What’s NDIR gas sensor?

A miniaturized NDIR gas sensor is manufactured using semiconductor micromachining techniques from a semiconductor material such as Si or GaAs. The NDIR gas sensor comprises an optical waveguide, a light source at one end of the waveguide, at least one light detector at the end of the waveguide opposite the light source, a diffusion type gas sample chamber formed within the waveguide and interposed in the optical path between the light source and light detector so that the light source and light detector are thermally isolated from the gas sample, and a separate bandpass filter interposed between the light source and each light detector. Because the NDIR sensor is fabricated out of a semiconductor material, the source driver and signal processing electronics may be added directly to the sensor using integrated circuit fabrication techniques. Particles and smoke and dust may be kept out of the sample chamber by application of a gas permeable membrane over apertures in the sample chamber walls.

Description
This is a continuation of co-pending application Ser. No. 08/195,523, filed on Feb. 14, 1994, now abandoned.

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to the field of gas sensing devices and, more particularly, to NDIR gas analyzers.
2. Description of the Prior Art
Nondispersive infrared (NDIR) gas analyzers are used for detecting the presence and concentration of various gases. The NDIR technique has long been considered one of the best methods for gas measurement. In addition to being highly specific, the NDIR gas analyzers are also very sensitive, stable and easy to maintain.

In contrast to NDIR gas sensors, which are by definition noninteractive, interactive optical gas sensors are less reliable, are generally nonspecific, and in some cases can be poisoned into a nonfunctional state.

Interactive gas sensors are generally nonspecific because the reagent being used to determine the concentration of the desired gas may react with other gases that are present. This will naturally result in false readings. Further, if the equilibrium of the reaction between the nonspecific gas and the reagent is such that the gas and reagent remain reacted even after the partial pressure of the gas drops in the environment being monitored, the sensor will no longer function properly and is poisoned.
The response time for NDIR gas sensors is also typically shorter than that for interactive gas sensors. The reason being that the kinetics of the reaction between the sample gas and reagent controls how quickly the sensor detects a change in the concentration of the gas in the environment being monitored.

Despite the fact that interactive gas sensors are unreliable and that the NDIR gas measurement technique is one of the best, NDIR gas analyzers have not enjoyed wide spread application because of their complexity and high cost of implementation.

In the past, NDIR gas analyzers typically included an infrared source, a motor-driven mechanical chopper to modulate the source, a pump to push or pull gas through a sample chamber, a narrow bandpass interference filter, a sensitive infrared detector plus expensive infrared optics and windows to focus the infrared energy from the source onto the detector.

In an attempt to reduce the cost and simplify the implementation of the NDIR technique, a low-cost NDIR gas sensor technique was developed. The low-cost NDIR technique employs a diffusion-type gas sample chamber of the type disclosed in U.S. Pat. No. 5,163,332, issued Nov. 17, 1992, to the present applicant, and hereby incorporated by reference. This diffusion-type gas sample chamber eliminates the need for: expensive optics, mechanical choppers, and a pump for pushing or pulling the gas into the sample chamber. As a result, a number of applications for the NDIR technique, which were previously considered impractical because of cost and complexity, have been opened.

The diffusion-type gas sample chamber of U.S. Pat. No. 5,163,332 uses an elongated hollow tube having an inwardly-facing specularly-reflective surface that permits the tube to function as a light-pipe for transmitting radiation from a source to a detector through the sample gas. A plurality of filtering apertures in the wall of the non-porous hollow tube permit the sample gas to enter and exit freely under ambient pressure. Particles of smoke and dust of a size greater than 0.1 micron are kept out of the chamber by use of a semi-permeable membrane that spans the apertures in the hollow tube, and condensation of the sample gas is prevented by heating the sample chamber electrically to a temperature above the dew point of the gas.

Although the low-cost NDIR gas sensor technique opened a wide variety of new applications, the gas sample chamber and the corresponding gas sensor of the low-cost NDIR technique are still too large for many potential gas sensor applications. As a result, applications in which low-cost NDIR gas sensors may be used remain limited. Furthermore, while the cost of gas sensors employing the gas sample chamber of U.S. Pat. No. 5,163,332 is less than previous NDIR gas sensors requiring expensive optics, pumps, and choppers, a further reduction in the cost of NDIR gas sensors would further increase the number of applications in which such sensors are used and the frequency of their use.
Therefore, while a need exists for a compact, inexpensive NDIR gas sensor, this need has gone unfilled. Accordingly, a goal of the present invention is to further advance the NDIR technique by providing a miniaturized, reliable, and low cost NDIR gas sensor.

SUMMARY OF THE INVENTION
The present invention is directed to an NDIR gas sensor for detecting the concentration of a predetermined gas. To this end, an optical waveguide is provided having a light source at one end and a light detector at the other end. A bandpass filter is interposed in the optical path between the light source and detector, so that the detector primarily receives radiation of a wavelength that is strongly absorbed by the gas whose concentration is to be determined. The waveguide is formed from two or more substrates of a semiconductor material, at least one of which has been micromachined. In addition, the light source and detector are directly manufactured on at least one of the semiconductor substrates used to form the optical waveguide. A pair of windows are also optically disposed between the light source and detector so as to define therebetween a sample chamber within the optical waveguide. The windows thermally isolate the light source and light detector from the sample gas, thus preventing the sample gas from cooling these elements. The gas whose concentration is to be determined diffuses into and out of the sample chamber in the optical waveguide through apertures or slots in the semiconductor substrates.

Because the walls of the optical waveguide are reflective, radiation is transmitted from the light source to the light detector through the sample gas without the need for expensive optics. Furthermore, because gas sensors according to the present invention employ a diffusion-type gas sample chamber, no pump is required to push or pull the sample gas into the sample chamber.

In a preferred embodiment, a gas permeable dielectric layer is deposited over the apertures to act as a filter and prevent dust or smoke particles from entering the optical waveguide sample chamber. Preferably, the gas permeable layer prevents particles larger than about 0.1 μm from entering.
In another preferred embodiment, at least a portion of the optical waveguide is metallized thereby improving its internal reflectivity and the overall efficiency of the NDIR gas sensor according to the present invention.

In yet another preferred aspect of the present invention, an NDIR gas sensor is provided that prevents condensation of gases or vapors on the walls of the sample chamber. To accomplish this object, means are provided for heating the gas sample chamber so that its temperature remains above the dew point of any gas or vapor that might have a tendency to condense on an inner surface of the sample chamber.

Other integrated circuit semiconductor devices can also be added wherever needed to further enhance the performance of the NDIR gas sensor according to the present invention. For example, temperature sensors, pressure transducers, and humidity sensors may be added. In addition, a micro-flow sensor may be added to detect the flow rate of the sample gas through the sample chamber.

In a particularly preferred embodiment of the present invention, an NDIR gas sensor is provided which can be used to simultaneously determine the concentration of a plurality of gases in the gas sample. The NDIR gas sensor according to this embodiment is comprised of a plurality of detectors and a plurality of bandpass filters. Each bandpass filter is interposed in the optical path between the light source and one of the plurality of detectors. The number of gases whose concentration is desired to be determined dictates the specific number of detectors and bandpass filters that are required. Each bandpass filter, therefore, is designed so that the detector it is associated with primarily receives radiation of a wavelength that is strongly absorbed by the gas whose concentration that detector is to determine.

Alternatively, in this embodiment, at least one of the detectors may be used as a reference detector. In this situation, the bandpass filter interposed in the optical path between the light source and the reference detector must be designed to pass a neutral wavelength. In other words, the bandpass filter must pass a wavelength of light that is not absorbed by the gas sample.

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