WO2005054836A1

WO2005054836A1 - Potentiometric measuring cell used in microprocessing technology

Info

Publication number: WO2005054836A1
Application number: PCT/EP2004/053151
Authority: WO
Inventors: Torsten Pechstein; Robert Scholz
Original assignee: Endress+Hauser Conducta Gesellschaft Für Mess- Und Regeltechnik Mbh
Priority date: 2003-12-01
Filing date: 2004-11-29
Publication date: 2005-06-16
Also published as: DE10356638A1

Abstract

Disclosed is a measuring cell (1) for detecting a potentiometric variable, comprising a potentiometric sensor (20), a measurement channel (10) for conducting a measurement medium, and at least one first reference half cell that is provided with a first reference chamber (28, 29, 31) and a first reference sensor (30) for supplying a reference potential. The measurement chamber encompasses a first final section with an inlet, a second final section with an outlet, and a central section which is located between the first and the second final section and is provided with at least one sensor aperture (15) on which the potentiometric sensor (20) is arranged. The reference chamber is connected to the measurement channel via a reference aperture (16).

Description

Description Potentiometric measuring cell for micro process engineering

Conventional sensors, which are integrated into the corresponding systems by special fittings, have hitherto been used in microprocessing and microreactor technology for measuring important process parameters. Here one easily comes up against limits with regard to the minimum cable cross-sections and the desired freedom from dreams, as well as particle deposits and material requirements. For the potentiometric parameters in process analysis (pH value, redox potential), no suitable sensors for miniaturized inline measurements are available. The use of conventional process analysis sensors in micro process engineering instead has the following disadvantages. It is necessary to use special fittings that have a large dead volume due to their design. This has an extremely negative effect, especially in the case of small available volumes of the measurement medium, because a relatively large minimum volume of measurement medium is required in order to be able to carry out meaningful measurements at all. In addition, a conventional sensor must be completely replaced in the event of defects or for maintenance; this usually includes both the actual sensor element and the reference system. Furthermore, unfavorable flow conditions, especially in flow shadows, can easily lead to deposits or the blocking of functional components such as diaphragms or the like. Finally, especially when examining "non-conductive liquids", static charges can occur due to the friction between the liquid and the pipelines, which can impair potentiometric measurements.

It is therefore the object of the present invention to provide a device which alleviates or overcomes the disadvantages described. This object is achieved by the measuring cell according to independent patent claim 1.

[003] The measuring cell according to the invention for detecting a potentiometric variable comprises

[004] a pentiometric sensor;

[005] a measuring channel for guiding a measuring medium,

[006] wherein the measuring channel has a first end section with an inlet opening, a second end section with an outlet opening and a middle section between the first and the second end section, and the middle one Section comprises at least one sensor opening at which the pentiometric sensor is arranged; and

[007] at least one first reference half-cell, which has a first reference chamber and a first reference sensor for providing a reference potential, the reference chamber being connected to the lumen of the measuring channel via a reference opening in the measuring channel.

In a currently preferred embodiment of the invention, the measuring cell comprises a base body in which the measurement channel and the reference chamber are formed in the base body, for example during production by machining processes (drilling) or casting processes (injection molding).

[009] The potentiometric sensor, a pH sensor or a sensor for determining a redox potential, is preferably a semiconductor sensor, in particular an ion-sensitive FET sensor, the principle of which is shown, among other things, in German patent DE 198 57 953 C2. To minimize dead volumes, it is currently preferred for the sensor to have an ion-sensitive section which is arranged at a minimal distance from the inner wall of the measuring channel in the region of the sensor opening. This means that no spur lines lead from the measuring channel to the ion-sensitive section. In particular, the ion-sensitive section is at most offset by the wall thickness of the measuring channel and the thickness of a sealing element between the ion-sensitive section and the outer wall of the measuring channel with respect to the inner wall of the measuring channel.

Furthermore, it is currently preferred that the ratio between the depth of the sensor opening and its diameter is minimized. The distance of the deepest point of the wettable surfaces in the sensor opening from the deepest plane, which runs perpendicular to an axis of the sensor opening and touches the inner wall of the measuring channel, is preferably no more than the maximum diameter of the sensor opening, preferably no more than the minimum diameter of the sensor opening. The distance of the deepest point of the wettable surfaces in the sensor opening from the deepest plane, which runs perpendicular to an axis of the sensor opening and touches the lumen of the measuring channel, is more preferably not more than half the maximum diameter of the sensor opening, particularly preferably not more than that half the minimum diameter of the sensor opening.

