Abstract:
A nephelometer for detecting the concentration of particulates in a sample aerosol is provided with a branched flow path with a sample aerosol input, a clean gas input and an output leading to an optical sensor unit. At least one of the inputs has periodic variable flow so that a concentration-modulated gas stream is supplied to the optical sensor unit. The detector output of the sensor unit is processed in synchrony with the concentration modulation to filter out DC components, such as 1/f noise and parasitic instrument noise.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to optical measuring and testing of aerosol samples by particle light scattering, as in a nephelometer instrument, and in particular to reducing DC and parasitic instrument background noise or otherwise separating such noise from the particle scatter signal. 
       BACKGROUND ART 
       [0002]    Nephelometer instruments for measuring the mass per unit volume of ambient aerosols have historically used a light source, such as a diode laser or light-emitting diode (LED), to illuminate a view volume containing particles suspended in a gas carrier. The amount of detected optical scatter from the illuminated particles in the view volume has then been taken as a measure indicative of the amount (mass) of particulate material in the aerosol. Continuous or near-continuous, near-real-time monitoring of the mass, for example in ambient air, can be accomplished by continuously or near-continuously sampling the ambient by pulling the particle carrier gas into and through the instrument view volume. The level of the detected optical signal is then at any time an instantaneous indicator of the mass of particles in the view volume. 
         [0003]    Electronic signals suffer from what is known as “1/f noise”, contributed for example by the detector. Electronic noise, including the 1/f noise, can be removed or filtered by a technique involving modulation of the light source intensity in time. The amount of light scattered by the illuminated particles in the view volume will thus also be modulated away from a near-DC level. Synchronous detection of the modulated scattering is a sensitive method of separating the modulated scattering signal from the essentially steady level of electronic noise. 
         [0004]    Unfortunately, the same detected optical signal also includes a background parasitic scattering from the nephelometer itself, such as scattering from internal optical components or from contaminants within the sensor instruments. Because this parasitic scattering will also be modulated in the same manner as that of the particle scattering, it cannot be separated out by the light modulation technique and forms a background noise level in the instrument underlying the desired particle scatter signal. 
         [0005]    What is needed is a nephelometer instrument and noise filtering technique that can not only remove the 1/f or electronic noise, but also the background parasitic scattering contributions of the instrument, so as to obtain a scattering signal that is a more accurate indicator of ambient particle concentration. 
       SUMMARY DISCLOSURE 
       [0006]    The invention is a nephelometer that provides a way to modulate the proportion of ambient sample gas and thus modulate the concentration of particulates that can enter the view volume. Such modulation can be achieved by periodically injecting clean filtered air into the stream of the particle aerosol sample, thereby diluting the particle concentration. Scattering from the particles is modulated synchronously with the proportion of ambient sample gas, while the parasitic instrument scattering is not. The detected scatter signal can then be readily filtered, e.g., with a lock-in amplifier using the sample modulation rate as a reference frequency, so as to separate the synchronously modulated particle signal from the various near-DC or asynchronous contributions of background noise. 
         [0007]    The optical source in this nephelometer can operate at a constant output level, i.e. with no modulation. Alternatively, the optical source could still be intensity modulated, e.g. to stabilize the light output against optical-feedback-induced laser noise, but with a modulation frequency much higher than the modulation of the particle concentration. As long as the different forms of modulation have highly disparate and incommensurate frequencies, all of the different noise contributions can be filtered by the synchronous detection technique. A higher signal-to-noise ratio and higher instrument sensitivity results. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]      FIG. 1  is a schematic diagram of a first concentration-modulation embodiment of a nephelometer in accord with the present invention. 
           [0009]      FIG. 2  is a schematic diagram of a second concentration-modulation embodiment of a nephelometer in accord with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    With reference to  FIG. 1 , a nephelometer instrument in accord with the present invention includes an optical sensor unit  11 , which may be any of the known particle scattering sensors for such instruments, including a light source, such as a diode laser or light-emitting diode (LED), to illuminate a view volume receiving an aerosol sample having particles suspended in a gas carrier, and a light detector positioned to detect light scattered from the illuminated particles in the view volume. The aerosol or particle-laden gas  13  is received through a sample intake  15 , flows through the view volume of the optical sensor unit  11 , and is then exhausted  19  from the sensor unit&#39;s output  17 . The particles in the aerosol sample may be solid or liquid and scatter the illuminating light. As in the prior instruments, the light source may provide either constant light output or be intensity modulated at some specified modulation frequency. Intensity modulation of the illumination would generally only be needed where optical-feedback-induced noise of the laser source is expected to be a significant problem. In such cases, the optical modulation frequency should be high enough to stabilize the average intensity against the noise. 
