Optical waveguide sensor for methane gas

• US 4,708,941 A
• Filed: 03/07/1985
• Issued: 11/24/1987
• Est. Priority Date: 03/07/1985
• Status: Expired due to Fees

• Current Assignee: The United States of America As Represented By The Secretary of Agriculture
• Original Assignee: the united states of america as represented by the secretary of the navy
• Inventors: Giuliani, John F.
• Primary Examiner(s): Richman, Barry S.
• Assistant Examiner(s): Hill, Jr., Robert J.
• Application Number:
  US06/709,251
• Time in Patent Office: 992 Days
• Field of Search: 436/139, 436/141, 436/164, 436/165, 436/167, 436/168, 436/169, 436/181, 422/55-60, 422/68, 422/69, 422/83, 422/86, 422/88, 422/91, 250/227, 350/96.3, 350/96.32, 350/96.15, 350/96.18, 356/73.1, 356/437, 356/445, 356/448
• US Class Current: 436/141
• CPC Class Codes: G01N 21/47 Scattering, i.e. diffuse reflection G01N21/25, G01N21/41 take precedence G01N21/55 takes precedence
• CPC Class Codes: G01N 21/552 Attenuated total reflection
• CPC Class Codes: G01N 33/0027 concerning the detector
• CPC Class Codes: G01N 33/0047 for organic compounds
• CPC Class Codes: Y10T 436/214 Acyclic [e.g., methane, octane, isoparaffin, etc.]
• CPC Class Codes: Y10T 436/25875 Gaseous sample or with change of physical state

Abstract

A device for detecting small amounts of alkanes such as methane, ethane, propane, and butane gases, comprising an optical waveguide and a light source for propagating light therethrough; apparatus for obtaining samples of air to be tested; apparatus for adding water vapor to these air samples to yield gas mixtures with at least a 30% relative humidity; and means for flowing these mixtures over the surface of the optical waveguide at a rate sufficient to favor condensation of the mixture on the waveguide surface to thereby form a thin film thereon. The presence of alkanes is determined by detecting the intensity of the light after propagation through the optical waveguide and then comparing that detected intensity to a reference. In a preferred embodiment, the optical waveguide is an optical glass capillary in which the surface is closed in a lens-like configuration at the detector end.

Claims

1. A method for detecting alkanes by forming and detecting clathrate structures of alkanes in a fluid at constant temperature and pressure comprising the steps of:

propagating light into an optical capillary waveguide having two ends and a longitudinal surface at end thereof which is closed with an optical stopper and with the other end thereof sealed with a lens-like curvature;

obtaining samples of a fluid to be tested;

hydrating any alkanes present in said fluid samples to be tested to yield gas mixtures with at least 30% relative humidity;

condensing any hydrated alkanes on the surface of said optical waveguide; and

detecting alkanes in the fluid to be tested by detecting a parameter of light affected by weak optical surface scattering of light at any condensed alkane-water-optical surface interfaces after propagation through said optical waveguide.

2. A method as defined in claim 1, wherein said detecting step includes the step of comparing said detected light parameter to a reference.

3. A method as defined in claim 2, wherein said fluid to be tested is a first gas;

and wherein said hydrating step includes the step of adding water vapor to the samples of said first gas to give said mixtures which comprise said first gas samples with at least 30% relative humidity.

4. A method as defined in claim 3, wherein said condensing step includes the step of flowing said mixtures over the surface of said optical waveguide at a rate sufficient to favor condensation on said surface.

5. A method as defined in claim 4, wherein said sample obtaining step includes the step of obtaining first gas samples of air.

6. A method as defined in claim 5, wherein said water vapor adding step comprises the step of bubbling said first gas air samples through water.

7. A method as defined in claim 6, wherein said comparing step includes the following steps:

substituting samples of a reference gas in place of the first gas samples to be tested and performing the previously recited steps; and

comparing intensity response obtained with the first gas samples to be tested to intensity response obtained with said reference gas.

8. A method as defined in claim 6, wherein said detecting step includes the step of detecting intensity of the light propagating through said optical waveguide.

9. A method as defined in claim 8, wherein said flowing step is performed at approximately 22°

C. over said optical waveguide with said surface being of glass.

10. A method for detecting alkanes by forming and detecting clathrate structures of alkanes in a fluid at constant temperature and pressure comprising the steps of:

propagating light through outer walls of an optical waveguide capillary having two ends and an uncoated surface, the propagated light being confined to the outer walls of the optical capillary;

obtaining a sample of a first gas to be tested;

adding water vapor to said first gas sanple to yield a gas mixuterwith at least 30% relative humidity;

flowing said gas mixture over the surface of said optical waveguide at a rate sufficient to favor condensation of saisd gas mixture on said surface to thereby form a thin film on said surface;

detecting intensity of light affected by weak optical surface scattering of light at any condensed alkane-water optical waveguide surface interfaces after propagation through said optical waveguide; and

detecting alkanes in the first gas to be tested by comparing said detected light intensity to a reference.

