Once the pump  348  has activated to obtain the sample, the controller  322  commands the first motor  314  to move the pipetting probe  302  via the probe positioner  312  away from the liquid  400  and/or the container  306 . After the pipetting probe  302  is removed from the container  306  and/or the passageway  510  of the cannula  304 , the controller  322  commands the second motor  318  to move the cannula  304  via the cannula positioner  316  away from the container  306  so that the cannula  304  is removed from the cavity  518  and/or spaced from the upper surface  614  of the seal  514 . 
     Alternatively, as noted above, the example pipetting system  301  may be configured with other types of signals to provide liquid level detection. For example, the pipetting system  301  may be configured with a digital signal to sense when the pipetting probe  302  contacts the liquid  400  (e.g., using a DC potential). For example, the pipetting probe  302  and the cannula  304  may be provided with substantially the same voltage. In some such examples, the signal generator  328  may be configured to provide a voltage to the pipetting probe  302  and the cannula  304  representative of the first signal  342  and the second signal  344 , respectively. For example, the pipetting probe  302  emits an electrostatic field potential (e.g., the first signal  342 ) that is substantially similar to an electrostatic field potential (e.g., the second signal  344 ) emitted by the cannula  304  as the pipetting probe  302  passes through the cannula  304  because the same voltage is applied to the cannula  304  and the pipetting probe  302 . Thus, in operation, the voltage or the first signal  342  applied to the pipetting probe  302  will not be affected when the pipetting probe  302  passes through the cannula  304  and/or through the air-filled portion of the container  306 . When the pipetting probe  302  contacts the liquid  400 , the electrostatic field emitted by the pipetting probe  302  shifts or changes (e.g., increases in amplitude). The change in the electrostatic field is sensed or received by the antenna  346 . For example, a change in the DC potential of the first signal  342  is detected when the pipetting probe  302  moves through the air-filled portion (e.g., a non-liquid filled portion) of the container  306  above the liquid and when the pipetting probe  302  contacts the liquid  400 . In turn, the signal analyzer  330 , via the amplitude detector  336  and/or the rate of change of amplitude detector  338 , determines if the pipetting probe  302  contacts the liquid  400 . For example, to determine when the pipetting probe  302  directly engages or contacts the liquid  400 , the signal analyzer  330  detects a shift or change (e.g., in amplitude) of the electrostatic field potential provided by the first signal  342  of the pipetting probe  302  when compared to, for example, (e.g., an amplitude of) the electrostatic field potential provided by the first signal  342  when the pipetting probe  302  is not in direct contact with the liquid  400 . 
     A flowchart  900  representative of example machine readable instructions for implementing the liquid level sensing system  300  and/or the pipetting system  301  of  FIGS. 3-8  is shown in  FIG. 9 . In this example, the machine readable instructions comprise a program for execution by a processor such as the processor  1012  shown in the example processor platform  1000  discussed below in connection with  FIG. 10 . The program may be embodied in software stored on a non-transitory computer readable storage medium such as a CD-ROM, a floppy disk, a hard drive, a digital versatile disk (DVD), a Blu-ray disk, or a memory associated with the processor  1012 , but the entire program and/or parts thereof could alternatively be executed by a device other than the processor  1012  and/or embodied in firmware or dedicated hardware. Further, although the example program is described with reference to the flowchart illustrated in  FIG. 9 , many other methods of implementing the example liquid level sensing system  300  and/or the pipetting system  301  may alternatively be used. For example, the order of execution of the blocks may be changed, and/or some of the blocks described may be changed, eliminated, or combined. Additionally or alternatively, any or all of the blocks may be implemented by one or more hardware circuits (e.g., discrete and/or integrated analog and/or digital circuitry, a Field Programmable Gate Array (FPGA), an Application Specific Integrated circuit (ASIC), a comparator, an operational-amplifier (op-amp), a logic circuit, etc.) structured to perform the corresponding operation without executing software or firmware. 
     As mentioned above, the example processes of  FIG. 9  may be implemented using coded instructions (e.g., computer and/or machine readable instructions) stored on a non-transitory computer and/or machine readable medium such as a hard disk drive, a flash memory, a read-only memory, a compact disk, a digital versatile disk, a cache, a random-access memory and/or any other storage device or storage disk in which information is stored for any duration (e.g., for extended time periods, permanently, for brief instances, for temporarily buffering, and/or for caching of the information). As used herein, the term non-transitory computer readable medium is expressly defined to include any type of computer readable storage device and/or storage disk and to exclude propagating signals and to exclude transmission media. “Including” and “comprising” (and all forms and tenses thereof) are used herein to be open ended terms. Thus, whenever a claim lists anything following any form of “include” or “comprise” (e.g., comprises, includes, comprising, including, etc.), it is to be understood that additional elements, terms, etc. may be present without falling outside the scope of the corresponding claim. As used herein, when the phrase “at least” is used as the transition term in a preamble of a claim, it is open-ended in the same manner as the term “comprising” and “including” are open ended. 