[011] The volume delimited by the wettable surface volume in the sensor opening is preferably constructed to be convex. That is, between any two points within this volume there are no constructive protrusions, which as Flow barriers could act. [012] Preferably, at least the wettable part of the sensor, which comprises the ion-sensitive section, has a planar surface. If the inner wall of the measuring channel is essentially cylindrical at least in sections and the sensor element has a planar surface, it is currently preferred that the outer wall of the measuring channel is adapted to the surface of the sensor element at least in the region of the sensor opening. For this purpose, for example, the outer wall in the area of the sensor opening can be guided planar in a straight cylindrical section in the axial direction parallel to the inner wall. On the other hand, the course of the measuring channel can have a curvature or a kink, in which case the sensor opening is expediently arranged in the area of the curvature or kink, in particular on the convex outside of the curved or kinking measuring channel. For example, the sensor opening can be arranged around the apex or the curvature. The sensor opening can be formed, for example, by a planar section through the measuring channel around the apex of the curvature, the section plane preferably being arranged essentially perpendicular to the radius of curvature of the course of the measuring channel.

[013] The measuring cell preferably has a modular structure, the sensor being manufactured as an exchangeable sensor module. The sensor module can have, for example, a semiconductor, in particular an FET sensor, which is pressed against the sensor opening of the measuring channel with a releasable clamping device. For this purpose, the main body of a measuring cell can have a sensor shaft which extends from a surface of the main body to the sensor opening of the measuring channel, the outer wall of the measuring channel around the sensor opening serving as an axial stop surface for the sensor module. The tensioning device can have an elastic element, for example a compression spring, with which the sensor module is pressed against the stop surface. The sensor module can be contacted, for example, via a sealing element which is arranged between the stop surface and the sensor module, the sealing element being anisotropically conductive. Details of this are disclosed in the unpublished application 10260961.6. Likewise, the sensor module can be contacted via ordinary cables or bond wires. [014] The reference opening in the middle section of the measuring channel preferably has a porous diaphragm, which for example comprise an organic material such as PTFE or a ceramic material such as corundum or zirconium dioxide can. In terms of a modular structure, it is expedient if the diaphragm is also interchangeable. In this case, it is currently preferred if a section of the outer wall of the measuring channel, which surrounds the reference opening, is designed as a stop surface. In order to hold the diaphragm in position, suitable clamping means are provided, for example a hollow screw, with which the diaphragm is pressed against the stop surface.

[015] KC1 is currently preferred as the reference solution, in which case a reference electrode with a silver wire is provided as the reference sensor.

In a further development of the invention, two reference half cells, each with a reference chamber and a reference electrode, are provided for providing a reference potential, wherein both reference half cells communicate with the measuring channel via reference openings and a reference diaphragm. In this case, a first reference opening of the first reference half-cell is arranged upstream of the sensor opening in the measuring channel, and a second reference opening of the second reference half-cell is arranged downstream of the sensor opening in the reference channel, the distance from the first reference opening from the sensor opening being substantially equal to the distance from the second reference opening from the sensor opening. Earlier investigations by the applicant have shown that the arrangement described enables reference potential errors due to electrostatic charges to be minimized by using the mean value between the first and the second reference potential as a reference potential for the pentiometric measurement, i.e. U = (U + U) / 2. ref refl ref2

[017] The measuring cell can optionally have further sensors, further sensor openings being provided for media-contacting sensors. To the extent that pentiometric measurements have a temperature dependency, the measuring cell according to a further embodiment of the invention has a temperature sensor. The temperature sensor can either be guided in a blind hole to the wall of the measuring channel, arranged in a separate sensor opening in contact with the media, or integrated in the sensor module of the pententiometric sensor.

[018] The connections to the measuring cell are preferably also designed such that no dead volumes between the supply line or discharge line and the measuring channel are avoided.

The invention is subsequently explained using the exemplary embodiment shown in the figures. It shows:

[020] FIG. 1: a schematic section through a measuring cell according to the invention in the Level of the measuring channel;

2: a perspective sectional view through a measuring cell according to the invention with cuts in the plane of the measuring channel and in the plane of the reference chamber; and

3: the linearity error of a pH measuring cell according to the invention.

