WO2008127041A1

WO2008127041A1 - A miniature electrical conductance sensor using dual concentric ring-disk electrodes with a flat face

Info

Publication number: WO2008127041A1
Application number: PCT/KR2008/002060
Authority: WO
Inventors: Sang-Ryoul Park
Original assignee: Korea Research Institute of Standards and Science
Current assignee: Korea Research Institute of Standards and Science
Priority date: 2007-04-11
Filing date: 2008-04-11
Publication date: 2008-10-23
Anticipated expiration: 2009-10-11
Also published as: KR100845343B1

Abstract

Disclosed is a miniature electrical conductance sensor m which a disk shaped receiving electrode formed of a section of the metal wire on the planarized plane, a ring shaped excitation electrode formed of a section of the fine metal tube on the planarized plane and an insulation film formed of a second of the insulation capillary tube on the planarized plane are laid on a single plane in a concentric structure.

Description

[DESCRIPTION]

[Invention Title]

A MINIATURE ELECTRICAL CONDUCTANCE SENSOR USING DUAL CONCENTRIC RING-DISK ELECTRODES WITH A FLAT FACE

)

[Technical Field]

The present invention relates to a miniature electrical conductance sensor, a method for manufacturing the same and a measuring circuit thereof, and more particularly, to a miniature electrical conductance sensor capable of measuring even when an amount of a sample is little, a method for manufacturing the same and a measuring circuit thereof .

[Background Art] > The present invention relates to a miniature electrical conductance sensor, a method for manufacturing the same and a measuring circuit thereof, and more particularly, to a miniature electrical conductance sensor capable of measuring even when an amount of a sample is little, a method for manufacturing the same and a measuring circuit thereof .

Measuring of an electrical conductance is performed by measuring a concentration of charged substance placed in a space formed between two facing electrodes. In general, an alternate voltage is applied between the electrodes of an electrical conductance sensor, the charged substance is transferred between the electrodes according to the applied voltage. As the result, the electrical conductance is measured by measuring redistribution of the charged material, i.e. f> alternate current.

The electrical conductance sensor may not be suitable to analyze a complex chemical composition, since the electrical conductance sensor measures transferring of every charged substance in a solution according to an applied electric field.

10 However, the electrical conductance sensor is used in many laboratories since it is fast and convenient to determine a normality of a specific solution having a fixed chemical composition compared with other chemical analyses. Because of this convenience, the electrical conductance sensor can also

If) be conveniently used to measure the purity of ultrapure water or to measure the salinity of food.

Various kinds of electrical conductance sensors are commercialized as described above, but an amount of a sample required for normal operation of the electrical conductance 0 sensors is relatively much. Therefore, it is generally difficult to apply the electrical conductance sensor to the case that the amount of the sample is little and limited as in a biology sample. That is to say, unless the size of the vessel into which the measuring electrodes are out is sufficiently large, a shape of the space between the electrodes may be varied by a wall of the vessel. Consequently, the result of measuring the electrical conductance may be varied even in the same solution.

) In order to solve the above problem, a structure that surrounds the measuring electrodes, i.e. a jacket may be additionally placed in the electrical conductance sensor. Therefore, by ensuring or limiting the space between the electrodes, it is possible to prevent that the result of

10 measuring the electrical conductance is influenced by the sample vessel. Nevertheless, since the size of the sensor itself including the jacket is relatively enlarged, it may still be impossible to apply the sensor in the case that the amount of the sample is little and limited as in a biology

11 sample. Also, a process of cleaning the sensor between the measurements may be a bit inconvenient due to a complicated structure including the jacket.

Another problem in the measuring the electrical conductance is that an area in which linearity is maintained i{) is narrow due to the electrical property of the sensor. Therefore, in currently commercialized electrical conductance sensors, a user adjusts a measuring area differently according to the electrical conductance of the solution to be measured, thereby allowing accurate measurement across the wide measurement range. That is to say, there may exist troublesomeness in that the measurement for the electrical conductance should be performed with the measuring area being segmented m order to improve the linearity.