     The example program of  FIG. 9  includes providing a first signal (e.g. the RF signal) to a probe to cause the probe to generate the first electrical field (block  902 ). For example, the signal generator  328  may be instructed or commanded to apply the first signal  342  to the pipetting probe  302  when the controller  322  receives a command to obtain a sample of liquid  400  from the cavity  518  of the container  306 . 
     The example program  900  also includes providing a second signal (e.g. a RF signal) to a cannula to cause the cannula to generate a second electrical field (block  904 ). In some examples, the signal generator  328  provides the second signal  304  to the cannula  344  to cause the cannula  304  to generate the second electrical field  606 . In some examples, the first signal  342  provided to the pipetting probe  302  by the signal generator  328  is substantially the same (e.g., within plus or minus 10%) or identical to the second signal  344  provided to the cannula  304  by the signal generator  328 . In some examples, the first signal  342  provided by the signal generator  328  is shifted out of phase relative to the second signal  344  generated by the signal generator  328 . 
     The example program  900  includes causing a cannula to pierce a seal of a container to provide an access to a liquid in the container (block  906 ). For example, the controller  322  may cause or command the second motor  318  to move the cannula  304  toward the container  306  via the cannula positioner  316  until the cannula tip  512  pierces or passes through the seal  514  and into the cavity  518  of the container  306 . 
     The example program  900  includes moving a probe through a passageway of the cannula and into the cavity of the container (block  908 ). For example, the controller  322  may cause or command the first motor  314  to move the pipetting probe  302  relative to the container  306  via the probe positioner  312  and toward the liquid  400  in the container  306  through the passageway  510  of the cannula  304 . 
     The example program  900  includes receiving, via an antenna, the first signal generated by the probe as the probe moves into the cavity of the container (block  910 ). For example, the first signal  342  applied to the pipetting probe  302  generates the first electrical field  602  that is detected or received by the antenna  346  as the pipetting probe  302  moves into the cavity  518 . 
     A signal analyzer detects a change in the first signal (block  912 ). The signal analyzer determines if the first signal is greater than a threshold (block  914 ). For example, the signal analyzer  330  employs the amplitude detector  336  to detect a change (e.g., an increase or decrease) in an amplitude of the first signal  342  and the rate of change of amplitude detector  338  to detect a rate of change (e.g., a rate of increase or decrease) of the detected first signal  342 . For example, the amplitude detector  336  determines an amplitude of the first signal  342  compared to, for example, a slope of a system noise envelope of the analytical system  100 . If the amplitude is greater than a threshold (e.g., a detected magnitude of the first signal  342  being greater than 10 percent of a magnitude of the slope of the system noise envelope curve), then the amplitude detector  336  determines that an amplitude change has occurred. When an amplitude change has occurred, the rate of change of amplitude detector  338  detects if a slope between the curve of the system noise envelope and a peak of the amplitude detected when the change occurred is greater than a threshold. 
     If the signal analyzer determines that the change in the detected first signal is not greater than a threshold at block  914 , then the process returns to block  912 . If the signal analyzer determines that the change in the detected first signal is greater than the threshold at block  914 , the signal analyzer determines that the probe is in contact with the liquid in the container (block  916 ). Thus, the liquid level sensing system  300  of the illustrated example determination that the pipetting probe  302  engages the liquid  400  is based on detection of both a signal amplitude detected by the amplitude detector  336  and a rate of change of signal amplitude detected by the rate of change of amplitude detector  338 . In other words, it is determined that the pipetting probe  302  has contacted the liquid  400  when both the signal amplitude and the rate of signal change indicate that the amplitude change and rate of change has occurred. 
     The controller stops movement of the probe when the probe is determined to be in contact (e.g., direct contact) with the liquid in the container (block  918 ). For example, the controller  322  commands or causes the first motor  314  to stop by removing power to the first motor  314 . 
     A liquid sample from the container is then aspirated (block  920 ). For example, with the tip  802  of the pipetting probe  302  in the liquid  400  after the first motor  314  is commanded to stop, the controller  322  activates the pump  348  to aspirate a sample of the liquid  400  in the container  306 . 
