Patent Publication Number: US-2020292374-A1

Title: Method for the verification of pipettes

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to European patent application 19161917.0, filed on 11 Mar. 2019, which is incorporated by reference as if fully recited herein. 
     TECHNICAL FIELD 
     The invention relates to a method for the verification of pipettes and to a device for the verification of pipettes. It is known that temperature, air pressure and humidity affect a balance itself. For this reason, in order to compensate for changes in the weighing value with changed environmental parameters, correction factors are stored in the device, for example in the form of curves or tables. For this purpose, in the environment of a loading cell, e.g., in the laboratory, temperature and air humidity sensors are arranged, by which then a correction of the balance itself takes place automatically depending on the changing environmental conditions. The evaporation of water during a measurement process is a frequently mentioned problem. As an indication of the magnitude of this effect, it can be shown based on the following example: A container with an opening of 40 mm diameter at 20° C. has an approximate evaporation rate of approx. 20 μg/s. 
     BACKGROUND OF THE ART 
     From EP 1 975 577 A1, a balance for the gravimetric calibration of pipettes is known, which has a windbreak and built-in temperature, air pressure and air humidity sensors. 
     In EP 3 066 430 B1, a method for calibrating a pipette with the aid of a balance is disclosed, in which during the calibration process, an evaporation rate is determined, and the measurement is corrected by the determined evaporation rate. Here, the evaporation rate is corrected by means of data which is recorded during the calibration process by a climate module which contains an air pressure sensor, an air humidity sensor and an air temperature sensor. In this way, the accuracy of measurement or the accuracy of the calibration process is increased because the calibration process does not have to be based on an assumed predefined evaporation rate, but on a realistic value which depends on the current climatic conditions during the calibration process. 
     In contrast to a method for calibrating a pipette, the method for the verification of a pipette is rather to be regarded as a proof of performance of a pipette. Based on a weight measurement, it is defined whether the volume to be tested is within a predefined range and a tolerance or not. From this, it can be regarded that the result is not a nominal value, but a status. A calibration is a time-consuming and costly process, which entails a precise documentation. In the interval between two calibrations, the user is uncertain whether the pipette volume will still meet the calibration standards or not. Failure to meet the calibration standards may put an entire series of measurements into question or have a critical impact on the user&#39;s business. 
     The aim of a pipette verification (pipette performance verification) is to give the user or handler immediate feedback—including a documented confirmation—that the pipette is still sufficiently accurate and ready for use. Expediently, it means that the feedback gives a statement as to whether the actual process accuracy of the pipetting is within the limit of a defined tolerance or not, which depends on the error of the pipetting instrument and possible influences of the user, the fluid and the environment. 
     Therefore, it is the object of the present invention to provide a user with a simplified and fast method, which requires minimal manual intervention and has a robust performance. 
     SUMMARY OF THE INVENTION 
     This object is achieved by a method that has the features specified in the independent claims. Advantageous embodiments of the invention are specified in the further dependent claims. 
     The object is achieved by a method for the verification of pipettes, in which the result of the verification is a release or a warning message, comprising the following steps: providing a liquid measuring container for receiving the pipette liquid volume to be verified, a loading cell connected in a force-transmitting manner to the liquid measuring container, which outputs a measurement signal corresponding to the weight force acting on the loading cell, and a processing unit for detecting and processing the measurement signal of the loading cell; determining a first weight force from the stable measurement signal at the time point t 1  of a last stable measurement point by the processing unit, in which said last stable measurement point is chronologically prior to the receiving of the pipette liquid volume to be verified; determining a second weight force from the stable measurement signal at the time point t 2  of a new stable measurement point by the processing unit, in which said new stable measurement point is chronologically after receiving the pipette liquid volume to be verified; calculating the pipette liquid volume by the processing unit according to the formula: V P =ρ −1 ×(G t2 −G t1 ); assigning the calculated pipette liquid volume to one of at least one pipette volume class having a defined class nominal value, in which the assignment by the processing unit takes place in such a way that the absolute value of a volume difference between the pipette liquid volume and the class nominal value is as small as possible; testing by the processing unit whether or not the absolute value of the volume difference is within a tolerance value for the class nominal value of the assigned pipette volume class; and outputting the test result by the processing unit as a release when the volume difference is within the tolerance value, or as a warning message when the volume difference is outside the tolerance value. 
