AU708240B2 - Sensor for measuring gas concentrations - Google Patents

Description

WO 97/04306 PCT/EP96/02835 mb\msoffice\specs\herpdoc2.doc Sensor for Measuring Gas Concentrations This invention relates to a sensor for measuring gas concentrations in a gaseous mixture including a small tube closed on one side on the outside of which an electrical sensor contact is arranged alongside the small tube and the end of which, opposite to the closed end is accommodated in a casing, an interspace surrounding the small tube between the casing and the small tube being sealed in gas-tight manner.

Such a gas sensor is known from US-A-3960693. A small tube including a layer acting catalytically is closed at the end pointing towards the gas to be measured and is open at its other end. It is formed thicker at this open end and is fastened in a casing. This casing is screwed into an opening of the wall of the chamber containing the gas to be measured, for example in internal combustion engines. The inside of the small tube communicates with the surrounding atmosphere, i.e. the reference gas. A gap is sealed off within the casing between the casing and the small tube so that the two different gases are separated from each other.

The seal is made of a metal ring like the ones usually used for example with spark plugs. Gas measurements of this kind are usually carried out at high temperatures (from approximately 150§C, in the case of internal combustion engines from approximately 500§C upwards). The casing which is made of metal for reasons of required temperature resistance is accordingly connected conductively to the small tube, that is to say to the sensor contact. This means that the casing is integrated in the measuring circuit; the sensor is a so-called monosensor. As the seal is in a region of the sensor array which during the measurement of a combustion gas is considerably heated up, the different expansion coefficients of the casing and small tube may result in leakage and thus in measuring inaccuracies as the seal cannot compensate reliably for the different expansions. The electrical connection of the sensor contact is also maintained in this hot region and can therefore only be realized with good reliability at considerable expense, because the contact materials tend to oxidise at high operating temperatures. The hot contact of the seal with the small tube, which may usually be made of a solid electrolyte, furthermore causes a parasitic voltage, the magnitude of which varies strongly with time because the slow oxidation or reduction of the seal cannot be controlled. This causes a considerable deterioration of the drift of the sensor characteristics. A similar sensor is also known from EP-A-0 520 528.

A slightly different arrangement is known as a galvanic solid electrolyte cell from US-A-3399233. In this case as well, the measuring circuit is integrated into the metal casing. A gas seal is not necessary in this case, as the measurement is performed between two liquids.

A gas sensor of a different kind is known from EP-A- 0398 579. In this case a plane sensor element is provided inside a cylindrical metal casing, a ceramics powder for fixing and sealing being provided between the sensor and the casing. Leakages can occur in this case as well on heating, due to the differences in geometries between the sensor element and the casing and due to the different expansion coefficients of the materials.

Especially the sealing in relation to the gas chamber to be measured is problematic. Apart *from that, the sensor element can be easily damaged by vibrations occurring during 15 operation.

Proceeding from the above described prior art the invention is based on the object to provide a sensor according to the generic type which ensures reliable sealing between the gas to be measured and the references gas even at high operating temperatures and with which several sensor contacts, which are electrically insulated 20 from the casing, can be realized simultaneously and where no parasitic voltages occur S* ;"which impair the drift characteristics.

According to one aspect of the present invention there is provided a sensor for measuring gas concentrations in a gaseous mixture, said sensor comprising: small tube being closed on one end, at least one electrical sensor contact arranged along an outside of said small tube, a gas-tight seal formed from electrically insulating elastic material and surrounding said small tube in an interspace provided between a said tube and a casing therefore, said seal being at an opposite end to that of the closed end which represents a cold end of the small tube, wherein: said electrical sensor contact is electrically insulated from the casing and ends by leading into the cold end and being connected to an electrical conductor.

The cold end or the cold region of the small tube is that region which is provided outside the chamber with the gas to be measured and which is removed far enough from Sthe chamber with the gas to be measured so that it, at the most, [R:\IBE]02277.doc:JJP.97 WO 97/04306 PCT/EP96/02835 heats up only slightly, so that the preferably used seal of electrically insulating, elastic material is not damaged and retains its elasticity. The actually required distance can be determined easily by a few tests and/or measurements. The connection of the sensor contacts to onward leading electrical wiring likewise takes place in such a cold region; here, the occurrence of oxidation at the contacts due to high temperature is avoided. In this case, different expansion coefficients of the materials used practically play no role, especially in the case of elastic seals practically no thermal stresses occur so that a high sealing effect can be attained. Furthermore, parasitic and electrochemical voltages developing in the contact region between conventional metallic seals, located in the hot region, and the solid electrolyte (the small tube) can be avoided by using electrically insulating seals in the cold region. It was furthermore found that vibrations, as occur when operating motor vehicle engines or their exhaust systems are not transmitted to the sensor so that a high freedom of interference of the sensor is achieved.

