DE19747875A1 - Method for measuring variable sizes and device for carrying out the method - Google Patents

Abstract

The invention relates to a method for measuring modifiable variables in a measuring medium using chemical or biological sensors. According to said method, at least one sensor is initially brought into contact with a calibration mixture and the values required for calibration are measured, the calibration medium is replaced by a measuring medium, the at least one sensor is a brought into contact therewith and the value of the modifiable variable is measured. A device for implementation of the inventive method includes a layered device consisting of the following layers: a supporting layer (1) with a surface upon which the electrodes (5,6) of at least one sensor, electric connections (9,10) for external connection of said electrodes (5,6) and conductors (7,8) connecting the electrodes (5,6) to the connections (9,10) are arranged; a covering (2) for said support with an opening (11) revealing the electrodes (5,6) and receiving a sensor membrane material (15); a duct carrier (3) forming a duct (12) in order to distribute and simultaneously separate or subsequently receive the calibration medium and measuring medium; and a duct covering (4) with at least one opening (13,14) to feed the calibration medium and measuring medium to said duct (12).

Description

The present invention relates to a method for Measuring variable sizes in a measuring medium with With the help of chemical or biochemical sensors as an apparatus for performing this procedure rens.

Such measurement methods are, for example, in the Medical technology, environmental protection, food technology and in many other areas leads.

It is known that chemical and biochemical Senso for measuring substance concentrations and ions activities are used (K. Cammann et al, Che mical and Biochemical Sensors, Ullmann's Encyclopedia of Industrial Chemistry, Volume B 6, pages 121-212). For measuring ionic activity in an aqueous  The medium to be measured is ion-selective electrodes in combination used with reference electrodes. The measurement of Ion activity and the ion cone derived from it concentration is carried out by potentiometric means Measurement of the electrical voltage between ionense selective electrode and reference electrode.

Amperometric chemosensors can e.g. B. for the measurement of concentrations of dissolved gases in aq solutions (F. Oehme, Chemical Senso ren, Vieweg-Verlag, Braunschweig, 1991). The measurement the substance concentration happens after creating one small electrical voltage (e.g. 600 mV) between Working electrode and reference electrode and measurement of the electric current.

It is also known that potentiometric and ampe rometric chemical sensors expanded to biosensors can be. For this, are known as substance Biocomponents such. B. Enzymes and microorganisms puts. Electrochemical immune sensors are also be knows.

A disadvantage of this prior art is that Che mo and biosensors are not that easy for Measurement purposes can be used like physical Sensors (temperature sensors, pressure sensors, etc.) A major reason for this arises from the immediate metabolism with the substance surface. This not only leads to ver dirt on the sensor, but also beyond to drift phenomena. The drift of a sensor be acts that between the input variable and the off output signal no permanently stable relationship be stands. This relationship is checked before the measurement  a calibration of the sensor is established. The drift The sensor must therefore be recalibrated regularly tion are taken into account.

This leads to complicated handling of Che mo and biosensors.

An additional problem arises during use ion selective electrodes. For measuring the Io A reference electrode becomes an additional activity needed, the constant potential relationships against ensures above the aqueous measuring medium. Such be pull electrodes are difficult to get into miniaturi realized form.

To overcome these problems, e.g. B. By flow systems are used. The most famous case The game for this is a system for flow injection lyse (FIA) (G. Schwedt, Pocket Atlas of Analytics, Georg Thieme Verlag, Stuttgart, 1992, pp. 190-195). However, such systems are technically complex and not suitable for every application.

It is therefore the object of the present invention change a method and an apparatus for measuring of such sizes in a measuring medium with the help of to indicate chemical or biochemical sensors at those in miniaturized form calibrate itself and the Ka librate with alternating insertion of the sensor in a calibration and a measuring medium are not required.

This object is achieved for the Procedure by the features in the characterizing part of claim 1 and for the device by the  Features in the characterizing part of claim 10. Advantageous further developments of the invention Method and the device according to the invention result from the assigned sub-sub sayings.

