Radiant Emission and Absorption Multigas Analyzer

The present invention relates generally to spectrometric detection or monitoring the concentration of constituents or substances of interest in the air or other fluid medium, and more particularly to infrared and ultraviolet point sampling devices, which draw into the instrument, from a point in space, a sample of a gas or other fluid or gaseous mixture for analysis.

The closest prior art appears to be my U.S. Pat. No. 3,827,675 entitled "Oscillating Bellows" and my U.S. Pat. No. 3,836,255 entitled "Spectrometric Substance Analyzer Employing Temperature Modulation."

Like the latter patent, the present invention maintains a substantially constant pressure in the optical chamber and utilizes blower and motor-driven valve means, along with external heating and cooling means, to recirculate fluid back to the optical chamber to accomplish the modulation of the temperature of the gaseous fluid in the optical chamber at substantially constant pressure, thereby producing cyclical radiant emission, primarily in the infrared, as a result of temperature modulation of a substance of interestion the fluid in the optical chamber, and cyclical radiant absorption, in the infrared and in other regions of the electromagnetic spectrum according to the output of radiant sources shining into the optical cavity, as a result of the concentration modulation of the substance of interest (concentration varies inversely with the temperature at constant pressure). Since the radiant emission and absorption signals are about 180.degree. out of phase but of opposite polarity, they add rather than tending to cancel each other, at any given wavelength, and the resulting augmented cyclical radiance in the optical chamber or cavity is monitored at wavelengths of interest by radiant detectors (one or more) looking into the optical cavity, utilizing techniques such as synchronous detection (see also my U.S. Pat. No. 3,516,745), whereby the concentrations of many substances or gases of interest may be monitored simutaneously and displayed in real time or recorded. The optical cavity walls always provide the cavity with some infrared radiation, which can be cyclically absorbed by a gas of interest. The motor-driven recirculation means facilitates more stable operation than do the thermally driven bellows and piston of my U.S. Pat. Nos. 3,827,675 and 3,516,745, respectively, and the constant fluid pressure feature, which produces radiant emission and absorption signals, which add algebraically as well as merely vectorially, facilitates operation at greater sensitivity over a broad spectral range than do the bellows and free piston devices, in a practical size gas analyzer.

The present invention differs from the gas analyzer of my U.S. Pat. No. 3,836,255 by more efficient modulation of the fluid temperature in the optical cavity, thereby facilitating greater sensitivity and frequency capability and less power required by the blower and the external heating and cooling means.

Basically these features and performance characteristics are provided, in the present invention, by an optical cavity highly reflective (on the inner surfaces of the walls of a substantially closed chamber) to electromagnetic radiation, e.g., infrared, visible, ultraviolet. The cavity includes first and second wall surfaces (more than two is also possible) pervious to fluid flow into and back out of the cavity via ports in the surfaces. The fluid recirculation means injects hot fluid (e.g., a portion of a gas sample to be analyzed) into the cavity through one of the surfaces while withdrawing cool fluid from the cavity through the other surface and recycling most of the cool fluid right back to the cavity as hot fluid injected through the one surface, by means of a blower or pump, motor-driven valves, a thermal regenerator which releases previously stored heat to the cool fluid, and a heating chamber external to the cavity and proximate the one surface. The cavity, surfaces and ports are formed and positioned with respect to each other so that the paths of fluid flowing substantially directly from the one surface through the optical cavity to the other surface encompass most, or preferably all, of the volume of the cavity, whereby most, or preferably all, of the cool fluid in the cavity is swept from the cavity, i.e., replaced, by the hot fluid replaces it (the cool fluid flows back out the ports in the other surface; each surface, of the two surfaces, has one or more ports). The portions of hot fluid flowing along the various flow paths through the cavity all begin to arrive at the other surface at about the same time, i.e., their leading edges or interfaces with the cool fluid ahead of them all arrive at the other surface at about the same time or phase of the cycle, whereupon, since the arrival of this "warm front" means that the replacement of the cold fluid by hot fluid in the optical cavity is basically complete or maximized, the fluid injection means terminates the injection of hot fluid and withdrawal of cool fluid (by means of the valves, for example), so that the warm front does not substantially pass through the other surface and substantially enter, and be substantially cooled by, the cooling chamber just beyond the other surface, which would waste heat if it occurred. Then, after an optional delay or dwell time of predetermined duration, which increases the amplitude of the Fourier component of the spectral signal at the fundamental frequency (the frequency of the fluid temperature modulation in the cavity) thereby improving the signal-to-noise ratio, the entire process is basically reversed, with hot fluid being withdrawn from the cavity via the ports in the one surface, flowing through the thermal regenerator for cooling (thereby storing its heat in the regenerator for use later when the fluid returns), thence through the blower and valves, through a cooling chamber proximate the other surface but external to the optical cavity, for cooling the fluid further, then through the pervious other surface as cold fluid, providing a "cold front" which sweeps substantially all of the volume of, and thus sweeps substantially all of the hot fluid from, the optical cavity by the time it reaches the one surface, i.e., arrives at the one surface, whereupon there is, optionally, another dwell of selected length, to complete the cycle. Since the fluid paths of the cold front, like those of the hot front, encompass most, or even substantially all, of the optical cavity, and since the fluid in each of the parallel paths of a given front (in the sense that the flow paths are in parallel) having leading edges, or interfaces with the fluid of different temperature ahead of it, that all arrive at the pervious surface ahead of it at approximately the same time, and since, further, there is a thermal regenerator in the recirculation circuit, with proper alternating flow through the regenerator, the present invention is more efficient, thermally and otherwise (e.g., size, weight, power requirement, cost), and is a more sensitive multigas analyzer (for reasons such as higher signal-to-noise, less possible thermal cycling of the optical cavity walls, and lower detector temperatures), than the multigas analyzer of my U.S. Pat. No. 3,836,255, which probably was potentially the most practical of my three previous gas analyzers (the devices of the three U.S. Patents mentioned above).

