Abstract:
A chemical sensor is disclosed. The sensor has a test chamber for receiving chemicals in a gaseous state, the test chamber having two substantially transparent windows at first and second ends of the test chamber. The sensor uses a pulse operated ultraviolet light emitting diode at the first end of the test chamber emitting at a wavelength close to a maximum in the absorption band of a test chemical, and an electromagnetic sensor at a second end of the test chamber, the sensor being sensitive to the light emitted by the light emitting diode.

Description:
FIELD OF THE INVENTION 
       [0001]    This disclosure relates to the field of noxious gas detection, and more specifically, to detection of gaseous chlorine dioxide. 
       BACKGROUND OF THE INVENTION 
       [0002]    Currently available commercial sensors exist for monitoring or measuring the concentration of chlorine dioxide. However, many of these sensors are developed for regulatory applications which require measurements at very low concentration levels (e.g., parts per billion). At higher concentrations and long exposure durations, existing electrochemical based sensors can be damaged such that they will become inaccurate or inoperative. Additionally, for some applications, the sensor needs to be able to operate accurately at high relative humidity. 
         [0003]    What is needed is a system and method for addressing the above, and related, issues. 
       SUMMARY OF THE INVENTION 
       [0004]    The invention of the present disclosure, in one aspect thereof, comprises a chemical sensor. The sensor has a test chamber for receiving chemicals in a gaseous state, the test chamber having two substantially transparent windows at first and second ends of the test chamber. A pulse operated ultraviolet light emitting diode is at the first end of the test chamber, emitting at a wavelength close to a maximum in the absorption band of a test chemical. An electromagnetic sensor is at a second end of the test chamber, the sensor being sensitive to the light emitted by the light emitting diode. 
         [0005]    In one embodiment, the pulsed light emitting diode is modulated at about 1 kilohertz to increase the signal-to-noise ratio at the electromagnetic sensor. The electromagnetic sensor may be a photodiode. In one embodiment, the photodiode is sensitive to radiation at a wavelength of about 370 nanometers. 
         [0006]    The windows may comprise a material that is resistant to degradation in the presence of chlorine dioxide. The window may comprise a material that is resistant to degradation in the presence of chlorine dioxide at relative humidities above 70%. In one embodiment, the windows material is polyethylene terephthalate (PET). In another embodiment, the windows comprise fluorinated ethylene propylene (FEP). 
         [0007]    The pulsed ultraviolet light emitting diode may operate in an on state for about 50 milliseconds per pulse and may remain in an off state for about 5 seconds between pulses. 
         [0008]    The test chamber may be elongated and have an inlet port and an outlet port. In some embodiments, a beam splitter interposes the pulsed ultraviolet light emitting diode and the window at the first end of the test chamber. The beam splitter directs a portion of the electromagnetic radiation from the ultraviolet light emitting diode to a reference diode and passes a remainder of the radiation into the test chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a cross-sectional view of one embodiment of a chlorine dioxide sensor according to aspects of the present disclosure. 
           [0010]      FIG. 2  is a schematic view of the chlorine dioxide sensor of  FIG. 1  with attached control and detection circuitry. 
           [0011]      FIG. 3  is a perspective view of the chlorine dioxide sensor of  FIG. 1  with attached control and detection circuitry. 
           [0012]      FIG. 4  is a perspective view of the chlorine dioxide sensor of  FIG. 1  with an opaque enclosure. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    Referring to  FIG. 1 , a cross-sectional view of one embodiment of a chlorine dioxide (ClO 2 ) sensor according to aspects of the present disclosure is shown. The chlorine dioxide sensor  100  is designed to detect chlorine dioxide in a gaseous state. Quantification may be achieved for minute (parts per billion) quantities up to very high concentrations. A test chamber  104  is defined by one or more test chamber walls  108 . Although the view of  FIG. 1  is a cross-sectional view, it will be appreciated that there may be four walls  108  providing a rectangular test chamber  104 . In other embodiments, the test chamber  104  may be tubular and the walls  108  may represent a single tubular circumferential wall surrounding the test chamber  104 . At opposite ends of the test chamber  104  are a source end  102  and a detector end  106 . 
         [0014]    In the present embodiment, an ultraviolet light emitting diode (UV LED) is provided for energizing the test chamber  104 . The presence of chlorine dioxide and/or the concentration thereof is determined by spectrographic analysis of the sample in the test chamber  104 . In the present embodiment, the UV LED  110  provides a beam of electromagnetic energy in the ultraviolet wavelength region. The beam is split by a beam splitter  112  that may comprise a quartz plate. A portion of the energy from the UV LED  110  may be directed by the beam splitter  112  to a reference diode  114 . The remainder of the energy from the UV LED  110  that is not reflected to the reference diode  114  will pass from the source end  102  through the test chamber  104  to the detector end  106 . The path of the beam is illustrated by line  115 . The detector end  106  is equipped with a detector diode  116 . 