[Disclosure]

[Technical Problem]

An object of the present invention is to provide an electrical conductance sensor that is provided with a

0 miniature electrical conductance sensor and thus is not limited by an amount of a sample, and a method for manufacturing the same.

Another object of the present invention is to provide a circuit for measuring an electrical conductance that is connected with the miniature electrical conductance sensor and is capable of maintaining a linearity of the measured result through a wide measurement range .

[Technical Solution]

-H) The present invention provides an electrical conductance sensor in which a disk shaped receiving electrode formed of a section of the metal wire on the planarized plane, a ring shaped excitation electrode formed of a section of the fine metal tube on the planarized plane and an insulation film formed of a second of the insulation capillary tube on the planaπzed plane are laid on a single plane in a concentric structure .

The electrical conductance sensor may further include an excitation voltage apply line and an excitation signal detection line connected with the excitation electrode and may further include a signal transfer line connected with the receiving electrode.

More specifically, the electrical conductance sensor is formed by inserting a metal wire into an insulation capillary tube and then fixing the metal wire and the insulation capillary tube,- fixing the insulation capillary tube fixed with the metal wire in a fine metal tube,- and planarizmg one ends of the metal wire, the insulation capillary tube and the fine metal tube, wherein the planarized sections of the metal wire, the insulation capillary tube and the fine metal tube are used as electrodes and an insulation film.

Most of all, the electrical conductance sensor composed of the receiving electrode, the insulation film and the excitation electrode respectively formed of a disk shape, a small ring shape and outermost ring shape uses the fine metal tube having an external diameter of less than lmm and thus has an advantage that the electrical conductance sensor can be miniaturized . In the manufacturing of the electrical conductance sensor, forming the excitation voltage apply line and the excitation signal detection line at the tip of the fine metal tube opposite to the electrical conductance measuring sensor unit > may be further included, and projecting the metal wire at the opposite side of the electrical conductance measuring sensor unit to form the signal transfer line may be further included

As the metal wire and the fine metal tube, any material may be used provided that it does not react with liquid or medium to be measured and has high electrical conductance.

Preferably, a platinum wire is used as the metal wire, and a stainless steel tube is used as the fine metal tube.

The fixation of the metal wire and the insulation capillary tube may be performed in such a manner that epoxy is filled and hardened between the metal wire and the insulation capillary tube.

The fixation of the insulation capillary tube into the fine metal tube may be performed m such a manner that epoxy is filled and hardened between the insulation capillary tube and the fine metal tube.

The miniature electrical conductance sensor further comprises a digital circuit unit and an analog circuit unit. The digital circuit unit includes a crystal oscillator, a counter IC and a selection switch, provides a negative signal, a positive signal and a sampling signal to the analog circuit unit and detects an excitation signal of a medium from the receiving electrodes, and the analog circuit unit synthesizes the signals of the digital circuit unit into an analog signal 7) and transfers a bidirectional pulse signal to the excitation electrode .

[Description of Drawings]

The above and other objects, features and advantages of

':() the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which:

Fig. 1 is a structural view illustrating a miniature electrical conductance sensor according to an embodiment of

17) the present invention;

Fig. 2 is a sectional view illustrating a signal transfer path and a measuring space of the miniature electrical conductance sensor according to an embodiment of the present invention;

21) Fig. 3 is a circuit diagram illustrating a measuring circuit of the miniature electrical conductance sensor according to an embodiment of the present invention,-

Fig. 4 is a timing diagram illustrating an excitation pulse and a signal sampling sequence in the electrical conductance measuring manner according to the present invention; and

Fig. 5 is a graph illustrating a result of measuring electrical conductance using the miniature electrical conductance sensor and the measuring circuit according to an experimental example of the present invention. [Detailed Description of Main Elements] 10: receiving electrode 20, 40: fixing epoxy 30: insulation capillary tube 50: excitation electrode 13: signal transfer line 53: excitation voltage apply line

55: excitation signal detection line

[Best Mode] Hereinafter, the embodiments of the present invention will be described in detail with reference to accompanying drawings .

