Reference electrode

A reference electrode is provided with an accommodation portion that is provided with a tube-shaped lead-out portion that can guide an accommodated internal liquid; a liquid junction portion that is connected to an end of the lead-out portion, and that allows the internal liquid to seep out; a liquid dripping portion that has a first end connected to the liquid junction portion, that has a second end that protrudes into the accommodation portion, and that guides the internal liquid to the liquid junction portion; and an internal electrode having at least a portion that is positioned further towards the first end side than the second end of the liquid dripping portion.

BACKGROUND OF THE INVENTION

The present invention relates to a reference electrode used for testing a given location.

One known method for measuring the pH of a given location, for example, is to use sensors, and one known type of sensor is one that measure pH with reference to the potential of a reference electrode.

The reference electrode is configured in a manner allowing a metal internal electrode to be soaked in an internal liquid and causing the internal liquid to permeate to an object for measurement through a liquid junction portion made of a porous material.

One known example of such a reference electrode is one in which the internal liquid and the liquid junction part are fixed to a sensor main body by a fitting structure (see, for example, Japanese Patent Application Laid-Open Publication No. H6-30913).

An accommodation portion that accommodates the internal liquid in the sensor main body is formed in the reference electrode, the internal electrode extends from one end of the accommodation portion, and the liquid junction portion is provided on the other end of the accommodation portion.

Another known reference electrode is one having the purpose of measuring the electrochemical indicator of high temperature/pressure boiler water or the like, and having a structure in which regular maintenance is required (see, for example, Japanese Patent Application Laid-Open Publication No. 2006-177678).

This reference electrode has an elongated accommodation space that accommodates the internal liquid, and the space between the internal electrode provided on one end of the accommodation space and the liquid junction portion on the other end is susceptible to bubbles forming therein. Thus, in order to maintain an electrical connection between the internal electrode and the internal liquid even when bubbles form in a portion of the space, the internal electrode and the liquid junction portion are connected by a braided body in which fibers are woven.

SUMMARY

However, in such reference electrodes, the internal electrode and the liquid junction portion are separated from each other. As a result, if the amount of internal liquid decreases while the reference electrode is being used in a state where the liquid junction portion is disposed towards the bottom, then regardless of the fact that the surface of the internal liquid is above the upper end of the liquid junction portion, conduction cannot be maintained between the internal electrode and the internal liquid. In such a case, there is a risk that the reference electrode cannot fulfill its function.

Also, in the latter reference electrode, the internal electrode and the liquid junction portion are connected by a braided body, but if there is a decrease in the internal liquid, then the internal liquid drawn upward by the braided body does not reach the internal electrode. In this case as well, there is a risk that the function of the reference electrode is diminished or cannot be fulfilled at all.

The present invention takes into account the above-mentioned problem, and an object thereof is to provide a reference electrode by which it is possible to mitigate a decrease in functionality resulting from a decrease in the amount of internal liquid.

A reference electrode according to the present invention includes: an accommodation portion that is provided with a tube-shaped lead-out portion that can guide an accommodated internal liquid; a liquid junction portion that is connected to an end of the lead-out portion, and that allows the internal liquid to seep out; a liquid dripping portion that has a first end connected to the liquid junction portion, that has a second end that protrudes into the accommodation portion, and that guides the internal liquid to the liquid junction portion; and an internal electrode having at least a portion that is positioned further towards the first end side than the second end of the liquid dripping portion.

According to the present invention, it is possible to provide a reference electrode by which it is possible to mitigate a decrease in functionality resulting from a decrease in the amount of internal liquid.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiment 1 of the present invention will be explained below with reference to the drawings.

FIG. 1is a cross-sectional view showing a sensor12including a reference electrode10according to the present embodiment. The sensor12is used in measuring the state of an object for measurement such as soil or a nutrient medium for hydroponic cultivation in which to grow a crop, and is configured to be able to measure the pH of the object for measurement.

