CALIBRATION REFERENCE ELEMENT FOR PH MEASURING INSTRUMENTS, CALIBRATION REFERENCE SYSTEM, HOLDER FOR CALIBRATION REFERENCE ELEMENT, AND METHOD FOR CALIBRATING A pH MEASURING INSTRUMENT - Patent application

Abstract

A calibration reference element (10) for a pH measuring instrument comprises a reference liquid (17) with a defined pH value and a tube bag (1) enclosing the reference liquid. The tube bag consists of a foil film sealed along at most three sealing edges (11, 12, 13). A calibration reference system comprises at least two such calibration reference elements (10) with reference liquids of different pH values. A holder (2) for the calibration reference element allows the calibration reference element to stand up so that the reference liquid is conveniently available for calibration. Correspondingly, a calibration set comprises the holder and the calibration reference element (10) or the calibration reference system.

Description

[Technical field]

[0001] The present invention relates to a calibration reference element of the kind characterized in the claims. The present invention further relates to a calibration reference system using a calibration reference element of the above kind, to a holder for such a calibration reference element, and to a method for calibrating a pH measuring instrument using the above object. [Background technology]

[0002] If a measurement transducer is used for measuring value acquisition, which converts a physical quantity into an electrical signal, it is essential to know the relationship between the generated electrical signal and the actual physical quantity. In many measuring value transducers or sensors, this relationship must be verified from time to time within the context of a calibration, i.e. determined anew, so that the exact value of the physical quantity can be determined on the basis of the measured electrical signal, i.e. displayed by the measuring device. For this purpose, a defined reference value or, in turn, several defined reference values ​​of the physical quantity to be determined are provided, and the corresponding electrical measured value is determined, i.e. the measuring device is adjusted so that the respective exact value is displayed for this physical quantity.

[0003] This also applies, for example, to sensors or measuring instruments for determining the pH value, where a pH value sensor is immersed in succession in buffer solutions with different defined pH values ​​and the measuring instrument is, for example, adjusted so that it displays the correct pH value in each case.

[0004] Alternatively, the electrical signal output by the sensor can be recorded as a function of the pH value, so that the pH value can then be inferred from the electrical signal if the pH value of the measured medium is unknown. The use of pH buffers as reference solutions ensures that the reference pH value is maintained. However, the handling of buffers is not entirely problem-free in practice. Thus, it is desirable that some strongly acidic or strongly basic, and therefore aggressive, buffer solutions are provided in containers that prevent spillage of the buffer solution as easily as possible when opening and handling them, at least with due care. It has also proven to be desirable if the container with the buffer solution is held securely during calibration and provides an access for the sensor that is as unobstructed as possible. For economic reasons and also for reasons of disposal of the buffer solution, which is not problem-free with regard to waste water, it is further desirable to provide the reference buffer solution in such a way that as little as possible of the respective amount is required, and that the reference buffer solution can be transported and stored space-savingly. Summary of the Invention [Means for solving the problem]

[0005] This specification relates to the subject matter described at the beginning. According to one aspect of this specification, a calibration reference element or a calibration reference system for a pH measuring device is presented, which allows for as simple and reliable handling as possible when calibrating the pH measuring device. Furthermore, a holder for such a calibration reference element is presented, which ensures reliable holding of the calibration reference element in a simple manner. Furthermore, a method for calibrating a pH measuring device or a pH sensor using the above subject matter is described.

[0006] Further advantages and benefits of the subject matter described herein, whether or not expressly stated, will be apparent in light of this specification.

[0007] This is achieved by the calibration reference element set forth in claim 1 and the subject matter described in the further independent claims. Particular embodiments of the claimed subject matter are evident from the dependent claims.

