Nondispersive optical monitor for nitrogen-oxygen compounds

• US 5,818,598 A
• Filed: 12/20/1996
• Issued: 10/06/1998
• Est. Priority Date: 12/20/1996
• Status: Expired due to Fees

• Current Assignee: Aerodyne Corporation
• Original Assignee: Aerodyne Corporation
• Inventors: Kebabian, Paul
• Primary Examiner(s): Rosenberger, Richard A.
• Application Number:
  US08/770,432
• Time in Patent Office: 655 Days
• Field of Search: 356/435, 356/437, 356/438, 356/439, 356/414, 250/339.13, 250/345
• US Class Current: 356/434
• CPC Class Codes: G01N 21/3518 Devices using gas filter correlation techniques; Devices using gas pressure modulation techniques

Abstract

A gas-correlation filter spectroscopy-based method and apparatus for determining the concentration of gas species having significant average broadband absorption incorporates a compensating filter having an absorption spectrum that varies with wavelength similarly to the spectrum of the gas species. The compensating filter eliminates interference due to differential absorption. The invention is especially suited to detecting nitrogen dioxide.

Claims

1. A method of determining a concentration of nitrogen dioxide in a sample, nitrogen dioxide having a transmission spectrum defined by at least one absorption line and an average broadband transmission varying according to a wavelength dependence, the method comprising the steps of:

a. providing in a first optical path a correlation filter, comprising a first cell for containing nitrogen dioxide at a correlation filter pressure;

b. providing in a second optical path a compensating filter, having an average broadband transmission that varies according to the wavelength dependence without the at least one absorption line;

c. supplying light from a first light source to the correlation filter;

d. supplying light from a second light source to the compensating filter;

e. merging light that has passed through the correlation filter and light that has passed through the compensating filter in a combined optical path;

f. measuring light in the combined optical path;

g. supplying light from the combined path to the sample; and

h. measuring light leaving the sample.

2. The method of claim 1 wherein:

a. the first and second light sources are alternately energized so as to supply light alternately to the correlation filter and to the compensating filter;

b. light is measured in the combined optical path synchronously with the alternate energizing so as to produce first and second reference signals corresponding to intensities of light that have passed respectively through the correlation filter and the compensating filter;

c. light leaving the sample is measured synchronously with the alternate energizing of the light sources, thereby producing first and second analytical signals corresponding to intensities of light that has passed through the sample respectively after passing through the correlation filter and after passing through the compensating filter, a difference between the first and second analytical signals indicating the concentration; and

further comprising the step of regulating operation of the second light source based on a difference between the first and second reference signals.

3. The method of claim 1 wherein the two light sources are two light-emitting diodes that emit light having a wavelength in the range of 400 nm to 550 nm.

4. The method of claim 1 wherein the compensating filter is of FG-13 glass.

5. The method of claim 1 wherein the compensating filter comprises a second cell containing nitrogen dioxide at a compensating filter pressure substantially identical to the correlation filter pressure, the second cell being located in a constant magnetic field having strength greater than approximately 1,000 Gauss.

6. The method of claim 1 further comprising the step of oxidizing any nitric oxide in the sample to nitrogen dioxide, the indicated concentration of nitrogen dioxide denoting a total concentration of nitric oxide and nitrogen dioxide.

7. The method of claim 6 wherein the step of oxidizing comprises supplying ozone to the sample.

8. The method of claim 1 further comprising the step of calculating a temperature based on the indicated concentration of nitrogen dioxide.

9. The method of claim 8 wherein the sample contains nitrogen dioxide at a pressure in the approximate range 40 to 100 Torr.

10. A method of determining a concentration of a gas species in a sample, the gas species having a transmission spectrum defined by at least one absorption line and an average broadband transmission varying according to a wavelength dependence, the method comprising the steps of:

a. filtering light alternately through a correlation filter comprising a first cell containing the gas species at a correlation filter pressure, thereby creating correlation-filtered light having a correlation-filtered intensity, and a compensating filter, thereby creating compensating-filtered light having a compensating-filtered intensity, the compensating filter having an average broadband transmission that varies according to the wavelength dependence without the at least one absorption line;

b. alternately passing into the sample compensating-filtered light and correlation-filtered light;

c. detecting light leaving the sample so as to alternately produce a first analytical signal proportional to an intensity of correlation-filtered light that has passed through the sample and a second analytical signal proportional to an intensity of compensating-filtered light that has passed through the sample; and

d. deriving the concentration of the gas species based on the difference between the first and second signals.