A case14of the sensor12is, for example, made of a synthetic resin. The bottom end of the case14has formed therein a first inclined surface16and a second inclined surface18that, moving towards the bottom ofFIG. 1, incline towards the center, and thus, the bottom end of the case14has a narrow tip.

A bottom surface20of the case14has formed therein a circular hole22, and the hole22communicates with an accommodation space24formed in the case14. A bottom surface26of the accommodation space24connected to the hole22has a counterbore28formed therein, and the counterbore28is formed so as to surround the hole22.

The second inclined surface18formed in the case14has formed therein a substrate accommodation opening30, and the edge of the substrate accommodation opening30has formed therein a flange32that extends inward. The substrate accommodation opening30has provided therein a sensor substrate34, and the sensor substrate34is fixed in place so as to be in close contact with the flange32via a ring-shaped sealing member36. As a result, a watertight seal is formed between the inside and the outside of the case14.

The sensor substrate34is provided with an ISFET (ion sensitive field effect transistor) chip38, which is a chip for measuring the pH of the object for measurement together with the reference electrode10to be described later.

The surface of the ISFET chip38has formed thereon an ion-sensitive film, which forms a sensitive surface38A. In a state where the sensor12is disposed in the object for measurement, when a reference potential from the reference electrode10is applied to the object for measurement, hydrogen ions (H+) in the object for measurement gather on the sensitive surface38A of the ISFET chip38. At this point, the ISFET chip38detects the interface potential between the object for measurement and the ion-sensitive film, and thus, by measuring the difference in potential between the interface potential detected by the ISFET chip38and the reference electrode, it is possible to measure the pH, which indicates the hydrogen-ion exponent of the object for measurement.

The reference electrode10is provided in the accommodation space24of the case14. The reference electrode10applies a potential that serves as a reference for calculation of electrode potential and electrochemical measurement, and in the present embodiment, the reference electrode applies a reference potential to the object for measurement.

An accommodation portion40of the reference electrode10is made of glass or vinyl chloride, for example. The accommodation portion40has formed therein a tank portion44in the form of a container that accommodates an internal liquid42, and a lead-out portion46that guides outward the internal liquid42inside the tank portion44.

The tank portion44has a peripheral surface48that forms a cylindrical wall, a first end face50that closes off one end of the peripheral surface48, and a second end face52that closes off another end of the peripheral surface48. The cylindrical lead-out portion46, which has a tube shape, extends from the first end face50, and the lead-out portion46is connected to the inside of the tank portion44.

The first end face50from which the lead-out portion46extends is inclined towards the one end from the outer peripheral portion towards the lead-out portion46, and is configured such that with the first end face50facing downward, the internal liquid42gathers towards the lead-out portion46.

The tip of the lead-out portion46is provided with a liquid junction portion54. The liquid junction portion54is made of a porous material having pores, and allows the internal liquid42supplied from the lead-out portion46to seep through. Also, the liquid junction portion54is made of a porous glass, and is durable against the alkalinity or acidity of the object for measurement as well as the internal liquid42.

In the present embodiment, an example will be described in which the liquid junction portion54is made of porous glass, but the configuration is not limited to this example. The liquid junction portion54can be made of a hydrophilic polypropylene, Vycor glass, or a porous ceramic, for example.

The liquid junction portion54is formed into a cylindrical shape with an outer diameter GS that is greater than an inner diameter NS of the lead-out portion46, and the outer diameter GS of the liquid junction portion54is substantially the same as the outer diameter of the lead-out portion46.

The outer peripheral surfaces of the liquid junction portion54and the lead-out portion46are provided with a thermal contraction tube56. The thermal contraction tube56covering the liquid junction portion54and the lead-out portion46contracts when heated, thereby fixing the liquid junction portion54to the lead-out portion46.

Examples are known of a thermal contraction tube56in which the internal surface thereof is provided with an adhesive. However, in the present embodiment, the thermal contraction tube56does not have an adhesive, thereby preventing seepage of an adhesive to the liquid junction portion54.