[0008] Thus, a calibration reference element for a pH measuring instrument is described, which comprises a reference liquid having a defined pH value and a tube bag, the reference liquid being enclosed in the tube bag. The tube bag is made of a foil film and is sealed along no more than three sealing edges, the tube bag being at least partially filled with the reference liquid. As explained above, a pH buffer is used inter alia as the reference liquid. It may be provided that the tube bag is only partially filled with the reference liquid, the remaining remaining volume in the tube bag being filled with air or a protective gas. This prevents the liquid from spilling out when the tube bag is opened at the end that is up. The protective gas can improve the shelf life of the reference liquid. Furthermore, it may be provided that the tube bag contains only an amount of liquid such that, when the tube bag is arranged in the configuration that is defined and realized during calibration, the filling level in the opened tube bag is between 30% and 90%, preferably between 50% and 80%, of the height of the internal volume of the tube bag thus arranged. The configuration that is routinely achieved during calibration can be derived, as explained below, in particular in relation to a holder provided for use with this calibration reference element. The height of the internal volume is known since a line along which the user should open the tube bag can be marked on the outer surface of the tube bag, or such a line, e.g. a tear line, can already be provided by a suitable device.

[0009] It may further be provided that the tube bag has a printing in a printing field indicating the pH value of the reference liquid, the printing area being arranged above the upper edge of the holder and below the line along which the user must open the tube bag when the tube bag is arranged in the configuration normally achieved during calibration. The configuration normally achieved during calibration may be derived in relation to a holder provided in particular for use with the calibration reference element, as will be explained below. In addition to this, it is preferred that the printing in the printing area is colored in a manner that corresponds to the color coding. In this way, the user can easily read the pH value of the reference liquid during measurement.

[0010] It is self-evident that in this context the calibration of a pH measuring device can also refer to the process of recording the relationship between the electrical output signal of the sensor described at the outset and the pH value. The calibration of a pH measuring device is therefore not limited to adjusting the measuring device so that it outputs the correct pH value, but also includes the recording of the relationship between the electrical output signal of the sensor described above and the pH value of the measured substance. The chosen expression should be understood broadly to include any process that serves to calibrate an apparatus used to measure pH values.

[0011] "ein" or "eine" are to be understood within the framework of this specification as indefinite articles and not as numerals, unless a different meaning is clearly indicated, e.g. by the use of "genau ein" or "exactly one".

[0012] It is further provided that the tube bag has a sealing edge on each of the two opposite end faces, each of which has a longitudinal extension that runs across the longitudinal extension of the tube bag and is respectively defined by a longitudinal edge of the end sealing edge. In the circumferential direction, the tube bag is in particular seamless or has one longitudinal sealing edge that is offset in the circumferential direction from the longitudinal edge of the end sealing edge or is folded in the circumferential direction away from the adjacent longitudinal edge of the end sealing edge. In this way, it is effectively avoided that the longitudinal sealing edge is a cause of shape rigidity that would cause the opened bag to assume a flat shape, as is the case with known packaging materials that have two longitudinal sealing edges running on opposite sides of the bag. In contrast, due to the pre-molding of the material of the tube bag in the region of the longitudinal sealing edges, the tube bag opened at the top side has only a relatively low shape stiffness in terms of the cross-sectional shape at the top edge and therefore spontaneously assumes an elliptical to at least approximately circular geometric shape when braced at the closed bottom side and otherwise properly held, which allows easy entry of the sensor into the reference liquid inside the opened tube bag. The circumference of the tube bag and its inside is in an exemplary embodiment ≧70 mm to ≦80 mm. This corresponds to an equivalent diameter in the range of approximately 23 to 27 mm, which allows good entry of a sensor with a diameter of, for example, 12 mm. The length of the fillable internal volume between the two end faces is in an exemplary embodiment ≧90 mm to ≦120 mm, and the filling amount of the liquid in a certain region is in the range of ≧15 ml to ≦30 ml. In particular, the length of the fillable internal volume is between 110 mm to 115 mm, and the filling amount is approximately 20 ml. These parameters result in a reference element that, when the bag is opened and positioned as specified, based on this filling height, provides a sufficient clearance for the upper edge of the open tube bag to prevent overflow of the liquid when the sensor is immersed, while the closed tube bag is compact enough for storage and transportation as well as handling. At the same time, a depth of immersion of the sensor in the reference liquid that ensures a correct measurement can be guaranteed.

