Gas discriminating gas detector system and method

• US 6,545,278 B1
• Filed: 04/23/1999
• Issued: 04/08/2003
• Est. Priority Date: 04/23/1999
• Status: Expired due to Term

• Current Assignee: Delphian Corporation (Meson Group, Inc.)
• Original Assignee: Delphian Corporation (Meson Group, Inc.)
• Inventors: Mottier, François, Bruce, Scott
• Primary Examiner(s): Epps, Georgia
• Assistant Examiner(s): HANIG, RICHARD E
• Application Number:
  US09/298,595
• Time in Patent Office: 1,446 Days
• Field of Search: 250/339.13, 250/339.12, 250/341.1, 250/341.5, 250/343
• US Class Current: 250/339.13
• CPC Class Codes: G01J 3/42 Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry beam switching arrangements G01J3/08
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis

Abstract

A gas detection system and method are provided for determining the type of gas present and the concentration of the gas in terms of a percentage of a specific level of the gas. This percentage level may be in terms of a lower explosive limit level of a combustible gas. The system generally includes a testing chamber, a detection channel apparatus, a memory for storing a plurality of gas signature tables, and a programmed controller for determining the identity and concentration level of a gas. The system and method further provide for the accurate determination of gas concentration level at very high and low concentration levels by weighted average computation.

Claims

1. An apparatus for detecting and measuring gases to be tested, comprising:

a chamber for receiving an air/gas mixture to be tested;

a detection channel apparatus comprising one or more light sources for directing light through the air/gas mixture to one or more light detectors to define a plurality of detection channels each responsive to a predetermined portion of the bandwidth of light passing through the air/gas mixture and for generating signals representing an amount of light received for each bandwidth portion;

a memory storing a plurality of gas signature tables one for each of a plurality of gases to be identified and each signature table comprising a plurality of signature values representing signals to be received from the detection channels at a plurality of air/gas concentrations and further representing gas concentration levels corresponding to a percentage of the lower explosive limit level for each gas; and

a programmed controller for receiving the signals from the plurality of detection channels, for comparing the received signals with the gas signature tables stored in the memory to identify a gas and the concentration of the identified gas, and for determining whether the concentration of the gas is at or near a lower explosive limit level.

2. The apparatus of claim 1 wherein the detection channel apparatus comprises

a light source for directing light through the air/gas mixture; and

a plurality of detectors each for receiving a predetermined portion of the bandwidth of light passing through the air/gas mixture and for generating signals representing an amount of light received.

3. The apparatus of claim 2 comprising a multiplexed analog to digital converter connected to receive signals from the plurality of detectors and apparatus for periodically sampling digital values from the analog to digital converter.

4. The apparatus of claim 1 wherein the memory comprises storage means for storing normalizing values representing the value of detection channel signals when no gas to be tested is present and the controller comprises an apparatus for normalizing detection channel signals when a test gas is expected by dividing the detector signals by the normalizing values.

5. The apparatus of claim 4 comprising means for initiating a start-up routine by the controller in which normalizing values are identified from the detection channel signals and stored in the memory.

6. The apparatus of claim 1 comprising a display connected to the controller for displaying an indicator of at least one gas present and the concentrations of the displayed gas.

7. The apparatus of claim 2 wherein the detectors each comprise a light filter having a predetermined center frequency and bandwidth and circuitry for producing a signal representing the amount of light in the frequency band defined by the filter.

8. The apparatus in accordance with claim 7 comprising at least a first infrared detector having a center wavelength substantially equal to 3.25 microns and a bandwidth substantially equal to 0.04 microns, a second infrared detector having a center wavelength substantially equal to 3.35 microns and a bandwidth substantially equal to 0.04 microns and a third infrared detector having a center wavelength substantially equal to 3.40 microns and a bandwidth substantially equal to 0.04 microns.

9. The apparatus of claim 8 comprising a fourth infrared detector having a center wavelength substantially equal to 3.00 microns and a bandwidth substantially equal to 0.4 microns.

10. An apparatus in accordance with claim 1 wherein the chamber comprises a gas input port and a gas output port, and an apparatus for causing gas flow through the chamber between the input port and output port.

11. An apparatus in accordance with claim 10 wherein:

the output port is arranged above the input port; and

the apparatus for causing gas flow is a heating apparatus.

12. The apparatus in accordance with claim 11 wherein the heating apparatus encircles the chamber at a point above the input port and above the one or more light sources.

13. The apparatus in accordance with claim 1 comprising at least one light diffuser between the light source and the detectors.

14. The apparatus in accordance with claim 13 wherein the at least one infrared diffuser comprises a PTFE diffuser having a thickness in the range of 10 microns to 5 millimeters.

