Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Reexamination Certificate
2001-11-16
2004-03-23
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
C356S302000, C250S226000, C250S339070, C250S343000, C250S345000
Reexamination Certificate
active
06710873
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to using spectroscopic techniques for gas concentration measurements, and in particular to using a ratio of gas concentrations to obtain temperature-independent measurements.
BACKGROUND AND PRIOR ART
Gas concentrations are measured in laboratory experiments, industrial plant operations as well as monitoring and sensing in public or private areas. In some of these cases the measurements are performed to determine what chemical reactions are taking place, in others they represent direct results, e.g., concentrations of pollutants in the atmosphere such as exhausts from stack and vehicles (smog check and on-road remote sensing).
Gas concentration sensors based on spectroscopic techniques and absorption spectroscopy in particular have been widely used for many industrial applications as well as vehicle exhaust monitoring. For general information on the use of absorption spectroscopy, including IR and UV absorption for monitoring vehicle emissions the reader is referred to: Bishop G. A., et al.,
IR long
-
path photometry: a remote sensing tool for automobile emissions
, Anal. Chem. A, 1989, Vol. 61, pp. 671-77; Cadle S. H., et al.,
Remote sensing of vehicle exhaust emissions
, Environ. Sci. Technol. A, 1994, Vol. 28, pp. 258-64; Stephens R. D., et al., Remote sensing measurements of carbon monoxide emissions from on-road vehicles, J. Air Waste Management Assoc., 1991, Vol. 41, pp. 39-46. In addition, U.S. Pat. No. 5,498,872 to Stedman et al. teaches an apparatus for remote analysis of vehicle emissions in vehicle exhaust including, for example, concentrations of CO, CO
2
, HC, NO and H
2
O by using wide-band radiation.
The above-mentioned references take advantage of known absorption techniques which measure gas concentrations by monitoring the attenuation of optical radiation passing through the sample containing the probe gas. Attenuation of the optical radiation is due to optical radiation getting absorbed at wavelengths corresponding to certain transitions in the molecules of the probe gas. In other words, when the incident radiation contains photons at wavelengths corresponding to absorbing transitions, also referred to as spectroscopic transitions of the probe gas molecules, then some of these photons will be absorbed by the probe gas molecules. The attenuation is generally proportional to the amount of the probe gas molecules encountered by the radiation along its path. In addition, the amount of attenuation suffered by optical radiation passing through a sample of the probe gas is dependent on the gas temperature and the gas mixture composition of the sample. That is because these parameters affect the linestrength and linewidth of the selected transition or transitions. In order to correct for these effects prior art methods require temperature and gas composition information.
Unfortunately, in many situations the temperature and gas composition data required by prior art methods to correct for linestrength and linewidth is unknown. In other cases, gas composition and/or temperature measurements are not feasible or difficult. In the example of on-road remote sensing of vehicle exhausts, the temperature and composition distributions along the optical beam path are non-uniform and unknown. Similar problems are encountered in monitoring emissions from stacks, especially into turbulent and hence non-uniform atmosphere.
These limitations lead to errors of traditional absorption spectroscopy techniques in determining concentrations of probe gases. These errors are especially large in samples exhibiting large non-uniformities and/or significant fluctuations of temperature and composition profiles along the probe beam path. It would be an advance in the art to provide a technique for measuring a concentration of a probe gas in a sample without the need to determine the temperature and gas composition along the path of the probe beam of the spectrometer.
OBJECTS AND ADVANTAGES
In view of the above, it is an object of the invention to provide a spectroscopic method for accurately determining the concentration of a probe gas in a sample without the necessity to determine the temperature and gas composition of the sample. Specifically, the method of the invention does not require knowledge of the temperature and gas composition along the path of the probe beam for line-of-sight absorption spectroscopy techniques. For point measurement spectroscopy such as laser induced fluorescence, the method does not require knowledge of the temperature and composition at that point.
It is another object of the invention to ensure that the method of the invention can be practiced in monitoring emissions in environments which are uncontrolled and in environments where gas temperature and composition are unknown. Specifically, it is an object of the invention to adapt the method for monitoring emissions from stacks and exhaust emissions from vehicles such as cars and airplanes.
Yet another object of the invention is to provide an apparatus for practicing the method of the invention.
These as well as other objects and advantages will become apparent upon review of the following detailed description.
SUMMARY
The objects and advantages of the invention are achieved by a method for temperature-independent determination of a concentration of a probe gas in a sample. First, a temperature range is selected. Preferably, the temperature range extends from a low temperature T
L
corresponding to a lowest temperature expected or found in the sample and a high temperature T
H
corresponding to a highest temperature expected or found in the sample. Next, a first spectroscopic technique is selected and a probe temperature function of the probe gas is determined over the temperature range using that first spectroscopic technique. Then, a second spectroscopic technique is selected and a reference gas is identified. A reference temperature function of the reference gas is determined using the second spectroscopic technique over the temperature range. It should be noted that the first and second spectroscopic techniques can be the same. The reference gas is identified such that a ratio of the probe temperature function and the reference temperature function is substantially constant over the temperature range. For example, the ratio of the temperature functions can be substantially equal to one over the temperature range. A probe reaction of the probe gas and a reference reaction of the reference gas is then measured by the first and second spectroscopic techniques and the concentration of the probe gas is derived from the probe reaction and reference reaction.
In one embodiment either one or both of the spectroscopic techniques are absorption spectroscopy employing a test beam. The test beam consists of light at several wavelengths with at least one wavelength for either probe transition or reference transition. The test beam passing through the sample causes the probe gas and the reference gas to absorb wavelength components of the light of the test beam corresponding to the probe and reference transitions. In other words, probe reaction is a probe absorption of a wavelength component corresponding to a probe absorption transition used for detecting the probe gas. Similiarly, reference reaction is a reference absorption of a wavelength component corresponding to a reference absorption transition used for detection of the reference gas.
At the detection side, the light of the test beam is separated by wavelength using appropriate optical components. For example, different wavelength components can be split and directed to separate photodetectors. The attenuations corresponding to the probe and reference transitions can then be obtained from signals detected at the different photodetectors.
The absorptive transitions of the probe gas and reference gas at which absorption occurs can be selected from any suitable transitions. For molecular gas species the transitions can be selected from rotational, rovibrational and rovibronic transitions. For atomic gas species
Hanson Ronald K.
Wang Jian
Brown Khaled
Font Frank G.
Lumen Intellectual Property Services Inc.
The Board of Trustees of the Leland Stanford Junior University
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