[0013] It should be noted that the longitudinal sealing edge extends from one end sealing edge to the other end sealing edge in such a way that the tube bag is hermetically closed only if both end sealing edges are present and closed. The opening of the tube bag is effected, in particular, by cutting the tube bag across its longitudinal extent near one of the end sealing edges. It should be noted further that in relation to the tube bag, its longitudinal and transverse directions as well as the end faces are implicitly predetermined by the tubular geometry.

[0014] In a particular embodiment, the tube bag consists of a foil film with at least two material layers, one of which is on the inside of the tube bag and a second layer on the outside of the tube bag. The inner layer consists of a first material, for example a first plastic, and the outer layer consists of a second material, for example a second plastic, which materials are in particular different from each other. The first material can be selected in particular to ensure stability towards the reference liquid, while the second material of the outer layer is selected in particular to resist environmental conditions and mechanical damage, and may also assume a plastic function. In this other embodiment, the layer on the inside of the tube bag and the layer on the outside of the tube bag may consist of the same material.

[0015] It may also be provided that an intermediate layer of metal foil film is arranged between the layer on the inside of the tube bag and the layer on the outside of the tube bag. The metal foil film is preferably an aluminum foil film. The metal foil film improves the stability of the reference solution, among other things, during storage and transport. In particular, the layer of foil film on the outside of the tube bag and the intermediate layer of aluminum foil film can be perforated, in particular by laser, so that a perforation line is generated. This allows for a simple opening of the tube bag along a defined line, from which the height of the internal volume is known. In contrast, at least the layer of foil film of the tube bag that is on the inside of the tube bag is not perforated at all or only incompletely.

[0016] A calibration reference system is also disclosed, which comprises at least two of the above mentioned types of calibration reference elements. At least two of the total number of all calibration reference elements of the calibration reference system contain reference liquids with different pH values ​​from each other. In particular, the calibration reference elements with reference liquids with different pH values ​​can be coded with different colors. This allows for quick recognition of the pH value of the individual reference elements during application. In a particular embodiment, the calibration reference system consists of three to five calibration reference elements with different pH values.

[0017] A suitable holder for a calibration reference element of the above-mentioned kind has at least partially the shape of a triangular prism. A triangular prism is a prism with a triangular base. Said prism has at least one recess, which extends from one side of the prism, through a side surface of the prism adjacent to said side, in the direction of a side surface of the prism opposite said side. In this respect, said side defines an upper edge of the holder, and the side surface of the prism opposite said side defines a lower surface of the holder. The recess is open at the upper edge. At the upper edge, the recess has a maximum width parallel to the upper edge, while this width decreases from the upper edge to a minimum width from the upper edge towards the lower surface. This minimum width remains constant in a lower region of the recess, which is distal to the upper edge, along a direction from the upper edge towards the lower surface. Due to the downward tapering shape of the upper region of the recess, a reference element of the above-mentioned type inserted in the recess can be wide open at its upper open end, while being reliably guided and supported in its lower region. The maximum width of the recess parallel to the upper edge corresponds in particular to 75% to 100% of the diameter of a circle whose circumference is the same as the inner circumference of the tube bag. It is intended that the holder is placed on the base with its lower side down and its upper side up, and the opened reference element is inserted in the recess with its closed lower side, the edge of the recess supporting the opened tube bag in a direction parallel to the upper edge. In this regard, the height of the holder measured from the lower side to the upper edge is determined in a specific embodiment in such a way that the inserted open tube bag protrudes from the holder at the upper side. This height is determined, for example, in such a way that the opened tube bag protrudes from the upper edge of the holder by 10% to 50% of its length. This geometry achieves that the open top end of the tube bag fitted into the holder has the above-mentioned elliptical or at least approximately circular cross-sectional shape. In particular, two or more recesses for accommodating reference elements with liquids of different pH values ​​can be arranged along the extension of the upper edge of the holder. The number of recesses can be the same as the number of reference elements with different pH values ​​in a set or system of reference elements. If the reference elements with different pH values ​​are color-coded as described above, the recesses in the holder can also be color-coded accordingly.

