Abstract:
A particle generating system includes an aerosol generator, an ejector diluter, and an aerosol diluter. The ejector diluter receives the generated aerosol and dilutes the aerosol to an expected raw concentration. The aerosol diluter further dilutes the aerosol to a concentration in the range of 0% to 100% of the expected raw concentration. The aerosol diluter includes a mini cyclone for diluting the aerosol. The particle generating system may be configured to provide variable concentrations of monodisperse or polydisperse aerosols for instrument calibration. The system may provide constant concentrations in the range of 0% to 100% of the raw concentration. The mini cyclone makes the system compact, and the system may be portable.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to particle generating systems and to calibrating particle instruments. 
   2. Background Art 
   Particle instruments have been widely applied to detect particulate matter level (mass and number) in ambient air, specific environments, and combustion engines, etc. To ensure that these instruments perform accurately, frequent calibrations with different constant concentration aerosols are extremely necessary. 
   Currently, aerosol generators, such as atomizer and propane burner, etc., have been widely used to generate particles. Many different types of diluters have been applied to dilute particles to different concentrations as well. However, the dilution ratio range is narrow, and does not provide concentration in the range of 0% to 100%. 
   By combining these two techniques, calibration aerosol is available to calibrate particle instruments. Since aerosol generators and diluters are in separate units, the units must be put together correctly to generate the expected concentration aerosol. The setup procedure is time-consuming, and not efficient. Many variations may be involved during the setup. As a result, greater uncertainties may be introduced into the calibrated instrument. 
   SUMMARY OF THE INVENTION 
   It is an objective of the invention to provide an improved particle generating system. 
   The invention involves generating well known concentrations of mono- or polydisperse aerosol for instrument calibration. In a preferred implementation, accurate aerosol concentration is available in the range of 0% to 100% of the raw aerosol by diluting the raw aerosol. 
   In accordance with the invention, a wide range constant concentration particle generating system provides calibration aerosol with well known characteristics. In one aspect, the invention may involve integrating an aerosol generator, ejector diluter, and aerosol diluter into a single system. 
   In an exemplary approach to carrying out the invention, an ejector diluter dilutes aerosol to the expected raw concentration. An aerosol diluter dilutes the aerosol further to a concentration in the range of 0% to 100% of the raw concentration. According to the invention, a mini cyclone on the aerosol diluter is used to mix aerosol, which dilutes raw aerosol from the ejector diluter and aerosol generator. Since the cyclone results in low particle losses and the aerosol diluter provides accurate dilution ratios, accurate concentration in the expected size range and similar size distributions are obtained in the range of 0% to 100% of the raw aerosol concentration. In this particular approach, the cyclone removes large size particles as well. And so as a result, this protects calibrated instruments from malfunctions. By using the mini cyclone instead of a traditional tunnel for mixing, the actual size of the diluter may be reduced. A PID loop controls the dilution ratio as constant on the aerosol diluter when a constant dilution ratio and concentration are expected. 
   The aerosol generator generates polydisperse aerosol under most circumstances. By feeding the aerosol generated by the aerosol generator into a size instrument, such as a differential mobility analyzer (DMA), monodisperse aerosol with 0% to 100% of raw concentration becomes available. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts a flow schematic for a wide range constant concentration particle generating system that provides polydisperse aerosol for calibration in accordance with a preferred embodiment of the invention; 
       FIG. 2  depicts a flow schematic for a wide range constant concentration particle generating system that provides monodisperse aerosol for calibration in accordance with a preferred embodiment of the invention; 
       FIG. 3  illustrates an ejector diluter; 
       FIG. 4  illustrates an aerosol diluter; and 
       FIG. 5  illustrates an alternative design of the aerosol diluter. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   As shown in  FIGS. 1 and 2 , in the preferred embodiment, the wide range constant concentration particle generating system includes aerosol generator  10 , aerosol conditioning unit  12 , neutralizer  14 , high efficiency particle filter (HEPA)  16 , ejector diluter  18 , aerosol diluter  20 , and control system  22 . The system also includes a suitable vacuum source, and particle-free compressed air, etc.  FIGS. 1 and 2  show the flow schematic of the system for poly- and monodisperse aerosol, respectively. 
   In  FIG. 1 , aerosol generator  10  generates aerosol by atomizing a liquid solution with compressed air, or combusting propane or diesel fuel on a burner, or other means. The type of generator used is determined by the calibrated instruments and their applications. For example, if the instrument is a condensation particle counter, which measures the particle number concentration, the atomizer is a good choice for the generator. 