     The method is advantageous for the user in that the calibration interval does not have to be defined by empirically determined data but can be tailored specifically to the requirements of the user. Thus, a necessary calibration of a pipette can be recognized when this is necessary in accordance with the impact of pipetting on the user&#39;s business and the pipetting process tolerance. For the user, this can lead to a prolonged calibration interval and thus to cost savings due to (properly) deferred calibrations. In the case of a shortened calibration interval, this leads to a quality improvement of the pipetting process, which, depending on the user&#39;s business, reduces or prevents material losses, results that are not true to specifications, time losses, reworking, product recalls or reputational damage. However, the calibration interval may also remain unchanged, whereby a defective pipette would be detected in time in order to be led to an earlier calibration. Undetected defective pipettes can put entire series of measurements in question, because e.g., it is often not apparent when a pipette is not working properly any more due to dropping. 
     The test by the processing unit ( 130 ) can be made based on a predefined number of times the pipette liquid volume (V P ) to be verified is received into the liquid measuring container. This means that the pipette liquid volume (V P ) to be verified is received multiple times in succession, in each case after signalling to the operator, and the testing takes place based on the multiple existing calculations of the pipette liquid volume. So to speak, a statistical multiple measurement is carried out. 
     When introducing the pipette liquid volume to be verified, care should be taken to ensure that it is introduced all at once from the pipette into the liquid measuring container, or at least dropwise, so that the measurement signal of the loading cell is indicated as stable by the processing unit only after the complete emptying of the pipette. 
     A signal is considered as a stable measurement signal, which is within a signal band with a defined time length and signal level. This means that from the time point of a measurement signal value, all measurement signal values, preceding over a certain time period, may deviate at most by a defined difference. 
     Advantageously, the method proceeds fully automatically, in which the receiving of the pipette liquid volume to be verified into the liquid measuring container triggers the verification by the processing unit, and in which after the output of the test result, the processing unit is ready to receive and verify the next pipette liquid volume to be verified. The method displays the test result without additional user interaction and, after displaying the result, switches into readiness to carry out additional verifications. 
     Further, the method may comprise the following steps: determining an evaporation rate of the liquid in the liquid measuring container by processing the measurement signal over a preceding time period which terminates at the latest at time point t 1 ; calculating the evaporation volume between time point t 1  and time point t 2 ; and calculating the pipette liquid volume according to the formula: 
         V   P =ρ −1 ×( G   t2   −G   t1   +c   v ×( t   2   −t   1 )).
 
     Advantageously, the time period is at least and inclusive of ten seconds, and/or at most and inclusive of ten times the time difference between time point t 1  and time point t 2 . It is further advantageous to check the validity of the evaporation rate based on a reference value. 
     In a further development of the method, the method is ended without an output of a result, when the weight force difference is less than a predetermined value, or is ended with an output of a warning message, when the weight force acting on the loading cell is greater than a predetermined value, or when the measurement signal has not reached a stable state within a defined time period. 
     A development of the method is characterized in that the output of the test result takes place in the form of a visual, acoustic and/or electrical signal. Advantageously, the test result is output as a visual signal by means of a multicolour LED for each pipette volume class. Further, after the output of the test result and a defined output time period of the visual signal, all LEDs can signal the verification as ended and readiness for another or a new verification by simultaneous flashing. 
     In a further development of the method, this further comprises the following steps: providing an identification sensor for identifying a pipette, and a database system for storing the test results of a verification; detecting an identification feature of the pipette, with which the pipette can be uniquely and unmistakably identified; storing the test result of the verification of the pipette in the database system. 
