GB2192462A

GB2192462A - Measuring probe for gas analysis

Info

Publication number: GB2192462A
Application number: GB08715047A
Authority: GB
Inventors: Prof Paul Profos
Original assignee: PROGRAMMELECTRONIC ENG AG
Current assignee: PROGRAMMELECTRONIC ENG AG
Priority date: 1986-06-27
Filing date: 1987-06-26
Publication date: 1988-01-13
Also published as: FR2600773B3; DE3621652C1; FR2600773A1; CH673710A5; GB8715047D0

Abstract

This probe comprises an assembly fitted with electrodes 5, 7 which are situated on the two sides of a wall section of a material which is selective from the point of view of the ions and separates a chamber 9 containing the gas to be measured from a chamber 3 containing a reference gas. The solid substances entrained by the gas to be measured do not influence the measurement, the probe works on the basis of diffusion and its response time is increased because the chamber 9 narrows from a device 16 for receiving the gas to be measured in the direction of the wall section, the chamber 9 being limited, outside the wall section, by heat-lagging bodies 11, 13 fitted with layers 11a, 13a preventing the diffusion of the gas. Application in particular to optimising combustion processes. <IMAGE>

Description

SPECIFICATION Measuring probe for the analysis of a gas and use of same The present invention relates to a measuring probe for the analysis of a gas, with at least one wall section of ion selective material provided on both sides with a tapped electrode arrangement, separating a measuring gas space with a measuring gas receiving device from a reference gas space, and use of the measuring probe, with at least two openings for receiving a measuring gas, whose axes, projected onto a plane perpendicular to direction A are perpendicular or nearly perpendicular to each other, for the analysis of a flowing gas.

Such measuring probes are used for gas analysis in power engineering and chemical engineering, such as for the optimization of burning processes. There, the problem, precisely in its use in processes in which the measuring medium flows and is contaminated by solid particles, consists in that solid particles are transported together with the gas to be analysed into the measuring gas space, leading to the possibility of incorrect measurements. Atypical example of a measuring medium mixed in this manner with solid particles is that of smoke during the use of such probes for 2 measurement in the surveillance of burning processes.

The invention sets as its primary aim a transmission of concentration within the measuring probe from the measuring gas receiverto the wall section, withoutthetransport of solid particles, in a measuring probeofthe kind described in the introduction.

This is solved according to the invention in that the cross-sectional surface of the measuring gas space expands in the direction of the wall section towards the measuring gas receiving device according to F(x) =k.xn where: x represents an observed distance from the wall section side end of the measuring gas space towards the measuring gas receiving device, F(x) represents the cross-sectional surface of the measuring gas space at point exposed to the measuring gas, k represents a coefficient, k > 0, n represents an exponant with no 2.

By this means, the transfer of concentration of the gas to be determined, for example 02, from the measuring medium to the wall section can occur exclusively by diffusion, without convection, without the transmission time or the reaction time between the change in concentration in the measuring medium and the change of the signal measured becoming unusably long in the process, so that such a measuring probe would only badly be suited as a variable receiver in a contrnl-loop.

By tapering the measuring gas space of the measuring gas receiving device, e.g. in the shape of a hollow cone, with n=2, in the direction of the wall section, the speed of reaction changes in concentration of the measuring gas in the measuring gas receiving device is raised by a factor of 3, compared with a cyclindrical, linear connection (n=0) between measuring gas receiving device and wall section. By providing this measure, the speed of reaction of the measuring probe is such that it can be used for control engineering purposes or control purposes which make necessary a quick intervention based on results detected by the probe, in spite of the sole use of diffusion. By the use of diffusion for the transmission of concentration within the measuring probe, however, the depositing of solid particles present in the measuring medium on the active measuring wall section is avoided.

In particular, if the measuring probe is to be used in a flowing measuring medium, it must be further ensured that as few solid particles as possible are blown into the measuring gas space.

This is preferably achieved by providing the measuring gas receiving device with at least two, but at most three openings for receiving a measuring gas, whose axes, projected onto a plane perpendicular to direction A are perpendicular or nearly perpendicular to each other.

If such a measuring probe is introduced into a flowing measuring medium in such a manner that, observed in the direction of flow, there is only one opening behind the measuring probe, and one to its left and to its right, i.e. there is no opening on the windward, or barrage side, then solid bodies in the measuring medium circulate around the measuring probe, without entering the measuring gas space. As against that, the circulation around measuring gas receiver causes the measuring gas to enter the opening provided in the rear side, the leeward side or flowing-away side, and, conditioned by the low pressure produced by the circulation, to be sucked out right and left of the measuring gas space. A gas flow free of solid bodies arises perpendicular to the hollow conical axis, conditioned by the aero-dynamic solid matter separation referred to.

