A method of measuring humidity by using an electrochemical cell

Abstract: In a method of measuring humidity of an oxygen-containing gas by using an electrochemical cell (1), firstly a voltage is applied across the pair of the electrodes (12), (13) to obtain a characteristic curve between an intensity of the voltage and that of electrical current, the characteristic curve continuously originating from a first flat portion in which a first diffusion limit current generally remains constant within a predetermined voltage range, and ending up in a second flat portion in which a second diffusion limit current generally remains constant within a predetermined voltage range. Secondly each value of the first diffusion limit current and the second diffusion limit current is read out, and an arithmetic ratio between the value of the first diffusion limit current and that of the second diffusion limit current is calculated to obtain the humidity of the gas.

Description: This invention relates to a method of measuring humidity of an oxygen-containing gas by using an electrochemical cell of an oxygen-ion conductive electrolyte on which a pair of electrodes are placed. A humidity measurement device using an electrochemical cell has been known in Japanese Patent Provisional Publication Nos. 60-222761, 62-150151 and 62-150152. In the Japanese Patent Provisional Publication No. 60-222761, partial pressure of an oxygen component in moisture-laden gas is measured with the function of limit currents, each value of which is obtained before and after removing aqueous vapor component from the gas. The Japanese Patent Provisional Publication No. 62-150151 teaches that the limit currents due to gas diffusion are obtained depending on an oxygen density and an aqueous density in the gas. The difference between values of the limit currents leads to humidity measurement. Further, the Japanese Provisional Publication Patent No. 62-150152 suggests that a first limit current due to an oxygen density and a second limit current due to an aqueous density are obtained. Then, a third limit current based on a moisture-laden gas is obtained. The humidity is measured based on the difference between the third limit current and, at least, one of the first and second limit currents. In each device according to the Japanese Patent Provisional Publication Nos. 60-222761, 62-150151 and 62-150152, however, a drying device is required to desiccate the gas to remove aqueous vapor. It is, simultaneously, necessary for the prior devices to measure the limit current. This necessitates a large scale measurement system as a whole, at the same time, resulting in a slow response to operation of the humidity measurement. On the other hand, porous electrodes or porous oxygen-ion diffusion limitting means is provided in the solid electrolyte, variation of the porosity inevitably occurs according to individual products. The same is true of thickness and density of the solid electrolyte. This causes variation in diffusion limit currents due to conductivity difference of the electrolyte and various gas diffusions under a certain voltage applied across the electrodes, thus resulting in discordance with humidity of the gas. Further, replacement of parts and elements of the device becomes difficult whilst ensuring the humidity measurement. Moreover, the electrolysis between the electrode and the solid electrolyte with the moisture tends to deteriorate an interface therebetween so as to gradually alter the characteristics of the electrochemical cell, thus making it possible to ensure a stable humidity measurement for an extended period of time. EP-A-0371774 discloses an apparatus for measuring humidity in accordance with the pre-characterising section of claim 2. In use, one of the first and second diffusion limit currents is obtained by applying across the electrodes a voltage corresponding with one of the generally flat portions of the characteristic curves. The measured diffusion limit current is then compared with an inflection current corresponding with an inflection point in the upwardly extending portions of the characteristic curve at which a second differential derivative of the characteristic curves falls to zero. EP-A-0166530 discloses prior art sensors of similar construction to the sensors illustrated in Figs. 8 and 9 of the present application. Fig. 1 of EP-A-0166530 discloses an oxygen sensor comprising a diffusion housing, an oxygen ion conductive plate of solid electrolyte bonded to the diffusion housing and a pair of electrodes secured to opposite sides of the conductive plate. The electrodes are porous and are connected to a DC power source. The diffusion housing has a gas diffusion aperture of small diameter formed through