[023] With reference to FIGS. 1 and 2, the measuring cell 1 according to the invention comprises a base body with a channel block 2 and a base plate 3. In this embodiment, the base body is made of PEEK, with other materials depending on the requirements with regard to temperature, pressure and pH range can be used, for example metallic materials, especially stainless steel.

A channel 10 runs in the channel block 2 and has a first channel section 11 and a second channel section 12 which adjoin one another, the measuring channel 10 having a kink of approximately 60 ° in the transition region between the first channel section and the second channel section. The measuring channel is made through bores with a diameter of 2 mm in the channel body 10. A sensor opening 15 is provided in the bend, the axis of the sensor opening 15 running in the plane spanned by the axes of the first channel section 11 and of the second channel section 12 and bisecting the angle between the axes of the channel sections. The sensor opening 15 has a diameter that is smaller than the diameter of the measuring channel. For example, the diameter of the sensor opening is approximately 1 mm. A sensor shaft 4 adjoins the sensor opening 15, which comprises a bore in the channel block 2, the bore having a larger diameter than the sensor opening 15. The outer wall of the measuring channel facing the sensor shaft 4 is planar around the sensor opening and serves as an axial stop surface for a sensor module 20 which comprises an FET sensor chip, in particular an isFET sensor chip. The currently preferred pH-isFET has Ta O as the gate material, with which long-term stable sensor modules with a broad pH, temperature and pressure specification can be provided. The wettable surface of the sensor module 20 is pressed against the sensor opening 15 by means of a compression spring 18, which is axially clamped between a receptacle 5 in the base plate 3 and the sensor module 20, with the sensor opening 15 and the outer wall of the measuring channel Sensor module 20 an annular flat gasket is clamped. The contacting of the sensor module 20 takes place via bond wires, which are led via a cable duct, not shown here, to an interface to which a transmitter can be connected. The structure of the reference half-cell can be seen from FIG. 2. The reference half-cell comprises a reference chamber which is formed by means of a first chamber bore 28 and a second chamber bore 29 in the channel block 2, and a third chamber bore 31 in the base plate 3. The first chamber bore 28 extends from a lateral surface of the channel block 2 horizontally to a region of the wall of the first channel section 11 of the measuring channel in which a reference opening 16 is arranged. The reference opening 16 has a diameter of approximately 1 mm. The diameter of the first chamber opening is a multiple thereof. At least in the axial section of the first chamber opening which borders on the reference opening 16, a thread is provided, into which a pierced clamping screw 27 is screwed in order to press a diaphragm 25 against an annular axial stop surface which passes through the wall of the measuring channel around the Reference opening 16 is formed. Between the clamping screw 27 and the diaphragm 25 an annular elastic element can also be arranged in order to keep the contact pressure on the diaphragm sufficiently constant. Currently, a diaphragm is preferred which has a large zirconium dioxide.

The second chamber opening 29 extends from the underside of the channel block 2 and extends essentially vertically and opens into the first chamber bore 28. The third chamber bore extends from the top of the base plate 3 as the first blind hole 31 into the base plate, the blind hole 31 being aligned with the second chamber bore 29 when the base plate is connected to the channel block. From a lateral surface of the base plate 3, a second blind hole 32 extends horizontally into the base plate, the axis of the second blind hole 32 intersecting the axis of the first blind hole 31. From the bottom of the second blind hole, an electrode opening 35 extends along the axis of the second blind hole into the first blind hole 31. The electrode opening 35 has a considerably smaller diameter than the second blind hole 32. Through the electrode opening, an end section of a silver wire electrode 30 is in the first blind hole 31 introduced. The outer surface of the second blind hole has a thread into which a conical screw 38 of a pinch seal is screwed, with which a truncated cone-shaped sealing element 37 of the pinch seal, through which the silver wire electrode 30 is guided, is pressed against the silver wire electrode and against the bottom of the first blind hole 32 , [028] For connecting leads and leads to the measuring cell are in the Channel block 2 Sacklcchbrrungen 13, 14 provided from the bottoms of the first and second channel section 11, 12 extend into the interior of the channel block 2. The blind holes have a considerably larger diameter than the channel sections, so that feed lines or discharges whose inner diameter corresponds to that of the channel sections can be fixed in the blind holes with crimped joint seals without dead space, as was explained in connection with the fixing of the silver wire electrode. Likewise, adapters for any other connection types can be screwed into the blind holes, for example Swagelock fittings.

