Optical monitor for water vapor concentration

• US 5,760,895 A
• Filed: 08/20/1996
• Issued: 06/02/1998
• Est. Priority Date: 08/20/1996
• Status: Expired due to Fees

• Current Assignee: Aerodyne Corporation
• Original Assignee: Aerodyne Corporation
• Inventors: Kebabian, Paul
• Primary Examiner(s): Font, Frank G.
• Assistant Examiner(s): STAFIRA, MICHAEL PATRICK
• Application Number:
  US08/700,026
• Time in Patent Office: 651 Days
• Field of Search: 356/73, 356/349, 356/350, 356/351, 356/346, 356/301, 356/303, 356/319, 356/322, 356/327, 356/316, 356/307, 356/311, 356/312, 356/315, 356/436, 356/437, 372/28, 372/29, 372/37, 372/32, 372/20, 372/22, 372/26, 250/339, 250/343, 250/344, 250/345, 250/339.13
• US Class Current: 356/307
• CPC Class Codes: G01N 33/18 Water

Abstract

A system for measuring and monitoring water vapor concentration in a sample uses as a light source an argon discharge lamp, which inherently emits light with a spectral line that is close to a water vapor absorption line. In a preferred embodiment, the argon line is split by a magnetic field parallel to the direction of light propagation from the lamp into sets of components of downshifted and upshifted frequencies of approximately 1575 Gauss. The downshifted components are centered on a water vapor absorption line and are thus readily absorbed by water vapor in the sample; the upshifted components are moved away from that absorption line and are minimally absorbed. A polarization modulator alternately selects the upshifted components or downshifted components and passes the selected components to the sample. After transmission through the sample, the transmitted intensity of a component of the argon line varies as a result of absorption by the water vapor. The system then determines the concentration of water vapor in the sample based on differences in the transmitted intensity between the two sets of components. In alternative embodiments alternate selection of sets of components is achieved by selectively reversing the polarity of the magnetic field or by selectively supplying the magnetic field to the emitting plasma.

Claims

1. A system for determining a concentration of water vapor in a sample, the system comprising:

a. an argon discharge lamp for supplying 1S₂ -2P₇ spectral line light, having a wavelength of approximately 935 nm, to the sample; and

b. means for evaluating the concentration of water vapor in the sample based on a change in intensity of the light supplied by the lamp to the sample.

2. The system of claim 1 further comprising:

a. a magnetic field source for applying a magnetic field to the lamp, the magnetic field splitting the spectral line supplied by the lamp into downshifted components having longer wavelengths that are readily absorbed by water vapor and upshifted components having shorter wavelengths that are less-readily absorbed by water vapor; and

b. selection means for selectively passing the downshifted components to the sample;

the means for evaluating the concentration of water vapor in the sample evaluating the concentration based on a change in intensity of the downshifted components passed to the sample.

3. The system of claim 2 wherein the means for evaluating the concentration of water vapor comprises:

a. a first detector for producing a first signal which is proportional to the intensity of the downshifted components before the downshifted components have passed through the sample;

b. a second detector for producing a second signal which is proportional to the intensity of the downshifted components after the downshifted components have passed through the sample; and

c. means for computing, from the ratio of the first signal to the second signal, the concentration of water vapor in the sample.

4. The system of claim 3 wherein the selection means selectively passes to the sample the upshifted components or the downshifted components, the first detector produces a first reference signal which is proportional to the intensity of the upshifted components before the upshifted components have passed through the sample and the second detector produces a second reference signal which is proportional to the intensity of the upshifted components after the upshifted components have passed through the sample, the means for computing determining, based on a difference between the first and second reference signals associated with the set of upshifted components, a correction to be included in computing the concentration of water vapor based on the first and second signals associated with the set of downshifted components.

5. The system of claim 4 wherein the first and second detectors are first and second photodiodes, the means for computing the concentration of water vapor comprises a bridge including the photodiodes, electrically balanced when the set of upshifted components are passed to the sample, and the means for computing produces a concentration signal that is proportional to the concentration of water vapor in the sample when the set of downshifted components are passed to the sample.

6. The system of claim 5 wherein the means for computing the concentration of water vapor determines whether the concentration exceeds or falls below a predetermined level by determining whether the concentration signal produced by the bridge exceeds or falls below a predetermined value.

7. The system of claim 2 wherein the means for evaluating the concentration of water vapor determines whether the concentration exceeds or falls below a predetermined level.

8. The system of claim 2 wherein the downshifted components have a downshifted circular polarization and the upshifted components have an upshifted circular polarization opposite to the downshifted circular polarization, the selection means selectively passing to the sample the downshifted components or the upshifted components based on their respective circular polarizations.

9. The system of claim 8 wherein the selection means comprises:

a. means for linearizing the respective circular polarizations of the downshifted and upshifted components thereby converting the downshifted circular polarization to a downshifted linear polarization and the upshifted circular polarization to an upshifted linear polarization, orthogonal to the downshifted linear polarization;

b. rotating means for selectively rotating the linearly polarized downshifted and upshifted components by 90 degrees, the rotating means alternately operating in a first mode, in which it refrains from rotating the components and in a second mode, in which it rotates each of the components such that each component has a linear polarization orthogonal to the respective linear polarization it has without rotation; and

c. a linear polarizer for passing components having a particular linear polarization and blocking components that do not have the particular polarization thereby passing upshifted components while blocking downshifted components and passing downshifted components while blocking upshifted components.