Since the recirculation circuit or recycling path is constantly connected to the atmosphere or other fluid medium being analyzed (except perhaps during the dwells), the temperature modulation at constant fluid pressure in the optical chamber is substantially achieved. The ports on at least one of the above-mentioned pervious surfaces are shaped and located such that most of the surface is pervious to fluid flow, in the sense that, if the at least one surface is considered as being made up of several sub-areas of roughly equal size, most of the sub-areas would each be considered pervious by virtue of containing at least a portion of a port. The at least one surface is also extended in area sufficiently to constitute a significant fraction of the internal wall area or surface area of the cavity, thereby facilitating fluid flow paths that encompass substantially all of the volume of the optical cavity, and thereby facilitating substantially complete sweeping out or replacement of a batch of fluid in the cavity twice each cycle by the alternating hot and cold fronts. The at least one surface, further, is preferably highly reflective, to at least the wavelengths of interest, in spite of the grid or array of ports or port portions distributed approximately uniformly over its surface area. Thus the port or ports (one extended port could conceivably be sufficient) in the at least one extended surface should be made as small in total area as possible consistent with faciliating adequate and properly directed flow needed for the substantially complete sweeping out and withdrawal, each cycle, of the gas filling the optical cavity.

The thermal regenerator can be formed in many different ways, e.g., stainless steel wool, metal screens, thin sheets of material. The regenerator can also serve as a particulate filter, or adsorber of undesirable chemicals or interferents. The regenerator can also be used as a catalytic converter, to chemically change interferents or other undesirable chemicals, such as chemicals that might degrade the optical cavity. By using two different types of regenerators separately, for example, one of which decomposes a substance of interest or an interferent and the other does not, and making a separate concentration measurement with each regenerator and subtracting the two, a much more accurate measurement of concentration could be made, and worth the extra effort, in certain difficult cases, using this difference method. Thus it might be worthwhile for the gas analyzer to include two or more thermal regenerators, of different types chemically, located and connected essentially in parallel in the recirculation means or recycling path, proximate the heating chamber and hot surface of the optical cavity, with the external heating chamber connected between the optical cavity and the regenerators, and further including a distributor valve for selecting one regenerator at a time, to facilitate ratio or differential measurements, such as the difference method mentioned above. One common and troublesome interferent in atmospheric, and especially in exhaust stack, measurements of chemical concentrations is water vapor and water droplets, e.g., an aerosol of water. In such a case, a thermal regenerator which could absorb or otherwise remove water from the system or from the measurement could be of great value. Of course, just the fact that this entire recycling circuit and optical chamber is kept warm by the external heat constantly applied, e.g., heat from an arc lamp or other radiant sources shining into the cavity, will in most cases be sufficient to avoid the water vapor condensation problem which sometimes occurs, since water vapor pressure increases rapidly with increasing temperature, and since, for example, the electromagnetic absorption strength of water vapor in the infrared is at least one order of magnitude (factor of ten) less than the infrared absorption strength of the same amount of liquid water in the optical path, due to the much smaller dipole moment of the H.sub.2 O molecule when it is dispersed as a vapor than when it is densely associated with other water molecules as a liquid. Thus, in this gas analyzer, the waste thermal energy from the radiant sources can be utilized in at least two different ways--facilitating development of the periodic spectral radiance in the optical chamber and on the detector flakes in the presence of a gas of interest in the gaseous mixture, and secondly, substantially reducing or eliminating the condensation problem that often occurs when a warm, vaporous, supersaturated mixture or aerosol is sampled and cooled by a gas analyzer. In addition, the cooling of some radiant sources is of value in proper operation of the sources, such as an arc lamp, and can be of value in cooling certain nearby instrument components, such as radiant detectors. Thus the cooling of the radiant sources or lamps by the recirculation circuit can, all at the same time, benefit the radiant sources, proximate instrument components, the temperature modulation of the sample, the condensation problem if it occurs, and the operation of the optional chemically active thermal regenerator mentioned above, e.g., the catalytic converter/thermal regenerator. However, for most purposes, an isobaric temperature modulation amplitude of only about 100.degree. C. would be adequate since, on a signal-to-detector noise basis, and assuming a one cubic foot gas analyzer, twenty second response time, and an ambient temperature thermal type detector, such as a pyroelectric detector, and further assuming, for the sake of simplified calculations, that there is no interference or drift, the sensitivities for various infrared absorbing (and therefore emitting) gases would typically be on the order of one part per billion, if the analyzer is properly designed. Therefore the thermal regenerator might not be hot enough to properly serve as a catalytic converter in certain cases.