In order further to secure a good local temperature constant in the region of the active measuring wall section and to prevent the measuring gas from diffusing into the side of the measuring gas space, which then acts as a long term store for momentary measuring gas states, falsifying subsequent measurement, it is suggested that the measuring gas space be partially limited by porous heat insulating material, of which the surface facing the measuring gas space is provided with a gas diffusion preventing layer. A ceramic material, coated by means of a glaze, as a gas diffusion inhibiting layer, may be used as a porous heat insulating material for example.

Because the wall section is formed by a pipe wall section, where a heating element is so centered in the pipe that an even admission of reference gas to the electrode arrangement is ensured, on the one hand, it is achieved that the wall section can be regularly wa rmed, without the heating element being exposed to the possibly corrosive measuring gas, wherein the securing of an even admission of reference gas to the electrode arrangement facing the reference gas space is essential for exact measurement with a reference constant in time. A heating element is provided, if the wall section consists, in known fashion, of a material that onlytakes on its ion-selective characteristic at higher temperatures, made for example of zircon oxide.

In order further to ensure, on the one hand, that no reference gas enters into the measuring gas space, and, further, to enable the wall section to expand or contract under thermal loading, it is suggested that the wall section be formed from the wall section of a pipe kept sealed in an electric insulating body by means of a spring-mounted packing-box.- By choosing an electric insulating body, e.g. of a ceramic material, the pipe with the wall section can be mounted in a metallic outer casing, which, for example, can be placed at earth potential as an electric screen.

If, as is further suggested, at least one lead is provided in the measuring gas space, provided of course, with a suitable shut off device, it is possible to introduce a calibrating gas into the measuring gas space in order to calibrate the measuring probe, or it is possible to undertake the removal of gasfrom the measuring gas space in operation, for further analysis. In order, further, notto falsify measurements taken with the measuring probe, it is suggested that electrical connections for the measuring probe are arranged at an end of the above-mentioned electrical insulating body turned away from the heating element,~bywhich means it is ensured that the electrical connections are as far as possible at the same temperature,-whereby falsifications, due to varying thermo-electric power at the connections, are prevented.

Further, it is suggested on the one hand, for reasons of safeguarding against explosions that a flame trap should sub-divide the measuring gas space in the region of the measuring gas receiving device, which dampens the measuring gas turbulences in the receiving device towards the wall section, whereby in addition to its actual function-the flame trap acts as a turbulence-dampening instrument and thereby additionally prevents solid particles from being driven into the measuring gas space against the active measuring wall section.

In order, further, to create the most isothermal conditions possible in the region of the active measuring wall section, it is suggested that the wall section be formed by a pipe section, wherein heat insulating material is provided axially in front of and behind the section, whose surface facing the measuring-gas space is provided with a gas diffusion inhibiting layer, for reasons explained above. Thereby, the active measuring wall section is practically surrounded by heat insulating material of the kind mentioned, whereby the thermal conditions referred to are realised.

In an application of the measuring probe with at least two openings for the reception of a measuring gas, whose axes, projected to a plane perpendicular to direction A are perpendicular or at least nearly perpendicular to each other,when perpendicular to a gas diffusion direction between measuring gas receiving device and active measuring wall section and for the analysis of a flowing gas it is now further suggested that none of the openings be arranged on the windward side in relation to the gas flowing around the measuring gas receiving device, whereby it is now ensured in use, as already explained above, that solid particles do not penetrate into the measuring gas space.

In the following, the invention will be explained by way of example, by means of Figures 1 to 3.

In the figures: Figure 1 shows a longitudinal section through a measuring probe according to the invention, Figure 2 shows a section according to line I-I through the measuring probe according to Figure 1.

Figure 3 shows a section according to line Ill-Ill through the measuring probe according to Figure 1.