The lead-out portion46has inserted therein one end of a liquid dripping portion58. The liquid dripping portion58is constituted of a porous material having pores, and is impregnated with the internal liquid42in the tank portion44. The liquid dripping portion58is made of a hydrophilic polypropylene, and is durable against the internal liquid42.

In the present embodiment, an example will be described in which the liquid dripping portion58is made of polypropylene, but the configuration is not limited to this example. The liquid dripping portion58can be made of another material that does not corrode in a chloride solution and does not deteriorate after being stored for long periods of time.

The liquid dripping portion58is formed as a quadrangular pole. The liquid dripping portion58is supported in a state where the corners thereof are in contact with an internal surface60of the lead-out portion46, and a flow path62through which the internal liquid42flows is formed between the outer surface of the liquid dripping portion58and the internal surface60of the lead-out portion46.

A first end64of the liquid dripping portion58is in planar contact with a base end face66of the liquid junction portion54, and a second end68of the liquid dripping portion58protrudes inside the tank portion44of the accommodation portion40. As a result, the internal liquid42in the tank portion44is supplied to the liquid junction portion54through the liquid dripping portion58.

The average diameter of the pores in the liquid dripping portion58is 10 μm or greater and less than or equal to the maximum size that would allow capillary action of the internal liquid42. Specifically, the average diameter is 10 μm to 100 μm, inclusive. In the present embodiment, the average diameter of the pores in the liquid dripping portion58is 17 μm.

The average diameter of the pores in the liquid junction portion54is set to 1 nm to 10 nm, inclusive, and in the present embodiment, the average diameter of the pores in the liquid junction portion54is 4 nm.

It is preferable that the average diameter of the pores of the liquid dripping portion58be 4000 times or greater the average diameter of the pores of the liquid junction portion54, and in the present embodiment as well, the average diameter of the pores of the liquid dripping portion58is set to 4000 times or greater the average diameter of the pores of the liquid junction portion54.

Thus, it is possible to quickly supply the internal liquid42to the liquid junction portion54through the liquid dripping portion58, the pores of which have a large diameter, and it is possible to suppress the amount of internal liquid42seeping from the liquid junction portion54, thereby enabling long-term use.

The second end face52of the tank portion44has fixed thereto a control substrate70, and an internal electrode72extends from the control substrate70. The internal electrode72penetrates the second end face52of the tank portion44and extends therein, and is inserted into the internal liquid42in the tank portion44. The tip of the internal electrode72approaches the first end face50of the tank portion44.

The internal electrode72is disposed at a position offset from a central axis CJ of the liquid dripping portion58, and the tip of the internal electrode72has a length that reaches further towards the first end64than the second end68of the liquid dripping portion58. As a result, at least a portion of the internal electrode72is positioned towards the first end64than the second end68of the liquid dripping portion58.

Examples of the internal liquid42include a potassium chloride (KCl) solution and a sodium chloride (NaCl) solution, for example, and examples of the material constituting the internal electrode72include silver and silver chloride (Ag/AgCl).

The control substrate70is provided with a connector74, and the portion of the case14facing the connector74has formed therein a connector insertion hole76. As a result, by connecting a communication cable to the connector74, it is possible to attain a configuration in which detection results from the sensor12can be outputted.

The lead-out portion46provided on the accommodation portion40of the reference electrode10has an O-ring78fitted therearound, and movement of the O-ring78in a direction approaching the first end face50is restricted by a stopper80provided on the first end face50of the accommodation portion40.

The lead-out portion46is inserted into the hole22provided in the case14, such that an end face82of the liquid junction portion54provided at the tip of the lead-out portion46is flush with the bottom surface20of the case14.

In the present embodiment, the liquid junction portion54is fixed to the lead-out portion46by the thermal contraction tube56, but the structure is not limited thereto. A stopper that abuts the end face82of the liquid junction portion54may be provided on the case14so as to prevent the liquid dripping portion58from separating.

In this insertion state, the O-ring78fitted around the lead-out portion46is fitted inside the counterbore28in the case14, and the O-ring78is in close contact with the internal surface of the counterbore28and the external surface of the lead-out portion46. As a result, watertightness is ensured, and it is possible to mitigate the entry of moisture into the interior.

