Abstract:
An impactor assembly comprises a base for supporting a plurality of cups that form impactor plates, and a nozzle above each of the cups through which a flow passes for classification. The cover is removable, and a test cover can be put into position for mounting in place and providing outlets connectable to pressure sensor for determining pressure drop across the nozzle plates at each impactor stage. The cover also is designed to be easily washed by having no blind cavities or moving parts on the cover, and the latch and other hinge assemblies are all supported on the base. The flow enters and exits the impactor without having external connections on the cover.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a Continuation-in-Part of U.S. patent application Ser. No. 09/360,466, filed Jul. 23, 1999 for HIGH ACCURACY AEROSOL IMPACTOR AND MONITOR, now U.S. Pat. No. 6, 431,014 and also is a Continuation-in-Part of U.S. patent application Ser. No. 09/679,936, filed Oct. 5, 2000, for METHOD AND APPARATUS FOR CASCADE IMPACTOR TESTING, both of which applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to improvements to a generally flat side by side cascade impactor, and includes apparatus for testing the integrity of seals, as well as the pressure drop across impactor nozzles or orifice to check the condition of the impactor nozzles. The cascade impactor also includes improvements in mounting and operational features. 
     In the prior art, it has been known to size classify and collect aerosol particles onto impactor plates. When series arranged nozzles are used, it is desirable to determine the condition of the nozzles, and whether or not they are becoming plugged or worn. This can be done by determining the pressure drop across nozzles, and in a cascade impactor, checking the pressure drop from the inlet to the outlet for total flow analysis is desirable. 
     Cascade impactors are widely used for size distribution analysis of aerosol particles, for example, for checking for air pollutants, and for also analyzing the chemical makeup of particles in the atmosphere. Size distribution is important, particularly in the drug delivery industry, where a metered dose of an inhaled drug delivered in aerosol form is tested for particle distribution. In such a cascade impactor it is important to insure the consistency of the test, which is related directly to the size of the nozzle or orifice or openings. The total nozzle or orifice opening in a nozzle or orifice plate can be analyzed by determining the pressure drop at a standard flow rate. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a drug metered dose or dry powder inhaler cascade impactor that has a separate test cover that can be used for determining the pressure drop across individual impactor nozzles, to in turn determine the condition of the nozzles and whether or not there is any plugging, wear or other abnormality. The separate test cover fits in place on the impactor, and when a standard flow rate of a gas, such as air, is passed through the impactor, the pressure drop across the individual nozzles can be sensed, and the total pressure drop also can be sensed. 
     The pressure inputs are provided to a set of pressure transducers, that provide outputs indicating pressure in each of the passageways on opposite sides of each nozzle plate, and thus the pressure drop across one or more of the nozzle plates or total pressure drop can be determined easily. The pressure drop can be used for determining an accurate total flow rate, which is important in the determination of particle distribution, particularly in dry powder inhalers. 
     The test cover that is utilized can also be used for applying a vacuum to the system utilizing a vacuum source, which indicates the integrity of the seals on the individual impaction chambers used in the cascade impactor. 
     Mechanical improvements shown include the placing of the hinges and latch part devices that have cavities on the base plate only, so that the cover, which has passageways for conducting samples can be washed fully between sample runs in automatic washers. The base that is used is not washed each time, and the parts that have hard to clean bores or recesses located on the base. 
     Additionally, a unique latch actuator is utilized that will release the latches using a cam arrangement. Further, the cover is made so it can be quickly removed and replaced. The hinges are constructed so the cover will not close if the cover is not properly positioned relative to the base. 
