Patent Application
20080035848
For over two decades, the present inventor diligently worked on simple, rugged and low-cost NDIR gas sample chambers culminating in the so-called “wave-guide” sample chamber invention disclosed in U.S. Pat. No. 5,163,332 (1992). Indeed, for gases with moderate to strong absorption bands in the mid-infrared spectral region like CO2 (4.26 μm), H2O (2.7 μm), CH4 (3.4 μm) etc., the optimum and simplest gas sample chamber is a hollow straight metallic tube with arbitrary cross-section but speculatively reflective inner walls very much likened to a standard microwave wave-guide. Today after over a decade has gone by since the advent of the “wave-guide” gas sample chamber (a.k.a., “The Tube”), most of the reliable, rugged, sensitive and low-cost CO2 sensors use this design, rendering a host of new applications, including faster and false-alarm resistant fire detectors and Demand Control Ventilation (DCV) strategies for saving energy in high-rise office and commercial buildings, many of which were not practical earlier because of high CO2 sensor costs.
The present invention discloses a new sample chamber that, like the prior “wave-guide” or The Tube, will create a host of new applications because it overcomes a number of technical impediments that have prevented the extension of such an approach into the arena of achieving compact, low-cost and long path-length NDIR and TDLAS gas sample chambers.
In U.S. Pat. No. 5,696,379 issued to Burkhard Stock in 1997, a two-dimensional configuration for a waveguide type NDIR gas sample chamber aimed at reducing the size of the subsequent gas sensor was disclosed. In this disclosure, a configuration utilizing two 90° bends and one with one 90° bend followed by a U-shape in one plane were described for achieving the overall size reduction for the designed NDIR gas sensors. Without an explicit description or reasoning, the author of this patent also claimed that his 2-dimensional configuration can also be extended to the third dimension with equal benefits. However, he emphasized with a detailed explanation that his teaching only worked for guiding the radiation through a maximum of a 180° bend such as two right angle (90°) bends or a U (180°) bend. Also, standard radiation sources (quasi blackbody) were used but no method teaching how to guide the emitted radiation down the two-dimensional hollow waveguide was advanced. There is no doubt that the use of curved hollow waveguides with speculatively reflective inner walls will render the design of an NDIR gas sensor more compact than the use of a straight hollow waveguide having the same path length. However, the gain in compactness for the designed gas sensor using either a 2- or 3-dimensionnal hollow waveguide configuration is severely limited by not being able to guide the radiation through a bend that is greater than 180°.
Contrary to the disclosure of U.S. Pat. No. 5,696,379 issued to Stock (1997), the current invention is a 3-dimensional configuration such as the shape of a many-turn circular coil (>360°) or a number of paper clip-shaped tubes stacked together to create a column by deforming the shape of the circular tube forming the coil into an elongated ellipse. The current invention called the “Super Tube” exploits the curved hollow waveguide concept in three dimensions by advancing methods to allow radiation to readily propagate through bends that are >180° thereby achieving a very long path length sample chamber that occupies the minimum of a 3-dimensional space, even though Stock teaches away from such a result.
The first problem that the present invention addresses is to make sure that the incident radiation entering a long waveguide sample chamber having many curves (such as a coil) or U-bends (in the form of a stack of paper clips) be able to “turn” corners inside with relative ease and thereby exiting without significant loss in radiation intensity. The present invention solves this first problem by reducing the angles of incidence for radiations emanating from a quasi-blackbody infrared source.
The second problem that the present invention addresses is to restore some of the inevitable losses that the radiation suffers as it traverses the 3-dimensioanl curved hollow tube or the “Super Tube.” The current invention addresses this problem with simple internal focusing elements installed inside the Super Tube. This is unique only to the innate structure of the current invention.
The third problem that the present invention addresses is the interference of the measurement by ubiquitous water vapor that can be present in the atmosphere in relatively small amounts (e.g. 5,000 ppm) on a dry day or in significantly larger quantities (>45,000 ppm) on a humid day. The current invention addresses this problem with a novel technique of in-situ water vapor correction.