The sensor12includes the reference electrode10, the ISFET chip38, and the substrates34and70, which control the foregoing, and measures to prevent water from adhering to the substrates34and70are necessary. By disposing the O-ring78at the base end of the lead-out portion46, it is possible to separate the inside of the case14from the outside thereof with ease, and it is possible to protect the substrates34and70from the external environment and for the sensor12to perform stable measurements over a long period of time.

Next, the operation of the present embodiment will be explained.

In the reference electrode10of the present embodiment, the second end68of the liquid dripping portion58, the first end64of which is in contact with the liquid junction portion54, protrudes into the accommodation portion40, and thus, it is possible to guide the internal liquid42in the accommodation portion40through the liquid dripping portion58to the liquid junction portion54.

The internal electrode72inserted into the tank portion44of the accommodation portion40has a length such that the tip protrudes further towards the first end64than the second end68of the liquid dripping portion58, and at least a portion of the internal electrode72is further towards the first end64than the second end68of the liquid dripping portion58.

Thus, in a usage state where the bottom surface20faces downward, even if there is a decrease in the amount of the internal liquid42and the liquid surface42A drops below the second end68of the liquid dripping portion58, electrical conduction can be maintained between the internal electrode72and the internal liquid42seeping out of the liquid junction portion54, through the internal liquid42that has impregnated the liquid junction portion54.

As a result, even if the liquid surface42A of the internal liquid42has dropped below the second end68of the liquid dripping portion58, it is possible to apply a reference potential to the object for measurement. Therefore, it is possible to mitigate a decrease in functionality resulting from a decrease in the amount of the internal liquid42. Thus, it is possible to attain stable measurement results over a long period of time.

Also, in the present embodiment, the liquid junction portion54is fixed to the tip of the lead-out portion46using the thermal contraction tube56.

Thus, according to the structure for fixing the liquid junction portion54in the case14by being fitted therein, the manufacturing cost can be reduced as compared to a situation in which the case14must be designed with consideration for the manufacturing tolerance of the case14and the liquid junction portion54. Also, it is possible to mitigate a situation in which the liquid junction portion54falls out due to differences in the coefficient of thermal expansion between the case14and the liquid junction portion54.

Here, if fixing the lead-out portion46to the liquid junction portion54using the thermal contraction tube56as in the present embodiment in order to reduce cost, there is a need to match the outer diameter of the lead-out portion46with the outer diameter GS of the liquid junction portion54. If, in order to achieve long-term use, the diameter of the liquid junction portion54were reduced in order to reduce the amount of seepage of the internal liquid42, the internal diameter NS of the lead-out portion46would be reduced.

In such a case, if air bubbles enter the lead-out portion46due to vibration during transit or the orientation during storage, it is difficult for air bubbles to escape from the lead-out portion46, which has a small diameter, resulting in conduction between the internal liquid42, which is in contact with the object for measurement, and the internal electrode72being cut off, which causes a defect that prevents measurement of pH.

However, in the present embodiment, the liquid dripping portion58is inserted into the lead-out portion46, and it is possible to supply the internal liquid42in the accommodation portion40through the liquid dripping portion58to the liquid junction portion54. Thus, it is possible to mitigate defects resulting from air bubbles that have formed in the lead-out portion46.

The liquid dripping portion58of the reference electrode10is made of polypropylene.

Thus, it is possible to mitigate corrosion or the like of the liquid dripping portion58by the chloride solution, which constitutes the internal liquid42, and to mitigate deterioration due to long-term storage. Also, polypropylene is durable against the alkalinity or acidity of the object for measurement, and thus, it is possible to mitigate degeneration of the liquid dripping portion58.

Also the internal electrode72is disposed at a position offset from a central axis CJ of the liquid dripping portion58.

Thus, it is possible to mitigate unwanted contact between the liquid dripping portion58and the internal electrode72. As a result, it is possible to mitigate peeling of the AgCl that has been plated onto the internal electrode72.