     After the liquid is aspirated, the probe and the cannula are removed from the container (block  922 ). For example, the controller  322  commands or causes the first motor  314  to operate to cause the pipetting probe  302  to move away from the container  306  via the probe positioner  312  and the controller commands or causes the second motor  318  to operate to cause the cannula  304  to move away from the container  306  via the cannula positioner  316 . 
     The example liquid level sensing system  300  disclosed herein may be used in any automated instrument where liquid level sensing is desired. 
       FIG. 10  is a block diagram of an example processor platform  1000  capable of executing the instructions of  FIG. 9  to implement the liquid level sensing system  300  or the pipetting system  301  of  FIG. 3 . The processor platform  1000  can be, for example, a server, a personal computer, a mobile device (e.g., a cell phone, a smart phone, a tablet such as an iPad™), a personal digital assistant (PDA), an Internet appliance or any other type of computing device. 
     The processor platform  1000  of the illustrated example includes a processor  1012 . The processor  1012  of the illustrated example is hardware. For example, the processor  1012  can be implemented by one or more integrated circuits, logic circuits, microprocessors or controllers from any desired family or manufacturer. The hardware processor may be a semiconductor based (e.g., silicon based) device. In this example, the processor implements the example liquid level sensing apparatus  300 , the example probe positioner  312 , the example cannula positioner  316 , the example controller  322 , the example positioner determiner  324 , the example processor  326 , the example signal generator  328 , the example signal analyzer  330 , the example amplitude detector  336 , the example rate of change of amplitude detector  338 , the example input/output interface  332  and/or, more generally, the example pipetting system  301 . 
     The processor  1012  of the illustrated example includes a local memory  1013  (e.g., a cache). The processor  1012  of the illustrated example is in communication with a main memory including a volatile memory  1014  and a non-volatile memory  1016  via a bus  1018 . The volatile memory  1014  may be implemented by Synchronous Dynamic Random Access Memory (SDRAM), Dynamic Random Access Memory (DRAM), RAMBUS Dynamic Random Access Memory (RDRAM) and/or any other type of random access memory device. The non-volatile memory  1016  may be implemented by flash memory and/or any other desired type of memory device. Access to the main memory  1014 ,  1016  is controlled by a memory controller. 
     The processor platform  1000  of the illustrated example also includes an interface circuit  1020 . The interface circuit  1020  may be implemented by any type of interface standard, such as an Ethernet interface, a universal serial bus (USB), and/or a PCI express interface. 
     In the illustrated example, one or more input devices  1022  are connected to the interface circuit  1020 . The input device(s)  1022  permit(s) a user to enter data and/or commands into the processor  1012 . The input device(s) can be implemented by, for example, an audio sensor, a microphone, a camera (still or video), a keyboard, a button, a mouse, a touchscreen, a track-pad, a trackball, isopoint and/or a voice recognition system. 
     One or more output devices  1024  are also connected to the interface circuit  1020  of the illustrated example. The output devices  1024  can be implemented, for example, by display devices (e.g., a light emitting diode (LED), an organic light emitting diode (OLED), a liquid crystal display, a cathode ray tube display (CRT), a touchscreen, a tactile output device, a printer and/or speakers). The interface circuit  1020  of the illustrated example, thus, typically includes a graphics driver card, a graphics driver chip and/or a graphics driver processor. 
     The interface circuit  1020  of the illustrated example also includes a communication device such as a transmitter, a receiver, a transceiver, a modem and/or network interface card to facilitate exchange of data with external machines (e.g., computing devices of any kind) via a network  1026  (e.g., an Ethernet connection, a digital subscriber line (DSL), a telephone line, coaxial cable, a cellular telephone system, etc.). 
     The processor platform  1000  of the illustrated example also includes one or more mass storage devices  1028  for storing software and/or data. Examples of such mass storage devices  1028  include floppy disk drives, hard drive disks, compact disk drives, Blu-ray disk drives, RAID systems, and digital versatile disk (DVD) drives. 
     The coded instructions  1032  of  FIG. 10  may be stored in the mass storage device  1028 , in the volatile memory  1014 , in the non-volatile memory  1016 , and/or on a removable tangible computer readable storage medium such as a CD or DVD. 
     At least some of the aforementioned examples include one or more features and/or benefits including, but not limited to, the following: 
     In some examples, an example liquid level sensing apparatus includes a cannula defining a body having a tip and an access channel. The tip is to pierce a container. The cannula is to be at least partially positioned in the container when the tip pierces the container. In some such examples, a probe is to be positioned in the access channel. In some such examples, a signal source is to electrically energize the probe and the cannula to cause the probe to emit a first signal and cause the cannula to emit a second signal. 
