the embodiments of the present invention described below are not intended to be exhaustive or to limit the invention to the precise forms disclosed in the following detailed description . rather , the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of the present invention . fig1 is a schematic representation of the components of an arrangement according to the invention for determination of a concentration of an analyte in a body fluid . the arrangement comprises an interface 1 , for example , a microdialysis probe , via which an analyte ( for example , glucose ) from a body fluid ( for example , interstitial fluid ) can pass into a stream of liquid 2 in the arrangement ( sample collection ). the perfusion liquid is pumped from a reservoir 3 through a first liquid conduit 4 to the interface 1 and from there through a second liquid conduit 5 , a flowmeter chamber 10 and a third liquid conduit 6 into a collecting container 8 . a transport device 9 is provided for pumping the liquid . in the flowmeter chamber 10 there is a detector arrangement 7 ( for example , a three - electrodes system containing a reference electrode , a working electrode and a counterelectrode with a potentiostat ), which is used to carry out measurements for determining the concentration of the analyte . in the area of the flowmeter chamber 10 , there is a sensor system 11 , which is used to measure values of at least two correlated system parameters . the sensor system 11 and the detector arrangement 7 are connected to a signal processor 12 . from the results of the measurements by the detector arrangement 7 , the signal processor 12 calculates ( if appropriate taking into account a calibration value ) the concentration of the analyte . the relationships between the measurement signal , the concentration of the analyte in the stream of liquid 2 , the concentration of the analyte in the body fluid with which the interface 1 is in contact ( for example , interstitial fluid ) and , if appropriate , the concentration of the analyte in another body fluid ( for example , blood ) are known in the prior art and are stored in the signal processor 12 , preferably in the form of calculation rules . the signal processor 12 is connected to a memory 13 in which , for example , measured or calculated values can be stored . moreover , a further storage unit 14 can be provided which contains reference values for comparison with the measured values of the sensor system 11 . the reference values stored in the further storage unit 14 can be limit values for the at least two determined system parameters , for which purpose the memory 13 can also be used . the signal processor 12 serves as a means for comparing the actual values of the at least two correlated system parameters with these limit values stored in the storage unit 14 . this in each case yields an actual combination of at least two comparison results . with defined combinations of comparison results , the signal processor 12 triggers a reaction of the arrangement . the reaction , for example , involves a display unit 15 displaying a warning ( for example , of a heightened measurement error of the detector arrangement 7 ), a problem report , a recalibration prompt , or a prompt to replace a component of the arrangement . by way of an interface 16 ( for example , a keypad ), a user can , for example , adjust the settings or functions of the arrangement or input a recalibration value . other possible reactions to an actual combination of at least two comparison results can be the triggering of an alarm tone generator 17 , which emits an acoustic alarm signal . furthermore , a regulator 18 can be activated , for example , for regulating a system parameter such as the cell voltage or the flow rate . such a regulator has the advantage that the arrangement can operate over a long period of time without intervention by the user . fig2 shows a flow chart of a first embodiment of a monitoring method according to the invention . this monitoring method is used to monitor the sensor voltages and currents of an amperometric detector arrangement . the cell voltage ( polarization voltage ) between the working electrode and the reference electrode is monitored by a comparison 19 of mean cell voltage values ū pol , determined from measured values of a voltage sensor of the sensor system , with limit values ū pol , min and ū pol , max . if the cell voltage lies outside the range set by the limit values ( comparison result n ), it must be assumed that there is a problem with the arrangement or with the coupling between arrangement and voltage sensor . consequently , an alarm is triggered / a warning report displayed 20 in order to prompt a trained user to eliminate the problem . if the comparison 19 reveals that the cell voltage values lie within the range set by the limit values ( comparison result y ), the battery voltage u batt is checked . the battery voltage measured by a voltage sensor of the sensor system is compared with a lower limit value u batt , min ( reference number 21 ). if the battery voltage lies below the limit value ( comparison result n ), an alarm is triggered / a report displayed 22 , in order to prompt the user to change the battery . if the comparison 21 reveals that the battery voltage values lie above the lower limit value ( comparison result y ), the countervoltage u ce between the counterelectrode and working electrode of the detector arrangement is checked . for this purpose , the countervoltage measured by a voltage sensor of the sensor system is compared with lower and upper limit values u ce , min and u ce , max ( reference number 23 ). if the countervoltage lies outside the range defined by the limit values ( comparison result n ), then the temperature t correlated with the countervoltage is checked by a comparison 24 with lower and upper limit values t min and t max . if the temperature measured by a temperature sensor of the sensor system lies outside the range set by the limit values ( comparison result n ), then the combination of the comparison results for countervoltage and temperature ( n , n ) reveals that a system error is present and an alarm 25 should be triggered . if the temperature lies within the range defined by the limit values ( comparison result y ), then , according to the invention , the mean flow rate f correlated with the countervoltage is checked by comparison 26 with limit values f min and f max . if the mean flow rate f determined from measured values of a flow rate sensor of the sensor system lies within the range set by the limit values ( comparison result y ), then the combination of the comparison results for countervoltage , temperature and mean flow rate ( n , y , y ) reveals that a system error is present and an alarm 27 should be triggered . if the mean flow rate f lies outside the range set by the limit values ( comparison result n ), then the combination of the comparison results for counter - voltage , temperature and mean flow rate ( n , y , n ) reveals that a flow obstruction is possibly leading to at least partial coating of the counterelectrode . therefore , in order to eliminate this problem , a pump variable p ( for example , the electric current of the pump ) is increased by δp ( reference number 28 ) for a time interval δt 5 in order to increase the flow rate . after δt 5 has elapsed ( reference number 29 ), the pump variable is lowered