(First Embodiment)

Fig. 1 is a structural view illustrating a miniature electrical conductance sensor according to an embodiment of the present invention.

As shown, the miniature electrical conductance sensor is provided with a cylindrical receiving electrode 10 and a ring shaped excitation electrode 50 that surrounds the receiving electrode 10. An insulation capillary tube 30 is interposed between the receiving electrode 10 and the excitation electrode 50. A lower gap (indicated by arrows in Fig. 2) between the two electrodes constituting a dual electrode is ) used as a measuring space.

Therefore, the miniature electrical conductance sensor has a concentric structure composed of the receiving electrode 10, the insulation capillary tube 30 and the excitation electrode 50. In order for fixing and insulation, epoxies 20 and 40 may be placed between the receiving electrode 10 and the insulation capillary tube 30 or between the insulation capillary tube 30 and the excitation electrode 50.

The excitation electrode 50 may be connected with an excitation voltage apply line 53 and an excitation voltage ) detection line 55, and the receiving electrode 10 may be connected with a signal transfer line 13.

The excitation voltage apply line 53, the excitation voltage detection line 55 and the signal transfer line 13 may be operationally connected with an external measuring circuit and preferably to the digital and analog circuit unit described above. Also, the signal transfer line 13 may be wrapped with a receiving electrode lead line shielding wire 15

Fig. 2 is a sectional view illustrating a signal transfer path and a measuring space of the miniature electrical conductance sensor according to an embodiment of the present invention .

As shown, in the miniature electrical conductance sensor, alternate voltage applied from the outside to the excitation ) electrode 50 is transferred to the receiving electrode 10 by charged substance between the two electrodes, i.e. an electricity transfer solution layer 60. The electricity transfer solution layer 60 may be a sample to be measured. A signal transferred to the receiving electrode 10 is processed '0 in the electrical conductance measuring circuit connected thereto at the outside, thereby capable of measuring the electrical conductance. The excitation electrode 50 may be formed with a diameter of less than lmm.

Therefore, the size of the sensor is minimized and thus

¹T) it is possible to obtain a reliable and reproducible measured result even in a sample with a volume level of lμL using tho miniature electrical conductance sensor of the present invention. Also, since the space for measuring the electrical conductance is formed close to the two electrodes due to the

/() structure of the sensor, the result of measuring the electrical conductance is not influenced by a sample vessel if a space of several mm is allowed under the sensor plane.

Therefore, the miniature electrical conductance sensor of the present invention has an advantage that, unlike the conventional electrical conductance sensor, a reliable measured result is obtained without limitation by volume or amount of the sample or a shape of the vessel.

Referring again to Fig. 1, a method for manufacturing the r> miniature electrical conductance sensor according to an embodiment of the present invention will be described.

At first, a metal wire 10 is inserted into the insulation capillary tube 30 and then the metal wire 10 and the insulation capillary tube 30 are fixed. The metal wire 10 and

'0 the insulation capillary tube 30 may be fixed in such a manner that the epoxy 20 is filled and hardened between the metal wire 10 and the insulation capillary tube 30. By using and hardening the epoxy resin, it is possible to fix and insulate at the same time.

\7> The metal wire 10 may be a platinum wire having low resistance, and the metal wire at an end opposite to the electrical conductance measuring sensor unit may be projected to form the signal transfer wire 13. In addition, the receiving electrode lead line shielding wire may be formed!ϋ around the outer periphery of the signal transfer wire 13 to protect the signal transfer wire 13 from external noises.

Next, the insulation capillary tube 30 fixed with the metal wire 10 is fixed in a fine metal tube 50. The fine metal tube 50 and the insulation capillary tube 30 may be fixed in such a manner that the epoxy 40 is filled and hardened between the insulation capillary tube 30 and the fine metal tube 50. Like the fixation of the metal wire, it is possible to fix and insulate at the same time by using and hardening the epoxy .^") resin.

