Optical gas analyzer

• US 4,069,420 A
• Filed: 03/08/1976
• Issued: 01/17/1978
• Est. Priority Date: 03/08/1976
• Status: Expired due to Term

• Current Assignee: Forney Corporation (Graham Holdings Company)
• Original Assignee: Cavitron Corporation
• Inventors: Ross, Thaddeus C.
• Primary Examiner(s): Church, Craig E.
• Application Number:
  US05/664,464
• Time in Patent Office: 680 Days
• Field of Search: 250/340, 250/341, 250/343, 250/344, 250/345, 250/346, 250/373
• US Class Current: 250/341.5
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis

Abstract

An optical gas analyzer is disclosed utilizing the infrared absorption principle having a sample chamber and reference chamber with infrared source and detector assembly spaced at opposite ends of and in alignment with the chambers with baffle means provided in cooperating relation with the infrared source to prevent cross-currents with respect thereto and masking means provided in cooperating relation with the detector assembly to mask out a gas having a partially overlapping infrared absorption band. The electronic processing system utilizes the total area under the signal waveform to derive the desired measurement. In addition barometric correction means is provided to permit on site calibration by the user of the gas analyzer.

Claims

1. In an optical gas analyzer including in combination, a sample chamber, a reference chamber, to provide dual optical paths one for analysis of a sample gas, the other for reference, radiation projector means mounted at one end of said chambers, means for chopping and directing the radiation beams alternately along the two paths, the improvement which comprises radiation detector means positioned at the other end of said chambers comprising:

A. housing means open at the front end thereof and defining a cavity,

B. a reflective surface within said cavity facing the front end of said housing means to reflect energy beams transmitted along said paths,

C. a spectral filter mounted in said cavity in spaced relation to the reflective surface for receiving reflected energy beams,

D. a detector mounted within said cavity adjacent the spectral filter to receive the radiation beams through said spectral filter,

E. sealing means cooperating with said housing means to provide an air tight seal to said cavity and permitting energy beams to enter through the front end of said housing means to engage the reflective surface and be reflected therefrom towards the spectral filter, and

F. masking means contained within said cavity in the form of a spectrally absorbing gas so as to eliminate the effect of an interfering gas that is contained in spectral proximity to the gas of interest in the sample gas such that the gas having spectral absorbing lines which overlap in the region covered by the spectral filter and specifically tuned to the absorption spectrum of the gas of interest is substantially eliminated.

2. In an optical gas analyzer as defined in claim 1, wherein the pressure of the spectrally absorbing gas in the cavity is in the range of 1 to 25 psi.

3. In an optical gas analyzer as defined in claim 2, wherein the pressure is approximately 5 psi.

4. In an optical gas analyzer as defined in claim 1, wherein the spectrally absorbing gas is carbon dioxide.

5. In an optical gas analyzer as defined in claim 1, wherein said masking means includes:

a. an inlet port opening into the cavity,

b. an inlet valve communicating with the inlet port,

c. an outlet port opening into the cavity, and

d. an outlet valve communicating with the outlet port, such that the cavity may be filled by having the gas enter through the inlet valve and the gas in the cavity purged through said outlet valve.

6. In an optical gas analyzer as defined in claim 1, wherein said sealing means includes

a. a plate extending across the open end of said housing means in sealed relation thereto and having spaced apart apertures extending substantially in axial alignment with the sample and reference chambers, and

b. a transparent member mounted in sealed relationship to each of said apertures that permits the energy beams to pass therethrough.

7. In an optical gas analyzer as defined in claim 6, wherein said outlet valve is calibrated to release gas from the cavity at a preselected pressure to avoid rupturing said transparent members.

8. In an optical gas analyzer as defined in claim 1, wherein the radiation projector means includes:

a. casing means open at the front end thereof and defining a cavity,

b. an infrared emitting element extending within the cavity,

c. an interior reflective surface within the cavity to reflect beams from the emitting element through the dual optical paths of the sample and reference chambers to the detector means, and

d. baffle means mounted in front of the infrared element to prevent buffeting by air currents so as to reduce to a minimum the optical noise which is generated by the infrared emitting element.