The main drawback for the present invention is the fact that the path-length of the sample chamber is no longer adjustable but depends only upon the actual length of the Super Tube. Comparing with the White and the Herriott cells, the present invention does not require any meticulous adjustments of the optical components prior to its use. It does not require any expensive component like the Pockels cell in the Wong cell which also limits its use by virtue of available electro-optical crystals for its construction. Furthermore, the Super Tube is very easy to build and at a significantly lesser cost. Finally, it is rugged enough for use outside of laboratories in both NDIR and TDLAS type transportable gas analyzers.
The Super Tube of the present invention is a hollow metallic tube, preferably made out of aluminum, brass or stainless steel with specularly reflective inner walls. The typical outside diameter for the Super Tube is not critical and a convenient value is 0.500″ with a typical wall thickness of ˜0.050″.
Depending upon the path length requirement together with an overall dimensional restriction for a particular sensor, the number of U bends forming the Super Tube can be increased to meet a particular demand.
The mechanism and efficiency of radiation propagating along a curved tube such as a circle, a U (180°) or right angle (90°) or a combination thereof is quite different from those propagating along a straight section of a tube. In both cases, radiation has to reflect repeatedly forward within the smooth inner walls of the tube in order to reach the detector, its intended target. For radiation propagating along a straight section of a tube, the result is simple and deterministic. However, for radiation propagating along a curved tubing, the result can be haphazard and unpredictable. In this case, the angle of incidence of the impinging radiation upon the inner wall surface plays a crucial role in determining whether the radiation will continue to go forwards, trapped into stationery resonant reflections or reversing its reflection direction going backwards altogether. Depending upon the size of the tube, there exists a critical angle of incidence for the impinging radiation above which the reflecting and forward-going radiation will reverse its direction along the tube and go backwards. Thus, in order to ensure that the radiation introduced into a waveguide sample chamber having a structure other than a simple straight section, the entrance angle of incidence, which depends upon the tube diameter, must be correctly designed and taken into consideration.
The present invention addresses this problem by reducing the angle of incidence 6 for radiations emanating from a quasi-blackbody infrared source 5 with a reflecting conical surface 7 having an inclined angle 8 to the longitudinal axis 9 of the Super Tube of ˜10° as depicted in
Since most infrared sources are quasi-blackbodies, the radiation pattern in a hemisphere follows closely to that of the Lambertian distribution. The Lambertian distribution of an infrared source specifies the magnitude of its spectral radiant emittance radiating into an hemisphere at any point in space as a function of the Euler angles subtended by the point at the geometrical center of the source. From the consideration of their subtended solid angles at the source, both obtuse (grazing) and normal (perpendicular to the source surface) direction radiations are much weaker than those radiating at angles between 30-60° as measured from the normal to the source's surface. Thus, a significant amount of radiation from an infrared source entering the “wave-guide” sample chamber or the Super Tube will normally have incident angles ranging from 30 to over 60°. Radiations traveling in a tube having this range of incident angles will not be able to turn circular corners having a relatively small radius (<2.0″) efficiently. The same holds true for traversing a 180° U turn. However, the installation of the hollow conical mirror 10 (see
Another approach to limit the launching angle for radiations emanating from an infrared source for the Super Tube is to install a parabolic mirror in front of the source as shown schematically in
Contrary to the disclosure of the U.S. Pat. No. 5,696,379 issued to Stock (1997), the current invention is a 3-dimensional configuration such as stacking a number of U-bends upon themselves much like forming a column of paper clips with one on top of the other as exemplified in
The final and extraordinarily difficult problem facing the Super Tube is a problem whose existence to date has not even been identified nor properly addressed by the NDIR gas sensor community, namely, the interference of the measurement of very weak absorbing gases, or gases that need to be detected down to very low levels (e.g. ppm-ppb ranges) or both by the presence of an unpredictable quantity of water vapor in the ambient air.