[0018] The calibration set comprises a calibration reference element and a holder according to the invention, which allows a particularly easy and reliable calibration of pH measuring instruments.

[0019] The calibration set preferably comprises a calibration reference system and a holder, and in particular preferably the holder has at least as many recesses as there are calibration reference elements with different pH values ​​in the calibration reference system, so that the user can easily and reliably use all the reference elements for the multi-point calibration.

[0020] A method for calibrating a pH measuring device or a pH sensor using the above object includes placing the holder with its upper edge facing up on a base. At least one calibration reference element is opened near one end face of the tube bag. The opened calibration reference element is inserted into at least one recess of the holder with the closed end face of the tube bag facing down, with the wall of the tube bag being supported by the edge of the recess. The tube bag in the recess thus becomes an open-topped container. A sensor for pH measurement is immersed in a reference liquid through the upper opening of the tube bag. The measured value read (as the electrical output of the sensor or already converted to a pH value in the measuring device) is then compared with a defined pH value of the reference liquid. The measured value assigned to the reference pH value can then be noted down or the measuring device can be adjusted to display a value corresponding to the reference pH value. This process can be repeated for further different reference pH values. To this end, the holder can be fitted with a number of reference elements with different reference pH values, into which the sensor is immersed in turn.

[0021] The individual embodiments listed above may be combined with one another, and further, not individually disclosed, embodiments of the teachings of this document can be easily inferred by those skilled in the art.

[0022] The facts described here will be explained in more detail below on the basis of selected exemplary embodiments illustrated in the drawings. [Brief description of the drawings]

[0023] [Figure 1] FIG. 2 shows two opposing faces of an example of a reference element as described herein, in this case with the tube bag of the reference element closed. [Diagram 2] FIG. 2 is a top view of the reference element from FIG. 1, in which the tube bag of the reference element has been opened near the top end. [Diagram 3] FIG. 13 is a top view of another embodiment of the reference element with the tube bag opened. [Figure 4] FIG. 2 is a side view of a holder arranged for use with reference elements of the type described herein, in this case with several reference elements fitted thereto as required; [Diagram 5] FIG. 5 is a front view of the holder from FIG. 4 with the reference elements properly fitted. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] The drawings are very simplified. Details not necessary for understanding the subject matter being described are omitted. Moreover, the drawings show only selected exemplary embodiments and should not be taken into account for the limitation of the subject matter rewritten in the claims. Embodiments not shown may be fully covered by the claims.

[0025] Figure 1 shows a reference element 10 of the type described here with a tubing bag 1 closed, Figures 1a and 1b showing two opposite faces of the reference element 10. The tubing bag 1 comprises two end side sealing edges 11 and 12.

[0026] The sealing edge 11 is also referred to below as the upper sealing edge, and the sealing edge 12 as the lower sealing edge. The end sealing edges 11 and 12 each have a longitudinal extension that runs transversely to the longitudinal extension of the tube bag, which therefore runs between the end sealing edges 11 and 12. This designation of the orientation or direction of the tube bag is given informally and in relation to FIG. 2, which shows a top view of the reference element 10, in which the tube bag has been opened along the tear line 14. The end sealing edges 11 and 12 are delimited in the direction of longitudinal extension by their longitudinal edges 111 and 121.