9. In an optical gas analyzer as defined in claim 8, wherein said baffle means includes:

a. a plate extending across the open end of said casing means in fixed relation thereto, having spaced apart apertures extending substantially in axial alignment with the sample and reference chambers, and

b. a transparent baffle member mounted in fixed relationship to each of said apertures that permits the energy beams to pass therethrough.

10. In an optical gas analyzer as defined in claim 9,

a. wherein said casing means is mounted in sealed relation to the plate, and

b. wherein the baffle members are mounted in sealed relation to the plate.

11. In an optical gas analyzer as defined in claim 10, wherein said baffle members are contained in seats on the plate.

12. In an optical gas analyzer as defined in claim 1,

a. wherein the sample chamber is essentially open to the atmosphere and subject to its pressure variations,

b. further including electronic means connected to the detector means for amplifying the detector signal and transforming it into an electronic signal for driving display means to measure the concentration of the sample gas passing through the sample chamber and absorbing the radiant energy in relation to the number of the gas molecules present along the optical path of the sample chamber, and dependent on the ambient atmospheric pressure to which effects the sample gas is subjected such that the number of sample gas molecules present in the optical path through the sample chamber will vary in inverse proportion to the ambient atmospheric pressure, and

c. attenuation means connected to said electronic means and so calibrated that the effect of adjusting of the attenuation means is to modify the electronic signal for driving the display means such that the indication shown by the display means is altered to read that value which would have been indicated under conditions of standard atmospheric pressure on the same sample gas.

13. In an optical gas analyzer as defined in claim 12, wherein said attenuation means includes:

a. an adjustable potentiometer having a dial for manually adjusting same, and

b. a calibrated dial having markings showing the correct settings for various ambient barometric pressures for which the dial may be positioned.

14. In an optical gas analyzer as defined in claim 13, wherein said potentiometer is connected to the output of the electronic means to modify the electronic signal driving the display means to a value normalized to unity at a potentiometer calibrated setting corresponding to a selected pressure in inches of mercury, and wherein the adjustment of the potentiometer above and below said calibrated setting alters the electronic signal driving the display in a ratio appropriate to compensate the reading for changes in ambient barometric pressure.

15. In an optical gas analyzer as defined in claim 13, wherein said selected calibrated setting corresponds to 30 inches of mercury at sea level.

16. In an optical gas analyzer as defined in claim 13, wherein said calibrated setting is adjustable in the range from approximately 25 to 31 inches of mercury.

17. In an optical gas analyzer as defined in claim 13, wherein said potentiometer has an arm extending exteriorally of the gas analyzer for manual adjustment by the user.

18. In an optical gas analyzer as defined in claim 1,

a. wherein said detector receives a first beam of energy through the sample chamber and a seconded beam of energy through the reference chamber for providing a sample signal voltage when said first beam impinges thereon and a reference signal voltage when said second beam impinges thereon;

b. wherein the means for chopping includes interrupter means disposed in the path of said first and second infrared beams for sequentially interrupting said beams;

c. automatic gain control means coupled to said detector for providing a constant system gain between successive signal measurements;

d. means for developing a first sync voltage displaced in time and related to said sample signal voltage and a second sync voltage displaced in time and related to said reference signal voltage;

e. means for combining said first and second sync voltages for providing a zero sync signal voltage;

f. clamping means coupled to said combining means and said automatic gain control means for clamping the output signal voltage from said automatic gain control means during the high level on the zero sync signal voltage;

g. inverting amplifier means coupled to said clamping means for providing said output signal in an inverted polarity;

h. first and second switching means, said first switching means being coupled to said inverting amplifier and being activated by said sample signal voltage, said second switching means being coupled to said clamping means and being activated by said reference signal voltage, said first and second switching means providing a time multiplexing of said direct and inverted signal voltages;

i. balancing means coupled to said direct and inverted signal voltages for equalizing the amplitudes of said direct and inverted signal voltages;

j. integration means coupled to said balancing means for providing a DC voltage proportional to the amplitude of said sample signal voltage;

k. display means coupled to said integration means for displaying said DC voltage in a suitable manner;

l. comparator means coupled to said balancing means and a reference voltage for comparing the amplitude of said direct signal voltage with said reference voltage and providing an output voltage related to the difference therebetween; and

m. error detector means coupled between said comparator means and said automatic gain control means for coupling the comparator means output voltage to said automatic gain control means in the proper polarity for stabalizing the gain thereof.

