An air quality monitor having multiple sensors deployed in an electric circuit returns a single sign indication of atmospheric impurity regardless of whether the impurity gas is of the oxidizing or reducing type. Each sensor employs a gas sensitive material that exhibits a response in the form of a change in electrical resistance of the material in the presence of a gas and that exhibits a negligible response to changes in the moisture content of the atmosphere. A powder is dried and calcined and the result is ground into a fine powder and pressed into a desired shape to make the gas sensitive material. Adding a binder during the pressing and firing the shaped powder results in a gas sensitive material with porosity. Gas is flowed to the gas sensitive material and the resulting change in resistance is measured and returned as a single sign indication of atmospheric impurity.
Description
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 shows the invention in a Wheatstone bridge circuit.
Claims
1. An air quality monitor apparatus, comprising:
a first sensor and a second sensor, andan electrical circuit,wherein the first and second sensors are deployed in the electrical circuit and the electrical circuit provides a unified, single sign indication of atmospheric impurity regardless of whether the impurity gas is of the oxidizing or reducing type.
2. The apparatus of claim 1, wherein each sensor includes a gas sensitive material that exhibits a response in the form of an increase or a decrease in electrical resistance of the material in the presence of a gas and that exhibits a negligible response to changes in the moisture content of the atmosphere.
3. The apparatus of claim 1, wherein the two sensors further comprise one p-type sensor and one n-type sensor.
4. The apparatus of claim 1, wherein the two sensors further comprise two n-type sensors.
5. The apparatus of claim 1, wherein the circuit is a Wheatstone bridge circuit.
6. The apparatus of claim 2, further comprising two or more electrodes in communication with the gas sensitive materials, wherein the gas sensitive materials are arranged so as to be capable of being contacted with a gas or gaseous mixture.
7. The apparatus of claim 6, wherein the gas sensitive material and the electrodes are in contact with the same gas.
8. The apparatus of claim 6, wherein the electrodes are applied by means of screen printing or sputtering.
9. The apparatus of claim 1, wherein the apparatus provides quantitative and/or qualitative determinations regarding gases or gaseous mixtures.
10. The apparatus of claim 1, wherein the sensors further comprise a temperature sensing means.
11. The apparatus of claim 1, further comprising a heating means to enable operating temperature to be adjusted or contaminants to be burnt off.
12. The apparatus of claim 2, wherein the resistance of the gas sensitive material is measured directly.
13. The apparatus of claim 2, wherein the gas sensitive material has porosity.
14. The apparatus of claim 2, wherein one of the sensors (for reducing gases) is a p-type sensor with a gas sensitive material comprising chromium oxide, Cr2O3, doped with a small amount of a transition metal ion having a valence of greater than 4.
15. The apparatus of claim 2, wherein one of the sensors (for reducing gases) is a p-type sensor with a gas sensitive material comprising one of the perovskites, LaFe0.95W0.05O3 or PrFe0.95W0.05O3.
16. The apparatus of claim 2, wherein one of the sensors (for oxidizing gases) is an n-type sensor with a gas sensitive material comprising W0.9Mo0.1O3.
17. A method of preparing the gas sensitive material of claim 1 from an oxide or from an appropriate precursor, comprising:
drying and calcining a powder at a temperature depending upon the particular composition of gas sensitive material being prepared,grinding a product of the drying and calcining as required to give a fine powder,repeating grinding and calcination as necessary to obtain a more suitable powder, andpressing the fine powder into any suitable shape.
18. The method of claim 17, further comprising firing the shaped powder.
19. The method of claim 17, wherein the drying a powder consists of spray drying a solution of appropriate starting material.
20. The method of claim 17, further comprising adding a binder during pressing.
21. The method of claim 20, further comprising firing the shaped powder and burning the binder out, giving rise to porosity.
22. The method of claim 18, wherein the firing takes place at the same temperature as the calcination.
23. A method of preparing the gas sensitive material of claim 1 from an oxide or from an appropriate precursor comprising gel processing.
24. The method of claim 23, wherein the gel processing comprises sol-gel processing.
25. The method of claim 23, wherein the gel processing comprises gel precipitation processing.
26. A method of gas sensing using the apparatus of claim 1, comprising:
flowing gas or a gaseous mixture to the gas sensitive material,measuring a response exhibited by the gas sensitive material in the form of an increase or a decrease in electrical resistance of the material,outputting from the electrical circuit a single sign indication of atmospheric impurity regardless of whether the impurity gas is of the oxidizing or reducing type.
27. The method of claim 26, wherein the measuring a response further comprises contacting the gas sensitive material with electrodes.
28. The method of claim 26, further comprising providing quantitative and/or qualitative determinations regarding gases or gaseous mixtures.
29. The method of claim 26, further comprising sensing the temperature.
30. The method of claim 29, further comprising heating the apparatus to adjust operating temperature or burn off contaminants.
31. The method of claim 26, wherein the measuring a response further comprises directly measuring the resistance of the gas sensitive material.
32. An air quality monitoring sensor comprising a p-type sensor with a gas sensitive material comprising chromium oxide, Cr2O3, doped with a small amount of a transition metal ion having a valence of greater than 4.
Patent Application
20070144237