11. The method of claim 10 wherein the step of alternately filtering comprises alternately passing light from a first of two light sources into the correlation filter and light from a second of the two light sources into the compensating filter, the first and second light sources being substantially identical.

12. The method of claim 11 further comprising the step of making the compensating-filtered light intensity and the correlation-filtered light intensity the same by controlling operation of the second of the two light sources based on a difference between the compensating-filtered light intensity and the correlation-filtered light intensity.

13. The method of claim 12 further comprising the step of alternately producing, synchronously with the alternate passing, a first reference signal proportional to the correlation-filtered intensity and a second reference signal proportional to the compensating-filtered intensity.

14. The method of claim 11 wherein the step of detecting light leaving the sample so as to generate first and second analytical signals is performed synchronously with the alternate passing of light to the respective filters.

15. The method of claim 11 wherein alternately passing light from each of the two light sources into the respective filters comprises alternately energizing the two light sources.

16. The method of claim 10 wherein the compensating filter consists of colored glass having thickness and composition chosen so that the average transmission of the compensating filter varies according to the wavelength dependence.

17. The method of claim 10 wherein the compensating filter passes the at least one absorption line.

18. The method of claim 10 wherein the gas species is nitrogen dioxide.

19. The method of claim 18 wherein the two light sources are light-emitting diodes that emit light having a wavelength in the range of 400 nm to 550 nm.

20. The method of claim 18 wherein the compensating filter comprises a second cell containing nitrogen dioxide at a compensating filter pressure substantially identical to the correlation filter pressure, the second cell being located in a constant magnetic field having strength greater than approximately 1,000 Gauss.

21. The method of claim 18 wherein the compensating filter is made of FG-13 glass.

22. The method of claim 18 further comprising the step of oxidizing any nitric oxide in the sample to nitrogen dioxide, the derived concentration of nitrogen dioxide indicating a total concentration of nitric oxide and nitrogen dioxide.

23. The method of claim 22 wherein the step of oxidizing comprises supplying ozone to the sample.

24. The method of claim 18 further comprising the step of calculating a temperature based on the derived concentration of nitrogen dioxide.

25. An apparatus for determining a concentration of nitrogen dioxide in a sample, nitrogen dioxide having a transmission spectrum defined by at least one absorption line and an average broadband transmission varying according to a wavelength dependence, the apparatus comprising:

a. a correlation filter, in a first optical path, comprising a first cell for containing nitrogen dioxide at a correlation filter pressure;

b. a compensating filter, having an average broadband transmission that varies according to the wavelength dependence without the at least one absorption line, in a second optical path;

c. a light generator comprising a first light source for supplying light to the correlation filter and a second light source for supplying light to the compensating filter;

d. combining means for merging light that has passed through the correlation filter and the compensating filter in a combined optical path, the combined path supplying light to the sample;

e. a reference detector for measuring light in the combined optical path; and

f. an analytical detector for measuring light leaving the sample.

26. The apparatus of claim 25 wherein the light generator comprises a controller for alternately energizing the first and second light sources so as to alternately supply light to the correlation filter and the compensating filter,

a. the reference detector being configured to measure light synchronously with the alternate energizing so as to produce first and second reference signals corresponding to intensities of light that have passed respectively through the correlation filter and the compensating filter;

b. the controller regulating operation of the second light source based on a difference between the first and second reference signals;

the analytical detector being configured to measure light leaving the sample synchronously with the alternate energizing of the light sources, thereby producing first and second analytical signals corresponding to intensities of light that has passed through the sample respectively after passing through the correlation filter and after passing through the compensating filter; and

c. a difference between the first and second analytical signals indicating the concentration.

27. The apparatus of claim 26 wherein the two light sources are light-emitting diodes that emit light having a wavelength in the range of 400 nm to 550 nm.

28. The apparatus of claim 27 wherein the compensating filter is FG-13 glass.

29. The apparatus of claim 26 wherein the compensating filter comprises a second cell containing nitrogen dioxide at a compensating filter pressure substantially identical to the correlation filter pressure, the second cell being located in a constant magnetic field having strength greater than approximately 1,000 Gauss.