it. The diffusion of oxygen from the monitored gas environment through the gas diffusion aperture into a chamber of the diffusion housing is effected by the application of a DC voltage across the two electrodes to pump oxygen present in the chamber through the oxygen ion conductive plate. The sensor illustrated in Fig. 3 of EP-A-0166530 differs from the oxygen sensor of Fig. 1 of EP-A-0166530 in that the diffusion housing is made of an open-cell porous structure and omits a gas diffusion aperture. With this construction of sensor, the oxygen diffuses into the housing chamber through the porous housing. According to a first aspect of the present invention there is provided a method of measuring humidity by using an electrochemical cell which includes a solid type oxygen-ion conductive electrolyte and a pair of electrodes tightly placed on the electrolyte, one of the electrodes being arranged to be exposed to moisture-laden gas so that an oxygen component of the gas is transferred from said one electrode to the other electrode in accordance with an intensity of voltage applied across the pair of the electrodes: said method comprising: consecutively applying a plurality of different voltages across the pair of electrodes to obtain at least part of a characteristic curve given by an intensity of the voltage against the resultant electrical current, the characteristic curve including a first generally flat portion (F1) in which a first diffusion limit current (IL1) generally remains substantially constant within a first, lower, voltage range (V1-V2), an upwardly extending portion representing an increase in current with increasing voltage (V2-V3), and a second, upper, generally flat portion (F2) in which a second diffusion limit current (IL2) remains substantially constant within a second voltage range (V3-V4), said at least part of the characteristic curve including at least part of each of said first and second generally flat portions; reading out a value of the first diffusion limit current (IL1) within the first voltage range (V1-V2) and reading out a value of the second diffusion limit current (IL2) within the second voltage range (V3-V4) calculating an arithmetic ratio (RI) of the read first and second diffusion limit currents (IL1, IL2); and using the arithmetic ratio (RI) to determine the humidity of the gas. According to a second aspect of the present invention there is provided apparatus for measuring humidity, the apparatus comprising: an electrochemical cell including a solid type oxygen-ion conductive plate and a pair of electrodes mounted on the oxygen-ion conductive plate, one of the electrodes being arranged to be exposed to moisture-laden gas whereby an oxygen component of the gas is transferable from said one electrode to the other electrode in accordance with an intensity of voltage applied across the pair of electrodes; a voltage source (E) for applying voltage across the electrodes; a voltage controller for varying the voltage applied across the electrodes by the voltage source (E) to obtain a characteristic curve, the characteristic curve including a first generally flat portion (F1) in which a first diffusion limit current (IL1) generally remains substantially constant within a first, lower, voltage range (V1-V2), an upwardly extending portion representing an increase in current with increasing voltage (V2-V3), and a second, upper generally flat portion (F2) in which a second diffusion limit current (IL2) remains substantially constant within a second voltage range (V3-V4); and current measuring means for reading a value of the first diffusion limit current (IL1) within the first voltage range (V1-V2) and a value of the second diffusion limit current (IL2); characterised by the apparatus being for measuring humidity in accordance with the method of the first aspect of the present invention and further comprising; calculator means for calculating an arithmetic ratio (RI) of the read first and second diffusion limit currents (IL1, IL2), whereby to determine the humidity of the gas exposed to said one electrode from the ratio (RI). The invention is made on the basis of the fact that the arithmetic ratio between the first and second diffusion limit current remains substantially uniform under a certain humidity, and changes depending on humidity of the moisture-laden gas when voltage is applied across the electrodes. In addition, this uniformity of the arithmetic ratio remains unchanged with the lapse of time regardless of the moisture-deterioration of the interfaces between the solid electrolyte and the electrodes in the electrochemical cell. The humidity of the vapor-laden gas is obtained as follows: Firstly the first diffusion limit current is read when the first flat