again by δp ( reference number 30 ). then , after a time interval δt 6 has elapsed ( reference number 31 ), a comparison 32 of the countervoltage u ce with the limit values u ce , min and u ce , max is again carried out . if the comparison 32 reveals that the countervoltage still lies outside the range defined by the limit values ( comparison result n ), an alarm 27 is triggered as a reaction . if it now lies within the range ( comparison result y ), then the ratio of the time change of the measured current di / dt to the time change of the countervoltage du ce / dt is next checked by a comparison 33 with lower and upper limit values . this comparison is also carried out if the original comparison 23 of the countervoltage has delivered the comparison result y ( countervoltage lying within the limit values ). if the ratio of the time change of the measured current to the time change of the counter - voltage lies between the limit values ( comparison result y ), then the method is carried out again , starting with the comparison 19 of the cell voltage . if the ratio lies outside the range set by the limit values , a distinction is made between two cases 34 , 35 . in the first case 34 , the lower limit value is undershot . as a reaction to this , the method , as described above , is continued by checking the flow rate ( comparison 26 ). in the second case 35 , the upper limit value is exceeded and , as a reaction , the arrangement is switched off 36 , since a short circuit is suspected . fig2 is shown enlarged , with further inscriptions , in both subsidiary fig2 a and 2 b . fig3 shows a flow chart of a second embodiment of a monitoring method according to the invention . the embodiments according to fig2 and 3 can be implemented singly or in combination in an arrangement according to the invention ( for example , according to fig1 ). the monitoring method according to fig3 is used to monitor the flow rate in an arrangement for determining the concentration of an analyte . the constancy of the flow rate of a stream of liquid is an important condition for a constant dead time , a constant substance exchange via the interface ( for example , membrane wall of a catheter ) and the concentration measurement in the flowmeter chamber ( for example , electrochemical measurement ). the mean flow rate is therefore preferably monitored by a comparison 37 of a flow rate f , determined from measured values of a flow rate sensor , with lower and upper limit values f min and f max . if the flow rate lies within the range defined by the limit values ( comparison result y ), then the pump variable p correlated with the flow rate is checked by a comparison 38 with lower and upper limit values p min and p max in order to test the transport device ( pump ). in the case of a pump variable lying between the limit values ( comparison result y ), the combination of the comparison results of flow rate and pump variables ( y , y ) reveals that the transport device ( pump ) is functioning correctly . the comparison 37 of the actual flow rate with the limit values is then repeated after a time interval δt 4 ( reference number 39 ). in the case of a pump variable lying outside the range defined by the limit values in the comparison 38 ( comparison result n ) ( for example , if the current consumption of the pump is too high ), an alarm is triggered / a report displayed 40 , in order to prompt a user to check the pump . if the comparison 37 of the mean flow rate reveals that it lies outside the range set by the limit values ( comparison result n ), then the battery voltage u batt correlated with the mean flow rate is checked . it is compared for this purpose with a lower limit value u batt , min ( reference number 41 ). if the battery voltage drops below the limit value ( comparison result n ), then an alarm or a message 42 prompts the user to replace the battery or accumulator . if the battery voltage lies above the limit value ( comparison result y ), then a distinction is made between two cases 43 , 44 for this combination of comparison results of flow rate and battery voltage ( n , y ). in the first case 43 , the mean flow rate lies below the lower limit value f min . in this case , the pump variable p correlated with the flow rate ( for example , the pump current or speed ) is checked by comparison 45 with limit values p min and p max . if the pump variable lies outside the range set by the limit values ( comparison result n ), then the combination of the comparison results for mean flow rate , battery voltage and pump variable ( n , y , n ) reveals that there is a defect of the transport device ( pump ), such that an alarm is triggered / a report displayed 46 in order to prompt the user to check the pump . if the pump variable lies within the range defined by the limit values ( comparison result y in comparison 45 ), then the combination of the comparison results for mean flow rate , battery voltage and pump variables ( n , y , y ) reveals that a bubble or a particle may be forming a temporary flow obstruction . in order to eliminate this problem , the pump variable p ( for example , the pump current ) is increased by δp ( reference number 48 ) for a time interval δt 1 in order to increase the flow rate . after δt 1 has elapsed ( reference number 49 ), a comparison 50 of the mean flow rate with the limit values f min and f max is carried out again . if the comparison 50 reveals that the mean flow rate still lies outside the range defined by the limit values ( comparison result n ), an alarm is triggered as a reaction or a warning report is displayed ( reference number 51 ). if it lies within the range ( comparison result y ), then the pump variable is lowered again by δp ( reference number 53 ) after a time interval δt 2 has elapsed ( reference number 52 ). the mean flow rate is then checked again ( comparison 37 ) after a defined time interval δt 3 has elapsed ( reference number 54 ). in the second case 44 , the mean flow rate lies above the upper limit value f max . in this case , an alarm 47 is triggered directly in order to warn of a system error . thus , embodiments of the method for monitoring an arrangement for determining the concentration of an analyte in a body fluid are disclosed . one skilled in the art will appreciate that the teachings can be practiced with embodiments other than those disclosed . the disclosed embodiments are presented for purposes of illustration and not limitation , and the invention is only limited by the claims that follow .
the invention relates to a method for monitoring an arrangement for determining a concentration of an analyte in a body fluid . the determination of the concentration of the analyte by means of the arrangement involves a procedure in which the analyte from the body fluid passes through an interface and is transported in a stream of liquid into a flowmeter chamber , in which a measurement is carried out to determine the concentration of the analyte . the evaluation of the measurement takes place in a signal processor . the monitoring of the arrangement comprises the following steps : measurement of measured values of at least two correlated system parameters of the arrangement by means of a sensor system , and comparison of the measured values with limit values stored for each of the system parameters in a storage unit , to obtain a combination of at least two comparison results .