Augmented Raman analysis using absolute Raman
A method for determining an amount of a Raman-invisible gas in a multi-component gas stream includes performing a first and second absolute Raman analysis on the gas stream. A decrease in the absolute Raman bands from the first analysis to the second analysis is attributed to an increase of the Raman-invisible gas in the gas stream. The amount of the Raman-invisible gas is calculated from the difference between the first and second sets of Raman bands. The calculation of the Raman-invisible gas is verified via a measurement and a calculation of a secondary property of the gas stream such as the thermal conductivity of the gas stream or the density of the gas stream.
1. A method for determining an amount of a Raman-invisible gas in a sample gas stream, the method comprising:
providing a Raman spectroscopic device, including a Raman laser;
performing a Raman analysis of a calibration gas stream, wherein the Raman analysis of the calibration gas stream includes measuring a pressure and a temperature of the calibration gas stream;
determining a first set of absolute Raman bands from the Raman analysis of the calibration gas stream, wherein the first set of absolute Raman bands correlate to Raman-visible gases within the calibration gas stream;
calculating, using the first set of absolute Raman bands, first amounts of each Raman-visible gas present in the calibration gas stream;
waiting a first time period;
performing a Raman analysis of the sample gas stream, wherein the Raman analysis of the sample gas stream includes measuring a pressure and a temperature of the sample gas stream;
determining a second set of absolute Raman bands from the Raman analysis of the sample gas stream, wherein the second set of absolute Raman bands correlate to the Raman-visible gases within the sample gas stream;
calculating, using the second set of absolute Raman bands, second amounts of each Raman-visible gas present in the sample gas stream;
comparing the amount of each Raman-visible gas in the sample gas stream to the respective amount of each Raman-visible gas in the calibration gas stream;
attributing decreases in the amounts of the Raman-visible gases in the sample gas stream to a displacement of the Raman-visible gases in the sample gas stream by the Raman-invisible gas in the sample gas stream; and
calculating the amount of the Raman-invisible gas in the sample gas stream using the decreases in the amounts of the Raman-visible gases in the sample gas stream.
2. The method according to claim 1 , further comprising:
measuring a first power of the Raman laser during the Raman analysis of the calibration gas stream;
measuring a second power of the Raman laser during the Raman analysis of the sample gas stream; and
adjusting the second set of absolute Raman bands to compensate for a difference between the first power of the Raman laser and the second power of the Raman laser.
3. The method according to claim 1 , further comprising:
measuring a first value of a secondary property of the sample gas stream during the Raman analysis of the sample gas stream;
calculating a second value of the secondary property of the sample gas stream using: the second amounts of each Raman-visible gas; the amount of the Raman-invisible gas; and reference values for the secondary property of each respective Raman-visible gas and of the Raman-invisible gas;
comparing the measured first value of the secondary property with the calculated second value of the secondary property;
when a difference between the measured value of the secondary property and the calculated value of the secondary property is less than or equal to a difference threshold, indicate the calculated amount of the Raman-invisible gas as verified; and
when the difference between the measured value of the secondary property and the calculated value of the secondary property is greater than the difference threshold, indicate the calculated amount of the Raman-invisible gas as unverified.
4. The method according to claim 3 , further comprising:
adjusting the contents of the calibration gas stream based on the second amounts of the Raman-visible components; and
repeating the method steps recited in claim 1 beginning with performing a Raman analysis of the calibration gas stream.
5. The method of claim 3 ,
wherein the Raman-invisible gas is Argon, and
wherein when the secondary property is a density of the gas sample.
6. The method of claim 3 ,
wherein the Raman-invisible gas is Argon, and
wherein the secondary property is a thermal conductivity of the gas sample.