Conveniently, the electrical wiring can be welded or soldered to the sensor contacts and/or be pressed /onto the sensor contact or contacts by means of an elastic ring. In this manner reliable contact formation is readily attainable.

Advantageously, a plurality of sensor contacts are provided on the outside of the small tube, of which one or more sensor contacts may be used as heating contacts. This avoids the use of a separate heating element which in the known apparatus is pushed into a small tube.

Efficiency and reaction rate of the heater are increased by this.

Conveniently, at least one electrical contact can be provided on the inside of the small tube. In an advantageous embodiment of the invention the small tube is made of cylindrical configuration and can therefore be produced very easily. It may furthermore be advantageous to provide an electrically insulating layer on the small tube between the casing and the sensor contacts. It is furthermore possible that at least part of the outside of the small tube includes a solid electrolyte material which is electrically connected to at least one sensor contact. It is also possible to form the entire small tube of a solid electrolyte material. It is possible as well that at least part of the outside of the small tube is made of a material the electrical resistance of which depends on the gas concentration. It is convenient in order to effectively cool the cold region that cooling ribs are arranged on the outside of the casing.

WO 97/04306 PCT/EP96/02835 The invention will be described in detail with reference to a drawing.

The drawing shows in: Figure 1 the small tube of the sensor with sensor contacts Figure 2 a cross section through the sensor Figure 3 the schematic representation of the sensor Figure 4 another cross section through the sensor Figure 5 the set up of some assembly elements.

Figure 1 shows the small tube I of the sensor. The small tube 1 is of cylindrical configuration and closed at its one end. It is made of an electrically insulating material, for example aluminium oxide or of a solid electrolyte material, for example zirconium dioxide. It may also be made of another material which is electrically insulated on its outside. Gas sensors 2 are provided as active elements on the outside of the small tube 1. Furthermore, an electrical heater 3 is applied onto the small tube 1. Onto these electrical sensor contacts, that is to say the gas sensors 2 and the heater 3, a thin, electrically insulating layer 4 which is, however, gaspermeable and which avoids contamination of the gas sensors 2, e.g. by exhaust constituents, has been applied. The electrical sensor contacts 2, 3 which are arranged alongside the small tube 1 end at the open side of the small tube 1, leading into so-called outer electrodes 5, by way of which the small tube 1 is electrically contacted. At the face of the open end of the small tube 1 a contact ring 6 is provided, which serves for contact formation with an electrical contact 7 provided inside of the small tube 1. This electrical contact 7 is necessary in case the gas sensor 2 is an electrochemical sensor acting with a reference gas. In this case, the reference gas is inside the small tube 1 and corresponds for example to the surrounding air.