portion appears on the characteristic curve. Secondly the second diffusion limit current is read when the second flat portion appears on the characteristic curve. The arithmetic ratio between the first and second diffusion limit current is calculated to readily insure humidity of the moisture-laden gas, according to the invention, since the calculated ratio is in accordance with the humidity of the gas: the arithmetic ratio shows a continuous and almost linear curve as the function of aqueous vapor pressure of the gas to be measured as shown in Fig. 7. If the arithmetic ratio curve is treated in advance by a linearized circuit and an amplifier or by a microcomputer, etc., reading of the ratio is the instant and accurate value of the humidity of the gas to be measured. The arithmetic ratio is insured regardless of deterioration and dimensional variation from which the electrochemical cell sustains, thus enables to maintain measuring the humidity for an extended period of time with high accuracy. Calculating the arithmetic ratio obviates the necessity of drying the vapor-laden gas, and insuring the humidity measurement in good respose with a relatively simple construction. The invention will be more clearly understood from the following description which is given by way of example only with reference to accompanying drawings in which Fig. 1 is a plan view of an electrochemical cell to serve as a humidity measurement device according to an embodiment of the invention, but partly broken away; Fig. 2 is a perspective view of the electrochemical cell shown as a humidity measurement device in Fig. 1; Fig. 3 is a cross-sectional view of the electrochemical cell taken along the line III-III; Fig. 4 is a perspective view of a ceramic heater illustrated incorporated into the electrochemical cell; Fig. 5 is a view of characteristic curves showing the relationship between an intensity of voltage and that of current according to the electrochemical cell; Fig. 6 is a view of characteristics showing the relationship between current and humidity according to the electrochemical cell; Fig. 7 is a view of a characteristic curve showing the relationship between an arithmetic ratio and aqueous vapor pressure; and Figs. 8 and 9 are cross sectional views of other types of electrochemical cells to which the present invention are applicable. Referring to Fig. 1 and 2 in which an electrochemical cell 1 is shown to serve as an oxygen concentration sensor. The electrochemical cell 1 comprises a sensor element 10 and a ceramic heater 20. The sensor element 10 includes an oxygen-ion conductive plate 11, and a porous anode electrode 12, a porous cathode electrode 13, an alumina porous layer 14 and a glaze layer 15. The oxygen-ion conductive plate 11 is made from a solid solution of zirconia partially stabilized by yttrium oxides to serve as a solid electrolyte. The oxygen-ion conductive plate 11 is in the shape of a rectangle, and measures 5 mm × 7 mm in square dimension and 0.3 mm in thickness. On an upper surface of the oxygen-ion conductive plate 11, the anode and cathode electrodes 12, 13 are located right and left at a predetermined interval which in turn has electrode portions 12a, 13a and connection portions 12b, 13b. Each of the electrodes 12, 13 is made of porous platinum sintered on the oxygen-ion conductive plate 11 at the temperature of 1500 degrees Celsius after they were printed on the plate 11. On the oxygen-ion conductive plate 11, the alumina porous layer 14 is coated in a manner to cover the electrode portion 13a, and only partly covering the connection portion 13b. The alumina porous layer 14 is covered by a glaze layer 15 together with the electrode portion 13a and the connection portion 13b to prevent moisture-laden gas from being in contact with the electrode portion 13a except for the connection portion 13b. The alumina porous layer 14 and the glaze layer 15, thus coated on the oxygen-ion conductive plate 11, are attached by baking them at the temperature of 850 ∼ 900 degrees Celsius. The electrode portion 13a is isolated from the moisture-laden gas, but the connection portion 13b is exposed outside from the glaze layer 15 as shown in Fig. 3. In this instance, the moisture-laden gas is often, referred to as a gas hereinafter in which its humidity level is to be measured according to the invention. At the connection portion 13b which is situated between a lower edge 15a of the glaze layer 15 and the oxygen-ion conductive plate 11, the connection portion 13b serves as a gas diffusion limiting aperture which works to limit an amount of oxygen gas diffusion and the moisture-laden gas diffusion so as to regulate the moisture-laden gas from entering the electrode portion 13a when voltage is applied