GB2112526A

GB2112526A - Oxygen gas analyzer using solid electrolyte

Info

Publication number: GB2112526A
Application number: GB08223968A
Authority: GB
Inventors: Masato Maeda
Original assignee: Yokogawa Electric Works Ltd
Current assignee: Yokogawa Electric Corp
Priority date: 1981-03-06
Filing date: 1982-08-20
Publication date: 1983-07-20
Also published as: GB2112526B; CA1180384A; GB2096772B; GB2096772A

Abstract

An oxygen meter for measuring the oxygen concentration in a measured gas, comprises a hollow probe detecting section 15 carrying at its end a solid electrolyte sensor 19, and a partition plate 34 which divides the internal cavity of the probe in the longitudinal direction to a location spaced from but near to the sensor in two sections. The sensor is heated at 25, and convection causes natural circulation of reference air as indicated by the arrows. <IMAGE>

Description

1 GB 2 112 526 A 1

SPECIFICATION

Oxygen gas analyzer using solid electrolyte The present invention relates to a solid electrolyte oxygen meter and, more particularly, to an oxygen meter having a probe detector adapted to produce a signal representing the difference in oxygen concen tration between the inner side and the outer side of a partition wall comprising a solid electrolyte sup ported in the end of the probe.

An object of the invention is to provide an oxygen meter having a particularly simple and reliable means for introducing a reference gas into the probe.

This object is achieved by a electrolyte oxygen meter having a hollow probe detecting section provided with a solid electrolyte disposed at or near one end thereof, a partition plate which divides the internal cavity of said probe in the longitudinal direction from the other end of the probe into two sections as far as a location spaced from the solid electrolyte where said sections separated from each other by said partition plate are in communication with each other and a communicating portion at the said other end of the probe providing communica tion between each of said sections and the ambient atmosphere to provide for natural circulation of air between the atmosphere and the solid electrolyte.

In order that the invention may be clearly under stood and readily carried into effect some prior art and examples of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is an elevation of a conventional oxygen 100 meter; Figure 2 is a sectional elevation of the detecting section of the convention oxygen meter of Figure 1; Figure 3 is a sectional elevation of a probe detecting section of a solid electrolyte oxygen meter 105 constructed specifically to achieve the aforesaid first object of the invention; Figure 4A and Figure 48 are enlarged sectional elevations of portions of the probe detecting section shown in Figure 3; Figure 5 is a sectional elevation of a detecting section of a solid electrolyte oxygen sensor con structed specifically to achieve the aforesaid second object of the invention; Figure 6 is an enlarged sectional elevation showing details of the detecting section of Figure 5; Figure 7 is a sectional elevation of an oxygen meter constructed specifically to achieve the aforesaid third object of the invention; Figure 8A is a cross section taken along the line X1-Xl of Figure 7; Figure 88 is a cross-section similar to Figure 8A showing the arrangement of various lead wires; Figure 9 is a graph illustrating a feature of the mode of operation of the oxygen meter as shown in Figure 7; Figure 10 is a sectional evaluation of a modification of the oxygen meter of Figure 7; and Figure 11 is a sectional elevation of another modification of the oxygen meter of Figure 7.

Figure 1 shows a typical known oxygen meter of the direct insertion type. This oxygen meter has a probe 1 having an end to be inserted into a stack or a furnace with a flange 2 thereof fastened to the furnace wall 5 by means of bolts. The probe 1 is provided at its inner end with a solid electrolyte such as zirconia, bismuth oxide or the like, detecting electrodes disposed across the solid electrolyte, a temperature sensor, a heater and so forth. The oxygen meter further has a terminal box 4 through which the detecting electrodes, temperature sensor and the heater are connected to external circuits. The oxygen meter also has a signal receiving portion 7 for delivery a signal corresponding to the oxygen concentration. The signal reeiving portion 7 has a linearizing section for linearizing the detection signal and a temperature adjusting section employing the heater as the control means. The oxygen meter is provided a Iso with a pump 6 adapted forcibly to supply air as a reference gas into the probe 1.

Thus, the conventional oxygen meter requires a pump for supplying the probe 1 with a reference gas (air), so that the cost of the production is raised uneconomicaliy. In addition, the realiability of opera- tion is lowered due to the employment of moving parts as in a mechanical pump.

