Abstract:
A single gas detector combines a dual cavity photo-acoustic gas sensor with a common microphone and common source. Electrical outputs from the microphone can be analyzed to determine an analyte gas concentration in the local region being monitored. Radiant energy from the common source can be directed into both cavities simultaneously. Alternately, the sensor can be used with two microphones to establish a concentration of each of two different gasses, or a gas and water vapor.

Description:
FIELD 
       [0001]    The application pertains to gas detectors. More particularly, the application pertains to such detectors which include multiple absorption detection bands in a photo-acoustic-type detector. Ammonia is a very desirable refrigerant as its GWP (Global Warming Potential) is zero because of its very low narrow IR absorption characteristics. It is abundantly available, found all throughout nature, and extremely efficient in refrigeration systems requiring less energy to be utilized per BTU output. The low absorption property makes it difficult for conventional low-cost IR type gas detectors to accurately read and detect low-ppm levels. 
       BACKGROUND 
       [0002]    Ammonia is the preferred low-cost environmentally friendly refrigerant used for most industrial food processes, cold storage, and pharmaceutical applications. Known commercially available ammonia detectors usually incorporate electrochemical sensor cells. 
         [0003]    Currently Electrochemical cells are the most effective approach to monitoring low-level NH 3  levels. Cells are costly as they degrade over time requiring frequent replacement and the associated electronics add an additional cost and complexity to the sensor. Additionally, electrochemical cells have a limited life time as the electrolyte eventually dries up and this is accelerated in dry hot environments. Also, applications that have zero oxygen environments can&#39;t use most electrochemical cells for NH 3  detection as the electrolytes redox reaction requires O 2  to be present. The redox reaction within the electrolyte is also non-regenerative causing the constituents to be consumed that eventually cause the cell to stop functioning should a large NH 3  exposure occur, or if a small long-term background is present. 
         [0004]    Additional applications where most electrochemical technology is not practical occur in chicken houses because of the constant background of NH 3  found in urine, live stock indoor air quality control because of the constant background of NH 3  in urine, dry/hot conditions. “Zero Oxygen” applications commonly found in fruit cold storage. Fruit storage or any environment requiring oxygen to be displaced using CO 2  or N 2  filled rooms to delay fruit ripening doesn&#39;t allow electrochemical cells to detect ammonia. Solar panel manufacturing also requires some oxygen to be displaced to reduce spontaneous explosions from chemicals used in the manufacturing process. 
         [0005]    Another problematic issue with electrochemical cells is the destruction of the electrolyte and the internal electrodes within the cell when exposed to VOC evaporates. A multitude of cleaning and solvent chemicals can destroy the integrity of most electrochemical cells slowing the response to NH 3  down or cause the cell to no longer detect NH 3 . This creates a safety hazard as there would not be sufficient time to warn individuals near the sensor of the presence of NH 3  causing a safety hazard should an NH 3  leak occur. 
         [0006]    Further, NDIR-type detectors that use the popular 3.3 μm band are costly and do not function as well because NH 3  has low IR absorption characteristics requiring high-gain sensitive components, highly polished surfaces, long path lengths, and complex precision optics. Ammonia has a very low absorption characteristic at this wavelength. In general, NDIR detectors are not as practical in ammonia gas detectors at the 3.3 μm, 10.4 μm, or 10.75 μm bands because the absorption bands very narrow, making it difficult to achieve stable signal levels for accurate detection readings near the 0-100 ppm range as the zero has a tendency to drift over time with this type of sensing technology. In addition, water vapor has IR absorption throughout the ammonia absorption spectra diluting signal levels and causing false alarms in wet humid environments. In a photo-acoustic system, the “zero” occurs naturally with no analyte present requiring minimal baseline signal correction compared to NDIR systems. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a graph illustrating absorption of ammonia at different wavelengths; 
           [0008]      FIG. 2  is a diagram of a single microphone sensor in accordance herewith; 
           [0009]      FIG. 3  is a block diagram of a single gas detector which incorporates the cavity of  FIG. 2 ; 
           [0010]      FIG. 4  is a diagram of a dual microphone sensor in accordance herewith; and 
           [0011]      FIG. 5  is a block diagram of a detector of two different airborne gases, or one gas and water vapor. 