The invention will be described by means of the example of an O2-measuring probe for arranging in a flue-channel. The probe according to the invention comprises a zircon oxide pipe 1, sealed at one, end on which it stands, and forming a reference gas space 3 inside. At the lower end of the zircon oxide pipe 1, a measuring gas electrode 5 in the form of a platinum layer is provided on its outer surface, and a reference gas electrode 7, also in the form of a platinum layer opposite it across the zircon oxide side. The zircon oxide pipe 1 projects at its tip into a measuring gas space 9, extending in the manner e.g. of a hollow cone, which is limited on the outside by a heat insulating body 11. The heat insulating body 11 consists preferably of a ceramic material.A second heat insulating body 13 is provided atthe standing end of the zircon oxide pipe 1. The surfaces of the insulating bodies 11 and 13 facing the-measuring gas space 9 are provided with a gas diffusion inhibiting layer 13a, or ila, e.g. a glass glazing. Only the active measuring section of the zircon oxide pipe covered by the two electrodes7, or 5, limited the bodies 11 and 13, lies free in the measuring gas space 9.

Thus isothermal conditions are ensured in the region of this section.

Starting from this section, the cross-sectional surface F of the measuring gas space 9 to the measuring gas extends, as a function of the distance x of the observed cross-sectional surface F from its section-side and, according to the expression f(x)=kxn where k is a coefficient and, for the exponent n nm2 holds for a hollow conical shape, n=2 is valid, whereas for a cylindrical shape n=0 would be valid.

By means of the extension, or narrowing, referred to, it is brought about that the diffusion in the X-direction becomes faster by a factor of 3 than if there were no narrowing. The measuring gas space 9 is separated across a flame trap 14 from a measuring gas receiving device 16. The body 11 and the flame trap 14 are mounted in a lower protective pipe 18, against which the heat insulating body 11 supports itself at the bottom, where the protecting pipe 18 projects downward past the flame trap 14 and a cleaning lid 15. The actual measuring gas receiving device 16 is defined by a measuring gas receiving space 21 between the flame trap 14 and the lid 15. As may be seen from Figure 2, two, at most three, openings 23, or 25, are provided in the lower protecting pipe at the measuring gas receiving device 16.These openings are arranged along the circumference of the pipe 18, each at 90 to another, in such a manner that the opening axis, projected onto a plane E perpendicular to the direction of X, or the pipe axis A, are at or nearly at right angles to each other.

Preferably three openings of this kind are provided. Above the heat insulating body 11 is an electrical insulating body 27, preferably also made of a ceramic material, with a flange 29. The flange 29 is fitted tight between the lower protecting pipe 18 and an upper protecting pipe 31 with the help of a muff 33 with counter-running threads. Flat packing 35 is provided at the flange flanks to guarantee good sealing. The zircon oxide pipe 1 projects through an appropriate bore through the electrical insulating body 27 and is braced there, by means of a packing box 43, mounted by a spring 37 between a stay 39 on the upper side of the body 27 and a packing box lid 41. The spring 37 thus keeps the packing box 43 compressed, even if-the latter goes slack at higher temperatures.

As may be seen from Figure 3, a cartridge type heater 45 is centered in the active region in the zircon oxide pipe 1 with the electrodes 5,7, with the help of a sheath thermal element 47 and two centering wires 49. The connections 5a, or 7a, to the measuring gas electrode, or reference gas electrode are led directly, preferably by means of platinum wires, to supporting points the electrical insulating body 27, from which they are externally conducted. The remaining electrical connections (not represented) 2, for the cartridge type heater 45 and the thermal element 47, are preferably led to such supporting points on the end of the electrical insulating body 27 facing away from the cartridge type heater 45.

The operation of this measuring probe is as follows: If the measuring probe is introduced into a gas stream P, then the opening 23 is turned leewards in relation to the direction of gas flow, so that both of the openings 25, observed in the direction of gas flow, lie to left and right. Solid particles flying along the gas stream P fly, as represented by the arrow F, past the measurement receiving space 21 surrounded by circulating gas, while gas enters the opening 23 and is sucked, by vacuum effect back out of the openings 25. By this means, solid particles flying along with the gas stream are prevented even from penetrating into the measuring gas receiving space 21.The gas that does penetrate through the current g diffuses according to concentration gradient into the measuring gas space 9, and, because of the tapering formation, by a factor of approx. 3 faster than if linearly bounded diffusion were obtaining. In order to prevent the gas from diffusing into the walls of the moulded articles 11 and 13, the latter then acting effectively as stores and falsifying subsequent measurements, said diffusion inhibiting layers 13a, 11 a are affixed to these articles.

Here it is to be stressed that the flame trap 14, besides its actual function, namely of preventing the measuring gas space 21 from being ignited by the active measuring parts of the probe, practically supresses, as a current rectifier, turbulences from spade 21 towards the active measuring region.

The aero-dynamic solid matter separation at the measuring gas receiving space 21, or at the measuring gas receiving device 16, thereby prevents solid particles from penetrating into the measuring gas space 9, wherein a good through-rinsing of the space 21 is guaranteed.