across the electrodes 12, 13. Each of the electrodes 12, 13 measures 2.5 mm × 2.5 mm in square dimension and 20 microns in thickness (t). The connection portion 13b has a width (W) of 1 mm, and a length (L) of 2 mm covered partly by the glaze layer 15. The amount of the gas diffusion toward the electrode portion 13a through the connection portion 13b, is in proportion to a cross sectional area (s) described hereinafter, but in inverse proportion to the length (L) of the connection portion 13b, wherein the cross sectional area (s) is given by the product of the width (W) and thickness (t) of the connection portion 13b which acts as a gas diffusion limiting aperture. An effective ratio (R) by which the diffusion limit current can be effectively controlled is determined. Where the cross sectional area (s) of the connection portion 13b, the surface area (S) of the electrode portion 13a and the length (L) are as follows : R = s/L/S = 1 × 10⁻⁵ ∼ 8 × 10⁻² According to the embodiment of this Invention, the formula R= 1.6 × 10⁻³ is obtained since the relationship among (s), (L) and (S) are s= 0.02, L= 2, S= 6.25 in turn. In the meanwhile, the sensor element 10 is bonded on the ceramic heater 20, with an aid of vitreous adhesive (not shown), and baked together at the temperature of approximately 800 degrees Celsius. The ceramic heater 20 is in the form of a substrate heater as shown in Fig. 4 which is manufactured by baking a green sheet 20A made of 96 % alumina on which an appropriate resistor pattern 20a is previously made by printing tungsten (wolfram) paste. On the green sheet 20A, another green sheet 20B is integrally laminated by simultaneously sintering the two sheets 20A and 20B. On the other hand, the resistor pattern 20a is connected at their respective ends to heater-electrodes 21, 22 provided on an outer surface 20d by way of conductive patterns 20b, 20c. Since the gas diffusion limiting action is effected at the connection portion 13b of the cathode electrode 13 in accordance with the invention, the resistor pattern 20a is adapted to locally heat the electrode portions 12a, 13a so as to prevent an oxygen pumping from occurring on the connection portion 13b which is so adapted to be normally kept comparatively at low temperature. In the meanwhile, the ceramic heater 20 has a central opening 23 to effectively heat the sensor element 10, and including a plurality of rows of perforations 24, 25 and 26 are made in the area where the sensor element 10 is placed. On the surface 20d of the ceramic heater 20, there is provided sensor electrodes 27, 28 which are in turn connected to the connection portions 12b, 13b by means of ruthenium printed pattern for the purpose of energizing the electrodes 12, 13. It is noted that the sensor electrodes 27, 28 are adapted to be printed on the surface 20d of the ceramic heater 20 at the same time when the sensor element 10 is baked on the ceramic heater 20. The electrochemical cell 1, thus constructed, is adapted to serve as a sensing portion of the humidity measurement device 100 to measure a relationship between voltage and current when voltage is applied across the sensor electrodes 27, 28 by means of a power source (E). The humidity measurement device 100 has a voltage controller 101, a current measurement portion 102, a calculator portion 103 and a controller unit 104. The humidity measurement device 100 is adapted to transform the power source (E) into a first voltage and a second voltage each corresponding to a first flat portion (F1) and a second flat portion (F2) depicted in a characteristic curve (A) in Fig. 5 as described in detail hereinafter. The first flat portion (F1) and the second flat portion (F2) are reduced to a first diffusion limit current (IL1) and a second diffusion limit current (IL2) also depicted in the characteristic curve (A) as described in detail hereinafter. On the basis of an arithmetic ratio (RI) between the first diffusion limit current (IL1) and the second diffusion limit current (IL2), humidity of the moisture-laden gas is to be measured. In this instance, the ceramic heater 20 is energized by an auxiliary power source 105 to keep the electrode portions 12a, 13a at the temperature of 300 ∼ 700 degrees Celsius. With the structure thus far described, the electrochemical cell 1 is placed in the vapor-laden gas which humidity is to be measured. The voltage applied across the electrodes 12, 13 causes to ionize oxygen gas in the electrode portion 13a which is covered by the glaze layer 15. As a result, the oxygen component in the moisture-laden gas which reaches through the gas diffusion aperture of the porous connecting portion 13b, is pumped from the cathode electrode 13 to the anode electrode through the sensor element 