19. In an optical gas analyzer as defined in claim 18,

a. wherein said comparator means includes adjustment means for setting the value of the reference voltage, and

b. wherein said preamplifier means is coupled between said photoelectric detector and said automatic gain control means for amplifying said sample and reference signal voltages.

20. In an optical gas analyzer as defined in claim 18, wherein said display means further includes means for introducing calibrated non-linear distortions and means for digitizing said DC voltage.

21. In an optical gas analyzer as defined in claim 1,

a. wherein the radiation means is an infrared source of energy disposed on one end of the sample chamber for emitting a first beam of infrared energy through the sample chamber, and the reference chamber is disposed proximate said infrared source of energy for receiving a second beam of infrared energy therethrough;

b. wherein said detector is disposed to receive said first and second beam of infrared energy for providing a sample signal voltage when said first beam impinges thereon and a reference signal voltage when said second beam impinges thereon;

c. wherein said means for chopping includes first and second infrared beams for sequentially interrupting said beams;

d. further including means for the generation of a signal waveform at the detector, and timing wave forms by said interrupter means including a trapezoidal waveform comprised of two similar lobes per chopping cycle having flat top portions corresponding to detector dark periods, flat bottom portions corresponding to maximum irradiance in each of the two lobes, one being due to energy passing through the reference chamber, the other the sample chamber, rise and fall slopes of essentially equal durations, separate timing waveforms developed from said interrupter means and arranged to extend over each of the reference and sample lobes separately and respectively, an auxiliary zero sync waveform synthesized from the reference and sample waveforms, all in such relationship that the reference lobe extends slightly into the dark level flat top portion ahead and behind the reference lobe, and the sample sync pulse extends over the entire duration of the sample lobe and slightly into the dark level flat top portion ahead and behind the sample lobe, and the zero sync pulse synthesized from the two exists within the time gaps between the two and consequently over periods of slightly less than the dark level durations manifested by the corresponding flat tops of the detector signal waveform, this arrangement being specifically valuable and necessary for providing adequate tolerances in mechanical relationship and electronic timing to make further adjustments uncritical,

e. means for electronically processing the detector signal in conjunction with the reference, sample, and zero sync signals, and

f. readout means in circuit relationship to the signal attenuation through the sample chamber to the unattenuated value when sample gas is not present, and calibrate the same to read the concentration of gas specific to the wave length of the detector with its associated filter proportional to the signal difference between the value corresponding to no attenuation without sample gas and the value with the sample gas filling the sample cell, thereby enabling the gas analyzer to read out the concentration of the sample gas.

22. In an optical gas analyzer including in combination, a sample chamber, a reference chamber, to provide dual optical paths one for analysis of a sample gas, the other for reference, radiation projector means mounted at one end of said chambers, means for chopping and directing the radiation beams alternately along the two paths, the improvement which comprises radiation detector means positioned at the other end of said chambers comprising:

A. housing means open at the front end thereof and defining a cavity,

B. a reflective surface within said cavity facing the front end of said housing means to reflect energy beams transmitted along said paths,

C. a spectral filter mounted in said cavity in spaced relation to the reflective surface for receiving reflected energy beams,

D. detector means mounted within said cavity adjacent the spectral filter,

E. sealing means cooperating with said housing means to provide an air tight seal to said cavity and permitting energy beams to enter through the front end of said housing means to engage the reflective surface and be reflected therefrom towards the spectral filter, wherein said sealing means includes;

1. a plate extending across the open end of said housing means in sealed relation thereto and having spaced apart apertures extending substantially in axial alignment with the sample and reference chambers, and

2. a transparent member mounted in sealed relationship to each of said apertures that permits the energy beams to pass therethrough, and

F. masking means contained within said cavity in the form of a spectrally absorbing gas so as to eliminate the effect of an interfering gas that is contained in spectral proximity to the gas of interest in the sample gas such that the gas having spectral absorbing lines which overlap in the region covered by the spectral filter and specifically tuned to the absorption spectrum of the gas of interest is substantially eliminated, wherein said masking means includes;