From my earlier discussion of hot fronts moving uniformly across the cavity from the pervious hot surface toward (in a fluid flow sense; not necessarily a perfectly straight line but guided by the cavity walls) the pervious cold surface and converging uniformly and simultaneously (ideally) on the pervious cold surface, alternating with cold fronts sweeping uniformly across the cavity in similar fashion from the pervious cold surface to the pervious hot surface, with the fronts sweeping, and the fluid flow paths encompassing, the entire volume of the optical cavity, it follows that the entire optical chamber, including the two pervious surfaces and all the ports, should be formed and positioned relative to each other, and the conduits leading to and terminating at these ports should be aligned and formed, such that the fluid flowing through either pervious surface into the cavity via the respective ports does flow substantially directly from this surface to the opposite pervious surface, with a minimum of eddy currents, random of circuitous flow, and stagnant regions or regions of "backwash", as might occur, for example, in corners of the optical chamber or cavity. It is also preferable to form the optical chamber and its conduits and ports to avoid directing a strong or high velocity jet of fluid directly against a wall surface of the cavity, especially where there might be a substantial temperature difference between the surface and the jet of fluid, in order to minimize any undesirable thermal cycling of, and therefore cyclical infrared emission from, the wall surface in that region. However, a wall of good thermal conductivity and specific heat, such as metal, would undergo very little temperature cycling. If there is any tendency of certain chemicals to coat the walls and undergo temperature cycling, that could be handled in at least two ways. First, since the chemicals would be in liquid or solid phase, rather than gaseous phase, their molecular vibration-rotation lines would be smeared out by collision broadening, whereby they would have broad, relatively structureless bands in the near and middle infrared, which would have relatively little effect, for example, on a selective pneumatic detector, such as disclosed in U.S. Pat. No. 2,924,713, or on most any detectors utilizing proper optical filtering, especially if gas chamber filters are used, e.g., the filtering technique disclosed in my U.S. Pat. No. 3,488,491. Since thermal cycling of a possible chemical coating on a cavity wall surface, or of the wall surface itself, if either is present and of significant and troublesome magnitude, would lag the phase of the temperature modulation of the gas sample, a second method of handling a possible interference from a chemical coating, as well as possible interference from a temperature cycling wall portion, is merely using standard synchronous rectification and smoothing after a.c. amplification of the detector signal, which probably would be used anyway for greatly improving the signal-to-noise ratio by filtering out noise and drift. Since the synchronous detection is phase sensitive, the interfering signal at lagging phase would tend to be cancelled out, depending on the phase lag. The degree of cancellation increases as a phase lag increases toward 90.degree., and becomes virtually complete at 90.degree.. If necessary, the desired spectral radiance signal from the gas can be synchronously rectified at a phase which leads the interference signal by 90.degree., whereby the interference signal from the coating or wall would be cancelled out (if present).

If desired, and if chemical coating of the cavity is not a problem, the optical chamber can be constructed to develop an out-of-phase greybody (nonspectral) signal from thermal cycling of the cavity wall surface, in order to continuously and automatically balance each analytical and reference detector pair monitoring the concentration of a gas of interest, or each pair of analytical and reference channels monitoring a gas of interest, by a synchronous rectification and nulling technique disclosed in my U.S. Pat. No. 3,516,745 (columns 5 and 6), whereby output drift can be further reduced.

It should be kept in mind when considering thermal cycling that, since the reflectivity of the cavity surface (inner surface of the optical chamber wall) is high at most wavelengths, its emissivity at the same wavelengths is low whereby, for a given temperature fluctuation, the infrared emission variation would be less than that of a blackbody. Similarly, the emissivity of a chemical coating, if present, would be less than that of a blackbody (i.e., less than unity).

Another way to handle thermal cycling, if it occurs and is undesirable, is to use an optical chopper or filter wheel operated at a frequency different from (probably higher than) the frequency of the gas sample temperature modulation. This can be helpful if common mode rejection of a large greybody signal is a limiting factor, or to more fully utilize the inherent sensitivity of a high speed detector. If, in this connection, the gas temperature modulation frequency is lowered sufficiently to produce substantially nonadiabatic conditions in the cavity, the pattern of operation of the injection means might have to be modified, as, for example, by continuing to supply hot or cold gas even after the hot or cold front reaches the pervious surface at the far end of the path through the cavity. When using two frequencies, synchronous rectification at both frequencies would generally be desirable, as discussed in my U.S. Pat. No. 3,836,255 (columns 12 and 13). The chopper or filter wheel can be located between a source and the cavity or between the cavity and a radiant detector, if shielded properly; usually a chopper is placed in front of the source. Some sources, such as arc lamps, can be chopped or pulsed electrically.

It will become evident, upon consideration of the accompanying drawings, that the above and still further objects, features and advantages are recognized and achieved to a significantly greater extent by the present invention than by the prior art.