15. The apparatus according to claim 13 comprising a first light diffuser in close proximity to the one or more light sources and a second light diffuser in close proximity to the one or more detectors and each of the first and second light diffusers comprise a PTFE diffuser having a thickness from 10 microns to 5 millimeters.

16. The apparatus according to claim 1 comprising a measuring head detachable from the programmed controller, the measuring head including the chamber, the detection channel apparatus and a data store for storing data from which signature tables may be derived for use by the programmed controller.

17. The apparatus according to claim 16 wherein the data store of the measuring head stores data describing signature tables for a set of gasses to be identified by detection channel signals from the detection channel apparatus of the measuring head.

18. The apparatus according to claim 1 wherein the detection channel apparatus comprises a plurality of light sources each for directing light in a predetermined portion of the light band through the air/gas mixture;

and

a detector responsive to light in all of the plurality of portions of the light band for generating detection channel signals.

19. The apparatus according to claim 18 comprising apparatus for turning the light sources on and off one at a time and the controller receives detection channel signals in synchronism with the turn on and turn off of the light sources.

20. The apparatus according to claim 1 comprising apparatus for measuring the temperature of the detectors and the controller adjusts signal values received from the detectors in accordance with the measured temperature.

34. The apparatus in accordance with claim 7 comprising at least a first infrared detector having a center wavelength substantially equal to 3.3 microns and a bandwidth substantially equal to 0.04 microns, a second infrared detector having a center wavelength substantially equal to 3.4 microns and a bandwidth substantially equal to 0.04 microns and a third infrared detector having a center wavelength substantially equal to 3.5 microns and a bandwidth substantially equal to 0.04 microns.

35. The apparatus in accordance with claim 7 comprising at least a first infrared detector having a center wavelength substantially equal to 3.28 microns and a bandwidth substantially equal to 0.04 microns, a second infrared detector having a center wavelength substantially equal to 3.38 microns and a bandwidth substantially equal to 0.04 microns and a third infrared detector having a center wavelength substantially equal to 3.46 microns and a bandwidth substantially equal to 0.04 microns.

21. In an apparatus for identifying gases and their concentrations comprising an infrared light source directing infrared light to a plurality of infrared bandwidth specific detectors through a gas under test and a controller, including a memory, for analyzing signals from the detectors representing absorption in a predetermined frequency bands by the gas under test, a method comprising:

storing in the memory a plurality of gas signature tables each representing a specific gas and expected signals from the detectors for various concentrations of the gas and further representing gas concentration levels corresponding to a percentage of the lower explosive limit level for each gas;

reading signals from the detectors to represent the current infrared absorption by a gas under test; and

comparing the signals read in the reading step with a plurality of the signature tables to identify the gas under test, to measure the concentration of the identified gas, and to determine whether the concentration of the gas is at or near a lower explosive limit level.

22. A method in accordance with claim 21, comprising

reading the signals from the detectors when no gas under test is present and storing normalizing values representing the detector signals when no gas under test is present.

23. A method in accordance with claim 22, comprising reading signals from the detectors when a gas under test is present and dividing the value of the signals so read by a normalizing value.

24. A method in accordance with claim 23 wherein the dividing step comprises dividing the value of signals detected from a given detector by a stored normalizing value corresponding to the same detector.

25. A method in accordance with claim 21 comprising identifying the altitude at which the apparatus is being used and correcting the concentration measured by a stored correction value.

26. A method in accordance with claim 21 comprising measuring the temperature at the detectors and correcting the concentrations measured by a stored correction value.

27. A method in accordance with claim 22 comprising storing in the apparatus a plurality of correction factors, each for a predetermined altitude of use;

and identifying to the apparatus the altitude of use to determine an appropriate correction for a measured concentration.

28. A method in accordance with claim 21 comprising:

storing in the gas signature tables a plurality of gas concentration levels and a value representing an infrared absorption level for each detector at each of the plurality of gas concentration levels.

29. A method in accordance with claim 28 comprising identifying a concentration level of a test gas by comparing normalized signal levels from the detectors with the values representing infrared absorption levels from the gas signature tables.

30. A method in accordance with claim 28 comprising identifying a gas concntration level by identifying a concentration level associated with each normalized signal level from the plurality of detectors and averaging the resultant concentration levels.

31. A method in accordance with claim 30 wherein the average of resultant concentration levels is derived by a weighted average method in which each detector level is assigned a weight determined by the change in absorption level divided by the change in gas concentration causing that change in absorption level within the bandwidth of the detector.

32. A method in accordance with claim 31 wherein a first, a second and a third detection channel are used;

the weight assigned to each detection channel W_K is computed by;

33. A method in accordance with claim 21 comprising assuming the identity of a detected gas when the comparing step fails to identify the gas under test and estimating the %LEL level of the detected gas from a signature table corresponding to the assumed gas.