   After aerosol flows into conditioning unit  12 , water or liquid drops or vapor is removed. Then, the aerosol moves into neutralizer  14 . By adjusting needle valve  15 , the extra flow can be vented from HEPA filter  16 , where particles in aerosol are removed, if the flow required in ejector diluter  18  is less than that generated by aerosol generator  10 . Under some circumstances, the flow is sucked into ejector  18  through HEPA filter  16 , where particles in ambient air are removed, if generator  10  does not generate enough flow for ejector diluter  18 . With an inlet open on HEPA filter  16 , the flow pressure in the aerosol can be stabilized. 
   In neutralizer  14 , the aerosol is charged to Boltzmann equilibrium. As a result, particle losses, which are caused by static charges on particles, are reduced. The aerosol is diluted in ejector diluter  18  to the expected concentration. Partial flow from ejector  18  moves into aerosol diluter  20 . A large fraction of the aerosol is vented. 
   When the dilution ratio is 1:1 on aerosol diluter  20 , the concentration of aerosol from ejector diluter  18  is measured by calibrated instrument  24 . This number is recorded and saved in computer  22  as the raw concentration of the aerosol. By inputting the expected percentage concentration from the computer, the computer and PID loop in control software control the aerosol diluter  20  to the expected dilution ratios. 100% concentration means no dilution on the aerosol, and 0% concentration means no aerosol into the calibrated instrument  24 . 
   In  FIG. 2 , the aerosol flowing from neutralizer  14  is connected to a differential mobility analyzer (DMA)  26  instead of directly to ejector diluter  18 . DMA  26  can output single size (monodisperse) particles by running at constant voltage. The monodisperse aerosol flows into ejector diluter  18 , which functions to vent or compensate the flow from DMA  26  while the aerosol from DMA  26  is higher or lower than that expected. Except for these noted differences, operation of the system for monodisperse aerosol is the same as operation of the system for polydisperse aerosol. 
     FIG. 3  illustrates the flow schematic of ejector diluter  18  in more detail. The flow schematic includes ejector  30 , orifice  32 , pressure regulator  34 , and pressure gauge  36 , HEPA filter  38 , as well as the particle free compressed air and by-pass. 
   Ejector  30  is operated by particle free compressed air. When compressed air flows through ejector  30 , vacuum is generated at the inlet side of ejector  30 . The vacuum sucks the aerosol flow, which is from neutralizer  14  or DMA  26 , into the ejector. Aerosol is mixed with particle free compressed air quickly and uniformly in the ejector. Most of the mixture from ejector  30  is vented, and a small fraction of the mixture flows into the aerosol diluter. 
   With a specific size orifice  32 , different dilution ratios can be obtained by adjusting the pressure of the compressed air. Under most circumstances, the greater the compressed air pressure is, the higher the dilution ratio is. Put another way, the lower the compressed air pressure is, the lower the dilution ratio is. 
   The size of orifice  32  is the other major factor to adjust dilution ratio on ejector  30 . With a larger size orifice, a smaller dilution ratio can be obtained. Put another way, a greater concentration of the aerosol can be obtained. With a smaller size orifice, a greater dilution ratio and lower aerosol concentration can be obtained. 
   In the case where polydisperse aerosol is expected, ejector diluter  30  receives the aerosol from neutralizer  14  directly. HEPA filter  38  should be closed by plug  40 , because HEPA filter  16  and needle valve  15  ( FIG. 1 ) upstream of the neutralizer can ensure the right amount of flow into the ejector diluter by venting or sucking extra flow. 
   In the case where monodisperse aerosol is expected, the aerosol from neutralizer  14  moves into differential mobility analyzer (DMA)  26  ( FIG. 2 ). DMA  26  selects single size particles by running at a fixed column voltage. A column voltage is related to a specific particle size. DMA  26  outputs constant air flow as well. This flow may be greater or less than that required by ejector diluter  18 . 
   With continuing reference to  FIG. 3 , by taking off the plug  40  connected to HEPA filter  38  on the ejector diluter, the flow into the ejector diluter can be adjusted automatically. For example, when the DMA is not able to provide enough flow to the ejector diluter, ambient air filtered by the HEPA filter  38  moves into and mixes with the aerosol from the DMA; when the DMA provides more flow than that required by the ejector diluter, the extra flow from the DMA is vented through the HEPA filter  38 . As a result, the adjustment of the dilution ratio on the ejector diluter does not influence the performance of the DMA. 
     FIG. 4  illustrates the flow schematic of the aerosol diluter in more detail. This includes mass flow controller  60 , mass flow controller  62 , mini cyclone  64 , and vacuum pump  66 . 
   Aerosol from the ejector diluter moves into aerosol diluter  20 , and uniformly mixes with particle free compressed air in mini cyclone  64 . Particles larger than  2 . 5  micrometers are removed by cyclone  64 , and cyclone  64  protects the calibrated instrument from malfunction caused by large size particles. Flow rates of the dilution air and total flow are controlled by the two mass flow controllers  60 ,  62 . The computer software and hardware control these flow rates to obtain the expected dilution ratio or aerosol concentration. The well known flow rate of aerosol moves into the calibrated instrument  24 . The extra flow is evacuated by vacuum pump  66 . 
   The following equations show the calculation of the dilution ratio and concentration: 
   