Then, one ends of the metal wire, the insulation capillary tube and fine metal tube 50 are planarized. By forming, through these processes, the metal wire 10 exposed on the planarized section to the receiving electrode, forming the

:0 fine metal tube 50 to the excitation electrode and forming the insulation capillary tube 30 interposed between the electrodes to an insulation film, an electrical conductance sensor, having a concentric structure in that the receiving electrode is formed at the center thereof and the excitation electrode

IJ is at the outermost periphery thereof, is formed.

In addition, the excitation voltage apply line 53 and the excitation signal detection line 55 are formed at an end of the fine metal tube 50 that is opposite to the electrical conductance measuring sensor unit. Therefore, the electrical 0 conductance measuring sensor unit can be connected to an external circuit so as to be applied with an external signal and detect a signal with respect to the sample to be measured.

Fig. 3 is a circuit diagram illustrating a measuring circuit of the miniature electrical conductance sensor according to an embodiment of the present invention.

As shown, the electrical conductance measuring circuit is preferably connected with the miniature electrical conductance sensor described with reference to Fig. 1. The electrical ) conductance measuring circuit includes a digital circuit unit and an analog circuit unit and may use a bidirectional pulse manner .

The digital circuit unit includes a crystal oscillator, a counter IC and a selection switch and provides a negative

'0 signal, a positive signal and a sampling signal to the analog circuit unit. Also, the digital circuit unit detects an excitation signal of a medium from the receiving electrodes.

The analog circuit unit synthesizes the signals of the digital circuit unit into an analog signal and transfers a

IT) bidirectional pulse signal to the excitation electrode.

In the miniature electrical conductance sensor according to an embodiment of the present invention has a very small serial conductance and a very large stray capacitance due to structural properties of the miniature electrical conductance Λ) sensor. Therefore, there occurs a problem that the measuring area maintaining the linearity is very narrow when applied with a conventional sign wave AC impedance measuring manner.

The above problem can be solved by a measuring circuit with a modified bidirectional pulse type electrical conductance measuring manner as shown in Fig. 4. A in Fig. 4 illustrates a positive pulse train according to time, B illustrates a negative pulse tram, C illustrates a bidirectional pulse train for the excitation, D illustrates a ) received signal, and E illustrates a signal sampling pulse train At this time, tl is an interval of the excitation pulse, t2 is a width of the excitation pulse, t3 is a delay time for sampling, t4 is a width of the sampling time, and t5 is a signal detection time. When the digital circuit unit produces

U) a positive excitation pulse A and a negative excitation pulse, produces a signal for the sampling E after delay for a suitable time t3 and then provides them to the analog circuit unit, the analog circuit unit synthesizes the positive pulse signal and the negative pulse signal into an analog signal to ) produce a bidirectional pulse signal C and then applies it to the excitation electrode. When the excitation signal is transferred to the receiving electrode, this current signal is converted into a voltage signal using a current/voltage converting circuit, and then a sampling t5 is performed

Λ) according to a sampling signal from the digital circuit unit after delay for a suitable time in a sample/hold circuit. The resulting detected signal is converted into a signal proportional to the electrical conductance through an amplification gain and a zero point adjusting circuit and is indicated as a value of the electrical conductance by a subsequent digital voltage meter.

Consequently, by using the miniature electrical conductance sensor and the measuring circuit and the r> bidirectional pulse signal of the above embodiment, it is possible not only to measure a little amount of the sample but also to maintain linearity and reproducibility of the measured result in wide measuring area.