[0027] The tube bag 1 or the reference element 10 further comprises a longitudinal sealing edge 13, which is folded away from the respective adjacent longitudinal edges of the end sealing edges 11 and 12, as is only visible in the representation of FIG. 1b, and will be further explained in connection with FIG. 2. The tube bag 1 shown in the example consists of a foil film element which is folded over and sealed along the sealing edges, for example welded or glued. Since the tube bag 1 consists of only one foil film element, it can be hermetically sealed by only three sealing edges. In a further, not shown, embodiment, the tube bag can be produced from a closed cylindrical foil film element, in which case only end sealing edges are required. In the tube bag 1, there is a reference liquid with a defined pH value, for example a pH buffer, which is filled before the tube bag is completely sealed. It is also possible for the tube bag not to be entirely filled with the reference liquid, but also to contain air or a protective gas. This makes it possible to open the reference element without spilling the reference liquid. The foil film forming the foil film element can be multi-layered. In this respect, the foil film layer which is on the inside of the tube bag can consist of a material which is chemically resistant to the reference liquid, whereas the foil film layer which is on the outside of the tube bag can consist of a material which is resistant to environmental and mechanical influences. Between the two mentioned layers, further layers can be arranged, for example of an aluminum foil film. This intermediate layer serves, among other things, to mechanically reinforce the tube bag. At least one of the outer layers of such a multi-layered tube bag is perforated, in the illustrated exemplary embodiment along a perforation line 14 near the upper sealing edge 11. Along this line, the tube bag is opened when applying the reference element, creating an access to the contained reference liquid. On one side of the tube bag 1, a printing field 18 is further arranged, in which information about the reference element can be written, for example the pH value of the reference liquid contained in the tube bag, the batch number, and the like.

[0028] 2 shows a top view of the reference element 10, in which the tube bag 1 has been opened along the cut line 18 in FIG. 1 and the sensor 3 has been inserted into the reference liquid 17 through the resulting opening at the top of the tube bag. As can be clearly seen in this figure, the longitudinal sealing edge is folded in the circumferential direction in such a way that this joint does not contribute at all or at least does not contribute excessively to the shape rigidity of the tube bag, which would result in the opened bag being flattened. This allows the opening to have an elliptical or at least approximately circular geometric shape, as can be seen, so that the sensor 3 can easily pass through the opening and be inserted into the reference liquid 17. In FIG. 3, an embodiment is shown in which the longitudinal sealing edge 13 is offset in the circumferential direction with respect to the longitudinal edge of the end sealing edge, which has a similar effect, i.e. the shape rigidity of the tube bag is likewise not or not significantly influenced undesirably by the longitudinal sealing edge 13.