The small tube 1 is accommodated in a casing 8 as shown in figure 2. The casing 8, at its tip WO 97/04306 PCT/EP96/02835 which holds the closed end of the small tube 1, has gas apertures 9 through which the gas to be measured reaches the gas sensors 2. In the region of the closed end of the small tube 1, the casing 8 has a nut 10 with a male thread 11, by means of which the sensor can be screwed into the wall of the chamber holding the gas to be measured. In the context of internal combustion engines this chamber may for example be part of the exhaust system (for example of motor cars). A seal 12 may furthermore be provided at the nut The small tube 1, by way of its closed end face, abuts against the corresponding end of the casing 8 and is pressed to this end by a spring 13. The small tube 1 is spaced from the wall of the casing 8 so that the electrical sensor contacts (gas sensors 2 and heater 3) have no electrical contact with the casing 8. An O-ring 14 made of an elastic and heat-resistant plastic, for example Teflon or Viton, is provided to serve as the gas seal. This O-ring serves as a seal of the gas chamber with the gas to be measured against the atmosphere surrounding the casing 8 so that for example the surrounding air will remain separated from the exhaust fumes of an engine. The O-ring 14 furthermore stabilises the position of the small tube 1 within the casing 8 and secures the electrical insulation of the small tube 1 and of the sensor contacts 2, 3 of the casing 8. The O-ring 14 furthermore avoids that vibrations from the casing 8 are transmitted to the small tube 1 and it compensates for the different expansion coefficients between the small tube 1 and the casing 8 so that even when heated up slightly thermal tensions cannot occur. A further O-ring 15 establishes contact of the outer electrodes 5 with the connecting leads 16. The O-ring 15 is also made of an elastic and heat-resistant plastic. The connecting leads 16 are reliably pressed onto the outer electrodes 5 by means of the O-ring 15 so that a reliable contact is ensured. It is also possible to weld the connecting leads 16 to the outer electrodes 5 or to bring a plug equipped with connecting leads 16 put on the small tube 1 into contact with the outer electrode The O-ring 15 is pressed against the O-ring 14 by the crimping piece 17 so that the small tube I is securely kept inside the casing 8 by it due to the additionally created tension. The pressure member 17 is pressed against the O-rings 14, 15 by means of the nut 18 via the washer 19 (of a heat-resistant plastic), the O-ring 14 being pressed against a stop face of the casing 8. The nut 18 serves to seal the inside of the casing 8, the connecting leads 16 as well as the WO 97/04306 PCT/EP96/02835 connecting lead 20 are passed through this nut. The connecting leads 16, 20 are centred by the washer 19 and led through the pressure member 17 into the inside of the casing. After assembly the connecting lead 20 has electrical contact with the spring 13 which bears electrically conductive against the contact ring 6 so that the connecting lead 20 contacts the electrical contact 7 within the small tube 1. This arrangement of pressure member 17, nut 18 and washer 20 is shown enlarged and from different views in figure The use of plastic seals as O-rings 14, 15 with the resulting above described advantages is possible in an area of the casing 8 which does not heat up significantly even at the operating temperature of the sensors, it only heats up so much that the plastic seals do not lose their elasticity. This region is, depending on the temperature of the gas to be measured, at least approximately 2 cm, especially approximately 4 cm to 6 cm distant from the seal 12 which seals off the chamber of the gas to be measured. The O-rings 14, 15 are usually fitted in a region in which the temperature is only so high that the materials of the O-rings 14, 15 can be used permanently. In order to increase the cooling effect cooling ribs 21 are arranged on the outside of the casing 8. In this manner it may be possible to decrease the distance between the rings 14, 15 and the closed end of the small tube 1. The number and shape of the cooling ribs 21 as well as the length of the casing 8 can be selected depending on the operating temperature of the gas to be measured to ensure that the so-called cold end of the small tube 1 at which the O-rings 14, 15 are fitted heats up only slightly, only to a point at which the Orings 14, 15 are not damaged and do not lose their function.

The arrangement of the cooling ribs is also shown in figures 3 and 4, figure 4 showing a longitudinal section through the sensor differing slightly from that of figure 2.

The sensor according to the invention has a very simple and thus cost-effective design; it has a low interference susceptibility combined with a capability to accommodate a plurality of sensors in one casing and is thus safe in handling.

Claims (12)

1. A sensor for measuring gas concentrations in a gaseous mixture, said sensor comprising: a small tube being closed on one end, at least one electrical sensor contact arranged along an outside of said small tube, a gas-tight seal formed from electrically insulating elastic material and surrounding said small tube in an interspace provided between a said tube and a casing therefore, said seal being at an opposite end to that of the closed end which represents a cold end of the small tube, wherein: said electrical sensor contact is electrically insulated from the casing and ends by leading into the cold end and being connected to an electrical conductor.

2. A sensor as claimed in claim 1, wherein the electrical conductor is 15 pressed onto the electric sensor contact by means of an elastic ring.

3. A sensor as claimed in claim 1, wherein said electrical conductor is welded to the electric sensor contact. 20

4. A sensor as claimed in any one of claims 1 to 3, wherein a plurality of electric sensor contacts are arranged on an outer surface of said small tube, and at least one electric sensor contact being adapted as a heater.

A sensor as claimed in any one of claims 1 to 4, wherein at least one electrical sensor contact is arranged on an inside of said small tube.

6. A sensor as claimed in any one of claims 1 to 5, wherein said small tube is cylinder-shaped.

7. A sensor as claimed in any one of claims 1 to 6, wherein an electrically insulating layer is provided on said small tube between the casing and the electrical sensor contacts. rR:\LIBE]02277.doc:JJP.97

8. A sensor as claimed in any one of claims 1 to 7, wherein at least part of an outer surface of said small tube includes a solid electrolyte material which is connected to at least one electrical sensor contact.

9. A sensor as claimed in any one of claim 1 to 7, wherein the entire small tube is made of a solid electrolyte material and is connected to at least one electrical sensor contact.

A sensor as claimed in any one of claims 1 to 7, wherein at least part of the outer surface of said small tube includes a material the electrical resistance of which depends on the gas concentration.

11. A sensor as claimed in anyone of claims 1 to 10, wherein cooling ribs o are provided on the outside of the casing. S

12. A sensor for measuring gas concentrations in a gaseous mixture, said sensor being substantially as herein described with reference to Figs. 1-3 and Fig. 5, or, Fig 1. and Figs. DATED this Twenty-sixth Day of May, 1999 20 Heraeus Electro-Nite International N.V. Patent Attorneys for the Applicant SPRUSON FERGUSON S [R:\LIBE]02277.do:JJP.97