In the present method, the minimum a sensor before the measurement with a calibration medium in contact via a channel with a small cross-section brought, and after connecting the sensors to an elek trical measuring device and measuring the electrical sensor signals the values for a calibration are won nen. Then the or at least one Sen sor with the measuring medium by exchange with the potash Brier medium brought into contact by diffusion, the measuring medium through microscopic openings, e.g. B. in a layer permeable to the analyte or membrane or a macroscopically opened, the Area of a channel opposite the sensor Sensor is fed. This can be done by immersing the permeable membrane or the macroscopically opened Range in the measuring medium or by giving up the measurement mediums on the permeable membrane or the macrosco open area. In this way is the calibration medium in front of the sensor by the Measuring medium replaced.

From the difference between the measurement signals before and after Bringing the sensor into contact with the measuring medium or from the difference between the measurement signals and the measurement medium to come in contact with a sensor and one of these Sensor farther away, the one with the meas dium has not come into contact, the Determine the substance concentration in the measuring medium.  

All known chemical elements can be used as sensor elements and biochemical sensors are used, e.g. B. ion-selective electrodes for the determination of ions activities, ion-selective electrodes for the concentration determination of dissolved gases in aqueous solutions, amperometric sensors and biosensors on the Basis of enzymes, microorganisms, antibodies and other biocomponents.

The channel can fill with the liquid calibration medium be filled out. However, it is also possible the channel in whole or in part with a holding matrix to be filled in, which holds the calibration medium. As Materials for such a holding matrix can Microfiber braids, papers, gels, textile braids , fabrics, knitted fabrics, foams and other suitable Fabrics are used.

The channel filling with the calibration medium can ent neither immediately after the measuring device has been manufactured or just before the measurement.

A system with two or multi-point calibration can can be realized in that two or more before directions of the present type with two or more calibration solutions with different concentrations trations of the analyte can be used in parallel.

The particular advantages of the invention are that the chemical and biochemical sensors before Measurement can be calibrated automatically. In particular for the use of potentiometric sensors by applying the principle of zero point potentioms trie with similar ion selective electrodes as  Measuring and reference electrodes on complicated counter electrodes toden be dispensed with.

The invention is described in the following in the Figures illustrated embodiments closer explained. Show it:

Fig. 1 sor an amperometric Chemosen,

Fig. 2 is a potentiometric chemical sensor with a reference and pseudo-reference electrode,

Fig. 3 shows the potentiometric Chemosen sor of FIG. 2 with a zusätz union capillary channel for receiving the medium to be measured,

Fig. 4 shows a variant of the chemotherapeutic sensor of FIG. 3,

Fig. 5 shows the amperometric chemical sensor of FIG. 1 with an addi tional membrane, and

Fig. 6 shows the amperometric chemical sensor according to FIG. 1 also with an additional membrane.

Fig. 7 is a potentiometric chemical sensor with an ion-selective electrode and a Referenzel ektrode,

Fig. 8 is a modification of the sensor of Fig. 7,

Fig. 9 is also a modification of the sensor of Fig. 7,

Fig. 10 shows a further modification of the sensor of Fig. 7, and

Fig. 11 is a doubled form of the sensor of Fig. 7.

A first embodiment of the invention is shown in FIG. 1. Herein, Fig. 1a, the single NEN layers of a sensor configuration in auseinan dergezogener representation, and Fig. 1b shows the con figuration of the assembled to the sensor Schich th.

The working electrode 5 and the counter electrode 6 of an amperometric sensor are realized on a carrier 1 . The electrodes 5 and 6 are connected via conductors 7 and 8 to electrical connections 9 and 10 . The working electrode 5 consists, for. B. made of platinum or gold. The counter electrode 6 can be realized as a silver film which can be chloride-coated on its surface. The conductor tracks 7 and 8 and the electrical connections 9 and 10 can be made of platinum, gold, silver or other materials.

The carrier 1 consists for example of a film made of polyester or another plastic. Its thickness is between 0.1 and 5 mm, preferably 0.2 mm. However, it can also consist of glass or ceramic.

The application of the working and counterelectrode 5 , 6 and the conductor tracks 7 , 8 and the electrical connections 9 , 10 is carried out in a known manner by screen printing processes, vapor deposition or sputtering processes with subsequent lithography or by known processes in the manufacture of printed circuit boards with subsequent electrodeposition the desired precious metal materials. The corresponding layer thicknesses are between 0.1 and 10 µm, preferably 1 µm.

The individual layers can be joined by known adhesive or laminating techniques, in particular also done by hot lamination techniques.