       
    
    
     DETAILED DESCRIPTION  
       [0012]    While disclosed embodiments can take many different forms, specific embodiments hereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles hereof, as well as the best mode of practicing same, and is not intended to limit the claims hereof to the specific embodiment illustrated. 
         [0013]    Embodiments hereof can implement multi-band multi-cavity photo-acoustic detectors which for example can utilize the two most intense absorption bands of NH 3  at 10.4 μm and 10.7 μm. verses the lower popular 3.3 μm single band for NH 3  gas detectors. NH 3  has a very low absorption characteristic. 
         [0014]    In another embodiment, the disclosed approach could also monitor H 2 O vapor levels as they exist throughout the entire NH 3  absorption bands. Alternately, two different analyte gasses could be measured simultaneously using a single source. 
         [0015]      FIG. 1  is a graph of NH 3  absorption spectra. It illustrates how much greater the absorption of NH 3  is in the 10 micron region compared to the 3 micron region. Thus, various substances, such as gasses, or water vapor can be detected using different absorption wavelengths. 
         [0016]    When implemented to sense NH 3 , the present photo-acoustic multi-band multi-cavity sensors include a photo-acoustic cavity for each of the two highest absorption bands; one at 10.4 μm and another at 10.75 μm. Advantageously, with the present multi-band multi-cavity structures, the absorption energy can be additive. This results in a greater signal-to-noise ratio for low NH 3  levels, hence, more accurate detection and lower LDL thresholds relative to known non-electrochemical technology detectors. 
         [0017]    By way of example, with respect to  FIGS. 2 ,  3  one embodiment would entail a single microphone and single source, with a mutually-coupled dual cavity. Each cavity is specific to one of multiple unique respective absorption band footprints of a single target gas. The photo-acoustic signal is combined from both cavities then coupled to a single microphone for an analyte gas measurement. This combined signal is stronger than the sum of individual signals which allow for weaker gas measurements because the signal to noise ratio is greater. 
         [0018]    Without the ability to combine the PA signal from the two cavities two individual microphones would be required. This configuration would add noise to the overall signal readings and the increased number seals and port openings may increase leakage. 
         [0019]    Use of one microphone in detectors of a single gas reduces background noise, pressure leakage, and volume required, allowing weaker signals to be detected. In the case for NH 3 , given the weak absorption characteristics, this arrangement allows lower concentrations to be detected. With the low-concentration detection capability, the dual-cavity, single microphone approach would be a low-cost solution to electrochemical cells and costly NDIR to measure NH 3  or other difficult to detect gases. In addition, this solution would not degrade over time and the life expectancy would be years longer than the current life expectancy of electrochemical cells. 
         [0020]      FIGS. 2 ,  3  disclose a single gas detector  10 , with a housing  10   a,  configured with a sensor  12 . The sensor  12  has first and second cavities  14   a,  b. Cavities  14   a, b  are covered by a common diffusion membrane  12   a,  shown partly broken away. 
         [0021]    Optical filters  16   a, b  correspond to two different absorption bands  1 ,  2  for a selected gas, for example NH 3  and provide an optical input port to the respective cavities  14   a, b.    
         [0022]    An infrared source  18  is positioned adjacent to both of the filters  16   a, b  as a common source of radiant energy. A common microphone  20 , coupled to the cavities  14   a, b  by a coupling channel  14   c,  additively receives acoustic signals indicative of the absorption from the cavities  14   a, b.  Output signals from the microphone  20 , as best seen in  FIG. 3 , can be filtered, and/or shaped by an analog bandpass filter/amplifier  22 . 
         [0023]    Filtered outputs from filter/amplifier  22  are coupled to an input to control circuits  26 . Control circuits  26  can include an analog-to-digital converter, and be implemented in part by a programmable processor  26   a.  Processor  26   a  can execute prestored instructions  26   b  in processing signals from the sensor  12 . 
         [0024]    Additional aspects of the detector  10  include a manually operable control pad  30 , and a driver  32  coupled to the source  18  in sensor  12 . A gas concentration readout device  34  can be coupled to the control circuits  26  along with a communications interface  36  which can communicate with a displaced monitoring system S via a wired or wireless medium. 