The liquid junction portion54and the liquid dripping portion58are made of porous materials having pores, and the average diameter of the pores of the liquid junction portion54less than an average diameter of the pores of the liquid dripping portion58. For example, in one embodiment, the average diameter of the pores of the liquid junction portion54is 1 nm to 10 nm, inclusive. The average diameter of the pores in the liquid dripping portion58is 10 μm or greater and less than or equal to the maximum size that would allow capillary action of the internal liquid42.

Thus, compared to a case in which the average diameter of the pores of the liquid junction portion54and the liquid dripping portion58were the same, it is possible to increase supply of the internal liquid42to the liquid junction portion54due to the larger pores of the liquid dripping portion58while suppressing seepage of the internal liquid42by the liquid junction portion54, which has smaller pores.

Therefore, compared to a case in which the structure requires regular maintenance, such as a reference electrode made for the purpose of measuring an electrochemical index of boiler water or the like, which is high temperature and pressure, it is possible to use the reference electrode of the present embodiment even without the expert knowledge required for maintenance.

FIGS. 2 and 3are cross-sectional views showing a sensor12including a reference electrode10according to Embodiment 2. Portions that are the same as or equivalent to Embodiment 1 are assigned the same reference characters and descriptions thereof are omitted. Only differing portions will be described.

The reference electrode10according to the present embodiment differs in terms of the lead-out portion46as compared to Embodiment 1.

Specifically, as shown inFIG. 3, the lead-out portion46has a jagged portion84formed on the internal surface60facing the liquid dripping portion58, which has been inserted therein. The jagged portion84has protrusions86that protrude in a mountain, saw-tooth, or triangular form towards the center CS of the lead-out portion46, and recesses88that are recessed towards the outside of the lead-out portion46, and the protrusions86and the recesses88are formed alternately in the length direction of the lead-out portion46.

Protrusions86that are adjacent to each other in the length direction of the lead-out portion46may be formed independently, or may be formed as a screw thread formed in a spiral along the internal surface60.

By forming the protrusions86of the liquid junction portion54as a screw thread, it is possible to insert the lead-out portion46while rotating the liquid dripping portion58, thereby enabling easy insertion.

Even with this configuration, similar operations and effects to Embodiment 1 can be attained.

Also, in the present embodiment, it is possible to increase frictional resistance between the protrusions86formed on the lead-out portion46and the liquid junction portion54, thereby enabling mitigation of unwanted detachment of the liquid junction portion54from the lead-out portion46.

FIG. 4is a cross-sectional view showing a sensor12including a reference electrode10according to Embodiment 3. Portions that are the same as or equivalent to Embodiment 1 are assigned the same reference characters and descriptions thereof are omitted. Only differing portions will be described.

The reference electrode10according to the present embodiment differs in terms of the liquid dripping portion58as compared to Embodiment 1.

Specifically, the second end68of the liquid dripping portion58is long enough to come into contact with the second end face52, which is an example of a wall surface of the accommodation portion40.

Even with this configuration, similar operations and effects to Embodiment 1 can be attained.

Also, in the present embodiment, a first end64of the liquid dripping portion58is in contact with a base end face66of the liquid junction portion54, and the second end68is in contact with the second end face52of the accommodation portion40. As a result, the liquid dripping portion58has one end surrounded by the lead-out portion46, and is held in a state of being sandwiched on both ends by the base end face66of the liquid junction portion54and the second end face52of the accommodation portion40.

Thus, it is possible to mitigate a situation in which the liquid junction portion54falls out.

FIG. 5is a cross-sectional view showing a sensor12including a reference electrode10according to Embodiment 4. Portions that are the same as or equivalent to Embodiment 1 are assigned the same reference characters and descriptions thereof are omitted. Only differing portions will be described.

The reference electrode10according to the present embodiment differs in terms of the positional relationship between the internal electrode72and the liquid dripping portion58as compared to Embodiment 1.