10 of oxygen-ion conductive electrolyte in accordance with the voltage applied across the electrodes 12, 13. At this time, only the electrode portion 13a is locally heated so that the oxygen component diffuses into the electrode portion 13a through the connection portion 13b which is not heated and normally kept comparatively at low temperature. The diffusion amount of oxygen component into the electrode portion 13a is limited by the gas diffusion limit aperture of the connection portion 13b in accordance with the density of the oxygen in the vapor-laden gas. The limited amount of gas diffusion accompanies with current restriction to be represented as the first diffusion limit current (IL1) or the second diffusion limit current (IL2) as seen in the range of the first flat portion (F1) or the second flat portion (F2) in Fig 5. At solid line in Fig. 5, the characteristic curve (A) is depicted between an intensity of the voltage and that of the current flow across the electrodes under a certain humidity level. The characteristic curve (A) continuously originates from the first flat portion (F1) which the first diffusion current (IL1) remains substantially constant within the first predetermined voltage range V1 ∼ V2, and passes through a point of inflection (P) at which its second differential derivative falls on zero, and ends up in the second flat portion (F2) in which the second diffusion limit current (IL2) remains substantially constant within the second predetermined voltage range V3 ∼ V4. The first diffusion limit current (IL1) which is lower than the inflection current (IM) decreases with the increase of aqueous or rather humidity density of the gas simply because a partial pressure of the oxygen component decreases with the increase of the aqueous density in the moisture-laden gas. When the impressed voltage increases such that its magnitude is higher than the inflection voltage (VM), the second diffusion limit current (IL2) increases with the increase of the aqueous density of the gas, because the aqueous component (H₂O) in the moisture-laden gas is dissolved to produce additional oxygen-ions which are pumped together to the anode electrode 12 from the cathode electrode 13. Although the aqueous component which diffuses into the electrode portion 13a through the connection portion 13b is controlled or rather limited to thereby make the current level (IL2) flat or constant in the predetermined voltage range, the aqueous density of the moisture-laden gas increases the level of the second diffusion limit current (IL2). Since the oxygen diffusion and aqueous diffusion are limited at the connection portion 13b of the cathode electrode 13, the second limit current (IL2) increases in proportion to the humidity, while the first limit current (IL1) increases in inverse proportion to the humidity under a constant density of oxygen component in the moisture-laden gas. The first and second limit current (IL1), (IL2) are determined depending on the oxygen diffusion limiting means, area of the electrode portion 13a, and density and thickness of the oxygen-ion conductive plate 11. Therefore, as shown at broken lines (B) and dot-dash lines (C) in Fig. 5, different characteristic curves are represented depending on dimensional variation, etc., when individual electrochemical cells are produced. However, it is found from indefatigable and strenuous efforts that the arithmetic ratio (i.e. RI = IL2/IL1) between the first diffusion limit current (IL1) and the second diffusion limit current (IL2) substantially remains uniform and unchanged under a certain humidity level, regardless of the deterioration or variation of the electrochemical cell with the lapse of time. Therefore the previous arrangement between the arithmetic ratio (RI) and the humidity level enables to stably measure a relative humidity level of the moisture-laden gas containing oxgen where the electrochemical cell 1 is placed. A method of measuring humidity level of the moisture-laden gas is as follows: Firstly a predetermined voltage is applied across the electrodes 12, 13 to obtain the characteristic curve (A). Secondly the first diffusion limit current (IL1) and the second diffusion limit current (IL2) are read out respectively. Then the arithmetic ratio (e.g. RI = IL2/IL1) between the first diffusion limit current (IL1) and the second diffusion limit current (IL2) is calculated to simply determine humidity level of the moisture-laden gas. Fig. 6 shows characteristics between the relative humidity level (RH %) and the diffusion limit current level under a constant density of oxygen component in the moisture-laden gas. In Fig. 6, the diffusion limit current levels (LI1) at the first flat portion (F1) are shown by solid line, while