         [0015]    In operation, the UV LED  110  will be active or illuminated in a pulsed fashion. Illuminating the test chamber  104  in a pulsed or periodic fashion will minimize photochemical reactions with chlorine dioxide within the test chamber  104 . In one embodiment, the UV LED  110  will be periodically activated. For example, activation for about 50 milliseconds approximately every 5 seconds. To increase the signal-to-noise of the detected radiation, the UV LED  110  can also be modulated at frequencies in the kilohertz range (e.g., about 1 to about 10 kilohertz). 
         [0016]    The UV LED  110  may provide electromagnetic radiation generally toward the ultraviolet portion of the spectrum but other wavelengths of light could be present in the beam as well. In the particular embodiment shown, the UV LED  110  will at least transmit around the wavelength of 370 nanometers. Both the reference diode  114  and the detector diode  116  will be sensitive to radiation at this wavelength. In this manner, the reference diode  114  may be activated to indicate that the UV LED  110  is active. Furthermore, the reference diode  114  may be used to compensate for any intensity variations in the output of the UV LED  110 . 
         [0017]    Due in part to the corrosive and photochemical effects that may occur with chlorine dioxide, the test chamber walls  108 , as well as any other components of the sensor  100  that come in contact with chlorine dioxide, will need to be made of a material that is suitably resilient to the effects of chlorine dioxide. The materials may also be resistant to the effects of chlorine dioxide in a high humidity environment (e.g., above 70% relative humidity). In one embodiment, the components may comprise polyethylene terephthalate (PET). In another embodiment, the components may comprise fluorinated ethylene propylene (FEP). PET and FEP are two examples of materials that would be suitable to construct the sensor  100 . 
         [0018]    It is contemplated that the sensor  100  may be installed and used more or less continuously for an indeterminate period of time. In the embodiment shown, one or more of the test chamber walls  108  will define opening  118 ,  120  for introduction of test gases into the test chamber  104  and for removal of the tested sample. In one embodiment, it is contemplated that gas flow through the test chamber will be more or less continuous 
         [0019]    In one embodiment, the source end  102  comprises the UV LED  110 , the beam splitter  112 , and the reference diode  114 . The construction of the beam splitter  112  in combination with the UV LED  110  and reference diode  114  may attach to an end plate  202 . The end plate  202  may define a first end of the test chamber  104 . The end plate  202  in the present embodiment is comprised of aluminum. 
         [0020]    The end plate  202  may define a passage  203  to allow passage of the beam from the UV LED  110 . A film holder  204 , which may comprise PET, attaches against the end plate  202 . The film holder also defines a passage  205 . Retained by the film holder  204  is a thin film  208 . In the present embodiment, the thin film  208  comprises PET. The thin film  208  may comprise high-purity PET having a thickness of about 0.004 inches. A high purity film of PET that is sufficiently thin will not interfere substantially with the optics and operation of the sensor  100 . The thin film  208  provides a substantially transparent window into the test chamber  104 . As described, the components of the sensor  100  coming in contact with the sample, which may include chlorine dioxide, must be made from a resilient material. Thus, the end plate  202  is protected by the film  208  and the film holder  204 . 
         [0021]    An O-ring  206  may be provided to seal the end of the test chamber  104 . The O-ring may comprise a fluorocarbon elastomer such as Viton®. Viton® will be less resilient against the corrosive effects of the chlorine dioxide than those components comprising PET. However, the Viton® will adequately seal the test chamber  104  and will be somewhat protected from the chlorine dioxide by the configuration of the end  102  of the sensor  100 . It can be seen that the end plate  202  and film holder  204  project inward to the chamber  104  against the walls  108 . This will prevent the O-ring  206  from being in the primary gas flow during operation and will serve to decrease the corrosive effects of the chlorine dioxide. 
         [0022]    The detector end  106  comprises another end plate  302 , which also positions the detector diode  116  in the center of the beam path  115 . The end plate  302  may comprise aluminum. Attached on the outside of the end plate  302  is the detector diode  116 . An end cap  304  attaches to the inside of the end plate  302 . The end cap  304  may comprise PET and define a passage  305 . The end cap also protects the end plate  302  from coming in contact with the sample, which may include chlorine dioxide. The passage  305  allows for maximum transmission of the test beam from the test chamber  104  to the detector diode  116 . In some embodiments, the end cap  304  retains another thin film  308  against the end plate  302  and the photodiode  116 . This film  308  may also comprise high purity PET having thickness of about 0.004 inches. This will allow the beam to exit the test chamber  104  and reach the detector diode  116  substantially unaffected. The thin film  308  provides a substantially transparent window into the test chamber  104 . 
         [0023]    As before, a Viton® O-ring  306  is provided to seal the end  106  of the sensor  100 . The configuration of the second end  106  of the sensor  100  will keep the O-ring  306  out of the main gas flow and serve as some protection against the corrosive effects of the chlorine dioxide on the O-ring. 