Specifically, an insertion hole58A formed along a center axis CJ is formed on a second end side of the liquid dripping portion58, and this insertion hole58A is formed in the second end68. Also, the internal electrode72is disposed on the central axis CJ of the liquid dripping portion58, and the tip of the internal electrode72is inserted into the insertion hole58A of the liquid dripping portion58.

As a result, at least a portion of the internal electrode72is inserted in the liquid dripping portion58.

In the present embodiment, in assembling the reference electrode10, the liquid dripping portion58has the internal electrode72inserted in advance into the insertion hole58A.

Even with this configuration, similar operations and effects to Embodiment 1 can be attained.

Also, in the present embodiment, a portion of the internal electrode72is inserted into the liquid dripping portion58, and thus, the internal electrode72can perform charge exchange with the internal liquid42that has impregnated the liquid dripping portion58. Thus, while the liquid dripping portion58is impregnated with the internal liquid42, it is possible to apply a reference potential, and thus, it is possible to lengthen the time over which the reference electrode10can be used.

Also, it is possible to attain effects resulting from the internal electrode72being closer to the liquid junction portion54.

FIG. 6is a cross-sectional view showing a sensor12including a reference electrode10according to Embodiment 5. Portions that are the same as or equivalent to Embodiment 1 are assigned the same reference characters and descriptions thereof are omitted. Only differing portions will be described.

The reference electrode10according to the present embodiment differs in terms of the arrangement of the internal electrode72as compared to Embodiment 1.

Specifically, the internal electrode72extends into the accommodation portion40from the first end face50of the accommodation portion40in which the lead-out portion46is provided. As a result, the internal electrode72protrudes inward towards the accommodation portion40from the first end face50, which is the wall surface of the accommodation portion40on the liquid junction portion54side.

Even with this configuration, similar operations and effects to Embodiment 1 can be attained.

Also, in the present embodiment, the internal electrode72protrudes from the first end face50on the liquid junction portion54side of the accommodation portion40. Thus, compared to a case in which the internal electrode72extends from the second end face52, which is opposite to the side with the liquid junction portion54, towards the liquid junction portion54, it is possible to shorten the separation distance between the position where the internal electrode72is fixed and the liquid dripping portion58.

In a usage state where the bottom surface20faces downward, even if there is a decrease in the amount of the internal liquid42and the liquid surface42A drops below the second end68of the liquid dripping portion58, electrical conduction can be maintained to a greater degree between the internal electrode72and the internal liquid42seeping out of the liquid junction portion54, compared to a case in which the internal electrode72extends into the tank portion44from the peripheral surface48of the tank portion44or the second end face52.

As a result, it is possible to stably maintain the position of the internal electrode72relative to the liquid dripping portion58. Also, it is possible to mitigate interference between the internal electrode72and the liquid junction portion54resulting from assembly, and it is possible to mitigate unexpected peeling of the AgCl that has been plated onto the internal electrode72.

FIGS. 7 and 8are cross-sectional views showing a sensor12including a reference electrode10according to Embodiment 6. Portions that are the same as or equivalent to Embodiment 1 are assigned the same reference characters and descriptions thereof are omitted. Only differing portions will be described.

The reference electrode10according to the present embodiment differs in terms of the shape of the liquid dripping portion58as compared to Embodiment 1.

Specifically, the liquid dripping portion58includes an insertion hole58A that is inserted into the lead-out portion46, and a bottom surface58B that extends from the insertion hole58A and that is arranged so as to be stacked on the first end face50of the accommodation portion40. Also, the liquid dripping portion58includes a cylindrical side wall58C that rises from the ends of the bottom surface58B and extends along the peripheral surface48of the accommodation portion40, and the insertion hole58A, the bottom surface58B, and the side wall58C are formed integrally.

As a result, at least a portion of the liquid dripping portion58extends along the peripheral surface48, which is an example of an internal wall of the accommodation portion40.

Even with this configuration, similar operations and effects to Embodiment 1 can be attained.