     In some examples, the first signal is electrically phase controlled relative to the second signal. 
     In some examples, the first and second signals are controlled via phase-lock loop control system. 
     In some examples, an antenna is adjacent the container to detect a transmission of the first signal when the probe contacts a liquid in the container and the probe is positioned through the access channel of the cannula. 
     In some examples, the cannula and the probe are composed of an electrically conductive material. 
     In some examples, the first signal and the second signal are radio frequency signals. 
     In some examples, the second signal emitted by the cannula is to prevent degradation of the first signal emitted by the probe when the probe is positioned in the access channel of the cannula. 
     In some examples, the cannula does not interfere with the first signal emitted by the probe. 
     In some examples, neither the probe nor the cannula are electrically grounded. 
     In some examples, the first signal is electrically in phase with the second signal. 
     In some examples, the first signal is electrically phase shifted from the second signal. 
     In some examples, both the probe and the cannula are electrically grounded. 
     In some examples, at least one of the first signal or the second signal includes a waveform having a sine wave, a square wave, a triangular wave, or a sawtooth wave. 
     In some examples, an example liquid level sensing apparatus includes a cannula composed of a conductive material. In some such examples, the cannula is to pierce a seal of a container. In some such examples, the cannula is movable relative to the container in a first direction and a second direction, where the first direction being different than the second direction. In some such examples, the cannula includes an opening passing through a first end of the cannula and a second end of the cannula to define a passageway. In some such examples, the cannula is to emit a first signal when at least a portion of the cannula is positioned in a cavity of the container. In some such examples, a probe is composed of a conductive material. In some such examples, the probe is movable relative to the cannula in the first direction and the second direction, where the probe to emit a second signal. In some such examples, the probe is to pass through the passageway provided by the cannula to aspirate a sample from the container. In some such examples, a signal generator operatively couples to the cannula and the probe. In some such examples, the signal generator is to provide the first signal to the cannula and the second signal to the probe. 
     In some examples, the first signal is electrically in phase with the second signal. 
     In some examples, the first signal is electrically phase shifted from the second signal. 
     In some examples, neither the probe nor the cannula are electrically grounded. 
     In some examples, both the probe and the cannula are electrically grounded. 
     In some examples, an antenna is to receive the second signal of the probe when the probe is inside the container and the passageway of the cannula. 
     In some examples, the cannula is to emit the first signal while the probe is moving through the passageway of the cannula. 
     In some examples, the first signal emitted by the cannula is to prevent degradation of the second signal emitted by the probe when the probe is positioned in the passageway of the cannula. 
     In some examples, a method for sensing liquid in a container includes providing a first signal to a probe to cause the probe to emit a first electrical field. In some such examples, the method includes providing a second signal to a cannula to cause the cannula to emit a second electrical field. In some such examples, the method includes moving the cannula to pierce a seal of a container to provide an access to a liquid in a cavity of the container. In some such examples, the method includes moving the probe through a passageway of the cannula and into the cavity of the container while the probe emits the first electrical field and the cannula emits the second electrical field. 
     In such examples, the method includes positioning an antenna adjacent a bottom surface of the container, the antenna to receive the first signal emitted by the probe. 
     In some such examples, the method includes detecting a change in the first signal and determining if the detected change in the first signal is greater than a threshold. 
     In some such examples, the method includes stopping movement of the probe when the detected change in the first signal is greater than the threshold. 
     In some such examples, the method includes electrically grounding neither the probe nor the cannula. 
     In some such examples, the method includes electrically grounding both the probe and the cannula. 
     In some examples, a non-transitory computer-readable medium includes instructions that, when executed, cause a machine to: provide a first signal to a probe to cause the probe to emit a first electrical field; provide a second signal to a cannula to cause the cannula to emit a second electrical field; move the cannula to pierce a seal of a container to provide an access to a liquid in a cavity of the container; and move the probe through a passageway of the cannula and into the cavity of the container while the probe emits the first electrical field and the cannula emits the second electrical field. 
     In some examples, the instructions when executed, further cause the machine to receive the first signal emitted by the probe via an antenna positioned adjacent a bottom surface of the container. 
     In some examples, the instructions when executed, further cause the machine to detect a change in the first signal and determine if the detected change in the first signal is greater than a threshold. 
     In some examples, the instructions when executed, further cause the machine to stop movement of the probe when the detected change in the first signal is greater than the threshold. 
     Although certain example methods, apparatus and articles of manufacture have been disclosed herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all methods, apparatus and articles of manufacture fairly falling within the scope of the claims of this patent.