On the carrier 1 , a carrier cover 2 z. B. applied by gluing. The carrier cover 2 can consist of the same material as the carrier 1 . An opening 11 is formed in the carrier cover 2 by punching out or drilling. After application of the carrier cover 2 to the carrier 1 , the opening 11 leaves the working electrode 5 and the counter electrode 6 free.

The opening 11 in the carrier cover 2 serves as a chamber for receiving a sensor membrane material. For this purpose, a membrane 15 is realized by filling a membrane solution into the opening 11 . For the production of a glucose sensor, this membrane material consists of a hydrogel with the immobilized enzyme glucose oxidase.

A channel carrier 3 is applied to the carrier cover 2 , for example by an adhesive process. The channel support 3 consists, for example, of a fil ter paper with a thickness of 100 microns, the fiber structure is sealed by means of the screen printing process with the exception of the inner region forming a channel 12 . This means that the channel carrier 3 only has paper properties in the area of the channel 12 .

It is also possible to produce the channel support 3 from a plastic film which has a cutout corresponding to the channel 12 . In this a Fil terpapier is inserted so that the channel 12 also contains a holding matrix made of filter paper.

The channel support 3 is covered by a cover 4 with two openings 13 and 14 . The cover 4 can be made of the same material as the Trägerab cover 2 and applied by an adhesive process.

To carry out, for example, a glucose measurement, the carrier 1 with the electrical connections 9 , 10 is inserted into a plug connector or another contact device of an electrical measuring device. In the case of an amperometric measurement, a small electrical voltage (e.g. 600 mV) is applied between the electrical connections 9 and 10 and the electrical current is measured. Before the measurement, a calibration liquid with a known glucose concentration is fed through the opening 14 in the cover 4 to the channel 12 . Due to the capillary action of the filter paper, the calibration solution is evenly distributed in channel 12 . In this way, the calibration liquid reaches the sensor membrane 15 . With the help of the electrical measuring device, the electrical current can now be measured, which is a measure of the glucose concentration in the calibration liquid.

After sensor calibration, the carrier 1 is brought into contact with the measuring medium. This can be done by submerging the carrier 1 with its lower end so far into the measuring medium that the opening 13 in the cover 4 is covered with the measuring medium. Through a material exchange by means of diffusion between the measuring medium and the channel 12 , the glucose concentration in the channel 12 above the sensor membrane 15 adjusts to that of the measuring medium. After complete exchange of substances, the glucose concentration in channel 12 is stable. The current measured between the electrical connections 9 and 10 is thus a measure of the glucose concentration in the measuring medium.

In the second embodiment according to FIG. 2, a potentiometric chemical sensor is used which works according to the principle of zero point potentiometry. On a carrier 1 made of polycarbonate back contacts 16 , 18 of ion-selective electrodes (ISE), the contact 17 of a pseudo reference electrode, conductor tracks 19 , 20 , 21 and electrical connections 22 , 23 , 24 are applied, for example, by a screen printing process. The contacts and conductor tracks mentioned exist for. B. from a silver film with a thickness of 1 micron.

A carrier cover 2 'made of polycarbonate is glued to the carrier 1 '. The carrier cover 2 'has three openings 25 , 26 and 27 . In the openings 25 and 27 , an ion-selective membrane 15 'of the same type is produced in each case by pipetting in a membrane solution. The breakthrough 26 remains open. A channel support 3 ', which is made as in the first embodiment, is glued to the support cover 2 '. According to the first embodiment, for example, the channel 12 is closed by a cover 4 'which is glued to the channel carrier 3 '.

After the production of this sensor configuration from the layers 1 'to 4 ', a calibration solution is introduced into the channel 12 . This is done, for example, by pipetting in the solution through the opening 13 '. For this purpose, there may also be a vent hole in the cover layer 4 '(not shown in the figure). It is also possible to fill the channel 12 using the vacuum filling method. For this purpose, the sensor configuration according to FIG. 2b is placed with the lower (front in the figure) end in a vessel with calibration solution such that the opening 13 'is completely covered by the calibration solution. If a vacuum is subsequently created in the environment, the air escapes from the channel 12 and the calibration liquid fills it up completely.

After filling the channel, the opening 13 'in the cover layer 4 ' z. B. be closed with an adhesive film ver, which can be easily removed before use (not shown in Fig. 2b).