Let us briefly explain why water vapor interference is such a nasty problem for making very low concentration measurements for weakly absorbing gases when using the NDIR gas detection technique. First of all, apart from its prominent infrared absorption bands at 1.87μ, 2.67μ, 3.2μ and 6.27μ, water vapor has numerous weaker absorptions literally everywhere throughout the middle infrared spectrum (2.0μ-16 μ). For measurement of gas concentrations such as CO2, methane etc. in the hundreds of ppm range using the NDIR technique, such weak absorptions of water vapor can be ignored because its interference, even within the absorption band of the gas in question, is equivalent to only a few ppm's and hence relatively small. However, for the measurement of very low concentration of gas species (ppb-ppm ranges) such as ethylene (C2H4), the interference of these weak but fluctuating water vapor absorptions within the pass band of C2H4 on the actual gas measurement is nevertheless significant. Even though its effect is also equivalent to just a few ppm's, its magnitude is comparable to the level of the gas measurement itself. For this reason, the water vapor interference must be corrected or compensated for. The current invention uses what is called an “in-situ” water vapor interference correction methodology to overcome this problem. Such a method actually measures the amount of water vapor present in the sample chamber with the use of a separate and in-situ calibrated detector so that the interference effect can be quantitatively deducted from the measured signal. As will be expounded below, such a methodology is feasible only with the advent of the current Super Tube sample chamber configuration and is therefore uniquely complementary to the current invention.
Apart from some very strong infrared absorbing bands, water vapor also has very weakly absorbing bands throughout the entire infrared region (2-16μ). In fact, that is the reason why water vapor is such a bother, but only in very special situations when very low concentration of gases need to be detected as mentioned above. However, there is a silver lining behind it. Because of the fact that the absorption is so weak in the regions other than its own strong absorbing bands, the concentration of water vapor can be measured almost anywhere provided that there are no known absorption bands for other gases (including the gas in question) present. In other words, whereas the spectral filter for the gas measurement has to be carefully selected, the spectral filter for the water measurement can be readily chosen as long as the filter band does not coincide with any known absorption bands of gases in the atmosphere. Thus it is possible to detect the actual presence of water vapor within a particular absorption band designed to measure gas X by providing a second detector that will simultaneously measure only water vapor and nothing else. Since the absorbing band for gas X also measures water vapor, the measurement of only water vapor by another detector could be used to subtract out the contribution of water vapor in the gas X measurement channel.
The Super Tube's unique sample chamber configuration provides a very convenient way to set up an optical arrangement for installing a second detector in line with the main “gas” detector for the measurement of water vapor as illustrated in
The optical arrangement concept disclosed above in the use of two in-line detectors for correcting the interference of water vapor in “gas” measurement can readily be extended to render the sensor into one that can detect multiple gas species. Such an optical arrangement is depicted in
The optics of the current Super Tube invention provide yet another very important sensor performance enhancement characteristic. Because the sensor optics comprises only a reflective hollow tube and radiation from the infrared source is made to travel forwards from one end of the tube to reach one or more detectors at the other end, there is no inherent restriction as to how much incident power from the source can be “pumped” into the Super Tube provided adequate heat-sinking means is available. In other words, the Super Tube allows, at least in principle, as much radiation as is needed to be used for the detection of one or more gases. Since the detectivities of the sensor detectors are not affected, the overall signal-to-noise (S/N) can be boosted at will. The only limitation appears to be the added implementation cost for the sensor.
It can be seen from the discussion above that the current Super Tube invention is capable of providing long path lengths for the detection of trace gases having a very weak infrared absorption band or required to be detected down to ppb-ppm ranges within a relatively compact space. Furthermore, the current invention offers great robustness and a low-cost implementation when applied to either NDIR or TDLAS type transportable gas analyzers.
While the invention has been described herein with reference to certain examples, those examples have been presented for illustration and explanation only, and not to limit the scope of the invention. Additional modifications and examples thereof will be obvious to those skilled in the art having the benefit of this detailed description. For example, while the Super Tube has been depicted in the drawings and discussed above as having a circular cross-section, it need not only have such geometry; instead, the Super Tube can have a cross section in many different geometries, such as a square, so long as it functions as a hollow waveguide. In addition, several detectors can be combined in one physical package and the Super Tube will also be useful in certain applications where only one detector is required. Further modifications are also possible in alternative embodiments without departing from the inventive concept.
Accordingly, it will be apparent to those skilled in the art that still further changes and modifications in the actual concepts described herein can readily be made without departing from the spirit and scope of the disclosed inventions as defined by the following claims.
