Abstract: A central electrode 12 is arranged in a coiled heater electrode 10. They are buried in a SnO2-based inner area 6, and the entirety is covered by a filter 8. The volume of the inner area 6 is set at from 1×10−3 mm3 to 16×10−3 mm3, the total volume of the bead 4 is set at from 15×10−3 mm3 to 70×10−3 mm3, and the ratio of the total volume of the bead 4 to the volume of the inner area 6 is set at from four to twenty to bring the sensor resistance in CO and that in methane closer to each other and increase the selectivity from hydrogen.

Abstract: After sintering of a CO sensor using an SnO2-based metal oxide semiconductor, Ir and Pt are added to the sensor at 5˜500 &mgr;g/g SnO2 each, the weight ratio of Ir and Pt being 5˜1/5. This addition is made by impregnating a mixed aqueous solution of an Ir salt and a Pt salt into the SnO2 sintered body and thermally decomposing these salts. Next, an aqueous solution of thiourea is impregnated into the SnO2 sintered body to add thiourea at 0.01˜10 mg/g SnO2 in reduction to simple S element. The temperature and humidity dependency of the CO sensor is suppressed, the CO concentration dependency of the output is increased, and the sensor resistance is brought to a more convenient range of use.

Abstract: A central electrode 12 is arranged in a coiled heater electrode 10. They are buried in a SnO2-based inner area 6, and the entirety is covered by a filter 8. The volume of the inner area 6 is set at from 1×10−3 mm3 to 16×10−3 mm3, the total volume of the bead 4 is set at from 15×10−3 mm3 to 70×10−3 mm3, and the ratio of the total volume of the bead 4 to the volume of the inner area 6 is set at from 4 to 20 to bring the sensor resistance in CO and that in methane closer to each other and increase the selectivity from hydrogen.

Abstract: A method of and a system for detecting a gas. A histogram of gas sensor outputs in the past is prepared to determine a gas detection threshold value from the histogram. The threshold value is determined, for example, based on the modal sensor output on the histogram. The threshold value is modified based on the width of distribution on the histogram since the width represents the degree of pollution of the background.

Abstract: An exhaust gas sensor containing BaSnO.sub.3 or the like and a process for producing the sensor. The BaSnO.sub.3 is made free of segregation and adapted to have acid-base strength of at least 6.8 as determined with Hammett's indicator. Causes to reduce the acid-base strength of the BaSnO.sub.3 includes presence of Si and B, and causes to increase the acid-base strength include addition of Mg or Ca. The desired BaSnO.sub.3 can be prepared by precipitating BaSnO.sub.3 hydrous crystals while preventing introduction of Si and B thereinto especially with use of a container lined with a synthetic resin to prevent introduction of Si and B from the container, adding Mg or Ca to the precipitate, and thermally decomposing the precipitate.

Abstract: A BaSnO.sub.3 exhaust gas sensor and a process for producing the same. Surface reformation of sensor electrodes is done for preventing the deterioration of the sensor response and the corrosion of the electrodes by the Sn in the BaSnO.sub.3. After depositing an oxide film such as La.sub.2 Ti.sub.2 O.sub.7 or MgO on the surface of the electrodes, the electrodes are embedded in BaSnO.sub.3, and the BaSnO.sub.3 is sintered. Through the sintering process, the film of La.sub.2 Ti.sub.2 O.sub.7 or MgO is converted to a conductive complex oxide film by reacting with BaSnO.sub.3. The resultant film contains Ba and Sn elements. CaSnO.sub.3 and SrSnO.sub.3, which are homologous compounds of BaSnO.sub.3, are usable in place of BaSnO.sub.3.

Abstract: When the output of a gas sensor is approximately constant, the sensor output is stored as a reference output. Evolution of a gas is detected from variations in the sensor output from the stored reference output. The sensor output which is constant is detected by establishing that the difference between a maximum and a minimum of sensor output in a predetermined section is not greater than a specified value.