Measuring and testing – Gas analysis – By vibration
Patent
1992-06-17
1994-02-15
Williams, Hezron E.
Measuring and testing
Gas analysis
By vibration
73 2402, G01N 2918
Patent
active
052856771
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention is in the field of gas sensors. It relates to a process and an apparatus for selectively measuring gases and vapors, in particular oxygen.
There is an increasing need for detecting gases selectively and reliably. On one hand the measurement of gases, in particular harmful compounds, represents an important field for ecologic investigations. In addition in medical techniques the measurement of gas concentrations is of increasing importance e.g. in connection with the respiration control.
There exist several methods for the selective detection of gases. The solid state sensors are very much worth their price, but usually they do not represent a very selective method. Those sensors partly are based on catalytic combustion, in which heat production (pellistor) or the creation and diminution of oxygen vacancies, respectively, are measured. Of great importance are the gas sensors based on field effect transistors which are equipped with a gas sensitive gate electrode. In addition chemical sensors are important where the ionic conduction of an electrolyte is used. In those cases, solubility of the gas in the electrolyte is required. The lambdaprobe, an important part in catalysator-equipped cars, is based on the high temperature solubility of oxygen in zirconium oxide and the corresponding conductivity of oxygen ions.
Optical gas sensors are characterized by their high selectivity and sensitivity. The optical emission capability of gases allows their detection even at extremely low concentrations.
Many gases and vapors can be detected by using the property of selective absorption of infrared radiation. For this reason there are a number of instruments on the market which are based on an extinction measurement in the infrared. Further, gases can be detected very reliably and selectively by utilizing the photoacoustic effect. The latter is based on a microphonic measurement of the gas pressure which is produced on the absorption of intensity-modulated infrared radiation. However, it has to be mentioned that when accomplishing photoacoustic measurements the acoustical disturbance given by the measuring principle can never be prevented completely.
The most important condition for using a detection process based on infrared spectroscopy is the absorption capability of the gas in this radiation range. Most gases and vapors have a very characteristic spectrum in the middle infrared, being produced by changes of the dipole moment on molecular-internal vibrations. However, monatomic gases, as well as homo-molecular two-atomic gases like N.sub.2 and O.sub.2 are not susceptible to infrared spectroscopic investigations. In particular, oxygen is very important in medical analysis and in heating techniques (control of heating installations).
Several possibilities are available for detecting oxygen selectively. Those processes which are based on the paramagnetic property of oxygen have proven very useful. While most gases are very weakly diamagnetic (the susceptibility is of the order of (-0.5.multidot.10.sup.-9 SI units), oxygen is characterized by a high paramagnetic susceptibility of +107.8.multidot.10.sup.-9 SI units as compared with NO (+48.7.multidot.10.sup.-9 SI-units) and NO.sub.2 (+3.3.multidot.10.sup.-9 SI units).
Conventional methods using the paramagnetism of oxygen for its selective detection are based e.g. on the temperature dependence of the magnetic susceptibility. Corresponding to the Curie law in so-called thermomagnetic devices the susceptibility of gas samples at two different temperatures are compared. The detection is based on a gas flow measurement, the so-called magnetic wind, which arises between the gas samples of different temperatues. Those devices are relatively slow because they are based on a thermal effect. In addition there is a position dependence of the sensor because of the disturbance by convective air flow. Although the paramagnetism is a specific property of oxygen, there exists an additional cross sensitivity with other gases because of the heat condu
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Farley Walter C.
Roskos Joseph W.
Williams Hezron E.
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