Referring now to the drawings, FIG. 1 is a cross-sectional. partly schematic illustration of one embodiment of the present invention, including an optical chamber near the top and a recirculation circuit below the optical chamber on the illustration. FIG. 2 is a view from inside the optical chamber of FIG. 1 of the hot pervious surface on the right side of the optical chamber. FIG. 3 shows a modified optical chamber, which is basically spherical in shape rather than cylindrical. FIG. 4 illustrates another alternative design for the optical cavity of the present invention, wherein the chamber is again spherical (basically) but wherein the hot and cold pervious surfaces are proximate each other on a single prong inserted into the optical chamber, rather than being on opposite sides of the cavity as in FIGS. 1 and 3.

Referring now in detail to the drawings, FIG. 1 illustrates an optical chamber or cavity 1 in the form of a right circular cylinder including a hot pervious surface 2 as its right endface and a cold pervious surface 3 as its left endface. hot surface 2 is basically a circular disc containing slit shaped ports 4 near its circumference and a sprayer 5 at the center of the disc containing short conduits 6 leading to additional hot ports 7. An arc lamp 10, including special hot, emissive material (not shown) close to the arc for emitting infrared and possibly visible radiation to augment the primarily ultraviolet radiant emission from the arc, radiates into optical cavity 1 via a window 11 in hot surface 2. Arc lamp 10 is mounted in, and provides heat for, heating chamber 14 which is external to cavity 1 and abuts the hot pervious surface 2. Optical chamber or cavity 1 is highly reflective (inside).

Cold pervious surface 3, on the opposite side of the cavity from the hot surface, includes slit-shaped cold ports 18 similar in shape to the hot ports 4 of hot surface 2. The cold surface 3 could have a cold sprayer similar to the hot sprayer 5. External cooling chamber 20, outside of the optical cavity 1 and abutting the cold surface 3, cools the gaseous fluid flowing into and out of cavity 1 via cold ports 18 in the cold pervious surface 3, and may require internal and external cooling fins (not shown) and a fan or blower (not shown), although ambient cooling of the gas analyzer case may provide sufficient cooling without the need for a separate cooling fan, especially if cooling chamber 20 is made part of the gas analyzer case, or at least connected to the case properly to promote good heat transfer to the case. Similarly, heating chamber 14 may have internal heat transfer fins (not shown) for improved heating of the gaseous fluid injected into, and withdrawn from, optical cavity 1 via ports 4 and 7 in the hot pervious surface 2, but heating chamber 14 would be thermally insulated from the ambient environment including certain instrument components. For example, the optical chamber can be constructed from several segments, such as segments 16 between the segments corresponding to the heating and cooling chambers, to provide some thermal insulation between the heating chamber and hot pervious surface on the right and the cooling chamber and cold pervious surface on the left. This insulation not only reduces the thermal, and therefore electrical, power requirement of the gas analyzer but also helps keep certain instrument components cool, such as radiant detector pair 24 and radiant detector pair 26 which are each connected to electronics box 28 utilzing synchronous detection techniques to monitor one gas of interest for each detector/filter pair. Radiant detector/optical filter pair 24 looks into optical cavity 1 via window 25 while radiant detector/optical filter pair 26 looks into the cavity through window 27. For simultaneous monitoring of many substances of interest in the gaseous sample, many filter/detector pairs can be mounted on the optical chamber walls and look into the cavity. A filter wheel can increase the number of substances monitored per detector, if desired, as well as offering other potential advantages discussed earlier, regarding possible thermal cycling and high speed detectors.

Heating chamber 14 is connected to the hot side of a recycling path or recirculation circuit 30 via thermal regenerator 34 and cooling chamber 20 is connected to the cold side of the recycling path. By means of rotary distributor valves 38 and 39, which are similar to the rotary valve 7 in my U.S. Pat. No. 3,836,255, and interconnecting conduits, a blower 40 recycles fluid alternately from the cooling chamber to the heating chamber via regenerator 34 (solid arrows show the cold fluid being recycled back to the optical cavity 1 as hot fluid injected through the hot pervious surface 2) and from the heating chamber 14 back to the cooling chamber 20 (dotted arrows show the hot fluid withdrawn from the cavity 1 flowing through the regenerator 34 and cooling chamber 20 and thus being recirculated back to the cavity as cold fluid injected into the cavity 1 via cold ports 18 in cold surface 3), with a preselected dwell between these two alternate portions of the temperature modulation cycle.

Rotary valves 38 and 39, driven by a motor (not shown), can each rotate in either direction at a frequency of rotation equal to the desired frequency of the isobaric temperature modulation of the gaseous mixture in optical cavity 1. Rotary valve 38 feeds gas upwardly toward the viewer from its single inlet below the plane of the paper, and distributes the gas either to the conduit on the right or the conduit on the left, according to whether the cutout portion 43 is to the right or to the left. Rotary valve 39 draws gas either from the conduit on its left or from the conduit on its right, according to whether its cutout portion 43 is to the left or right, and feeds the gas downwardly from the viewer to its single outlet below the plane of the paper. The conduits on the left and right of each valve have widened or funnel-shaped portions 45 where they connect to the valve body, which are optional. The optional funnel portions 45 slightly extend the portion of the cycle during which each of the four conduits is operational, thereby slightly extending and "rounding off" the injection/withdrawal portions of the temperature modulation cycle, and thus also decreasing the dwell time between the periods of fluid injection/withdrawal in optical cavity 1. The dwell times occur when the valve cutout portions are close to their midway positions between the left and right conduits where the cutout portions do not, for a short duration of time, overlap the funnel portions 45 of the conduits on the left and right. The optional "wave-shaping", by the "rounding off" and the adjustment of the duration of the dwell, can produce an approximately sinusoidal gas temperature modulation in the optical cavity, for improving the signal-to-noise ratio, and therefore the sensitivity of the fluid analyzer.