24. In an optical gas analyzer as defined in claim 22, wherein the radiation projector means includes:

a. casing means open at the front end thereof and defining a cavity,

b. an infrared emitting element extending within the cavity,

c. an interior reflective surface within the cavity to reflect beams from the emitting element through the dual optical paths of the sample and reference chambers to the detector means, and

d. baffle means mounted in front of the infrared element to prevent buffeting by air currents so as to reduce to a minimum the optical noise which is generated by the infrared emitting element, wherein said baffle means includes;

1. a plate extending across the open end of said casing means in fixed relation thereto, having spaced apart apertures extending substantially in axial alignment with the sample and reference chambers, and

2. a transparent baffle member mounted in fixed relationship to each of said apertures that permits the energy beams to pass therethrough.

25. In an optical analyzer as defined in claim 22,

a. wherein the sample chamber is essentially open to the atmosphere and subject to its pressure variations,

b. further including electronic means connected to the detector means for amplifying the detector signal and transforming it into an electronic signal for driving display means to measure the concentration of the sample gas passing through the sample chamber and absorbing the radiant energy in relation to the number of the gas molecules present along the optical path of the sample chamber, and dependent on the ambient atmospheric pressure to which effects the sample gas is subjected such that the number of sample gas molecules present in the optical path through the sample chamber will vary in inverse proportion to the ambient atmospheric pressure, and

c. attentuation means connected to said electronic means and so calibrated that the effect of adjusting of the attenuation means is to modify the electronic signal for driving the display means such that the indication shown by the display means is altered to read that value which would have been indicated under conditions of standard atmospheric pressure on the same sample gas.

27. In an optical gas analyzer as defined in claim 25, wherein said baffle members are contained in seats on the plate.

28. In an optical gas analyzer as defined in claim 25,

a. wherein the infrared emitting element disposed on one end of the sample chamber emits a first beam of infrared energy through the sample chamber, and a second beam of energy through the reference chamber;

b. wherein the detector means is disposed to receive said first and second beam of energy for providing a sample signal voltage when said first beam impinges thereon and a reference signal voltage when said second beam impinges thereon;

c. wherein the chopping means includes interrupter means disposed in the path of said first and second infrared beams for sequentially interrupting said beams;

d. automatic gain control means coupled to said detector means for providing a constant system gain between successive signal measurements;

e. means for developing a first sync voltage displaced in time and related to said sample signal voltage and a second sync voltage displaced in time and related to said reference signal voltage;

f. means for combining said first and second sync voltages for providing a zero sync signal voltage;

g. clamping means coupled to said combining means and said automatic gain control means for clamping the output signal voltage from said automatic gain control means during the high level on the zero sync signal voltage;

h. inverting amplifier means coupled to said clamping means for providing said output signal voltage in an inverted polarity;

i. first and second switching means, said first switching means being coupled to said inverting amplifier and being activated by said sample signal voltage, said second switching means being coupled to said clamping means and being activated by said reference signal voltage, said first and second switching means providing a time multiplexing of said direct and inverted signal voltages;

j. balancing means coupled to said direct and inverted signal voltages;

k. integration means coupled to said balancing means for providing a DC voltage proportional to the amplitude of said simple signal voltage;

l. display means coupled to said integration means for displaying said DC voltage in a suitable manner;

m. comparator means coupled to said balancing means and a reference voltage for comparing the amplitude of said direct signal voltage with said reference voltage and providing an output voltage related to the difference therebetween; and

n. error detector means coupled between said comparator means and said automatic gain control means for coupling the comparator means output voltage to said automatic gain control means in the proper polarity for stabalizing the gain thereof.

29. In an optical gas analyzer as defined in claim 27, wherein said comparator means includes adjustment means for setting the value of the reference voltage.

30. In an optical gas analyzer as defined in claim 27, wherein said preamplifier means is coupled between said first photoelectric detector and said automatic gain control means for amplifying said sample and reference signal voltages.

31. In an optical gas analyzer as defined in claim 27, wherein said display means further includes means for introducing calibrated non-linear distortions and means for digitizing said DC voltage.