10. The system of claim 2 wherein the downshifted components have a downshifted circular polarization and the upshifted components have an upshifted circular polarization, the selection means selectively passing to the sample the downshifted components or the upshifted components, the selection means comprising:

a. means for controlling the magnetic field source, such that the magnetic field source selectively reverses the polarity of the magnetic field, thereby interchanging the upshifted circular polarization and the downshifted circular polarization; and

b. a circular polarizer, for passing components having a particular circular polarization and blocking components that do not have the particular circular polarization, thereby passing upshifted components while clocking downshifted components and passing downshifted components while clocking upshifted components.

11. The system of claim 2 wherein the selection means comprises means for alternately turning on and off the magnetic field source, such that the light produced by the lamp is alternately (i) split into the sets of upshifted and downshifted components and (ii) not split into the sets of components, the selection means alternately passing to the environment of interest split components and the unsplit light.

12. The system of claim 11 further comprising means for blocking the upshifted components from entering the sample.

13. The system of claim 2 wherein the magnetic field has a strength of approximately 1575 Gauss.

14. The system of claim 3 wherein the selection means selectively passes to the sample unsplit light or downshifted components, the first detector produces a first reference signal which is proportional to the intensity of the unsplit light before the unsplit light has passed through the sample and the second detector produces a second reference signal which is proportional to the intensity of the unsplit light after the unsplit light has passed through the sample, the means for computing determining, based on a difference between the first and second reference signals associated with the unsplit light, a correction to be included in computing the concentration of water vapor based on the first and second signals associated with the set of downshifted components.

15. The system of claim 14 wherein the first and second detectors are first and second photodiodes, the means for computing the concentration of water vapor comprises a bridge including the photodiodes, electrically balanced when the unsplit light is passed to the sample, and the means for computing produces a concentration signal that is proportional to the concentration of water vapor in the sample when the set of downshifted components are passed to the sample.

16. A system for determining a concentration of water vapor in a sample, the system comprising:

a. an argon discharge lamp for supplying light of the 1S₂ -2P₇ spectral line, having a wavelength in the vicinity of 935 nm, to the sample; and

b. a magnetic field source for applying a magnetic field to the lamp, the field splitting the spectral line supplied by the lamp into downshifted components having longer wavelengths that are readily absorbed by water vapor and upshifted components having shorter wavelengths that are less readily absorbed by water vapor, the downshifted components having a first circular polarization and the upshifted components having a second circular polarization, opposite to the first circular polarization;

c. control means for selectively controlling the magnetic field source so as to selectively apply a magnetic field having a polarity equal to one value or to an opposite value, the polarity determining the respective circular polarizations of the sets of upshifted and downshifted components;

d. a circular polarizer for selectively passing to the sample set of upshifted components or the set the downshifted components, depending on the orientations of the respective circular polarizations; and

e. means for determining the concentration of water vapor in the sample based on a change in intensity of the light including the set of downshifted components supplied to the sample.

17. A system for determining a concentration of water vapor in a sample, the system comprising:

a. an argon discharge lamp for supplying 1 S₂ -2P₇ spectral line light, having a wavelength approximately 935 nm, to the sample;

b. a magnetic field source for applying a magnetic field to the lamp, the field splitting the spectral line supplied by the lamp into downshifted components having longer wavelengths that are readily absorbed by water vapor and upshifted components having shorter wavelengths that are less-readily absorbed by water vapor;

c. control means for selectively turning on and off the magnetic field source; and

d. means for determining the concentration of water vapor in the sample based on (i) a change in intensity of light being supplied to the sample while the magnetic field source is turned on, and (ii) a change in intensity of light supplied to the sample while the magnetic field source is turned off.

18. A system for determining a concentration of water vapor in a sample, the system comprising:

a. an argon discharge lamp for supplying 1S₂ -2P₇ spectral line light, having a wavelength of approximately 935 nm, to the sample; and

b. a magnetic field source for applying a magnetic field to the lamp, the field splitting the spectral line supplied by the lamp into downshifted components having longer wavelengths that are readily absorbed by water vapor and upshifted components having shorter wavelengths that are less readily absorbed by water vapor, the downshifted components having a first circular polarization and the upshifted components having a second circular polarization, opposite to the first circular polarization;

c. a polarization modulator for selectively passing upshifted and downshifted components to the sample, the polarization modulator comprising

i. a quarter-wave plate for linearizing the respective circular polarizations of the downshifted and upshifted components thereby converting the downshifted circular polarization to a downshifted linear polarization and the upshifted circular polarization to an upshifted linear polarization, orthogonal to the downshifted linear polarization;

ii. a liquid-crystal cell for selectively rotating the linearly polarized downshifted and upshifted components by 90 degrees, the cell alternately operating in a first mode, in which it refrains from rotating the components and in a second mode, in which it rotates each of the components such that each component has a linear polarization orthogonal to the respective linear polarization it has without rotation;

iii. a linear polarizer for passing components having a particular linear polarization and blocking components that do not have the particular polarization; and

d. means for determining the concentration of water vapor in the sample based on a change in intensity of the light including the set of downshifted components supplied to the sample.