The recirculation circuit constantly communicates with the atmosphere or other fluid medium being sampled by means of inlet conduit 48, which is connected to the low pressure side of blower 40, and outlet conduit 49, which is connected to the high pressure side of the blower. Therefore, if the pressure of the atmosphere or other medium being sampled is constant, the temperature modulation of the sample in the optical chamber will be substantially at constant pressure, i.e., isobaric. The sampling conduits 48 and 49 allow the recirculation circuit to "breath out and in" as the gaseous sample is alternately heated and cooled isobarically. In other words, except for the dwells, the optical cavity and recirculation circuit are in constant communication with the fluid medium being sampled, and the sampled fluid is not being substantially heated or cooled during the dwells. Therefore the periodic temperature variation of fluid in the optical cavity, i.e., sampled fluid, occurs at substantially constant pressure if the pressure of the atmosphere or other fluid mixture being sampled is substantially constant during the temperature variation cycle.

It follows from the above description of the spectrometric substance analyzer in FIG. 1 that, alternately and isobarically, a hot front sweeps across the optical cavity from the hot surface to the cold surface and a cold front sweeps across the cavity from the cold pervious surface back to the hot pervious surface, with an optional dwell or time lag between each sweep. Thereby, the fluid in the optical cavity at the current temperature is substantially completely swept out and replaced by fluid at the new or succeeding temperature, twice each cycle, at substantially constant fluid pressure, and with a minimum of wasteful sweeping, e.g., nonuniform or multiple sweeping of the same area, or sweeping far beyond the pervious surface at the far end of the cavity (sweeping far beyond the pervious surface through which the old fluid is being withdrawn from the cavity, wherever in the optical cavity that surface, with its one or more ports, may be). Waterful sweeping degrades system performance, regarding especially power/required, sensitivity and response time.

Typically, a detector/filter pair, such as detector/filter pair 24, would have two identical detector flakes, one of which looks into the optical cavity 1 via an analytical filter which passes wavelengths of one or more characteristic absorption or emission bands (generally characteristic absorption and emission are at the same wavelengths) of the substance or gas that that detector/filter pair is supposed to monitor the concentration of, while the other detector flake looks into the cavity through a reference filter designed to transmit an equal amount of cyclical radiant energy but at nearby or reference wavelengths not characteristically absorbed or emitted by the gas of interest. The two detector flakes are typically in a bridge circuit, which, if the gas is present in the sample, produces a difference signal which is amplified and synchronously rectified at the proper phase to produce a d.c. signal, which is smoothed by an RC filter (resistance-capacitance), which smoothed d.c. signal is a measure of the concentration of the gas of interest in the sampled medium, and at low concentrations is proportional to gas concentration, as described in slightly more detail in my above-mentioned U.S. patents. If the gas of interest monitored by that detector/filter pair is not present in the sample, the difference signal and d.c. output are supposed to be zero, but spectral interferences can sometimes be a problem, and optical filters containing gases can help, especially in the infrared. A phase sensor (not shown) provides phase information to electronics box 28, so that the a.c. difference signal can be rectified at the proper phase. The phase sensor can take any of several forms, such as, for examples, a switch or relay on one of the synchronized rotary valves; a radiant detector looking into the optical cavity through a wideband optical filter; a temperature sensor such as a thermopile or thermistor disposed in the optical cavity or recirculation circuit.

Thermal regenerator 34, by containing thin pieces of material which store heat from, and release almost all of the stored heat to, the fluid flowing through the regenerator from and to the heating chamber 14 via the associated conduits of the recycling path, facilitates a greater amplitude and frequency of the periodic isobaric temperature variation of sampled gaseous fluid in optical cavity 1 for a given thermal input to the fluid in heating chamber 14 and a given rate of cooling of the fluid, primarily in cooling chamber 20. Thermal regenerators, which are designed to minimize thermal transfer through the regenerator material from the hot end to the cold end of the regenerator, are well known in the field of Stirling cycle engines and Stirling cycle cryogenic coolers, but are not known, or at least not well known, in the field of gas analyzers.

Thermal regenerators are also designed to introduce into the fluid path a minimum of additional volume and a minimum of fluid drag. This can be accomplished by providing properly designed multiple fluid flow paths arranged in parallel. One regenerator design that has been used experimentally in Stirling cycle engines is a stack of loosely wound spools or spirals of metal foil. Multiple thin plates or sheets, arranged in parallel and properly spaced, and possibly cut into segments or strips to reduce axial heat flow (parallel to the fluid flow axis), can thus serve as a decent regenerator. Radiant heat transfer should also be kept to a minimum. Because of its low infrared emissivity, metal foil serves well in this respect, too. And metal foil can take a high temperature, also.