As an alternative, such an oxygen meter has been proposed as adapted to introduce instrumentation air into the probe 1. This system, however, requires a specific pipe work and, therefore, is not preferred from an economical point of view.

Figure 2 shows the construction of the detecting section of the conventional oxygen meter. A probe 11' has a flange 2'to be secured to a furnace wall Wwith its end inserted into the furnace. Reference numeral 8 designates a test-tube shaped solid electrolyte such as zirconia or bismuth oxide. The solid electrolyte 8 is secured to the inner end of the probe Vwith its closed end located at the end of the probe 1' so as to form a partition separating the end portion of the probe Vfrom other portions of the probe. Electrodes 9 and 10 are closely secured to the inner and outer surfaces of the solid electrolyte 8. Atemperature sensor 11 senses the temperature of the solid electrolyte 8 which is heated by a heater 12. The temperature sensor 11, heater 12 and the temperature adjusting section (not shown) in combination constitute a temperature control system for maintaining the solid electrolyte at a constant tempera- ture. In operation, a signal corresponding to the difference of oxygen concentration between the inner side and the outer side of the solid electrolyte 8 is derived from the electrodes 9 and 10 through respective leads which are not shown. A reference numeral 13 designates a filter preventing dust from attaching to the wall surface of the solid electrolyte 8, while a reference numeral 14 denotes a conduit having one end located at the closed end (outer surface) of the solid electrolyte 8 and the other end located at the outer end of the probe 1' so as to be able to introduce a calibration gas to the closed end of the solid electrolyte 8.

In this type of oxygen meter, it is necessary occasionally to renew the solid electrolyte 8 in order to maintain the desired precision of measurement.

2 GB 2 112 526 A 2 The work for renewal, however, is quite troublesome and time-consuming.

Preferred embodiments of the invention will be explained hereunder with reference to the accom panying Figures 3 to 11.

In Figures 3,4A and 4B reference numeral 15 designates a probe made of an electrically conduc tive materia 1 and having a flange 16 welded to a barrel portion thereof. The probe 15 is secured to the wall 17 of a stack or a furnace by means of the flange 16. Reference numeral 18 designates a cell unit provided at the end of the probe 15 and including a test tube shaped part 19 consisting of zirconia and a flange 20. The zirconia part has porous platinum electrode films 21 and 22 provided on the outer wall surface and the inner wall surface of its closed end, a lead 23 consisting of a ring-shaped portion 23a formed on the outer wall surface by baking platinum paste and a connecting portion 23b through which the ring-shaped portion 23a is connected to the 85 electrode film 21, and a lead 24 consisting of a contact surface 24a formed by baking platinum paste on the open end surface of the zirconia part and a connecting portion 24b formed on the inner wall surface and formed by baking platinum paste and providing an electric connection between the contact surface 24a and the electrode film 22.

Reference numeral 25 denotes a heater for heating the zirconia part 19. This constitutes, in combination with a temperature sensor (not shown) provided on the closed end of the zirconia part 19 and a temperature controller (not shown), a temperature control system for maintaining the zirconia part 19 at a predeterined constant temperature. Reference numeral 26 denotes a contact ring provided with a recessed inner peripheral surface, 27 denotes a coil contactfitted in the recess of the contact ring 26 and consisting of a nichrome wire or other wire of a heat-resistant and anti-oxidation metal, 28 denotes a highly insulative adhesive such as that known commercially as Sumiceram for fixing the contact ring 26 on the probe 15, and 29 denotes a wire lead covered by an insulator 30 and disposed in the probe with one end thereof connected to the contact ring 26 while the other end is connected to an external terminal (not shown).