Because-the zircon oxide pipe, apart from its active part with the electrodes, is thermally insulated, on the one hand a locally constant temperature is guaranteed at this active region, and, additionally, only a minimal expenditure of heat for warming up the zircon oxide pipe by means of the cartridge type heater 45 is necessary. By arranging the cartridge type heater within the reference gas space 3, it is further protected from flue gas corrosion.

A gas tight separation of the measuring gas space 9 from the surroundings, in this case from the surroundings delivering the reference gas, is guaranteed on the one hand by the spring-compressed packing box 43 and on the other hand by the flange seals between the braced protecting pipes 31 and 18. The admission of air to the reference electrode occurs evenly through the annular gap visible in Figure 3 between the centered cartridge type heater 45 and the inner side of the zircon oxide pipe 1. Further, a main 51 is represented in Figure 1, which joins into the measuring gas space 9 and with whose help either reference gas for the calibration of the measurement can be introduced, or a gas sample extracted for the examination of the measuring probe functioning. Of course, a suitable locking valve (not represented) is provided at the main 51.

In order to prevent Seebeck voltages, as at soldering points of the electrical taps, from falsifying the result of the measurement, especially the electrode taps are arranged at the end of the electrical insulating body 27 directed away from the cartridge type heater 45.

With the help of the measuring probe described, it is possible even in solid body-contaminated and flowing gases, to undertake measurement based only on the gas diffusion, wherein the reaction speed of the measuring probe to concentration variations in the flowing gas medium is significantly better than would be the case with linearly bounded diffusion. Storing effects of the construction elements coming into contact with the measuring gas are avoided, so that a relatively cheap, quick-reacting and exact measuring probe is realised.

Claims

1. A Measuring probe for the analysis of a gas, with at least one wall section of ion-selective material provided on each side with a tapped electrode arrangement, separating a measuring gas space with a measuring gas reception device from a reference gas wherein the cross-sectional surface of the measuring gas space expands in the direction of the wall section towards the measuring gas receiving device according to F(x)=k.xn where: x represents an observed distance from the wall-section end of the measuring gas space in the direction of the measuring gas receiving device, F(x) represents the cross-sectional surfaceof the measuring gas space at position x exposed to the - measuring gas, k represents a coefficient, k > 0, n represents an exponant with na2.

2. A measuring probe, according to claim wherein the measuring gas receiving device comprises at least two, and at most three openings for the reception of a measuring gas, whose axes, projected to a plane (E) perpendicular to the direction (A) are perpendicular or very nearly perpendicular to each other.

3. A measuring probe, according to claim 1 or 2, wherein the measuring gas space is partly limited by porous heat insulating material, wherein its surface facing the measuring gas space is provided with a gas diffusion inhibiting layering.

4. A measuring probe, according to at least one of claims 1 to 3, wherein the wall section is formed by a pipe-wall section, wherein a heating element is so centered in the pipe that an even admission of reference gas to the electrode device arrangement is ensured.

5. A measuring probe, according to at least one of claims 1 to 4, wherein the wall section is formed by a wall- section of a pipe, which is sealed in an electrical insulating body by means of a spring mounted packing box.

6. A measuring probe, according to at least one of claims 1 to 5, wherein at least one of the mains flows into the measuring gas space, for a calibrating gas or for gas extraction.

7. A measuring probe, according to at least one of claims 4 and 5, wherein electrical connections for the measuring probe are arranged at one end of the electrical insulating body pointing away from the heating element.

8. A measuring probe, according to at least one of claims 1 to 7, wherein a flame trap sub-divides the measuring gas space in the region of the measuring gas receiving device, which flame trap damps down the measuring gas turbulence in the receiving device towards the wall section.

9. A measuring probe, according to at least one of claims 1 to 8, wherein the wall section is formed by a pipe section, wherein heat insulating material is provided axially in front of and behind the section, whose surface facing the measuring gas space is provided with a gas diffusion inhibiting layer.

10. Use of the measuring probe, according to at least claim 1, with at least two openings for receiving a measuring gas, whose axes, projected to a plane (E) perpendicular to the direction (A) are perpendicular or nearly perpendicular to each other, for the analysis of a flowing gas, wherein none of the openings are arranged on the windward side in relation to the gas flowing around the measuring gas receiving device.

11. A measuring probe substantially as herein described and as illustrated in the accompanying drawings.

12. Use of the measuring probe substantially as herein described with reference to the accompanying drawings.