Thermal regenerators in Stirling cycle engines have been operated satisfactorally at frequencies up to at least fifty Hertz, which is more than enough for the gas analyzer of the present invention. For even an optical chamber as small as one cubic inch, assuming its surface area-to-volume ratio is not much higher than that of a sphere, the gas temperature in the chamber, with respect to thermal losses to the chamber walls, would be substantially adiabatic at frequencies above about ten Hertz, so a larger chamber of low surface area-to-volume ratio would be substantially adiabatic at even lower frequencies, whereby a regenerator with 50 Hertz capability would be more than fast enough to accommodate a temperature modulation frequency likely to be selected for the gas analyzer of the present invention.

Of course, since phenomena special to the gas analyzer will have to be carefully considered, such as chemical adsorption, absorption and reactivity, a selected thermal regenerator design will likely be a compromise between thermodynamic considerations and the special needs of the gas analyzer of the present invention. And of course the usual practical considerations, such as cost, size, weight, reliability and lifetime, will be factors in this selection as well as for the entire gas analyzer.

FIG. 2 illustrates the hot pervious surface 2, including hot slits 4, the hot sprayer 5 containing hot slits 7, and the window 11 of radiant source 10, as viewed from the center of the optical chamber 1. The hot slits serve as hot ports of the hot pervious surface 2, and thereby serve as openings through which hot gas is injected into, and later withdrawn from, the optical cavity each cycle by the recirculation circuit or injection and withdrawal means.

FIG. 3 illustrates an alternative shape for the optical chamber, wherein the cavity now is basically spherical rather than the substantially cylindrical shape of the cavity in FIG. 1. Thus, substantially spherical, highly reflective optical cavity 60 includes a highly reflective sprayer head 61 inserted slightly into the right side of cavity 60 and serving, by virtue of hot ports 62 of hot conduits 63 within the sprayer head, as a hot pervious surface of cavity 60. Hot sprayer 61 has a convex, substantially spherical shape where it enters cavity 60 (radius of curvature much less than the general radius of curvature of cavity 60). The hot sprayer head 61 is highly reflective except, perhaps, for hot ports 62, but the hot conduits 63 can be made metallic or otherwise (the entire hot sprayer 61 can be metallic) for high reflectivity and low infrared emissivity, to help conserve the electromagnetic radiation in cavity 60 and to reduce any possible cyclical infrared emission from the hot sprayer due to possible thermal cycling of the hot conduits (unless such a signal is desired for calibration, analytical and reference channel balancing, or otherwise). The solid arrows show hot gas or other hot compressible fluid being injected into cavity 60 while cold fluid (dotted arrows) is being withdrawn from the opposite side of the cavity through cold sprayer head 67 which is inserted slightly into the left side of optical chamber 60 and is constructed similarly to the hot sprayer 61. Thus the cold fluid is withdrawn through cold ports 68 of cold conduits 69 of the cold sprayer 67 and is cooled therein or flows to a cooling chamber (not shown). Cold sprayer 67 thereby serves as the cold pervious surface of the optical cavity 60. By substantially eliminating corners, the spherical cavity 60 makes the flat disc-shaped hot and cold pervious surfaces 2 and 3, respectively, of FIG. 1 unnecessary. Thus the sprayers 61 and 67 of FIG. 3 are sufficient to efficiently sweep the fluid from the spherical optical cavity twice each cycle, in the manner discussed earlier.

FIG. 4 shows a highly reflective spherical optical cavity 80 into which is inserted a highly reflective dual sprayer head 81 containing hot conduits 82 branching out from a central, axial hot passageway 83 within the dual sprayer. The hot conduits terminate in hot ports 84 in the spherical tip of the sprayer 81, whereby the spherical tip serves as the hot pervious surface through which hot fluid (solid arrows) is injected into the optical cavity 80 while cold fluid (dotted arrows) is being swept out or withdrawn from the cavity through cold ports 87 of cold conduits 88 branching out from a cold passageway 89 running just below the cylindrical surface of the rod-shaped dual sprayer 81 in an axial direction (parallel to the axis of the rod or cylinder formed by the sprayer surface). Thus a segment of the cylindrical surface near the sprayer tip serves, by virtue of cold ports 87, as the reflective cold pervious surface of optical chamber 80. This is a very simple design, as far as the optical chamber is concerned, but thermal insulation is especially important here because of the close proximity of the hot and cool passageways and conduits. Cryogenic or Dewar techniques, such as the use of evacuated, internally reflective chambers or envelopes between the hot and cold portions of the sprayer 81, are of value in this dual sprayer design. As with FIG. 3, twice each cycle the arrows reverse. (The arrows in FIG. 1 have a different meaning; the solid and dotted arrows there represent gas flow direction during two different portions of the cycle.)