[029] The reference chamber can be filled with a reference electrolyte through the opening of the first chamber opening 28 on the surface of the channel block 2, KC1 being currently preferred. The opening of the first chamber opening can be closed with a screw plug 34.

[030] Between the base plate 3 and the channel block 2, sealing rings 39 and 40 are arranged around the reference chamber and around the sensor shaft. [031] The exemplary embodiment fulfills the requirements summarized in the following Table I:

[032] Table I [033] Table 1 pH measuring range 0 ... 14 temperature range -20 ... + 80 ° C pressure range up to 6bar geometric requirements inner diameter lines 1,5-2 mm, dead volume free, flow geometry with low turbulence volume flow 0.5 to 101 / h

The linearity of the measuring cell is shown in FIG. 3. In the investigated pH range from pH 1 to pH 12.45, the acid and alkali errors are less than 0.015 pH.

Overall, the tests carried out showed a very stable measuring behavior of the measuring cell. It was striking that when using measuring media with low electrical conductivities (<10 μS / cm) a strong flow dependency of the measured values was recorded. This behavior is caused by electrostatic charges along the flow channel between the sensor element and the reference. However, this can be compensated for by a second reference half-cell which is arranged symmetrically to the first. A summary of the test results and boundary conditions is given in Table II.

Table II Table 2

Long-term stability is given in Table III

Table in

Change over time of the calibration values:

Table 3

The results are satisfactory and demonstrate the performance of the measuring cell according to the invention.

Claims

Claims [001] 1. Measuring cell (1) for detecting a pentiometric quantity, comprising: [002] a pentiometric sensor (20); A measuring channel (10) for guiding a measuring medium, the measuring channel (10) having a first end section with an inlet opening, a second end section with an outlet opening and a middle section between the first and second end sections, and the middle section comprises at least one sensor opening (15) at which the pententiometric sensor (20) is arranged; and

[005] at least one first reference half-cell, which has a first reference chamber (28, 29, 31) and a first reference sensor (30) for providing a reference potential, the reference chamber being connected to the measuring channel (16) via a reference opening (16) is.

2. Measuring cell according to claim 1, further comprising a base body (2, 3) in which the measuring channel and the reference chamber are formed.

3. Measuring cell according to claim 1 or 2, wherein the distance of the lowest point of the wettable surfaces m of the sensor opening (15) from the deepest plane, which is perpendicular to an axis of the sensor opening and touches the lumen of the measuring channel, no more than the value of the maximum diameter of the sensor opening, preferably not more than the value of the minimum diameter of the sensor opening.

4. Measuring cell according to claim 1 or 2, wherein the distance of the lowest point of the wettable surfaces in the sensor opening from the deepest plane, which is perpendicular to an axis of the sensor opening and touches the lumen of the measuring channel, no more than the value of half maximum diameter of the sensor opening, preferably not more than the value of half the minimum diameter of the sensor opening.

5. Measuring cell according to one of the preceding claims, wherein the course of the measuring channel has a curvature or a kink, the sensor opening being arranged in the region of the curvature or kink.

[010] 6. Measuring cell according to one of the preceding claims, wherein the measuring cell preferably has a modular structure, the sensor comprising an exchangeable sensor module.

7. Measuring cell according to claim 6, wherein the sensor module is an FET sensor has, which is pressed with a releasable clamping device against the sensor opening of the measuring channel.

8. Measuring cell according to one of the preceding claims, wherein in the reference opening (16) has a protruding diaphragm (25) which comprises organic or a ceramic material.

[013] 9. Measuring cell according to claim 8, wherein the diaphragm is interchangeable.

10. Measuring cell according to one of the preceding claims, wherein the measuring cell has two reference half cells, each with a reference chamber and a reference electrode for providing a reference potential, wherein both reference half cells communicate with the measuring channel via reference openings and a reference diaphragm.

11. Measuring cell according to claim 10, wherein a first reference opening of the first reference half cell is arranged upstream of the sensor opening in the measuring channel, and a second reference opening of the second reference half cell is arranged downstream of the sensor opening in the measuring channel, and wherein the distance from the first reference opening from the sensor opening is substantially equal to the distance of the second reference opening from the sensor opening.

12. Measuring cell according to one of the preceding claims, wherein the measuring cell has a temperature sensor.

[017]