     The inventive method as described above can be performed by a device, said device comprising a liquid measuring container for receiving a pipette liquid volume to be verified, a loading cell connected to the liquid measuring container in a force-transmitting manner, which outputs a measurement signal corresponding to the weight force acting on the loading cell, and a processing unit being used for detecting and processing the measurement signal of the loading cell in order to perform the method. Advantageously, the opening of the provided liquid measuring container has a minimum opening cross section of and including 100 mm 2 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Details of the force measuring device according to the invention, the force measuring module according to the invention, and the method according to the invention are given based on the description of the embodiment examples shown in the drawings. Shown are:  FIG. 1  a schematic structure of an embodiment of the inventive device; 
         FIG. 2  a measurement signal course when receiving the pipette liquid volume to be verified; 
         FIG. 3  determination of a stable measurement signal by the processing unit; and 
         FIG. 4  a diagram of the calibration- and verification interval of a pipette. 
     
    
    
     Features with the same function and similar configuration are provided with the same reference numbers in the following description. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  schematically shows the device  100  for the verification of pipettes. The device  100  shown here consists of a liquid measuring container  110 , a loading cell  120 , and a processing unit  130 . The liquid measuring container  110  has an opening  111 , through which the pipette liquid volume V P  to be verified can be introduced by the pipette P. Care must be taken here to ensure that the pipette liquid volume V P  to be verified is introduced all at once from the pipette P into the liquid measuring container  110 , or at least dropwise, so that the measurement signal ms of the loading cell  120  is indicated as stable by the processing unit  130  only after the complete emptying of the pipette P (see description relating to  FIG. 3 ). The liquid measuring container  110  is connected to the loading cell  120  in a force-transmitting manner, so that it exerts a weight force F G  acting on the loading cell  120 , which is continuously output to a processing unit  130  as a measurement signal m S . The processing unit  130  is used for detecting and processing the measurement signal m S  of the loading cell  120  and for performing the method. The test result (R P ) is output in the form of a visual, acoustic and/or electrical signal. 
     The introduced pipette liquid remains in the liquid measuring container  110 , which is filled more and more by each further verification. When the weight force F G  of the liquid measuring container  110  acting on the loading cell  120  reaches an upper limit, this is recognized by the processing unit  130  based on the measurement signal ms and signalled to the user. Until the upper limit is reached, the method runs uninterruptedly, that is, the processing unit  130  assesses the measurement signal ms of the loading cell  120  without interruption. Thus, the user only introduces the pipette liquid volume V P  into the liquid measuring container  110 , whereby the measurement signal m S  of the loading cell  120  is triggered. This triggering starts the verification method as described below with reference to  FIG. 2 . 
     Based on the measurement signal course from  FIG. 2 , the inventive method will now be described in more detail. The measurement signal course is shown before and after the receiving of the pipette liquid volume to be verified, which is recognized by the fact that the measurement signal between time point t 1  and time point t 2  is unsteady or that it is triggered. 
     The process is started by the user introducing the pipette liquid volume V P  to be verified into the liquid measuring container  110 . The triggering of the measurement signal now starts the measurement signal assessment. From the stable measurement signal ms at the time point t 1  of a last stable measurement point M t1 , a first weight force G t1  is determined by the processing unit. This weight force corresponds to the amount of liquid already present in the liquid measuring container  110 . From the stable measurement signal ms at the time point t 2  of a new stable measurement point M t2 , a second weight force G t2  is determined by the processing unit  130 . This weight force corresponds to the sum of the liquid amount already present in the liquid measuring container  110  plus the introduced pipette liquid volume V P  to be verified. The pipette liquid volume V P  can now be calculated according to the formula V P =ρ −1 ×(G t2 −G t1 ). 
     The calculated pipette liquid volume V P  is assigned to a pipette volume class K i  having a defined class nominal value V Ki . The device may be pre-set to a few volumes, e.g., 20 μl, 100 μl, 200 μl and 1000 μl, each with its own tolerance. The assignment takes place in such a way that the absolute value of a volume difference ΔV between the pipette liquid volume V P  and the class nominal value V Ki  is as small as possible. Now it can be checked whether or not the absolute value of the volume difference ΔV is within a tolerance value T for the class nominal value V Ki  of the assigned pipette volume class K i . When the volume difference ΔV is within the tolerance value T, a release takes place as an output of the test result, otherwise a warning message takes place when the volume difference ΔV is outside the tolerance value T. 