The coil contact 27 is held in contact with the ring-shaped portion 23a of the lead 23 on the zirconia part 19 so that an electric path is formed by the electrode film 21, lead 23, coil contact 27, contact 115 ring 26 and the wire lead 29. Reference numeral 31 designates a mesh filter made of stainless steel, and 32 denotes a flange. The mesh filter 31 is pressed and secured to the flange 20 of the zirconia part 19 by means of bolts 33 acting on the flange 32, and the 120 mesh 31 is arranged so that an electric path is formed by the electrode film 22, lead 24, mesh filter 31, flange 20, flange 32 and the probe 15. Reference numeral 34 (Figure 3) designates a partition plate adapted to divide the internal cavity of the probe 15 other than the portion near the zirconia part 19 into an upper section and a lower section. Reference numeral 35 denotes a cup-shaped member provided at the outer end of the probe 15 and made of a conductive metal, while 36 denotes a wire lead 130 connected to the member 35. The metal member 35 is so mounted as to permit the upper and lower sections of the internal cavity of the probe 15 to be in communication with the outside independently of each other. The metal member 35 is electrically connected to the probe 15 so that the electric path including the electrode film 22 and the probe 15 is connected to an external terminal (not shown) through the wire lead 36.

In the detecting action of the oxygen meter, the end of the probe 15 is held in a stream of hot flue gas at about 200 to 50WC, the oxygen content of which is to be measured. This gas enters the zirconia part through the filter 31 by diffusion and convection continuously to make contact with the inner surface of th zirconia part 19. The gas contacting the outer wall surface of the zirconia part 19, i. e. the air in the probe 15, is heated to a high temperature of about 700 to 8000C by the heat derived from the gas being measured and the heat produced by the heater 25. Circulation due to natural convection of the air is set up from the ambient atmosphere, through the lower section of the internal cavity of the probe 15, the part of the probe 15 near the zirconia part 19 of the upper section of the internal cavity of the probe 15 and back to the atmosphere. Therefore, the reference gas side of the zirconia part 19 is always held in contact with fresh air, so that it is possible to obtain stably a detection signal between the two electrode films 21 and 22. The signal is then derived thorugh the wire leads 29 and 30. Figure 3 shows the probe 15 held in a horizontal position. It is to be understood, however, that sufficient convection of air is maintained even if the probe 15 is inclined with the end thereof directed downwardly even as far as a substantially vertical position.

As has been described above, the cell unit 18 is secured to the end of the probe 15 by means of the flange 32 and bolts 33, and the electric connection between the electrode films 21, 22 and the external terminals is made through the leads 23,24, coil contact 27 and the mesh filter 31. It is therefore, possible to easily mount and demount the cell unit 18.

Thus, there is provided a solid electrolyte type oxygen meter having a cell unit devoid of lead wires such that the renewal of the solid electrolyte can be made easily to reduce the maintenance cost.

Referring to Figures 5 and 6, reference numeral 37 denotes a probe having a flange 38 welded thereto and mounted on a flue or a furnace wall 39 by means of the flange 38. Reference numeral 40 denotes a cell unit consisting of a testtube shaped zirconia part 41 and a flange 42 mounted on the end of the probe 37, while 43 designates a heater for heating the zirconia part 41. Although not shown, porous electrode films of platinum are forned on both surfaces of the closed end of the zirconia part 41. A signal corresponding to the difference in the oxygen concentration between the two portions of the probe separated by the wall of the zirconia part 41, is taken out of the probe by means of the electrode films. A temperature sensor provided at the end of the zirconia part 41 constitutes in combination with a temperature controller and a heater 43, a temperature control system which k 3 GB 2 112 526 A 3 serves to maintain the zirconia part at a predetermined constant temperature.

Reference numeral 44 denotes a conduit consisting of a straight portion 44a and a U-shaped portion 44b, 45 denotes a mesh filter provided at the open end of the zirconia part 41 and 46 denotes a flange. The flange 46 supports the U-shaped portion of the conduit 44 by way of a bracket 47 and is secured to the probe 37 by means of bolts 48 thereby fixing the cell unit 40 to the end of the probe 37.

In this state, the free end of the U-shaped portion 44b of the conduit 44 is positioned in the cavity of the zirconia part 41, while the other end of the U-shaped portion is connected to the adjacent end of the straight portion 44a of the conduit 44. Reference numeral 49 denotes a partition plate which divides the portion of the internal cavity of the probe 37 beyond the vicinity of the zirconia part into an upper section and a lower section. A cup-shaped member 50 is secured to the outer end of the partition plate 49. The member 50 permits the upper and lower sections of the internal cavity of the probe 37 to be in communication with the ambient atmosphere independently of each other.