33. An optical gas analyzer as defined in claim 31, wherein said interrupter means includes a chopper disc having circumferentially displaced apertures therein, said apertures being adapted to sequentially interrupt said infrared beams as said disc is journaled about its axis.

35. An optical gas analyzer as defined in claim 33, further including a second and third light source and a second and third detector, said light sources and said detectors being disposed to cooperate with said reflective stripes to provide said first and second sync voltage.

37. An optical gas analyzer as defined in claim 31, wherein said display means further includes means for introducing calibrated non-linear distortions and means for digitizing said DC voltage.

38. An optical gas analyzer as defined in claim 31, and further including:

a. housing means open at the front end thereof and defining a cavity,

b. a reflective surface within said cavity facing the front end of said housing means to reflect the first and second beams of energy,

c. a spectral filter mounted in said cavity in spaced relation to the reflective surface for receiving reflected energy beams.

d. said detector mounted within said cavity adjacent the spectral filter,

e. sealing means cooperating with said housing means to provide an air tight seal to said cavity and permitting the energy beams to enter through the front end of said housing means to engage the reflective surface and be reflected therefrom towards the spectral filter, and

f. masking means contained within said cavity in the form of a spectrally absorbing gas so as to eliminate the effect of an interfering gas that is contained in spectral proximity to the gas of interest in the sample gas such that the gas having spectral absorbing lines which overlap in the region covered by the spectral filter and specifically tuned to the absorption spectrum of the gas of interest is substantially eliminated.

39. An optical gas analyzer as defined in claim 37, wherein said masking means includes:

a. an inlet port opening into the cavity,

b. an inlet valve communicating with the inlet port,

c. an outlet port opening into the cavity, and

d. an outlet valve communicating with the outlet port, such that the cavity may be filled by having the gas enter through the inlet valve and the gas in the cavity purged through said outlet valve.

40. An optical gas analyzer as defined in claim 37, wherein said sealing means includes:

a. a plate extending across the open end of said housing means in sealed relation thereto and having spaced apart apertures extending substantially in axial alignment with the sample and reference chambers, and

b. a transparent member mounted in sealed relationship to each of said apertures that permits the energy beams to pass therethrough.

41. An optical gas analyzer as defined in claim 31, and further including:

a. casing means open at the front end thereof and defining a cavity,

b. said infrared source extending within the cavity,

c. an interior reflective surface within the cavity to reflect means from the emitting element through the sample and reference chambers to the detector, and

d. baffle means mounted in front of the infrared source to prevent buffeting by air currents so as to reduce to a minimum the optical noise which is generated by the infrared source.

42. An optical gas analyzer as defined in claim 40, wherein said baffle means includes:

a. a plate extending across the open end of said casing means in fixed relation thereto, having spaced apart apertures extending substantially in axial alignment with the sample and reference chambers, and

b. a transparent baffle member mounted in fixed relationship to each of said apertures that permits the energy beams to pass therethrough.

44. The method as defined in claim 42, and further including the step of baffling said source of infrared energy to prevent buffeting by air currents, whereby the optical noise is reduced to a minimum.

23. an inlet port opening into the cavity,an inlet valve communicating with the inlet port,an outlet port opening into the cavity, andan outlet valve communicating with the outlet port, such that the cavity may be filled by having the gas enter through the inlet valve and the gas in the cavity purged through said outlet valve.

26. In an optical gas analyzer including in combination, a sample chamber, a reference chamber to provide dual optical paths one for analysis of a sample gas, the other for reference, means for alternately chopping the two paths, detector means positioned at one end of the dual optical paths, the improvement which comprises radiation projector means positioned at the other end of the dual optical paths comprising:

A. casing means open at the front end thereof and defining a cavity,

B. an infrared emitting element extending within the cavity,

C. an interior reflective surface within the cavity to reflect means from the emitting element through the dual optical paths of the sample and reference chambers to the detector means,

D. a plate extending across the open end of said casing means in fixed relation thereto, having spaced apart apertures extending substantially in axial alignment with the sample and reference chambers,

E. means for mounting said plate in sealed relation to said front end of said casing means,

F. baffle means mounted on said plate in front of the infrared element to prevent buffeting by air currents so as to reduce to a minimum the optical noise which is generated by the infrared emitting element, and

G. said baffle means comprises a transparent baffle member mounted in fixed sealed relationship to each one of said apertures such that each said baffle member permits the energy beams to pass therethrough.

