Method and apparatus for negating measurement effects of interferent gases in non-dispersive infrared analyzers

• US 4,355,233 A
• Filed: 01/31/1980
• Issued: 10/19/1982
• Est. Priority Date: 02/22/1979
• Status: Expired due to Term

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Beckman Instruments, Inc. (Danaher Corporation)
• Inventors: Keenan, Charles A., Cederstrand, Carl N., Warnke, Dale F.
• Primary Examiner(s): Smith, Alfred E.
• Assistant Examiner(s): Howell, Janice A.
• Application Number:
  US06/117,175
• Time in Patent Office: 992 Days
• Field of Search: 250/343, 250/344, 250/345, 250/339, 250/340, 250/373
• US Class Current: 250/343
• CPC Class Codes: G01N 21/37 using pneumatic detection opto-acoustic detection G01N21/1702

Abstract

A method and apparatus is disclosed for selectively determining by non-dispersive infrared techniques the concentration of a gaseous constituent in a sample gas mixture while determining and discounting deleterious effects of spectrally interfering gases in the sample gas mixture.

Claims

1. In a non-dispersive infrared analyzer having a pneumatic detector measuring the concentration of a preselected component gas by infrared energy absorption and producing a first signal representing component concentration, apparatus for negating the measurement effects of interferent gases comprising:

response means responsive to infrared energy for detecting the infrared energy absorbed by an interferent gas which absorbs infrared energy in the bandwidth of said preselected component gas and producing a second signal in response thereto;

filter means associated with said response means for permitting passage of an infrared bandwidth absorbed by said interferent gases while rejecting infrared bandwidths absorbed by said preselected component gas;

rectifier means connected to said response means for receiving said second signal and producing a substantially rectified third signal;

scaling means connected to said rectifier means for receiving said third signal and scaling said third signal to be compatible with said first signal; and

combining means connected to said scaling means and said pneumatic gas analyzer for combining said scaled third signal and said first signal and producing an output representative of said preselected component gas.

2. The apparatus according to claim 1 wherein said response means comprises a pyroelectric sensor.

3. The apparatus according to claim 1 wherein said response means comprises a black body detector.

4. The apparatus of claim 2 or 3 wherein said filter means comprises an interference filter on a quartz substrate responsive to a bandwidth of wavelengths having its center wavelength at 2.7 microns.

5. The apparatus of claims 2 or 3 wherein said filter means comprises an interference filter on a quartz substrate responsive to a bandwidth of wavelengths having its center wavelength at 2.6 microns.

6. The apparatus according to claims 2 or 3 wherein said rectifier means comprises a synchronous rectifier.

7. A method for increasing accuracy in a system having a non-dispersive infrared analyzer with a pneumatic detector for measuring the concentration of a preselected component gas by infrared energy absorption and producing a first signal representing component concentration, said method comprising the steps of:

permitting passage of an infrared bandwidth absorbed by an interferent gas to be incident upon a detector while rejecting infrared bandwidths absorbed by said preselected component gas;

detecting the infrared energy absorbed by said interferent gas which absorbs infrared heat energy in the bandwidth of said preselected component gas and producing a second signal in response thereto;

receiving said second signal and producing a substantially rectified third signal;

receiving said third signal and scaling said third signal to be compatible with said first signal; and

combining said third signal and said first signal to provide an output representative of said preselected component gas.

8. The method of claim 7 including the additional steps of detecting the infrared energy absorbed by a second interferent gas which absorbs infrared energy in the bandwidth of said preselected component gas and producing a fourth signal in response thereto;

permitting passage of an infrared bandwidth absorbed by said second interferent gas while rejecting infrared bandwidths absorbed by said preselected component gas;

receiving said fourth signal and producing a substantially rectified fifth signal;

receiving said fifth signal and scaling said fifth signal to be compatible with said first signal; and

combining said fifth signal with said scaled third signal and said first signal and producing an output representative of said preselected component gas.

9. In a nondispersive infrared analyzer for measuring the quantity of carbon monoxide with spectrally interferring component gases, an apparatus for negating effects of interferent gases comprising:

first means for measuring said quantity of said carbon monoxide with said effects of said interferent gases and producing a first output representative thereof;

solid state detector for measuring said effects of said interferent gases and producing a second output representative thereof; and

second means for combining said first output and said second output and producing a signal representative of said quantity of said carbon monoxide.

10. The apparatus of claim 9 wherein said second means comprises a process circuit including a rectifier, and scaler associated therewith.

11. The apparatus of claim 10 wherein said solid state detector comprises a pyroelectric sensor.

12. The apparatus of claim 10 wherein said solid state detector comprises a black body detector.

13. The apparatus of claim 10 wherein said process circuit also includes a linearization circuit.

14. In a dual-beam infrared radiant energy analyzer of the type having a reference and sample radiation path and a pneumatic detector sensitized to detect radiation absorbed by gas of interest in a first selected wavelength region and producing a detector signal proportional to the concentration of said gas of interest plus any interferent gas present in the sample having an absorption band in said first selected wavelength region, the improvement comprising:

filter means disposed in said sample radiation path, said filter means having a narrow infrared radiant energy band pass in a second wavelength region where said interferent gas also has an absorption band and said constituent gas has no appreciable absorption band;

solid state detector means disposed adjacent said filter means and producing an electrical signal proportional to the concentration of said interferent gas; and

circuit means connected to said pneumatic detector and to said solid state detector means for producing an output signal proportional to said constituent gas.

15. The infrared radiant energy analyzer according to claim 14 further comprising:

second filter means disposed in said sample radiation path, said second filter means having a narrow, infrared radiant energy band pass in a third region where a second interferent gas has an absorption band and said constituent gas has no appreciable absorption band;

second solid state detector means disposed adjacent said second filter means and producing a second electrical signal proportional to the concentration of said second interferent gas; and

means for connecting said second solid state detector means to said circuit means whereby said circuit means produces an output signal proportional to said constituent gas.

16. The infrared radiant energy analyzer according to claim 15 wherein said first filter means has a bandwidth with a center point of 2.694 microns and a bandwidth of 0.04 microns.

17. The infrared radiant energy analyzer according to claim 16 wherein said second filter means has a bandwidth having a center point of 2.6 microns with a bandwidth of 0.03 microns.