Before the measurement, the carrier 1 'with its electrical connections 22 , 23 and 24 is inserted into a plug-in direction of an electrical measuring device.

Since the membranes 15 'of the ion-selective electrodes are in contact with the rear contacts 16 , 18 and the contact 17 of the pseudo reference electrode and with the calibration medium in the channel 12 , a reference electrode with the electrical connection 22 can be made between the electrical connections 23 and 24 against the pseudo electrical voltage can be measured. Due to the homogeneous distribution of the calibration medium in the channel 12 , this electrical voltage is initially zero volts for the same type of ion-selective membranes 15 '. If the voltage is not equal to zero due to manufacturing tolerances of the ion-selective electrodes, the measured voltage value can be set to zero when evaluating the electrical measuring signals. This completes the calibration of the sensor.

For measurement in a measuring medium, the adhesive film, not shown, is peeled off from the opening 13 'on the cover 4 '. Subsequently, the sensor configuration is brought into contact with the measuring medium in that the opening 13 'is immersed in the measuring medium. By mass transfer between the measuring medium and the channel 12 , the analyte concentration in the area above the opening 25 adjusts to the concentration of the measuring medium. Since the holding matrix (filter paper) in channel 12 was already completely filled with calibration solution, the ion concentration in the area of the opening 27 remains stable over a longer period.

Due to the different ion concentrations in the area of the openings 25 and 27 , different potentials are obtained at the ion-selective electrodes with the rear contacts 16 , 18 and the electrical connections 24 , 23 , which are measured relative to the pseudo reference electrode 17 and the connection 22, respectively.

In addition, it is possible to apply a layer of filter paper or other material above the opening 13 'on the cover 4 ' which receives the measuring medium (not shown in FIG. 2).

A third embodiment is shown in FIG. 3. The arrangement of carrier 1 ', carrier cover 2 ' and channel carrier 3 'corresponds to that in FIG. 2. In addition, in this embodiment, a capillary channel carrier 28 is provided, which is glued to the channel carrier 3 '. An existing in the Kapil larkanalträger 28 capillary channel 30 is closed by the cover 4 ". The capillary channel 30 can be brought into contact with the measuring medium at the lower (in FIG. 3 front) end of the sensor configuration, which medium is drawn into the channel 30 due to the capillary forces. The channel 30 is vented via the opening 19 in the cover 4 ″.

Such a sensor design with a capillary channel is particularly advantageous if only very small Amounts of the measuring medium are available for the measurement stand.

In the fourth exemplary embodiment according to FIG. 4, the capillary channel 30 'for receiving the measuring medium does not emerge from the end face of the sensor arrangement. The measuring medium is applied here via an opening 33 in the cover 4 '''. An opening 29 in the cover 4 '''serves to vent the capillary channel 30 '.

The fifth exemplary embodiment shown in FIG. 5 is based on the sensor configuration according to FIG. 1. In order to implement an amperometric sensor for measuring concentrations of the dissolved oxygen in aqueous media, an additional membrane 32 is introduced as a gas-permeable layer between the carrier cover 2 and the channel carrier 3 . In the opening 11 of the carrier cover 2 , a KCl gel 15 is introduced as an electrolyte layer. In this way, a sensor configuration is obtained which is structured analogously to the Clark principle.

The sixth exemplary embodiment shown in FIG. 6 is also based on the configuration according to FIG. 1. Deviating from this, the channel carrier 3 ″ here consists of a polyester film from which the channel region 12 ′ is punched out. The channel 12 'is covered by a dialysis membrane 32 '. Cover 4 again forms the upper end. The calibration solution is introduced into the channel 12 'here according to the principle of vacuum filling.