Also, in the present embodiment, the liquid dripping portion58is formed along the peripheral surface48of the accommodation portion40. Thus, as shown inFIG. 8, when the sensor12is used in an inclined state, even if the liquid surface42A of the internal liquid42does not reach the opening of the lead-out portion46, it is possible to supply the internal liquid42through the liquid dripping portion58to the liquid junction portion54.

Therefore, even when using the sensor12in an inclined state with a reduced amount of the internal liquid42, it is possible to apply the reference potential, enabling long-term use.

FIGS. 9 and 10are cross-sectional, perspective views showing a sensor12including a reference electrode10according to Embodiment 7. Portions that are the same as or equivalent to Embodiment 6 are assigned the same reference characters and descriptions thereof are omitted. Only differing portions will be described.

The reference electrode10according to the present embodiment differs in terms of the structure of the internal electrode72as compared to Embodiment 6.

Specifically, the internal electrode72includes a core72A that penetrates the second end face52(not shown inFIGS. 9 and 10, due to the perspective view) of the accommodation portion40(seeFIG. 7) and extends linearly, an extension portion72B that extends towards the side from the core72A, and a cylindrical portion72C having a cylindrical shape that is connected to the extension portion72B and that extends in contact with the side wall58C of the liquid dripping portion58.

As a result, the internal electrode72is in contact with the liquid dripping portion58.

Even with this configuration, similar operations and effects to Embodiments 1 and 6 can be attained.

Also, the internal electrode72of the present embodiment includes a cylindrical portion72C that extends along the side wall58C of the liquid dripping portion58and that is in contact with the side wall58C.

Thus, as shown inFIG. 10, when the sensor12is used in an inclined state, even if the liquid surface42A of the internal liquid42does not reach the opening of the lead-out portion46, it is possible to supply the internal liquid42through the liquid dripping portion58to the liquid junction portion54.

Even if the liquid surface42A of the internal liquid42does not reach the core72A of the internal electrode72, the internal liquid42is in contact with the cylindrical portion72C of the internal electrode72, and thus, it is possible for the internal electrode72to exchange charge with the internal liquid42.

Thus, compared to Embodiment 6, even when using the sensor12in an inclined state with an even more reduced amount of the internal liquid42, it is possible to apply the reference potential, enabling long-term use.

In the present embodiment, by having a structure in which the cylindrical portion72C of the internal electrode72is in contact with the liquid dripping portion58, it is possible to ensure conduction between the internal electrode72and the internal liquid42that has impregnated the liquid junction portion54, thereby enabling a more preferable form.

FIG. 11is a cross-sectional view showing a sensor12including a reference electrode10according to Embodiment 8. Portions that are the same as or equivalent to Embodiment 1 are assigned the same reference characters and descriptions thereof are omitted. Only differing portions will be described.

The reference electrode10according to the present embodiment differs in terms of the structure of the integrated liquid dripping portion59compared to the separate liquid dripping portion58and liquid junction portion54in Embodiment 1.

Specifically, the integrated liquid dripping portion59includes a liquid dripping region59A and a liquid junction region59B, which has a larger diameter than the liquid dripping portion59A, are formed integrally of the same material, and an example of such a material is polypropylene. In the present embodiment, the viscosity of the internal liquid42is adjusted according to the average diameter of the pores of the integrated liquid dripping portion59.

Thus, the liquid dripping region59A and the liquid junction portion59B are formed integrally in an integrated liquid dripping portion59.

Even with this configuration, similar operations and effects to Embodiments 1 and 6 can be attained. For example, in one embodiment, the liquid dripping region59A has an average pore diameter greater than the liquid junction portion59B.

Also, in the present embodiment, the integrated liquid dripping portion59makes it possible to prevent defects such as an offset in cores between the liquid dripping portion59A and the liquid junction portion59B.

Thus, it is possible to reduce the cost of material acquisition as well as to reduce the number of assembly steps.

Therefore, it is possible to reduce the cost of the sensor12.

DESCRIPTION OF REFERENCE CHARACTERS

42A liquid surface

58C side wall

59integrated liquid dripping portion

59A liquid dripping portion

59B liquid junction portion