In operation, the end portion of the probe 37 is held in a stream of hot flue gas to be measured having a temperature of about 200 to 5000C, and the gas enters the zirconia part 41 through the filter 46 by diffusion and convection, thereby to make con- tinuous contact with the inner wall surface of the zirconia part 41. On the other hand, the gas contacting the outer wall surface of the zirconia part 41, i.e. the air inside the probe 37, is heated to a high temperature of 700 to 8500C by the heat from the gas to be measured and the heat produced by the heater 100 43. In consequence, circulation by natural convection of air occurs as shown in Figure 5, from the ambient atmosphere, through the lower section of the internal cavity of the probe 37, round the end of the plate 49, through the upper section of the internal cavity of the probe 37 to the atmosphere.

Therefore, the reference gas side of the zirconia part 41 is always kept in contact with fresh air, so that a stable signal is derived from the electrodes. In the measuring operation stated above, the conduit 44 is normally kept closed so that the gas in the conduit 44 is stagnant. The presence of the conduit 44, therefore, does not adversely affect the state of flow and composition of the gas flowing into the zirconia part 37 for measurement. It is, however, possible to improve the response by making use of the conduit 44. For this purpose the introduction of the measured gas into the zirconia part 37 is accelerated to promote the circulation of the gas, by connecting one end of the conduit 44 to the suction port of an ejector or the like.

An explanation is given below of a calibrating operation and a cleaning operation. The calibration is conducted by introducing, through the conduit44, a calibration gas having a pressure somewhat higher 125 than the pressure of the gas which enters the zirconia part 37 for measurement. Since the calibration gas has a pressure higher than that of the measured gas, the substitution of one gas forthe other in the zirconia part 37 can be made in quite a smooth manner to permit an accurate substitution of the two gases.

By using a cleaning gas, e.g. air, in place of the calibration gas, it is possible to remove the dust attaching to the filter 46 and the zirconia part 37 by a blow-back.

For renewing the zirconia part 37, the bolts 48 are withdrawn to separate the straight portion 44a and the U-shaped portion 44b of the conduit 44 f rom each other, and the cell unit 40 is withdrawn from the end of the probe 37. It is, therefore, possible to renew the zirconia part 37 in quite an easy manner.

As has been described, in the solid electrolyte type oxygen meter the cell unit consisting of a test tube shaped solid electrolyte and a flange integral therewith is inserted into and fixed to the end of the probe with the closed end of the solid electrolyte directed inwardly. In addition, the conduit for introducing the calibration gas can be divided at a portion thereof near the end of the probe. It will be seen that the described arrangement of the oxygen meter remarkably facilitates the protective maintenance work for renewing the solid electrolyte.

Figure 7 shows an oxygen meter constructed in accordance with still another embodiment of the invention. The oxygen meter of this embodiment includes probe 51 having a flange 57 welded to the barrel thereof and provided with holes 55 and 56 formed in the side wall near the end 51 b thereof, a test tube shaped zirconia part 52 mounted on the end 51 a of the probe 51, a heater 53 for heating the closed end of the zirconia part 52, a partition plate 54 which divides the internal cavity of the zirconia part 52 into two sections 51 d and 51 e while leaving a communicating portion 51 c in the vicinity of the end of the zirconia part 52 as shown in Figure 8A, a cup-shaped member 59 which closes the end 51 b of the probe 51, detecting electrodes formed on both surfaces of the end wall of the zirconia part 52, a temperature sensor (not shown) for sensing the temperature of the end portion of the zirconia part 52, a heater 53, terminal box 60 for connecting the electrodes, temperature sensor and the heater to external circuits, a linearizing portion for linearizing the detection signal derived through the terminal box 60 and delivering the linearized signal as its output, and a signal receiving portion (not shown) having the temperature sensor providing its input and the heater 53 to receive its output. The probe 51 is fixed to the stack or the furnace wall 61 by means of bolts 58 with the end 51 a inserted into the flue to measure the oxygen concentration in the measurement gas.

Although not shown in Figure 7, the wiring to the detecting electrodes, temperature sensor and the heater 53 is supported by making use of the partition plate 54, as shown in Figure 8B. Thus, the heater lead wire 62 and the electrode leads 63 are placed on the partition plate 54 and are located and fixed by means of a leaf spring 65. A temperature sensor lead 64 is fixed to the leaf spring 65 by spot welding.