As discussed in my U.S. Pat. No. 3,836,255, it is difficult to increase the versatility of a multigas analyzer by broadening the wavelength range of a spectrometric gas analyzer to include, simultaneously, those spectral regions best suited, primarily on a sensitivity/cost/size basis, to radiant absorption techniques and those spectral regions best suited to radiant emission techniques. Generally, radiant absorption measurements are best in the ultraviolet, visible and sometimes the near infrared regions of the electromagnetic spectrum, while radiant emission measurements are best in the middle infrared and, in many cases, the near infrared. A very simple, compact and sensitive way of making the infrared emission measurements (to monitor gas concentration) is by periodic compression of a gas sample in or into a highly reflective, random path, optical chamber or cavity. The resulting periodic concentration increase and adiabatic temperature increase of the gaseous sample in the cavity combine to produce a relatively large, periodic, characteristic spectral radiant emission by the substance or gas of interest in the sample in the optical cavity, and the resulting periodic characteristic radiance in the cavity can be monitored by, for example, one or more infrared detectors looking into the cavity through infrared filters appropriate to the gas of interest, whereby, using synchronous rectification, the concentration of the substance of interest in the sampled gaseous medium may be continuously monitored in real time, and recorded or displayed. Various techniques for accomplishing this periodic compression of gas samples and infrared monitoring of the gas of interest are disclosed in my U.S. Pat. Nos. 4,110,618; 4,063,094; and 3,516,745.

If, however, with such an analyzer, one attempts to cover more gases of interest or cover them better by extending the spectral coverage to shorter wavelengths, such as into the ultraviolet, by increasing the compression ratio, one generally finds that thermal decomposition of gases of interest, especially organic molecules, as well as pumping and other instrumentation problems, limit the wavelength extension to the near infrared. If, alternatively, one attempts the wavelength extension toward shorter wavelengths by shining infrared, visible and ultraviolet sources into the optical cavity, with the intent to monitor the modulated radiant absorption by the gases of interest resulting from their periodic concentration variation in the optical cavity, one finds that this can work well in the ultraviolet, visible, and possibly the near infrared portions of the electromagnetic spectrum bu that, in the near and middle infrared, a periodic infrared emission signal is present at the characteristic infrared absorption, and therefore emission, wavelengths of the substances of interest present in the gas sample, as a result of the periodic temperature variation of these substances in the sample, due to the periodic, substantially adiabatic, compression of the gaseous sample in the optical cavity. The periodic infrared emission signal is in phase with the periodic infrared absorption signal, but the two signals are of opposite polarity (as far as the infrared detector is concerned), so they tend to cancel each other (their rectified output signals are of opposite polarity and therefore subtract), thereby reducing sensitivity of the gas analyzer in the infrared. Due to the difference in the blackbody curves at the gas temperature and at the much higher radiant source temperature, and due also to the relatively small size of the radiant source as compared with the size of the optical cavity (e.g., on a surface area basis, weighted by emissivity), the infrared emission signal gradually becomes larger, and the infrared absorption signal gradually becomes smaller, as the wavelengths of observation are shifted from the visible toward longer wavelengths in the infrared. Thereby, the canceling increases toward longer wavelengths and the sensitivity decreases, until, at a crossover wavelength in the infrared, the amplitudes of the periodic emission and the periodic absorption are equal, whereby the cancelation is complete and the sensitivity is zero. At wavelengths longer than the crossover wavelength, the emission signal predominates over the absorption signal, and the sensitivity gradually climbs back up. But over a substantial spectral interval including, and extending on each side of, the crossover wavelength, the sensitivity of such a gas analyzer would not be adequate for most purposes.

The location of the crossover wavelength, at which sensitivity is zero, depends on instrument parameters, but would typically be in the near or middle infrared, both valuable spectral regions for detecting and monitoring gases of interest. If one attempts to solve the canceling problem by increasing the radiant power of the radiant source in the infrared, which will shift the crossover wavelength to longer wavelengths of the infrared, the increased size, weight, power requirement and cost of the radiant source, as well as the increased heat load from the now powerful radiant source make the instrument less desirable and practical, especially as a portable multigas analyzer, and especially also when taken in conjunction with the already substantial power requirement of the pump or compressor needed for the cyclical compression of the sample.

