Patent Application
20130111975
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.
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.
Currently Electrochemical cells are the most effective approach to monitoring low-level NH3 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't use most electrochemical cells for NH3 detection as the electrolytes redox reaction requires O2 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 NH3 exposure occur, or if a small long-term background is present.
Additional applications where most electrochemical technology is not practical occur in chicken houses because of the constant background of NH3 found in urine, live stock indoor air quality control because of the constant background of NH3 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 CO2 or N2 filled rooms to delay fruit ripening doesn'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.
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 NH3 down or cause the cell to no longer detect NH3. This creates a safety hazard as there would not be sufficient time to warn individuals near the sensor of the presence of NH3 causing a safety hazard should an NH3 leak occur.
Further, NDIR-type detectors that use the popular 3.3 μm band are costly and do not function as well because NH3 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.
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.
Embodiments hereof can implement multi-band multi-cavity photo-acoustic detectors which for example can utilize the two most intense absorption bands of NH3 at 10.4 μm and 10.7 μm. verses the lower popular 3.3 μm single band for NH3 gas detectors. NH3 has a very low absorption characteristic.
In another embodiment, the disclosed approach could also monitor H2O vapor levels as they exist throughout the entire NH3 absorption bands. Alternately, two different analyte gasses could be measured simultaneously using a single source.
When implemented to sense NH3, 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 NH3 levels, hence, more accurate detection and lower LDL thresholds relative to known non-electrochemical technology detectors.
By way of example, with respect to
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.
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 NH3, 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 NH3 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.
Optical filters 16a, b correspond to two different absorption bands 1, 2 for a selected gas, for example NH3 and provide an optical input port to the respective cavities 14a, b.
An infrared source 18 is positioned adjacent to both of the filters 16a, b as a common source of radiant energy. A common microphone 20, coupled to the cavities 14a, b by a coupling channel 14c, additively receives acoustic signals indicative of the absorption from the cavities 14a, b. Output signals from the microphone 20, as best seen in
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 26a. Processor 26a can execute prestored instructions 26b in processing signals from the sensor 12.
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.
Embodiments hereof include two adjoining cavities, such as 14a, 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.
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
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.
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 CO2 and CO measurements which can be made simultaneously without the necessity for a multitude of individual gas sensors when multiple-gas sensing is required.
Optical filters 56a, b correspond to two different absorption bands 1, 2 for two different selected gases, for example CO2, CO and provide an optical input port to the respective cavities 54a, b.
An infrared source 18 is positioned adjacent to both of the filters 56a, b as a common source of radiant energy. First and second microphones 58a, b, are coupled to the respective cavities 54a, b by respective coupling channels 58c, d. The microphones 58a, b each receive acoustic signals indicative of the absorption from the cavities 54a, b. Output signals from the microphones 58a, b, as best seen in
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 60a. Processor 60a can execute pre-stored instructions 60b in processing signals from the sensor 12-1.
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.
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.
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.
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.
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.
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.