In the measuring state, the end 51 a of the probe 51 is held in the stream of the gas to be measured which has a temperature of about 200 to 50M and one wall surface of the zirconia part 52 is held in 4 GB 2 112 526 A 4 continuous contact with this stream of gas. The space inside the probe 51 is in communication with the atmosphere through the through holes 55 and 56 and, therefore, is always filled with fresh air. There- fore, the detection electrodes provided on both surfaces of the wall of the zirconia part produce a detection signal in accordance with the Nernst's equation using the air as the reference gas. The detection signal is delivered to the signal receiving portion which performs a predetermined arithmetic operation to provide a signal corresponding to the oxygen concentration of the measured gas. The air in the probe 51 is heated to a high temperature of about 70OcC bythe heat derived from the measured gas and the heat produced by the heater 53, so that a natural convection circulation of air takes place from the ambient atmosphere, through hole 55, section 51 e, communicating portion 51c, section 51d, through hole 56 to the atmosphere, as shown in Figure 7. In consequence, the other wall surface of the zirconia part 52 is maintained in contact with fresh air serving as the reference gas, so that the oxygen meter provides a stable output signal.

The inventor has conducted the following test to confirm the presence of the convection of air.

Atmospheric air was introduced into the area where Figure 7 shows the measured gas to be flowing and both sides of the wall of the zirconia part 52 were immersed in the atmospheric air. The space inside the probe 51 was maintained at a high temperature of about 70WC. Then N2 gas containing about 1% of 02 gas was supplied for 2 to 3 seconds to the area X2 in the vicinity of the hole 55. Figure 9 is a graph showing the change of the detection signal.

The time to represents the time length over which the 1% 02 gas was blown. As will be understood from Figure 9, the change in the 02 gas concentration appearing at the area X2 is transmitted to the end of the zirconia part 52 in about 8 seconds. Since the volume of section 51 e in the probe 51 used in the experiment was about 150 mi, the flow rate of air moved by the convection can roughly be calculated to be about 1125 mi/min.

Many modifications within the scope of the follow- ing claims may be made to the embodiments described above. For instance, it is possible to employ the air ducts shown in Figures 10 and 11. In Figure 10, the probe 51 is provided with a discharge sleeve 56' connected to the hole 56 so that the convection in the probe 51 is promoted by the chimney-like action of the discharge sleeve 56'. In contrast, in the embodiment shown in Figure 11, the probe 1 is devoid of the holdes 55 and 56 shown in Figure 7 but, instead, the convection circulation passes through the gap between the cup-shaped member 59 and the portion 51 b of the probe.

As has been described, in the oxygen meter the internal cavity of the probe is divided into two sections by a partition plate and the air is introduced into the cavity divided into two sections, to eliminate the necessity for a pump for supplying the air or piping for introducing the air for the measuring instrument, thereby reducing the cost of production of the oxygen meter while remarkably improving the reliability of its operation.

Claims

1. Asoiid electrolyte oxygen meter having a hollow probe detecting section provided with a solid electrolyte disposed at or near one end thereof, a partition plate which divides the internal cavity of said probe in the longitudinal direction from the other end of the probe into two sections as far as a location spaced from the solid electrolyte where said sections separated from each other by said partition plate are in communication with each other, and a communicating portion atthe said other end of the probe providing communication between each of said sections and the ambient atmosphere to provide for natural circulation of air between the atmosphere and the solid electrolyte.

2. An oxygen meter according to Claim land substantially as hereinbefore described with refer- ence to Figures 3,4A and 413 of the accompanying drawings.

3. An oxygen meter according to Claim land substantially as hereinbefore described with reference to Figures 5 and 6 of the accompanying drawings.

4. An oxygen meter according to Claim land substantially as hereinbefore described with reference to Figures 7,8A, 8B and 9 of the accompanying drawings.

5. An oxygen meter according to Claim 1 substantially as hereinbefore described with reference to Figure 10 of the accompanying drawings.

6. An oxygen meter according to Claim 1 substantially as hereinbefore described with reference to Figure 11 of the accompanying drawings.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1983. Published byThe Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.

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