30. The apparatus of claim 26 further comprising a system for oxidizing any nitric oxide in the sample to nitrogen dioxide, the indicated concentration of nitrogen dioxide denoting a total concentration of nitric oxide and nitrogen dioxide.

31. The apparatus of claim 30 wherein the system for oxidizing comprises an ozone generator.

32. The apparatus of claim 26 wherein the sample contains nitrogen dioxide at a pressure in the approximate range 40 to 100 Torr, the indicated concentration of nitrogen dioxide denoting a temperature.

33. An apparatus for determining a concentration of a gas species in a sample, the gas species having a transmission spectrum defined by at least one absorption line and an average broadband transmission varying according to a wavelength dependence, the apparatus comprising:

a. a correlation filter comprising a first cell for containing the gas species at a correlation filter pressure;

b. a compensating filter having an average broadband transmission that varies according to the wavelength dependence without the at least one absorption line;

c. a light generator;

d. means for supplying light alternately to the correlation filter and the compensating filter;

e. means for passing light, alternately from the correlation filter and the compensating filter, to the sample;

f. an analytical detector for alternately producing a first analytical signal proportional to the intensity of light that has passed through the sample from the correlation filter and a second analytical signal proportional to light that has passed through the sample from the compensating filter; and

g. means for evaluating the concentration based on the difference between the first and second analytical signals.

34. The apparatus of claim 33 further comprising a reference detector for alternately producing a first reference signal proportional to a light intensity leaving the correlation filter and a second reference signal proportional to a light intensity leaving the compensating filter.

35. The apparatus of claim 34 wherein the light generator comprises a controller operable to regulate operation of the generator, based on the difference between the first and second reference signals, so that the intensity of light leaving the correlation filter and the intensity of light leaving the compensating filter are the same.

36. The apparatus of claim 33 wherein the light generator comprises two substantially identical light sources, the means for alternately supplying light alternately directing light from a first light source to the correlation filter and directing light from a second light source to the compensating filter.

37. The apparatus of claim 36 further comprising a reference detector for alternately producing, synchronously with the alternate directing of light from the two light sources, a first reference signal proportional to a light intensity leaving the correlation filter and a second reference signal proportional to a light intensity light leaving the compensating filter, the light generator comprising a controller operable to regulate operation of the generator, based on the difference between the first and second reference signals, so that the intensity of light leaving the correlation filter and the intensity of light leaving the compensating filter are the same.

38. The apparatus of claim 36 wherein the analytical detector is configured to produce the first and second analytical signals synchronously with the alternate supplying of light to the correlation filter and the compensating filter.

39. The apparatus of claim 36 wherein the means for alternately supplying light alternately energizes the two light sources.

40. The apparatus of claim 36 wherein the two light sources are two light-emitting diodes.

41. The apparatus of claim 33 wherein the compensating filter consists of colored glass having thickness and composition chosen so that the average transmission of the compensating filter varies according to the wavelength dependence.

42. The apparatus of claim 33 wherein the compensating filter passes the at least one absorption line.

43. The apparatus of claim 33 wherein the gas species is nitrogen dioxide.

44. The apparatus of claim 43 wherein the two light sources are light-emitting diodes that emit light having a wavelength in the range of 400 nm to 550 nm.

45. The apparatus of claim 43 wherein the compensating filter comprises a second cell containing nitrogen dioxide at a compensating filter pressure substantially identical to the correlation filter pressure, the second cell being located in a constant magnetic field having strength greater than approximately 1,000 Gauss.

46. The apparatus of claim 43 wherein the compensating filter is made of FG-13 glass.

47. The apparatus of claim 43 further comprising a system for oxidizing any nitric oxide in the sample to nitrogen dioxide, the evaluated concentration of nitrogen dioxide indicating a total concentration of nitric oxide and nitrogen dioxide.

48. The apparatus of claim 47 wherein the system for oxidizing comprises an ozone generator.

49. The apparatus of claim 43 wherein the sample contains nitrogen dioxide at a pressure in the approximate range 40 to 100 Torr, the evaluated concentration of nitrogen dioxide indicating a temperature.