the diffusion limit current levels (LI2) at the second flat portion (F2) shown by broken lines when the temperature of the vapor-laden gas in turn changes to 40, 60 and 80 degrees Celsius. Dot-dash lines in Fig. 6 represent the current level (IM) corresponding to the point of inflection (P). On the other hand a relationship between the arithmetic ratio (RI) and aqueous vapor-pressure (mmHg) of the moisture-laden gas is given as shown in Fig. 7 which indicates that the arithmetic ratio (RI) is generally in proportion to the aqueous vapor-pressure (mmHg). When each voltage corresponding to the first flat portion (F1) and the second flat portion (F2) is applied by the voltage controller 101, while the diffusion limit current levels (IL1), (IL2) corresponding to the first flat portion (F1) and the second flat portion (F2) are determined by the current measurement portion 102, on the basis of the diffusion limit current levels (IL1), (IL2) the arithmetic ratio (RI) is given by the calculator portion 103, and the ratio (RI) is function of the aqueous vapor pressure which leads to the humidity of the gas. A relative humidity of the gas at a certain temperature is given by dividing the aqueous vapor pressure by a saturated aqueous vapor pressure of that temperature. As understood from the foregoing description, the invention is made on the basis of our discovery that the arithmetic ratio given by the first and second diffusion limit current remains substantially uniform under a certain humidity, and changes only depending on humidity of the gas regardless of the deterioration of the electrochemical cell when voltage is applied across the electrodes. In other words, the arithmetic ratio remains uniform and unchanged with lapse of time, etc., regardless of the solid electrolyte and the electrodes which are under the influence of the electrochemical cell which deteriorates with the passage of operating period of time. Since it is also found that the arithmetic ratio between the first and second diffusion limit current is substantially immune to the dimensional variation of the electrochemical cell, it enables to replace the sensor element so as to make it interchangeable when the sensor element was broken down. The arithmetic ratio between the first and second diffusion limit current is calculated to readily insure humidity of the moisture-laden gas, since the calculated ratio is in accordance with the humidity of the moisture-laden gas. The characteristic curve between the arithmetic ratio and the aqueous vapor pressure of the gas to be measured is insured regardless of deterioration and dimensional variation from which the electrochemical cell sustains, thus enables to maintain measuring the humidity of the gas for an extended period of time at high accuracy. This is therefore electronically useful. Since the relation between the aqueous vapor pressure and the arithmetic ratio (RI) calculated by the analog circuit is on a constant and smooth curve as shown in Fig. 7, the humidity measurement of the gas can be easily done if the calculated (RI) is converted by a linearizing circuit to be an output of the vapor pressure. If a microcomputer is available, the relation between the vapor pressure and the ratio (RI) as shown in Fig. 7 can be put into an approximation equation or a direct table memory in advance thereby to quickly output the humidity level from the microcomputer only by feeding the arithmetic ratio of IL2/IL1. Further, calculating the arithmetic ratio obviates the necessity of drying the moisture-laden gas, insuring the humidity measurement in good respose with a relatively simple construction. It is appreciated that electrodes 31, 32 may be placed in a manner to sandwich an oxygen-ion conductive plate 30 as shown in Fig. 8 wherein the cathode electrode 32 is covered by a coverlet 34 provided with a central throughhole 33. The central throughhole 33 works to effect a vapor diffusion limiting action and an oxygen gas diffusion limiting action toward a hollow space 35. In this instance, the cathode electrode 32 may be covered by a porous coverlet 36 to effect a moisture diffusion limiting action and an oxygen gas diffusion limiting action toward a hollow space 35 as shown in Fig. 9. It is further noted that the arithmetic ratio (RI) between the first diffusion limit current (IL1) and second diffusion limit current (IL2) may be IL1/IL2 instead of IL2/IL1. While the invention has been described with reference to the specific embodiments, it is understood that this description is not to be construed in a limiting sense as much as various modifications and additions to the specific embodiments may be made by skilled artisan without departing from the spirit and scope of the invention.

