Measuring and testing – Gas analysis – With compensation detail
Patent
1994-10-21
1996-10-08
Williams, Hezron E.
Measuring and testing
Gas analysis
With compensation detail
73 3107, 73 2331, 55270, 261104, G01N 2135
Patent
active
055633307
DESCRIPTION:
BRIEF SUMMARY
In non-dispersive infrared (NDIR) gas analysis, the problem of cross sensitivity arises when the measuring gas not only contains the detector gas, whose concentration in the measuring gas is supposed to be measured, but also contains an interfering gas, whose spectral absorbance bands substantially overlap those of the detector gas. This happens, for instance, when nitrogen oxides are measured in exhaust gases, which contain water vapor as an interfering gas. To remove water vapor and other interfering gases as well, the book Messen, Steuern und Regeln in der chemischen Technik [Measuring, Controlling and Adjusting in Chemical Technology], 3rd edition, volume II, p. 611, Springer publishing house, 1980, discloses cooling the measuring gas and subsequently directing it over a drying agent that has to be renewed from time to time.
To measure nitrogen oxides in the exhaust gas of motor vehicle engines, the International Patent Application W0 89/03029 also discloses cooling the measuring gas in order to remove the water vapor contained in the gas and, in this manner, to facilitate the nitrogen oxide measurement. The disadvantage of the known measures for removing water vapor is that an expensive cooler is needed, which is subjected to wear, and that operating and maintenance costs accrue.
The underlying object of this invention is to indicate a method and a device for measuring the concentration of a detector gas in a measuring gas containing an interfering gas, which is simpler than known methods and will ensure better operational reliability.
The idea behind the invention is to add so much interfering gas to the measuring gas, the zero gas, and the test gas that the same interfering gas concentration will be reached in each case. When the original concentration of interfering gas in the measuring gas is more or less the same or is higher, it can be necessary to lower the concentration by means of cooling. For this purpose, it generally suffices to simply cool the measuring gas to about the ambient temperature. It is beneficial to more or less retain the original concentration of the interfering gas or to only change it slightly, because any such change also affects the concentration of the detector gas, and this generally has to be allowed for in a corrective calculation.
The measured value determined with the zero gas corresponds to a zero concentration of the detector gas and, therefore, constitutes the zero point. This value is subtracted from the measured values of the measuring gas and, in fact, either arithmetically or, in the case of double-beam NDIR analyzers, physically. When the test gas, which contains the detector gas in a known concentration, is measured, the increase in the measuring effect is determined.
In the case that the concentration of nitrogen oxides in water-vapor-containing exhaust gas is measured using a single-beam NDIR analyzer, the measuring, zero, and test gases are directed one after another through a single humidifier. This has the advantage that the gases are moistened in the same manner. However, a measuring pause must be observed from time to time to redefine the zero point and the increase. This generally does not interfere, since when measuring exhaust gases from motor vehicle engines, for example, the measuring pauses that are required anyway can be utilized. In the case of a double-beam analyzer, the zero gas can be used as a reference gas, so that the zero point does not need to be determined separately. In this case, however, two humidifiers are needed, preferably with a combined construction, so that largely conforming conditions, for example the same temperatures, will prevail for the moistening operation.
In one advantageous specific embodiment, the humidifier contains a humidifying chamber, through which the gases are directed and which has a moistening wall made of a material, which absorbs the water and releases it to gases that are not saturated with water vapor. In one specific embodiment, which is especially simple, but is less suited for a higher measuring ac
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Patent Abstracts of Japan, vol. 9, No. 62 19 Mar. 1985 & JP-A-59 196 461 (Shimazu Seisakusho) 7 Nov. 1984.
Book Messen, Steuern und Regeln in der Chemischen Technik, 3rd ed., vol. II, p. 611, J. Hengstenberg et al.
Larkin Daniel S.
Siemens Aktiengesellschaft
Williams Hezron E.
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