Hereinafter, the miniature electrical conductance sensor 10 and the measuring circuit according to the present invention will be described through following experimental examples. However, the following experimental examples are only illustrative of the present invention and do not limit the present invention. IJ (Experimental Example 1)

Manufacture of a Miniature Electrical Conductance Sensor

A receiving electrode was formed using a platinum wire with a diameter of 0.1mm. The receiving electrode was inserted into an insulation capillary tube formed of fused silica and 0 having an internal diameter of 0.15mm and an external diameter of 0.37mm to be electrically insulated from the outside. Epoxy adhesive agent was filled in the insulation capillary tube and then hardened to fix the receiving electrode at the inserted position. In order for the electric connection, a length of the platinum wire was regulated so that the platinum wire projects towards the opposite side of the sensor plane, thereby forming a signal transfer line.

Into a stainless steel injection needle having an ) internal diameter slightly larger than the external diameter of the insulation capillary tube, i.e. the internal diameter of more than 0.4mm, the insulation capillary tube formed with the receiving electrode was inserted. After that, an epoxy adhesive agent was filled to fix the insulation capillary tube

'0 in the injection needle. After the epoxy adhesive agent was sufficiently hardened, a tip of the injection needle was polished so that the tip of the needle, the epoxy, a tip of the insulation capillary tube and the receiving electrode form an even plane. At this time, it was confirmed whether there is

IT) a pit at the epoxy portion. As the result, an electrical conductance sensor composed of the stainless steel injection needle as an excitation electrode, the insulation capillary tube and the receiving electrode was completed.

Two electric wires were wound around a tip portion of the0 injection needle that is opposite to the sensor plane and then permanently fixed thereto by a soldering. The electric wires were respectively used as the excitation voltage applying line and the excitation signal detection line. The projected platinum wire was connected by a shielded electric wire and thus formed as the signal transfer line. At this time, an epoxy adhesive agent, etc. was protectively wrapped around the projected portion of the platinum wire so that the platinum wire is not broken by external shock. "i (Experimental Example 2)

Design and Manufacture of an Electrical Conductance Measuring Circuit

An electrical conductance measuring circuit of a modified bidirectional pulse type was realized by using the digital

10 circuit unit and the analog circuit unit together. The digital circuit unit produced a reference frequency using the crystal oscillator and then allowed to select a frequency suitable to measure by properly combining the counter IC and a selection switch. Positive excitation signal and negative excitation

¹1 signal were produced based on the selected reference frequency and then, after delay for a suitable time, a signal for sampling was produced and outputted. As such, the digital circuit unit provides three kinds of signals including the positive excitation signal, the negative excitation signal and

2.0 the sampling signal. The analog circuit unit synthesized the positive excitation signal and the negative excitation signal into an analog signal to produce a bidirectional pulse signal and applied it to the excitation electrode.

The excitation signal was transferred to the receiving electrode through conductive medium of the sample. Then, this current signal was converted into a voltage signal using a current/voltage converting circuit and detected after suitable delay in a sample/hold circuit according to the sampling 1 signal from the digital circuit unit. The detected signal was converted into a signal proportional to the electrical conductance through an amplification gain and a zero point adjusting circuit and was indicated as a value of the electrical conductance by a subsequent digital voltage meter. ¹O (Experimental Example 3)

Experiment for measuring Electrical Conductance

An experiment for performances of the miniature electrical conductance sensor and measuring circuit manufactured in the Experimental Examples 1 and 2 was IS performed as follows.

At first, a sample to be measured was prepared. Purified deionized water was used as a base solution, and sodium chloride (NaCl) with a purity of 99.99% was dissolved in the base solution to prepare 1.000M sodium chloride solution. Then, 'Λ) the sodium chloride solution was properly diluted by using gravimetric method to prepare eight kinds of sodium chloride standard solutions across concentration range from 0.5mM to 10OmM, thereby completing the sample to be measured.

The concentrations of the standard solutions were converted into theoretical values of the electrical conductance based on electrical conductance data of the sodium chloride in CRC handbook. For example, the conductance of the 9 97mM sodium chloride solution is converted into 1.1814mS/cm. > The miniature electrical conductance sensor was calibrated with the theoretical electrical conductance value of the 9.97 mM sodium chloride solution, and a state in that the miniature electrical conductance sensor is left m air was set to the zero point The sample solution was placed in a water tank set at 25.0⁰C so as to establish thermal equilibrium, thereby maintaining the exact measuring temperature. Measurement for the electrical conductance was performed by five times respectively to the standard solutions manufactured above.