11. A method as defined in claim 10, wherein said obtaining step includes the step of obtaining a sample of air as said first gas.

12. A method as defined in claim 11, wherein said water vapor adding step comprises the step of bubbling said first gas sample through water.

13. A method as defined in claim 12, wherein said flowing step is performed at approximately 22°

C.

14. A method as defined in claim 13, wherein said comparing step comprises the steps of:

substituting a sample of a reference gas in place of said first gas sample and performing the previously recited propagating, adding, flowing, and detecting steps; and

comparing intensity response obtained with said first gas sample to intensity response obtained with said reference gas.

15. An optical detection device for detecting alkanes by forming and detecting clathrate structures of alkanes in a fluid at constant temperature and pressure, comprising;

a light source;

an optical capillary waveguide having two ends and a surface, with light from the light source directed into said capillary at one end thereof which is closed with an optical stopper and with the other end thereof sealed with a lens-like curvature;

means for obtaining samples of a fluid to be tested;

means for adding water molecules to the fluid samples to be tested to form gas mixtures with at least 30% relative humidity;

means for flowing the mixtures formed by said water adding means on the surface of said optical waveguide at a rate sufficient to favor condensation; and

17. An alkane detection device as defined in claim 15, wherein said detecting means includes means for comparing said detected light parameter to a reference.

19. An alkane detection device as defined in claim 17, wherein said comparing means includes:

means for substituting samples of a reference gas in place of the first gas samples to be tested in said device; and

means for comparing the detected light parameter obtained with the first gas samples to be tested to the detected light parameter obtained with the reference gas.

20. An alkane detection device as defined in claim 17, wherein said sample obtaining means includes means for obtaining first gas samples of air.

21. An alkane detection device as claimed in claim 19, wherein said water vapor adding means includes means for bubbling the first gas air samples through water.

22. An alkane detection device as defined in claim 20, wherein said detecting means includes means for detecting intensity of light after propagation through said optical waveguide.

23. An alkane detection device as defined in claim 21, wherein said flowing means comprises apparatus for flowing the mixtures over the surface of said optical waveguide at approximately 22°

C.

24. An alkane detection device as defined in claim 22, wherein said optical waveguide surface is soda-lime glass.

25. An alkane detection device as defined in claim 21, wherein said flowing means includes a gas pump.

26. An alkane detection device as defined in claim 24, wherein said water vapor adding means includes mixing means for mixing the first gas air samples after they have been bubbled through water.

27. An alkane detection device as defined in claim 24, wherein said comparing means comprises a chart recorder.

28. An alkane detection device as defined in claim 24, wherein said comparing means includes:

means for substituting samples of a reference gas in place of the first gas air samples to be tested in said device; and

means for comparing intensity response obtained with the first gas air samples to be tested to intensity response obtained with the reference gas.

30. An optical detection device as defined in claim 28, further including means for sampling air.

32. A optical detection device as defined in claim 30, wherein said flowing means includes a mixing apparatus for mixing the first gas samples after they have been bubbled through water.

16. means for detecting a parameter of light affected by a weak optical surface scattering of light at a condensed alkane-water optical waveguide surface inteface, after propagation through said otpical waveguide.

18. An alkane detection device as defined in claim 16, wherein the fluid to be tested is a first gas;

and wherein said water molecule adding means includes means for adding water vapor to the samples of the first gas to obtain the mixtures which comprise the first gas samples having at least 30% relative humidity.

29. An optical detection device for detecting alkanes by forming and detecting clathrate structures of alkanes in a fluid at constant temperature and pressure, comprising:

a light source;

an optical capillary waveguide having two ends and a longitudinal surface, with light directed into said capillary at one end thereof which is closed with an optical stopper and with the other end thereof sealed with a lens-like curvature;

means for sampling a first gas to be tested to obtain first gas samples;

means for adding water vapor to the first gas samples to yield gas mixtures with at least a 30% relative humidity;

means for flowing the mixtures formed by said water adding means over the surface of said optical waveguide at a rate sufficient to favor condensation of the mixtures on said surface to thereby form a thin film on said surface;

means for detecting intensity of light affected by weak optical surface scattering of the light at a condensed alkane-water optical waveguide surface interface after propagation through said optical waveguide; and

means for comparing the detected light intensity to a reference.

31. An optical detection device as defined in claim 29, wherein said longitudinal surface of said optical waveguide is soda-lime glass.

33. An optical detection device as defined in claim 31, wherein said comparing means includes:

means for substituting a sample of a reference gas in place of the first gas samples in said device; and

means for comparing intensity response obtained with the first gas samples to intensity response obtained with the reference gas.