     
       
         
           Dr 
           = 
           
             
               
                 Q 
                 totalflowrate 
               
               + 
               
                 Q 
                 instrument 
               
             
             
               
                 ( 
                 
                   
                     Q 
                     totalflowrate 
                   
                   + 
                   
                     Q 
                     instrument 
                   
                 
                 ) 
               
               - 
               
                 Q 
                 dilutionair 
               
             
           
         
       
     
     
       
         
           C 
           = 
           
             
               
                 C 
                 raw 
               
               Dr 
             
             = 
             
               p 
               * 
               
                 C 
                 raw 
               
             
           
         
       
     
     
       
         
           p 
           = 
           
             1 
             Dr 
           
         
       
     
   
   Where, Q totalflowrate  is total flow through the flow controller; Q instrument  is well defined flow rate to the calibrated instrument; Q dilutionair  is the dilution air flow rate; C raw  is aerosol concentration from the ejector diluter; Dr is the dilution ratio on the aerosol diluter; C is expected concentration; p is the percentage concentration in 0 to 100%. All flow rates above are at standard condition or the same reference condition. 
   To have 100% concentration, the dilution air flow is zero. As a result, raw aerosol from the ejector only moves into the cyclone. To have 0% concentration aerosol into the calibrated instrument, Q dilutionair  should equal to or be larger than Q totalflowrate +Q instrument  in the above equations. As a result, no aerosol flow moves into the aerosol diluter. 
   When the constant concentration of the aerosol is expected, the dilution ratio on the aerosol diluter needs to keep as constant. A PID loop ( FIGS. 1 and 2 ) has been built to control the dilution ratio at the constant. By comparing the set point of the dilution ratio or the percentage concentration to the real value, the PID loop adjusts the flow rate of the dilution air. As a result, constant dilution ratio is maintained. 
     FIG. 5  shows the alternative design of the aerosol diluter at  70 . The critical orifice  72  and a mass flow meter  74  replace the mass flow controller  62  shown in  FIG. 4 . This provides the same function as the mass flow controller for the total flow control. By changing the size of the critical orifice  72 , different total flows can be obtained. 
   While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.