Claims: A method of measuring humidity by using an electrochemical cell (1) which includes a solid type oxygen-ion conductive electrolyte (11) and a pair of electrodes (12, 13) tightly placed on the electrolyte, one of the electrodes (13) being arranged to be exposed to moisture-laden gas so that an oxygen component of the gas is transferred from said one electrode to the other electrode (12) in accordance with an intensity of voltage applied across the pair of the electrodes: said method comprising: consecutively applying a plurality of different voltages across the pair of electrodes (12, 13) to obtain at least part of a characteristic curve given by an intensity of the voltage against the resultant electrical current, the characteristic curve including a first generally flat portion (F1) in which a first diffusion limit current (IL1) generally remains substantially constant within a first, lower, voltage range (V1-V2), an upwardly extending portion representing an increase in current with increasing voltage (V2-V3), and a second, upper, generally flat portion (F2) in which a second diffusion limit current (IL2) remains substantially constant within a second voltage range (V3-V4), said at least part of the characteristic curve including at least part of each of said first and second generally flat portions; reading out a value of the first diffusion limit current (IL1) within the first voltage range (V1-V2) and reading out a value of the second diffusion limit current (IL2) within the second voltage range (V3-V4) calculating an arithmetic ratio (RI) of the read first and second diffusion limit currents (IL1, IL2); and using the arithmetic ratio (RI) to determine the humidity of the gas. Apparatus for measuring humidity, the apparatus comprising: an electrochemical cell including a solid type oxygen-ion conductive plate (11) and a pair of electrodes (12, 13) mounted on the oxygen-ion conductive plate (11), one of the electrodes (13) being arranged to be exposed to moisture-laden gas whereby an oxygen component of the gas is transferable from said one electrode to the other electrode in accordance with an intensity of voltage applied across the pair of electrodes; a voltage source (E) for applying voltage across the electrodes (12, 13); a voltage controller (101) for varying the voltage applied across the electrodes (12, 13) by the voltage source (E) to obtain a characteristic curve, the characteristic curve including a first generally flat portion (F1) in which a first diffusion limit current (IL1) generally remains substantially constant within a first, lower, voltage range (V1-V2), an upwardly extending portion representing an increase in current with increasing voltage (V2-V3), and a second, upper generally flat portion (F2) in which a second diffusion limit current (IL2) remains substantially constant within a second voltage range (V3-V4); and current measuring means (102) for reading a value of the first diffusion limit current (IL1) within the first voltage range (V1-V2) and a value of the second diffusion limit current (IL2); characterised by the apparatus being for measuring humidity by the method of claim 1 and further comprising; calculator means (103) for calculating an arithmetic ratio (RI) of the read first and second diffusion limit currents (IL1, IL2), whereby to determine the humidity of the gas exposed to said one electrode (13) from the ratio (RI). Apparatus according to claim 3, wherein said electrodes (12, 13) are porous. Apparatus according to claim 2 or claim 3, wherein said one electrode (13) is covered by an alumina porous layer (14) such that a connection portion (13b) of said one electrode (13) is, in use, exposed to moisture laden gas. Apparatus according to any of claims 2 to 4, wherein said one electrode (13), and the alumina porous layer (14) where provided, are covered by a glaze layer (15) to prevent moisture-laden gas from being in contact with the electrode portion (13a) of said one electrode (13). Apparatus according to any of claims 2 to 5, wherein a heater (20) is provided to heat the electrodes (12, 13). Apparatus according to claim 6, wherein said heater (20) is arranged to maintain the electrodes (12, 13) at a temperature between 300°c and 700°c. Apparatus according to claim 2, wherein said electrodes (31, 32) sandwich said oxygen-ion conductive plate (30). An apparatus according to claim 8, wherein one of said electrodes (32) is covered by a cover (34, 36).

Inventors: HORII KATSUHIKO, YAGI HIDEAKI, HORII, KATSUHIKO, YAGI, HIDEAKI
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1991
Main classification: G01N27
IPC codes: G01N27