         [0024]    The detector diode  116  is configured to detect absorption around the ultraviolet absorbance maximum of the chlorine dioxide spectra. Thus, the LED beam traveling from the UV LED  110  through the test chamber  104  and striking the detector diode  116  will have been altered in the presence of chlorine dioxide gas. This alteration may present itself in the form of a loss in intensity of the test beam. 
         [0025]    As can be seen in  FIG. 1 , various plastic screws may be used to hold the major components of the sensor  100  in place. In one embodiment, the assembly screws will be nylon. However, in other embodiments, the components may be assembled and epoxied or glued in place. For example, the walls  108  may be epoxied together and against the end plates  202 ,  302 . The film holder  204  and end cap  304  may also be epoxied in place against the insides of the end plates  202 ,  302 , respectively. 
         [0026]    Referring now to  FIG. 2 , a schematic diagram of the chlorine dioxide sensor of  FIG. 1  with attached control and detection circuitry  402  is shown. The control and detection circuitry  402  provides power and control to the UV LED  110  as needed. The control and detection circuitry  402  connects to the UV LED  110  via wire leads  410 . The control and detection circuitry  402  connects to the reference diode  114  and the detector diode  116  via wire leads  412  and  414 , respectively. 
         [0027]    The detection circuitry  402  may read and/or compare the output from the LEDs  114 ,  116  to determine whether there is a significant absorption that would be indicative of chlorine dioxide within the test chamber. The concentration of chlorine dioxide in the test chamber  104  may also be discernable. In one embodiment, detection and control circuitry may comprise one or more integrated circuits and/or discrete analog or digital components. 
         [0028]    The functionality of the circuitry  402  could also be provided in software operating on a general purpose computer or integrated circuit. In addition, the circuitry  402  may provide the necessary signal conditioning and amplification to ensure adequate and usable readings from the diodes  114 ,  116 . The output from the circuitry  402  may be an analog voltage or a digital reading. In some embodiments, the circuitry  402  will be equipped to log or record readings for later retrieval. 
         [0029]    In one embodiment, the photocurrent of the detector diode  116  will be measured by the circuitry  402 . A change in photocurrent of the detector diode  116  may correspond to the presence and/or concentration of chlorine dioxide. Enhanced signal-to-noise ratio may be obtained at the detector circuit  402  by modulating the UV LED  110  at several kilohertz. In some embodiments, the reference diode  114  will measure the output of the UV LED  110  to correct for possible variations in the output of the UV LED  110 . The photocurrent of the reference diode  114  will be proportional to the output of the UV LED  110  regardless of the presence of chlorine dioxide because the UV light incident on the reference diode  114  will not pass through the test chamber. 
         [0030]    In  FIG. 2 , environmental gas is shown as a cloud  404 . The environmental gas  402  may be ambient atmosphere that is to be tested for chlorine dioxide. The gas  402  may also be the output or input of a specific process for which monitoring is desired. In some embodiments, a conduit or input port  406  may be provided for delivering the test sample to the sensor  100 . An outlet conduit or port  408  may be provided for exhausting the tested sample. In some embodiments, the ports  406 ,  408  provide a continuously refreshed test sample into the openings  118 ,  120  ( FIG. 1 ) of the test chamber. Positive pressure and/or vacuum may be used depending upon the needs of the user. 
         [0031]    Referring now to  FIG. 3 , a perspective view of the chlorine dioxide sensor of  FIG. 1  with attached control and detection circuitry is shown. Here, the overall physical shape of the sensor  100  can be seen. In the embodiment shown, the test chamber is defined in part by a single cylindrical wall  108 . The detection and control circuitry  402  has been mounted to the wall  108  outside the test chamber. The leads  414  connecting the control circuitry  402  to the detector diode  116  can be seen leading to the detector end  106  of the sensor  100 . Similarly, the leads  410 ,  412  connecting the circuitry  402  to the UV LED  110  and the reference diode  114 , respectively, can also be seen leading to the source end  102 . 
         [0032]    Referring now to  FIG. 4 , a perspective view of the chlorine dioxide sensor of  FIG. 1  with an opaque enclosure  420  is shown. Due to the detrimental photochemical reactions that are possible with chlorine dioxide, the test chamber  104  may need to be shielded from ambient light. Depending upon the wavelength of the ambient light, the diodes  114 ,  116  may also benefit from shielding. The detection electronics may also benefit from shielding to reduce possible electromagnetic interferences. In the present embodiment, an opaque casing  420  has been fitted around the entire sensor  100 . In order to provide test samples to the sensors  100 , ports  406 ,  408  are provided in communication with the openings  118 ,  120  of the wall  108  of the test chamber  104 . Using positive pressure and/or vacuum, a continuous or intermittent test sample may be provided. In order to power the circuitry  402  and to read the output or results of the test, a communication and power port  412  may be provided. The enclosure  420  itself may comprise an appropriate metal, plastic, or other material of suitable opacity and resiliency to the environment in which the sensor  100  will be used. 
         [0033]    Thus, the present invention is well adapted to carry out the objectives and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those of ordinary skill in the art. Such changes and modifications are encompassed within the spirit of this invention as defined by the claims.