The embodiment of FIG. 7 shows a sensor configuration with an ion-selective electrode and a conventional reference electrode. The rear side contact 16 of an ion-selective electrode, a conductor track 21 'and an electrical connection 24 ' are applied to a carrier 1 '. Rücksei tenkontakt 16 , trace 21 'and electrical connection to 24 ' exist z. B. from a silver film. In addition, a silver contact 37 with an interconnect 20 'and an electrical connection 22 ' is applied to the carrier 1 '. The silver contact 37 and parts of the conductor track 20 'consist of silver, which is chloridized on the surface. This AgCl contact forms the derivation for a conventional reference electrode. The carrier 1 'is covered by a carrier cover 2 ''with the help of an adhesive process. The carrier cover 2 ″ has 2 openings 25 , 27 . An ion-selective membrane material 15 'is introduced into the opening 25 . As in the example according to FIG. 2, the arrangement is supplemented by a channel carrier 3 'and a cover 4 '. The channel 12 in the channel carrier 3 'z. B. with the help of the vacuum filling process with a calibration liquid through the opening 13 'through. If the calibration liquid has a defined chloride ion concentration, a defined potential difference is established at the phase boundary between the chloridized silver contact 37 and the calibration liquid. In this way, the silver contact 37 acts as a conventional reference electrode. If a measuring medium is supplied to the channel 12 via the opening 13 ', the calibration medium is replaced by the measuring medium. This leads to a change in the electrical potential between the back contact 16 of the ion-selective electrode and the liquid in the channel 12 above the membrane 15 '. As the medium to be measured over an extended period can not reach the area of the chloridised silver contact 37, the electric potential at the conven tional reference electrode remains constant.

In the exemplary embodiment according to FIG. 8, the sensor configuration according to FIG. 7 is shown, in which the opening 13 ′ is closed with the aid of a sealing film 36 . The sealing film 36 closes the sensor arrangement after filling with a calibration medium. It can be removed in a measuring medium before the measurement.

Fig. 9 also shows an embodiment based on Fig. 7, here the channel carrier 3 'is modifi ed. The channel carrier 3 'can, for. B. consist of a paper matrix, which was sealed with the exception of the Kanalbe area 12 using a plastic paste. Thus, the channel support 3 'has retained its paper properties only in the area of the channel 12 . In addition, an opening 34 was produced by punching out in the region of the channel 12 . In this exemplary embodiment, the entire channel 12, including the breakthrough region 34, is filled with a calibration liquid. When the sensor comes into contact with the measuring medium later, the mass exchange takes place particularly quickly in the area of the opening 34 . Here, too, the opening 13 'in the cover 4 ' can be closed with the aid of a sealing film, as shown in FIG. 8.

Another Konaltrager modification is shown in the exemplary embodiment of FIG. 10. A channel carrier 3 'consisting of a paper matrix is provided with 2 openings 12 , 12 ' and, with the exception of a region 35 between the openings 12 , 12 ', is sealed with the aid of a plastic material. In this way, the channel carrier 3 'has retained its paper properties only in the region 35 . The filling of the channel with a calibration fluid is carried out as in the previous exemplary embodiments.

In the exemplary embodiment according to FIG. 11, the sensor configuration according to FIG. 7 is doubled. This sensor configuration has 2 similar ion-selective membrane materials 15 'and 15 '''. Calibration solutions with different analyte concentrations are introduced into the channels 12 , 12 '. In this way, only one parameter can be measured with this doubled sensor configuration, but in this example the measurement is based on a two-point calibration, which relates to the two analyte concentrations of the calibration liquid in the channels 12 , 12 '.

In a further exemplary embodiment (without illustration), sensor elements of the type of the double matrix membrane sensors can be used instead of the membrane materials 15 , 15 ', 15 '', 15 ''' in the openings 25 , 25 '. Such sensors are known from patent DE 41 37 261.

Claims (24)

1. A method for measuring variable sizes in a measuring medium with the aid of chemical or biochemical sensors, at least one sensor being brought into contact with a calibration medium and the values required for calibration being measured, and then the at least one sensor by exchanging the calibration medium brought into contact with it by a measuring medium and the value of the variable variable is measured, characterized in that the calibration medium is brought into contact with the sensor via a channel with a small cross section and the calibration medium is replaced by the measuring medium by means of diffusion, and that the measuring medium is brought into contact with the sensor via a layer or a membrane with microscopic openings permeable to the analyte or via a macroscopically open area of the channel. 2. The method according to claim 1, characterized in net that an amperometric measurement leads. 3. The method according to claim 1, characterized in net that a potentiometric measurement by to be led. 4. The method according to any one of claims 1 to 3, because characterized in that the abge the sensor turned side of the membrane or the macroscopic  open area immersed in the measuring medium becomes. 5. The method according to any one of claims 1 to 3, because characterized in that the measuring medium over a capillary channel facing away from the sensor th side of the membrane or the macroscopic open area is led. 6. The method according to any one of claims 1 to 5, there characterized in that the variable size ß from the difference of the calibration dium and the Sen obtained from the measuring medium sor signals is determined. 7. The method according to any one of claims 1 to 6, there characterized in that the sensor signals sequentially by one first with the calibration medium and then with the Get the medium in contact with the sensor become. 8. The method according to any one of claims 1 to 6, there characterized in that the sensor signals simultaneously by two sensors, one of which is the one with the calibration medium and the other with are brought into contact with the measuring medium become. 9. The method according to any one of claims 1 to 7, there characterized by two different Calibration solutions with their own sen be brought into contact.   10. Device for performing the method according to one of claims 1 to 9, characterized in that it consists of a layered arrangement with the following layers:

• a) a carrier layer ( 1 ) on whose surface the electrodes ( 5 , 6 ) of at least one sensor, electrical connections ( 9 , 10 ) for connecting the electrodes ( 5 , 6 ) to the outside and the electrodes ( 5 , 6 ) with the connections ( 9 , 10 ) connecting conductor tracks ( 7 , 8 ),
• b) a carrier cover ( 2 ) with at least one opening ( 11 ) which releases the electrodes ( 5 , 6 ) and which receives a sensor membrane material ( 15 ),
• c) a channel support ( 3 ) which forms at least one channel ( 12 ) for distribution and simultaneous separate or sequential reception of the calibration medium and the measuring medium, and
• d) a channel cover ( 4 ) with at least one opening ( 13 , 14 ) for the supply of the calibration medium and the measuring medium to the channel ( 12 ).

11. The device according to claim 10, characterized in that the channel cover ( 4 ) contains two openings ( 13 , 14 ), each of which one for supplying a medium to the channel ( 12 ) and the other for simultaneous ventilation of the channel ( 12 ) serve. 12. The apparatus of claim 10 or 11, characterized in that the channel ( 12 ) holds a Haltema trix ent for the medium recorded in each case. 13. The apparatus according to claim 12, characterized records that the holding matrix from a microfa serer braid, paper, gel, textile braid, Fabric, knitted fabric or foam. 14. The apparatus according to claim 12 or 13, characterized in that the holding matrix is removed at least over one of the openings ( 25 , 27 ) in the carrier cover ( 2 ''). 15. The device according to one of claims 10 to 14, characterized in that at least one of the electrodes on the carrier layer ( 1 ') an ion-selective electrode ( 16 , 18 ) and at least one of the electrodes on the carrier layer ( 1 ') a reference electrode ( 17 , 37 ). 16. The device according to one of claims 10 to 15, characterized in that the layers ( 1 , 2 , 3 , 4 ) are glued together. 17. The device according to one of claims 10 to 16, characterized in that the layers ( 1 , 2 , 3 , 4 ) made of plastic, glass or ceramic be. 18. Device according to one of claims 10 to 17, characterized in that between the channel carrier ( 3 ') and the channel cover ( 4 '', 4 ''') a capillary channel carrier ( 28 , 28 ') to form a capillary channel ( 30th , 30 ') for supplying the measuring medium to the channel ( 12 ) is arranged. 19. The apparatus according to claim 17, characterized in that the capillary channel ( 30 ) up to egg ner front outer edge of the capillary channel carrier ( 28 ) is guided. 20. The apparatus according to claim 17, characterized in that the capillary channel ( 30 ') is formed in the interior of the capillary channel carrier ( 28 ') and is connected to at least one opening ( 29 , 33 ) in the channel cover ( 4 '''). 21. Device according to one of claims 10 to 20, characterized in that a gas-permeable membrane ( 32 ) between the carrier cover ( 2 ) and the channel carrier ( 3 ) is arranged for measuring the concentration of a gas dissolved in a liquid. 22. Device according to one of claims 10 to 20, characterized in that the channel ( 12 ') does not contain a carrier matrix for the received medium and between the channel carrier ( 3 '') and the channel cover ( 4 ) a dialysis membrane ( 32 ' ) is arranged. 23. Device according to one of claims 10 to 22, characterized in that the channel filled with calibration medium ( 12 ) through a via the at least one opening ( 13 , 14 ) in the channel cover ( 4 ) releasably attached film ( 36 ) is closed on the outside. 24. Device according to one of claims 10 to 23, characterized in that the channel carrier ( 3 '') has two separate channels ( 12 , 12 ') for receiving a calibration medium.