     A measurement signal ms is indicated as stable by the processing unit  130  (see  FIG. 3 ) when it is within a signal band S having a time-defined length ts and having a maximum signal difference or signal level Δm S . This means that from the time point of a measurement signal value, all measurement signal values, preceding over a certain time, may deviate at most by a defined difference. The definition of these parameters significantly affects how fast the processing unit  130  determines a stable measurement point M ti . Here, it should be considered that the user should introduce the pipette liquid volume V P  to be verified into the liquid container  110  without interruption as much as possible. Also, in the case of an accidental interruption, the processing unit  130  should not output the measurement signal ms as stable too early (by the parameter selection). The measurement signal ms at time point to is not indicated as stable, because said signal has not always been within the signal band S A  over the preceding time period ts. On the contrary, it differs at time point ts and at time point t 2 , at which the measurement signal m S  is indicated as stable. 
     Furthermore, the method can further determine an evaporation rate c v  of the liquid present in the liquid measuring container  110  by processing the measurement signal m S , by detecting the weight loss over a preceding time period Δtv which terminates at the latest at time point t 1 . With this evaporation rate c v , the evaporation volume V v  between time point t 1  and time point t 2  can now be determined. Taking these values into account, the pipette liquid volume V P  can be calculated more accurately according to the formula: V P =ρ −1 ×(G t2 −G t1 +c v ×(t 2 −t 1 )). 
     The determination of the evaporation rate cv always takes place by means of a stable measurement signal after time point t 2  and before time point t 1 , in which the duration over a time period Δtv is at least and inclusive of ten seconds, and/or at most and inclusive of ten times the time difference between time point t 1  and time point t 2 . In the time period Δtv, the determination of the evaporation rate c v  can also be carried out continuously. Based on a reference value, the validity of the evaporation rate c v  can be checked. 
     Now it can happen that the weight force difference ΔG t  is smaller than a predetermined value. In this case, the method is ended without an output of a result. Also, a weight force F G  acting on the loading cell  120  may be greater than a predetermined value, in which case the method is ended with an output of a warning message. If these parameters are defined too narrowly for the determination of a stable measurement point M ti , there is the possibility that the measurement signal ms cannot assume a stable state for the processing unit  130  within a defined time period. The method is then ended with an output of a warning message. 
     Each of the predefined volumes may be displayed by three LEDs, e.g., green, orange and red. When a new result is output, the corresponding LED is switched to a steady light for a certain time period. After the expiration of this time period, the LED is switched to a flashing mode, which further signals the result to the user, but also displays readiness for the next test. When the liquid measuring container  110  is full, the result is displayed as described above, but in addition all other LEDs flash, for example in red, with a short-on long-off pattern. 
     The method is fully automated and only the operation of the pipette by the user is required. The device  100  recognizes the process and displays the result without any additional user interaction. 
     In  FIG. 4 , the calibration and verification interval of a pipette is shown. A pipette is calibrated at the factory before it is delivered to the user. Between two regular calibrations of a pipette, verifications—also called quick tests—are carried out according to the inventive method. The frequency of these quick tests is recommended to the user through a risk assessment which is based on two specific criteria: A) the impact of pipetting on the user&#39;s business and B) the pipetting/pipetting process tolerance. From this, it can be deduced at which time interval a pipette must be subjected to a calibration, and what tolerance is to be maintained during the pipetting process. 
     Between the calibrations of a pipette, the inventive method is now used to give the user an immediate feedback that the pipette is still sufficiently accurate and ready for use. If the feedback is negative, the pipette can be subjected to a calibration already before the expiration of the calibration interval. Such early detection can prevent critical impact on the user&#39;s business. 
     Depending on the user&#39;s business, it may also be considered to extend the calibration interval in order to reduce the cost of a more expensive calibration and in order to reduce the time when the pipette is not available.