Method of calibrating a wavelength-modulation spectroscopy apparatus using a first, second and third gas to determine temperature and pressure values to calculate concentrations of analytes in a gas
Several methods of calibrating a wavelength-modulation spectroscopy apparatus configured to measure a concentration of an analyte in a sample gas are disclosed. Each of the methods allows for calibration and recalibration using a relatively safe gas regardless of whether the sample gas for which the concentration of the analyte can be determined is a hazardous gas. In one embodiment of the invention, calibration that is sample-gas specific is accomplished by determining a first slope coefficient and calibration function for the sample gas, after which a scaling factor can be determined based on the first slope coefficient and a second slope coefficient for the same or a different sample gas and used in a subsequent calibration (or recalibration) to scale the calibration function. In other embodiments of the invention, calibration that is not sample-gas specific is accomplished to allow for the determination of the analyte concentration in variable gas compositions and constant gas compositions.
1. A method of calibrating a wavelength-modulation spectroscopy apparatus configured to measure a concentration of an analyte in a sample gas, said method comprising the steps of:
determining an intercept coefficient reflecting contribution from the noise floor, said determination based on a plurality of harmonic spectra of known concentrations of said analyte at a reference pressure and a reference temperature of a first sample gas;
determining a temperature-dependent constant reflecting spectral line intensity corresponding to a known temperature of a second sample gas, said determination based on said intercept coefficient and at least one harmonic spectrum of known concentration of said analyte at a known pressure and said known temperature of said second sample gas;
determining an effective pressure-broadening coefficient of a third sample gas, said determination based on said intercept coefficient, said temperature-dependent constant, a pressure of said third sample gas, a temperature of said third sample gas, and at least one harmonic spectrum of said analyte in said third sample gas; and
determining a concentration of said analyte in said third sample gas, said determination based on said intercept coefficient, said temperature-dependent constant, said effective pressure-broadening coefficient, the pressure of said third sample gas, the temperature of said third sample gas, and at least one harmonic spectrum of said analyte in said third sample gas.
2. The method of claim 1 , wherein said steps of determining said intercept coefficient and said temperature-dependent constant are performed during the development and manufacturing stage of said apparatus.
3. The method of claim 1 , wherein said first sample gas and said second sample gas are relatively safe.
4. The method of claim 1 , wherein said first sample gas and said second sample gas are the same.
5. The method of claim 1 , wherein said first sample gas, said second sample gas, and said third sample gas are the same.
6. The method of claim 1 , wherein said step of determining said effective pressure-broadening coefficient of said third sample gas is performed once when said apparatus is employed to measure said third sample for the first time.
7. The method of claim 1 , wherein said step of determining said concentration of said analyte in said third sample gas is performed in real time.