Saturation Filtering NDIR Gas Sensing Methodology

• US 20100258728A1
• Filed: 04/13/2010
• Published: 10/14/2010
• Est. Priority Date: 04/14/2009
• Status: Active Grant

• Current Assignee: Airware, Inc.
• Original Assignee: Airware, Inc.
• Inventors: Wong, Jacob Y.
• Granted Patent: US 8,003,944 B2
• Time in Patent Office: Days
• US Class Current: 250/339.130
• CPC Class Codes: G01N 2021/3527 and using one filter cell as attenuator
• CPC Class Codes: G01N 21/274 Calibration, base line adjustment, drift correction
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis
• CPC Class Codes: G01N 21/61 Non-dispersive gas analysers G01N21/3504 takes precedence
• CPC Class Codes: G01N 2201/0231 Thermostating

Abstract

NDIR gas sensing methodology is advanced which renders the output of an NDIR gas sensor, when implemented with this new methodology, to remain stable or drift-free over time. Furthermore, the output of such a sensor will also be independent of the temperature of an environ wherein the sensor is in physical contact. This method utilizes the same narrow band-pass spectral filter for the detection of the gas of interest for both the signal and the reference channels. By so doing, the two channels always receive radiation of the same spectral content from the infrared source of the sensor convoluted with that from any external elements exposed to the sensor. While the same sample chamber through which the gas of interest to be detected flows is shared by the two channels, the detector package for the reference channel is hermetically sealed with 100% of the gas to be detected instead of 100% N₂ as for the signal detector. In so doing, the reference channel is rendered almost completely “blind” to the presence or absence of the gas of interest flowing in the common sample chamber thereby creating an absorption bias or difference between the two channels enabling the concentration of the gas of interest to be detected by ratioing the outputs of the two channels via calibration.

Claims

1. Apparatus for detecting a gas of interest utilizing a non-dispersive infrared sensor, the apparatus comprising:

a signal channel with a signal detector;

a reference channel with a reference detector and a saturation cell containing the gas of interest; and

a single narrow band-pass spectral filter used for both the signal channel and the a reference channel;

wherein both the signal detector and the reference detector are mounted on a single thermal platform.

2. The apparatus of claim 1 wherein a reference channel signal remains substantially unchanged irrespective of the absence or presence of the gas to be detected in any concentration level in the sample chamber.

3. The apparatus of claim 1 wherein the saturation cell has a length L′

and contains a concentration c′

of the gas of interest such that a resulting saturation cell concentration of the gas of interest of (L′

)×

(c′

) is at least an order of magnitude greater than a sample cell concentration of the gas of interest of (L)×

(c) where L is a length of a sample cell of the signal channel and c is a maximum concentration of the gas of interest in the sample cell.

4. The apparatus of claim 1 wherein the reference detector and the signal detector are mounted in a single structure of a heat transmitting material.

5. The apparatus of claim 4 wherein the heat transmitting material is comprised of aluminum.

6. A non-dispersive infrared sensor, comprising:

a sample chamber through which a gas of interest flows;

an infrared source;

a spectral filter;

a reference detector for detecting a reference channel signal;

a saturation cell;

a signal detector for detecting a signal channel signal; and

electronics for processing a ratio of the signal channel signal to the reference channel signal;

wherein the spectral filter has a narrow spectral pass-band coincident with a chosen absorption band of the gas of interest;

wherein the infrared source, the spectral filter, the saturation cell, the sample chamber and the reference detector form a reference channel;

wherein the infrared source, the spectral filter, the sample chamber and the signal detector form a signal channel; and

wherein the reference detector and the signal detector are mounted on a single thermal platform so that their temperatures track one another.

7. The non-dispersive infrared sensor of claim 6 wherein the reference channel signal remains substantially unchanged irrespective of the absence or presence of the gas to be detected in any concentration level in the sample chamber.

8. The non-dispersive infrared sensor of claim 6 wherein the saturation cell has a length L′

and contains a concentration c′

of the gas of interest such that a resulting saturation cell concentration of the gas of interest of (L′

)×

(c′

) is at least an order of magnitude greater than a sample cell concentration of the gas of interest of (L)×

(c) where L is a length of the sample cell and c is a maximum concentration of the gas of interest in the sample cell.

9. The non-dispersive infrared sensor of claim 6 wherein the chosen absorption band of the gas of interest is a strong absorption band.

10. The non-dispersive infrared sensor of claim 9 wherein the gas of interest is water vapor.

11. The non-dispersive infrared sensor of claim 6 wherein the gas of interest is a hydrocarbon.

12. The non-dispersive infrared sensor of claim 6 wherein the gas of interest is carbon dioxide.

13. The non-dispersive infrared sensor of claim 6 wherein the reference detector and the signal detector are mounted in a single structure of a heat transmitting material.

14. The non-dispersive infrared sensor of claim 6 wherein the single thermal platform is an aluminum structure.

15. A method comprising utilizing a single narrow band-pass spectral filter in a non-dispersive infrared sensor for both a signal channel and a reference channel used to detect a gas of interest wherein the reference channel also contains a saturation cell containing the gas of interest and both a signal detector and a reference detector are mounted on a single thermal platform.

16. The method of claim 15 wherein a reference channel signal remains substantially unchanged irrespective of the absence or presence of the gas to be detected in any concentration level in a sample chamber of the non-dispersive infrared sensor.

17. The method of claim 15 wherein the saturation cell has a length L′

and contains a concentration c′

of the gas of interest such that a resulting saturation cell concentration of the gas of interest of (L′

)×

(c′

) is at least an order of magnitude greater than a sample cell concentration of the gas of interest of (L)×

(c) where L is a length of a sample cell of the signal channel and c is a maximum concentration of the gas of interest in the sample cell.