         [0025]    Embodiments hereof include two adjoining cavities, such as  14   a, b , combined with a common infrared source, such as  18 , and, only one microphone, such as  20 . Increased magnitudes of acoustic energy are additively sensed at the microphone  20  from two channels of absorption response. This results in an increased signal-to-noise ratio. Use of a single source reduces power requirements. 
         [0026]    Sensors similar to sensor  12  can be used to simultaneously sense different substances such as two different gases, or a selected gas and water vapor. Again with reference to  FIG. 1 , a cavity can be irradiated at an H 2 O vapor band (outside of the absorption band for the selected gas) to compensate for variance in H 2 O vapor inside the absorption band of a selected gas. 
         [0027]    One embodiment of a dual gas detector includes dual microphones, dual cavities, and a single source. Each cavity is specific to a selected target gas footprint absorption band. The photo-acoustic signal from each individual cavity is directed to a respective microphone for individual analyte gas measurement. 
         [0028]    The above described embodiment allows multiple gasses to be simultaneously measured in one simple sensor configuration that utilizes common hardware. Two different gases can be detected simultaneously by use of a single source. Utilizing a dual cavity sensor in combination with a single source reduces processing power requirements, and hardware costs. Examples include CO 2  and CO measurements which can be made simultaneously without the necessity for a multitude of individual gas sensors when multiple-gas sensing is required. 
         [0029]      FIGS. 4 ,  5  illustrate an exemplary multiple substance detector  50 . Elements common to those in  FIGS. 2 ,  3  have been given the same identification numerals as they were discussed above and need not be described further. 
         [0030]      FIG. 4  discloses a detector  50 , with a housing  50   a,  configured with a dual substance sensor  12 - 1 . The sensor  12 - 1  has first and second cavities  54   a, b.  Cavities  54   a, b  are covered by a common diffusion membrane  12   a,  shown partly broken away. 
         [0031]    Optical filters  56   a, b  correspond to two different absorption bands  1 ,  2  for two different selected gases, for example CO 2 , CO and provide an optical input port to the respective cavities  54   a, b.    
         [0032]    An infrared source  18  is positioned adjacent to both of the filters  56   a, b  as a common source of radiant energy. First and second microphones  58   a, b , are coupled to the respective cavities  54   a, b  by respective coupling channels  58   c, d.  The microphones  58   a, b  each receive acoustic signals indicative of the absorption from the cavities  54   a, b.  Output signals from the microphones  58   a, b,  as best seen in  FIG. 5 , can be filtered, and/or shaped by an analog bandpass filter/amplifiers  22 - 1 ,  22 - 2 . 
         [0033]    Filtered outputs from filter/amplifiers  22 - 1 , - 2  are coupled to inputs to control circuits  60 . Control circuits  60  can include analog-to-digital converters, and be implemented in part by a programmable processor  60   a.  Processor  60   a  can execute pre-stored instructions  60   b  in processing signals from the sensor  12 - 1 . 
         [0034]    Additional aspects of the exemplary detector  50  include a manually operable control pad  30 , and a driver  32  coupled to the source  18  in sensor  12 - 1 . A gas concentration readout device  34  can be coupled to the control circuits  26  along with a communications interface  36  which can communicate with a displaced monitoring system S via a wired or wireless medium. 
         [0035]    In yet another embodiment, water vapor can be sensed and measured. Water vapor is very problematic when attempting to measure a multitude of gasses as water vapor is present over most of the refrigerant band and can cause false readings and humidity signal errors, can be measured. For example, in circumstances where water vapor exists in the analyte absorption band, one cavity can respond to the water vapor absorption band and the other cavity can respond to the target gas absorption band. 
         [0036]    The water vapor signal is then subtracted from the analyte gas signal to produce a gas concentration parameter without water vapor cross sensitivity which results from humidity variations. The water vapor cavity can also be used as a reference cavity for ambient noise cancelation in a dual microphone configuration. 
         [0037]    Those of skill will recognize that multi-substance sensors in accordance herewith can have a variety of physical configurations. All such configurations fall within the spirit and scope of this application. 
         [0038]    From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims. 
         [0039]    Further, logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. Other steps may be provided, or steps may be eliminated, from the described flows, and other components may be add to, or removed from the described embodiments.