[0029] Figures 4 and 5 illustrate the application of the described reference element 10 in a holder 2 prepared and adapted for use in conjunction with the reference element described above. As is evident from Figures 4 and 5 relative to one another, the holder 2 has the basic shape of a triangular prism, i.e. a prism with a triangular base. A side surface 29 of the prism defines the lower surface of the holder, while a side edge 28 opposite this side surface 29 defines the upper edge of the holder. As is evident from Figure 4, the exemplary holder shown has three recesses 21a, 21b, and 21c each extending from the upper edge 28 of the holder in the direction of the lower surface. Each of the recesses has an extent 23 parallel to the upper edge 28 of the holder, which is smaller in the region above the recess towards the lower surface, but is constant in the lower region 22 of each recess. It can be seen that the recesses do not pass completely to the lower surface of the holder. In the recesses 21a and 21b, the fitted reference elements 10a and 10b are shown with the upper side of the tube bag unsealed. These reference elements are braced against the underside of the holder at the bottom sealing edge 12 of the tube bag and are guided in the lower area in the relatively narrow lower area 22 of the respective recess. In the upper area of ​​the recess, on the other hand, the reference elements are supported in a direction parallel to the upper edge 28 of the holder by the edge of the recess which flares upwards and can flare upwards in a funnel-like manner due to the avoided shape rigidity. In this respect, the maximum width of the recess in the area of ​​the upper edge 28 is determined in such a way that at the upper end of the tube bag an ellipse results, the axial ratio of the minor axis and the major axis of the ellipse being, for example, in the range between 0.5 and 1, and is adapted to the geometry of the tube bag. As a result, an opening of the tube bag is formed at the upper end, which allows the sensor to reach the inside of the reference element through the opening without any problems. In the tube bag there is a volume of reference liquid 17, and it may be provided, inter alia, that the reference liquids in the tube bags 10a and 10b have different pH values. A tube bag thus used as specified and fitted into the holder 2 has a maximum available filling height marked 16. As is evident from the reference element 10b, the volume of reference liquid 17 is selected such that the actual filling height 15a, at which the level 15 of the reference liquid 17 is present, is lower than the maximum filling height 16.In the illustrated exemplary embodiment, the volume of the filled reference liquid 17 is determined, for example, in such a way that the actual filling height 15a with a defined positioning of the reference element in the holder 2 is approximately 60% to 75% of the maximum filling height 16. When the sensor is immersed in the reference liquid 17, the displacement of the sensor causes the liquid level 15 to rise, as shown by the reference element 10a. However, the safety margin between the actual filling height 15a and the maximum filling height 16 is selected to be sufficiently large so that even in this case an overflow of the liquid beyond the upper edge of the tube bag is prevented. In FIG. 5 the holder 2 is shown in a front view of the holder with the fitted reference element 10 and the sensor 3 immersed in the reference liquid 17 of the reference element.

[0030] Although the subject matter herein has been described based on selected exemplary embodiments, these exemplary embodiments are not intended to limit the invention as set forth in the claims, which include embodiments not expressly set forth, and embodiments that differ from the examples set forth are still covered by the claims.

Claims

1. A calibration reference element (10) for a pH measuring device, comprising a reference solution (17) having a specified pH value and a tube bag (1), wherein the reference solution is enclosed in the tube bag, the tube bag is made of a foil film and is sealed along one or more sealing edges (11, 12, 13), the number of the sealing edges is at most three, the tube bag is at least partially filled with the reference solution, and the calibration reference element (10) is opened by cutting the tube bag across its longitudinal extension near one of the end face side sealing edges.

2. The calibration reference element according to claim 1, wherein the tube bag (1) is only partially filled with the reference solution (17), and the residual volume is filled with air or a protective gas.

3. The calibration reference element according to claim 1, wherein the tube bag (1) has one sealing edge (11, 12) at each of two opposite end faces, each of the sealing edges runs across the longitudinal extension of the tube bag and has a longitudinal extension defined by the longitudinal edges (111, 121) of the end face side sealing edges respectively, the tube bag has no seam in the circumferential direction or has one longitudinal side sealing edge (13), and the longitudinal side sealing edge (13) is displaced circumferentially from the longitudinal edges of the end face side sealing edges or is folded circumferentially away from the adjacent longitudinal edges (111, 121) of the end face side sealing edges (11, 12).

4. The calibration reference element according to claim 1, wherein the tube bag (1) is made of a foil film having at least two material layers, the layer inside the tube bag is made of a first material, and the layer outside the tube bag is made of a second material.

5. The calibration reference element according to claim 1, wherein an intermediate layer made of a metal foil film, especially an aluminum foil film, is arranged between the layer inside the tube bag and the layer outside the tube bag.

6. Due to the pre - forming of the material of the tube bag in the region of the longitudinal sealing edge, the tube bag opened at the upper side has a relatively low shape rigidity with respect to the cross - sectional shape at the upper edge. Thus, when it is stretched at the closed lower side and properly held otherwise, it will spontaneously change from an elliptical shape to at least approximately a circular geometry, and the geometry enables easy access of the sensor to the reference liquid inside the opened tube bag. The calibration reference element according to claim 1.

7. The calibration reference element according to claim 1, wherein the perimeter of the tube bag and its interior is 70 mm or more and 80 mm or less.