It was primarily to greatly broaden the spectral range of coverage of a practical, portable multigas analyzer that the device of my U.S. Pat. No. 3,836,255 was invented, and the present invention achieves this also, since both inventions utilize periodic temperature variation of a gaseous sample at constant pressure in a highly reflective optical cavity, which eliminates the canceling problem and facilitates substantial reduction of the pumping and other practical instrument problems mentioned above. The isobaric temperature modulation does cause the gas concentration to be at a minimum in the optical cavity at the time of maximum gas temperature and desired maximum infrared emission by the gas of interest, which drop in concentration does reduce the number of molecules of the gas of interest which can contribute to the infrared emission at this time or phase of the cycle, thereby reducing the amplitude of the spectral infrared emission signal that otherwise could have been generated in the optical cavity had the same amplitude of temperature modulation occurred without the inverse concentration modulation, but, due to the nonlinearlity of the blackbody curve with respect to temperature, the temperature factor is much more important than the concentration factor in determining infrared emission from a gas, so that the periodic spectral infrared emission by the gas of interest in the optical cavity is more than strong enough, in spite of the concentration varying inversely with the gas temperature at constant pressure, to facilitate the high sensitivity discussed above. In other words, in the infrared, and especially in the near and middle infrared, e.g., 1-15.times.10.sup.-6 meters in wavelength (usually 0.72-3.0.times.10.sup.- 6 meters is considered the near infrared; 3.5-20.times.10.sup.-6 meters the intermediate or middle infrared; and 20-1000.times.10.sup.-6 meters the far infrared region), the radiant emission from a gas at very low concentrations (which is when sensitivity is so important) varies with the first power of (linearly with) gas concentration, while the radiant emission varies much faster than this with gas temperature (a power much greater than unity). Thus, in a constant pressure system, the radiant emission is in phase with the gas temperature variation and, in spite of the gas concentration varying at 180.degree. out-of-phase with respect to the temperature variation, the periodic spectral emission signals by the gases of interest in the optical cavity typically are large enough in amplitude to permit, on a signal-to-detector noise basis, part-per-billion detection of many gases of interest simultaneously, in real time, by a practical, portable, multigas analyzer, assuming that the instrument is properly designed. To this end, for example, the optical cavities in FIGS. 1, 3 and 4 (optical cavities 1, 60 and 80, respectively), including the hot plate 2 and hot sprayer 5 in FIG. 2, are made as highly reflective (internally, of course) as possible and practical, considering the ports, the spectral range of the gas analyzer, and the chemical stability needed for the material of the surfaces of the optical cavities. But if the instrument is built properly, the inherent high sensitivity of the random path optical chamber, with its long effective optical path length (many times its diameter) and with its relatively large quantity of gas sample within it, facilitates sensitive, simultaneous monitoring of many substances of interest in a gaseous medium, in almost real time (relatively short response time) by a practical, rugged, portable device which is a multigas analyzer in the full sense of one device being able to sensitively detect and monitor the concentrations of virtually any gases of interest in a complex gaseous mixture, in a practical, economical way. At least that is the primary goal of the present invention.

Generally, but depending on instrument parameters (especially the nature of the radiant sources selected, and the temperature range of the gas sample temperature modulation), the periodic emission signal by the gaseous sample predominates over its periodic absorption signal in the middle and far infrared (the far infrared is seldom used for these purposes, primarily because of low sensitivity and strong atmospheric interferences, e.g., by water vapor and carbon dioxide; but an emission type gas analyzer, such as the present invention, is better able to use the far infrared than a portable spectrometric gas analyzer not utilizing infrared emission by the sample), while the periodic radiant absorption predominates over the periodic radiant emission by the sample in the ultraviolet and visible and perhaps in the near infrared. But no matter in what spectral region the periodic emission and absorption signals by the gas are simultaneously present in the optical cavity, the two signals add rather than subtract from each other, since, in this isobaric temperature modulation system, the two signals are 180.degree. out of phase (one half cycle out of phase) and of opposite polarity. Thereby, in spectral regions common to both periodic emission and periodic absorption of electromagnetic radiant energy by the gaseous sample in the optical cavity, the emission and absorption signals are phased to augment each other, in algebraic fashion, rather than subtracting and tending to cancel each other, as would be the case if unchopped (unmodulated) radiant sources were used in conjunction with a compressive type gas sample modulation system (chopped radiant sources would avoid the canceling problem but would require double synchronous rectification, i.e., at the chopping frequency and again at the gas sample compression frequency, an additional complication and cost; and compressive type pumps generally introduce pumping type problems, such as vibration, lifetime, servicing power requirement, size, weight, cost and, especially in the case of a piston type compressor, possible generation of unwanted, undesirable radiant signals at the gas compression frequency, e.g., by the changing geometry). Therefore the present invention facilitates relatively high gas analyzer sensitivity throughout the ultraviolet, visible and infrared regions of the spectrum, except for regions of strong interference by constitutents of the gaseous sample, and limited, of course, by the availability of suitable optical, electrical and mechanical components and materials - but these appear to be available.

As disclosed in U.S. Pat. No. 3,319,071, a random path optical cavity should not be perfectly spherical but should be randomized by distortions, such as dimples in the spherical surface while still maintaining high reflectivity of the surface. The randomization minimizes normal modes of reflection which never reach the filter/detector pair window and thus can not contribute to the sensitivity of the gas analyzer. The randomization increases the effective optical path length of the cavity to a value which can be many times the cavity diameter, depending mainly on reflectivity and port and window areas in addition to the randomization, and can increase the radiant signal power at the detectors. In the present invention, the hot and cold sprayers serves as randomizers, in addition to their other function of facilitating the proper injection and withdrawal of hot and cold gas, respectively (hot sprayers inject and withdraw hot gas and cold sprayers inject and withdraw cold gas, into and from the cavity). But dimples can be added if desired (to the surfaces of the substantially spherical cavities 60 and 80 in FIGS. 3 and 4, respectively).

In view of all the above, it should be evident that the present invention, to a significantly greater extent than the prior art, facilitates the difficult task of building a practical, portable, sensitive, truly multigas analyzer in the fullest sense.

It should also be evident, especially to those skilled in the art, that numerous modifications and variations of the embodiments of the present invention specifically described herein can obviously be made that would still fall within the above teachings and within the scope of the appended claims.

It should further be evident that the present invention can also be used as an oscillatory pressure source, e.g., as in my burglar alarm patents, U.S. Pat. Nos. 3,968,482; 3,990,069 and 3,990,063; or as a combined burglar and gas detector (burglar, gas and fire alarm, if products of combustion are detected).