Fig. 5 is a graph illustrating the result of measuring > electrical conductance using the miniature electrical conductance sensor and the measuring circuit according to an experimental example of the present invention, and the result was obtained from Experimental Example 3.

As shown, it can be appreciated that very good linearity (R2=0.99998) was ensured in the measuring area even with one point calibration (the point indicated by an arrow m Fig. 5 is the point at which the measuring apparatus is calibrated) . The reproducibility of the measurement was as good as less than 0.6%. Though it was increased to 1.5% at less than ImM m which a signal to noise ratio is lowered, the performance was enhanced compared with the conventional sensors.

[industrial Applicability]

The miniature electrical conductance sensor according to the present invention is not only minimized in size to minimize the amount of the sample required for the measurement but also enhanced in a portability by arranging two electrodes for measuring the electrical conductance in a concentric structure and the gap between the two electrodes as the space for measuring the electrical conductance. Also, there is an advantage of enhancing the accuracy of measurement by designing and applying a measuring circuit suitable for the electrical conductance sensor with ultra fine structure. Therefore, it is possible to measure an electrical conductance without limitation by volume or amount of the sample unlike the conventional electrical conductance sensor,

Claims

[CLAIMS] [Claim 1]

A miniature electrical conductance sensor, formed including the steps of: inserting a metal wire into an insulation capillary tube and then fixing the metal wire and the insulation capillary tube ; fixing the insulation capillary tube fixed with the metal wire m a fine metal tube; and

0 planarizmg one ends of the metal wire, the insulation capillary tube and the fine metal tube, wherein a disk shaped receiving electrode formed of a section of the metal wire on the planarized plane, a ring shaped excitation electrode formed of a section of the fine ¹ ") metal tube on the planarized plane and an insulation film formed of a section of the insulation capillary tube on the planarized plane are laid on a single plane in a concentric structure .

[Claim 2]

^;ϋ The miniature electrical conductance sensor as set forth m claim 1, wherein the fine metal tube has an external diameter of less than lmm.

[Claim 3]

The miniature electrical conductance sensor as set forth 2?

in claim 1, wherein the metal wire is a platinum wire.

[Claim 4]

The miniature electrical conductance sensor as set forth m claim 1, wherein the fine metal tube is a stainless steel

1 tube .

[Claim 5]

The miniature electrical conductance sensor as set forth m claim 1, further comprising an excitation voltage applying line and an excitation signal detection line connected with '0 the excitation electrode.

[Claim 6]

The miniature electrical conductance sensor as set forth in claim 1, further comprising a signal transfer line connected with the receiving electrode.

IT)

[Claim 7]

The miniature electrical conductance sensor as set forth in claim 1, wherein the fixation of the metal wire and the insulation capillary tube is performed in such a manner that epoxy is filled and hardened between the metal wire and the

O insulation capillary tube.

[Claim 8j

The miniature electrical conductance sensor as set forth in claim 1, wherein the fixation of the insulation capillary tube into the fine metal tube is performed in such a manner that epoxy is filled and hardened between the insulation capillary tube and the fine metal tube.

[Claim 9]

The miniature electrical conductance sensor as set forth ) m claim 1, wherein the miniature electrical conductance sensor further comprises a digital circuit unit and an analog circuit unit, wherein the digital circuit unit includes a crystal oscillator, a counter IC and a selection switch, provides a

:0 negative signal, a positive signal and a sampling signal to the analog circuit unit and detects an excitation signal of a medium from the receiving electrodes, and the analog circuit unit synthesizes the signals of the digital circuit unit into an analog signal and transfers a bidirectional pulse signal to

I! the excitation electrode.