Optical waveguide vapor sensor

• US 5,315,673 A
• Filed: 03/09/1992
• Issued: 05/24/1994
• Est. Priority Date: 03/09/1992
• Status: Expired due to Fees

• Current Assignee: Transducer Research, Inc. (TSI Incorporated)
• Original Assignee: Transducer Research, Inc. (TSI Incorporated)
• Inventors: Ballantine, David S. Jr., Stetter, Joseph R., Maclay, G. Jordan
• Primary Examiner(s): Bovernick, Rodney B.
• Assistant Examiner(s): Ngo, John
• Application Number:
  US07/848,240
• Time in Patent Office: 806 Days
• Field of Search: 385/12, 385/141, 385/144, 385/145, 385/13, 385/126, 385/127, 250/227.11, 250/227.14, 250/227.18, 250/227.21
• US Class Current: 385/12
• CPC Class Codes: G01N 2021/773 Porous polymer jacket; Polymer matrix with indicator
• CPC Class Codes: G01N 21/7703 using reagent-clad optical fibres or optical waveguides using measurement of total internal reflection or attenuated total reflection G01N21/552; optical fibres or waveguides per se G02B
• CPC Class Codes: G01N 21/783 for analysing gases
• CPC Class Codes: G01N 21/81 Indicating humidity

Abstract

An optical waveguide sensor for the detection of acid vapors comprises a chemically sensitive reagent coating of bromothymol blue or thymol blue indicator suspended in a Nafion polymer film. The sensor uses a 562-nm light-emitting diode source and a phototransistor detector. The response to HCl and H₂ S vapors is both rapid and reversible, with an estimated detection limit for H₂ S of less than 15 ppmv (parts per million by volume). The sensor exhibits generalized response to protonic acid vapors, but does not produce an indicator response to CO₂, even at large concentrations (60 volume-%) in the presence of water vapor. The sensor exhibits a systematic interference from water vapor which may be corrected by a differential approach, either using a reference sensor (Nafion without an indicator) or by monitoring the sensor responses at two wavelengths.

Claims

1. A sensor for detecting a selected air constituent comprising:

an optical waveguide having a front end and a rear end;

a light source for transmitting light of a selected wavelength to one end of the waveguide; and

means for measuring the intensity of light emitted from the other end of the waveguide, said waveguide comprising a film of material disposed on a substrate such that multiple internal light reflections occur at the interfaces between the film and the substrate and between the film and air, said material having optical properties that are susceptible to change in the presence of said air constituent, wherein said film comprises a reagent embedded in or forming a part of a polymer, said reagent yielding a substance having characteristic light-transmissive or light-adsorptive properties upon exposure to said air constituent.

2. The sensor of claim 1, wherein:

said light source is a light-emitting diode;

said photodetector is a phototransistor; and

said reagent is a colorimetric reagent.

3. The sensor of claim 2, wherein said polymer comprises a fluorinated ethylene sulfonic acid compound or derivative.

4. The sensor of claim 3, comprising signal-processing means for detecting the presence of at least one air constituent, wherein the presence of water vapor is deduced from an increase in the transmissivity of said waveguide.

5. The sensor of claim 4, wherein the presence of an acidic constituent is deduced from a change in the absorption of light of a selected wavelength range.

6. The sensor of claim 5, wherein said reagent comprises a pH indicator.

7. The sensor of claim 6, wherein said pH indicator comprises bromothymol blue or thymol blue.

8. The sensor of claim 7, wherein the selected wavelength range comprises the wavelength of 562 nm.

9. The sensor of claim 2 for the detection of basic air constituents, wherein said polymer comprises polyvinylpyrrolidone and said reagent comprises bromcresol green, said film having been first treated with acid so as to turn the reagent to its yellow form.

10. The sensor of claim 2, wherein said light-emitting diode and phototransistor are controlled by an electric circuit, said circuit causing the diode to be turned on and off in repetitive cycles.

11. The sensor of claim 1, wherein said film is disposed on a thin transparent substrate.

12. A sensor for detecting a selected air constituent comprising:

an optical waveguide having a front end and a rear end;

a light source for transmitting light of a selected wavelength to one end of the waveguide;

means for measuring the intensity of light emitted from the other end of the waveguide; and

signal-processing means for detecting the presence of at least one air constituent, wherein the presence of water vapor is deduced from an increase in the transmissivity of said waveguide and the presence of an acidic constituent is deduced from a change in the absorption of light of a selected wavelength range;

said waveguide comprising a film of material that comprises a reagent embedded in or forming part of a polymer;

said reagent yielding a substance having characteristic light-transmissive or light-adsorptive properties upon exposure to said air constituent;

wherein;

said light source is a light-emitting diode;

said photodetector is a phototransistor; and

said reagent is a colorimetric reagent;

said polymer comprises a fluorinated ethylene sulfonic acid compound or derivative; and

said reagent comprises a pH indicator;

wherein said signal-processing means includes means for distinguishing between the effects of water vapor and of acidic constituents.

13. The sensor of claim 12, wherein said distinguishing means comprises a second light-emitting diode whose range of emitted wavelengths is outside said light-absorption range.

14. The sensor of claim 13, wherein said distinguishing means comprises a timing circuit that causes the two diodes to go on and off in alternating cycles.

15. The sensor of claim 13, wherein said distinguishing means comprises a second waveguide and a second phototransistor operating in conjunction with the second diode.

16. The sensor of claim 13, wherein:

said pH-sensitive substance comprises bromothymol blue;

said light-absorption range comprises the wavelength of 562 nm; and

said second light-emitting diode emits at a wavelength of about 680 nm.

17. The sensor of claim 12, wherein said distinguishing means comprises a second waveguide and a second phototransistor, the film in said second waveguide being free of said pH indicator.

18. The sensor of claim 17, wherein said indicator is bromothymol blue or thymol blue.

19. A sensor for detecting a selected air constituent comprising:

an optical waveguide having a front end and a rear end;

a light source for transmitting light of a selected wavelength to one end of the waveguide; and

a photodetector for measuring the intensity of light emitted from the other end of the waveguide;

said waveguide comprising a film of material whose optical properties are susceptible to change in the presence of said air constituent;

wherein said film comprises a reagent embedded in or forming part of a polymer, said reagent yielding a substance having characteristic light-transmissive or light-absorptive properties upon exposure to said air constituent;

wherein said film is coating a substrate that is opaque to said selected wavelengths.

20. The sensor of claim 19, wherein said substrate comprises a rod that is opaque to said selected wavelengths and has two end surfaces and transparent lenses affixed onto each of said surfaces.