         [0011]    The nephelometer instrument also comprises a sample concentration modulation mechanism. In the embodiment of  FIG. 1 , this mechanism includes a HEPA filter  23  for cleaning an intake gas  21 , a non-steady-state pump  25 , such as a diaphragm pump, and flow path  27  for the clean air leading into a Y or T junction  29  coupled to the sample intake  15 . The relative positions of the filter  23  and pump  25  in the gas flow path can be reversed, particularly if the chosen variable pump  25  is likely to create particles of its own. 
         [0012]    In the absence of such a mechanism, a steady or near-steady flow of sample would be pulled through the view volume. However, when activated, the pump  25  periodically introduces pulses of clean air or other gas through one branch of the junction  29  into the flow, thereby periodically diluting the sample. As a result, the input  31  to the optical sensor unit  11  downstream of the junction  29  contains a concentration-modulated aerosol stream as the relative proportions of sample and clean air vary. 
         [0013]    In alternative embodiment, seen in  FIG. 2 , the nephelometer instrument again includes an optical sensor unit  51  into the view volume of which is received an aerosol, or particle-laden gas  53  through a sample intake  55 . The aerosol sample is thereafter exhausted  59  from the sensor unit&#39;s output  57 . This instrument likewise includes a sample concentration modulation mechanism, which in this case comprises a steady-state pump  65 , such as a rotary vane pump, with a gas intake  61 , a HEPA filter  63 , and a variable flow valve  66 , all leading to a Y or T junction  69  coupled to the sample intake  53 . Again, the relative positions of the filter  63 , pump  65 , and valve  66  can be varied as needed to ensure that gas or air in the flow path  67  is clean. When activated, the pump  65  periodically introduces pulses of clean air or other gas through one branch of the junction  69  into the flow, thereby periodically diluting the sample. As a result, the input  71  to the optical sensor unit  51  downstream of the junction  69  contains a concentration-modulated aerosol stream as the relative proportions of sample and clean air vary. While the supply of clean gas is shown here in a preferred embodiment as being varied using the pump  25  or valve  66 , the flow of particle-laden sample aerosol  13  might be varied in addition to or instead of the flow of clean gas. 
         [0014]    In either embodiment, the concentration modulation may have an amplitude that swings from all sample to all clean gas (unity modulation depth), or can have lesser modulation depths (non-unity modulation depth) where the concentration varies from all sample to a mixture of sample and clean gas, or from all clean gas to a mixture of sample and clean gas, or from different mixture proportions of sample and clean gas. The non-unity modulation depths would have a reduced signal-to-noise benefit compared to the unity modulation case, so unity modulation depth is preferred. 
         [0015]    The detector output  37  or  77  from the optical sensor  11  or  51 , representing the detected particle scattering plus noise, is processed in a manner essentially similar to the case of modulated light intensity in order to separate the signal from the noise. In particular, with modulated sample concentration, the particle scattering will modulate according to the relative proportion of sample air. Accordingly, the scattering contribution from the sample can be computed. In particular, the signal processing carried out by the electronics associated with the optical sensor unit  11  or  51  may employ a lock-in amplifier  35  or  75  wherein the detector output  37  or  77  in the sensor unit is mixed with a reference frequency f REF  (from oscillator  33  or  73 ) which is chosen to be the same frequency as the sample concentration modulation rate. Any contribution to the detection signal that is not at the same frequency as the reference frequency will be attenuated essentially to zero by the lock-in amplifier&#39;s low pass filter. While any sample modulation profile provided by the diaphragm pump  25  or variable flow valve  66  could be used, the sample modulation will ideally approach a single frequency sinusoidal profile so that signal and noise are separated sufficiently in the frequency domain. As a lock-in amplifier is phase sensitive, the reference frequency f REF  may also serve as a variable flow control CTRL for the variable pump  25  or variable flow valve  66 , thereby ensuring that the noise filtering by the lock-in amplifier is synchronized with the concentration modulation. The now-filtered detection signal  39  or  79  is output from the lock-in amplifier and may be further processed as in other known nephelometer instruments to obtain particle data for the aerosol sample. 
         [0016]    A nephelometer in accord with the invention will provide effective filtering of both 1/f noise and intrinsic instrumental scattering and give a more accurate and more sensitive particle-sensing instrument.