32. An optical gas analyzer for determining the concentration of a gas in a sample chamber, comprising:

A. an infrared source of energy disposed on one end of said sample chamber for emitting a first beam of infrared energy through said sample chamber;

B. a reference chamber disposed proximate said infrared source of energy for receiving a second beam of infrared energy therethrough;

C. a first photoelectric detector disposed to receive said first and second beam of infrared energy for providing a sample signal voltage when said first beam impinges thereon and a reference signal voltage when said second beam impinges thereon;

D. interrupter means disposed in the path of said first and second infrared beams for sequentially interrupting said beams;

E. automatic gain control means coupled to said first detector for providing a constant system gain between successive signal measurements;

F. means for developing a first sync voltage displaced in time and related to said sample signal voltage and a second sync voltage displaced in time and related to said reference signal voltage;

G. means for combining said first and second sync voltages for providing a zero sync signal voltages;

H. clamping means coupled to said combining means and said automatic gain control means for clamping the output signal voltage from said automatic gain control means during the high level on the zero sync signal voltage;

I. inverting amplifier means coupled to said clamping means for providing said output signal voltage in an inverted polarity;

J. first and second switching means, said first switching means being coupled to said inverting amplifier and being activated by said sample signal voltage, said second switching means being coupled to said clamping means and being activated by said reference signal voltage, said first and second switching means providing a time multiplexing of said direct and inverted signal voltages;

K. balancing means coupled to said direct and inverted signal voltages for equalizing the amplitudes of said direct and inverted signal voltages;

L. integration means coupled to said balancing means for providing a DC voltage proportional to the amplitude of said sample signal voltage;

M. display means coupled to said interruption means for displaying said DC voltages in a suitable manner;

N. comparator means coupled to said balancing means and a reference voltage for comparing the amplitude of said direct signal voltage with said reference voltage and providing an output voltage related to the difference therebetween; and

O. error detector means coupled between said comparator means and said automatic gain control means for coupling the comparator means output voltage to said automatic gain control means in the proper polarity for stabalizing the gain thereof.

34. An optical gas analyzer as defined in claim 32, wherein said chopper disc further includes reflective stripes.

36. An optical gas analyzer as defined in claim 34, wherein three sets of apertures and three sets of stripes are provided on said chopper disc.

43. The method of obtaining a DC output voltage related to the amount of concentration of a gas in a sample chamber comprising the steps of:

A. providing a source of infrared energy disposed on one end of said sample chamber, said energy source emitting a first beam of energy through said chamber;

B. providing a reference chamber disposed proximate said source of energy for receiving a second beam of infrared energy therethrough;

C. providing a sample signal voltage and a reference signal voltage when said first and second beam respectively impinges upon a first photoelectric detector;

D. interrupting said first and second infrared beam in a sequential manner;

E. stabilizing the gain between successive signal measurement of the first detector output voltage by automatic gain control circuit means;

F. developing a first sync voltage displaced in time and related to said sample signal voltage and a second sync voltage displaced in time and related to said reference signal voltage;

G. combining said first and second sync voltages to provide a zero sync signal voltage;

H. clamping the output signal voltage from said automatic gain control circuit means during the high level on the zero sync signal voltage;

I. inverting said output signal voltage;

J. providing a time multiplexing of said direct and inverted signal voltages;

K. equalizing the amplitudes of said direct and inverted signal voltages;

L. integrating said sample signal voltage to provide a DC voltage proportional to the amplitude of said signal voltage;

M. displaying said DC voltage;

N. comparing the amplitude of said direct signal voltage with said reference voltage to provide an output voltage related to the difference therebetween; and

O. coupling said difference voltage to said automatic gain control means in the proper polarity for stabilizing the gain thereof.

45. The method as defined in claim 43, wherein said step of baffling includes:

a. providing a plate extending across the source of infrared energy, and

b. selecting said plate so that the beams of energy can pass therethrough.