8. The calibration reference element according to claim 1, wherein the length of the fillable internal volume between the two end faces is 90 mm or more and 120 mm or less, and the liquid filling amount in a certain region is in the range of 15 ml or more and 30 ml or less.

9. The calibration reference element according to claim 1, wherein a line is marked on the outer surface of the tube bag, or a suitable device, for example, a cut - off line is provided, and the tube bag should be opened by the user along the line.

10. A calibration reference system for a pH measuring device, comprising at least two calibration reference elements, each including a reference liquid (17) having a specified pH value and a tube bag (1). The reference liquid is enclosed in the tube bag, the tube bag is made of a foil film and is sealed along one or more sealing edges (11, 12, 13), the number of the sealing edges is at most 3, the tube bag is at least partially filled with the reference liquid, at least two of the calibration reference elements contain reference liquids with different pH values, and the calibration reference elements having reference liquids with different pH values are coded with different colors, especially.

11. A holder (2) for a calibration reference element according to any one of claims 1 to 9, wherein the holder has at least partially the shape of a triangular prism, the prism has at least one recess (21a, 21b, 21c), the recess extends from one side (18) of the prism, through the side surface of the prism adjacent to the side, in the direction of the side surface (29) of the prism facing the side, and the side (28) defines the upper edge of the holder, and the side surface (29) of the prism facing the side (28) defines the lower surface of the holder. The holder (2) in which the recess opens at the upper edge, the width (23) of the recess parallel to the upper edge decreases from the upper edge to the minimum width towards the lower surface (29), and the minimum width is maintained along the direction from the upper edge to the lower surface in the lower region (22) of the recess distal from the upper edge.

12. The holder according to claim 11, wherein the maximum width of the recess parallel to the upper edge is 75% or more and 100% or less of the diameter of a circle having a circumference equal to the inner circumference of the tube bag.

13. A calibration reference element for a pH measuring device, comprising a reference liquid (17) having a specified pH value and a tube bag (1), the reference liquid being enclosed in the tube bag, the tube bag being made of a foil film and sealed along one or more sealing edges (11, 12, 13), the number of the sealing edges being at most three, and the tube bag being at least partially filled with the reference liquid, in particular a calibration reference element according to any one of claims 1 to 9 or a calibration reference system according to claim 10, and a holder according to claim 11, a calibration set.

14. A calibration method for a pH measuring device or a pH sensor using at least one calibration set according to claim 13, comprising the steps of placing the holder (2) on a base with the upper edge (28) facing up, opening at least one calibration reference element (10) near one end face of the tube bag, fitting the opened calibration reference element into at least one recess (21a, 21b, 21c) of the holder with the closed end face of the tube bag facing down, wherein the wall of the tube bag is supported by the edge of the recess, whereby the tube bag in the recess forms an open-top container, and immersing a sensor (3) for pH measurement through the upper opening of the tube bag into the reference liquid (17), and comparing the measured value read with the specified pH value of the reference liquid.

15. At least one calibration reference element for a pH measuring device, comprising a reference liquid (17) having a specified pH value and a tube bag (1), wherein the reference liquid is enclosed in the tube bag, the tube bag is made of a foil film and sealed along one or more sealing edges (11, 12, 13), the number of the sealing edges is at most three, and the tube bag is at least partially filled with the reference liquid. In particular, the use for calibrating a pH measuring device or a pH sensor of at least one calibration reference element according to any one of claims 1 to 9, wherein the material of the tube bag in the region of the longitudinal sealing edge is preformed so that the tube bag opened at the upper side has a relatively low shape rigidity with respect to the cross-sectional shape at the upper edge. The calibration reference element is stretched at the closed lower side and otherwise properly held, so that the tube bag opened at the upper side has a geometric shape that changes from an ellipse to at least approximately a circle at the upper edge, and the geometric shape allows easy access of the sensor to the reference liquid inside the opened tube bag.

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