     A method of milling undercut seal grooves in a seal carrying plate, which are within the boundaries of the plate and do not open to edges where a tool can be introduced is shown. Also, a final filter for the exhaust air is disclosed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of an impactor made according to the present invention; 
     FIG. 2 is a top plan view thereof with parts broken away; 
     FIG. 3 is a top plan view thereof with the top cover removed; 
     FIG. 3A is a bottom plan view with cups and a seal plate removed to show interstage passages on the underside of a cover; 
     FIG. 4 is a sectional view taken as on line  4 — 4  in FIG. 3; 
     FIG. 5 is a sectional view taken as on line  5 — 5  in FIG. 3; 
     FIG. 6 is a sectional view taken as on line  6 — 6  in FIG. 3; 
     FIG. 7 is an enlarged fragmentary perspective view of the cover partially assembled on the hinge to the base; 
     FIG. 8 is a perspective view of the discharge end of the impactor, with the cover in an open position to show a portion of a seal plate; 
     FIG. 9 is a fragmentary perspective view of a latch assembly, including a latch plate and handle plate in an exploded view; 
     FIG. 10 is a side elevational view of right hand latch links and latch lever shown in position for latching and releasing the cover in place on the base; 
     FIG. 11 is a top view of a test fixture cover in place on the base shown in FIGS. 1-10; 
     FIG. 12 is a bottom view of the test fixture of FIG. 11 with parts removed, similar to FIG. 3A, to show the passageways in the test fixture cover; 
     FIG. 13 is a schematic representation of pressure sensors used with the test fixture of claim 11; 
     FIG. 14 is a section view similar to FIG. 4 showing a modified discharge arrangement utilizing a final filter; 
     FIG. 15 is a perspective view of the final filter shown in FIG. 14; 
     FIG. 16 is a fragmentary view of a portion of a seal plate showing a plug used for cutting seal grooves in the seal plate; 
     FIG. 17 is a sectional view taken as on line  17 — 17  in FIG. 16; and 
     FIG. 18 is a schematic representation of a vertically stacked cascade impactor having differential pressure measurements made according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A general form of the invention illustrated in FIGS. 1 through 6 comprises an impactor assembly  10 , which has a housing assembly  12 , with an aerosol inlet  14  of standard size described in USP 24, Section 601. The inlet can be a standard USP type inlet tube. A pre-separator  16  is illustrated on the inlet in FIG. 1, and it is used to separate out large particles. 
     The aerosol that is passed through the impactor  10  is an aerosol generated by a hand-held inhaler  17  or other device that may be a liquid or dry powder drug inhaler, such as those used to control asthma and similar problems. The amount of flow from each charge is small, so the internal volume of the impactor  10  must be kept low. The flow rate through the impactor will be generated in a selected manner, for example by providing a vacuum pump such as that shown at  20  on an exhaust or flow outlet opening  22  (see FIG. 2) from the impactor housing  12 . This type of impactor is described in U.S. patent application Ser. No. 09/679,936, filed Oct. 5, 2000. 
     The impactor  10  has a lid or cover  24  that is sufficiently thick to include flow passageways on the underside. The lid or cover  24  has an opening for the inlet pipe to pass through to seal plate  30 . The lid or cover  24  is hinged along one edge to a base frame  25  that has a number of egg shaped or tear drop shaped openings that receive and support tear drop shaped impactor particle collection chambers or cups as will be shown. 
     As shown in FIGS. 3,  4 ,  5  and  6  a seal plate  30  is positioned just below the cover or lid  24  and has seals in grooves on both sides to seal passageways on the underside of the cover  24  and, on the opposite or bottom side of the seal plate  30 , to seal around lips of each of the impaction chambers or cups to define sealed passageways for forming the flow path. The impaction chambers or cups will be individually numbered in this description. The first cup at the inlet is shown at  34 . The inlet tube  14 A passes through cover  25  and is joined to the seal plate  30  and opens through an inlet opening  32  that is sealed to carry the aerosol through the seal plate  30  into a chamber or passageway  34 A defined by the first impaction stage cup  34 . Cup  34  fits through an opening in a cup retainer tray or frame  36 . The cup  34  has a peripheral flange  34 B that rests on the tray or frame  36 . The cup also fits in an opening  34 E in the frame  25 . The tray or frame  36  is supported on the top of the base  25 . 
     The impaction cups are tear drop shaped as shown. The bottom wall at the large end  34 F of the first stage cup (and all cups) forms the impactor surface and underlies the inlet opening  32 . The flange  34 B of the cup  34  is sealed with a seal  34 D on the seal plate and extends transversely of the impactor to a vertical passageway  38  that opens through the seal plate  30  to interface or crossover passage  40  formed on the underside of the cover  24 . 
     FIG. 3A is a bottom view of the base, with the cups and seal plate removed, so the interstage passages on the underside of the cover  24  can be seen. The openings in the cups on the base frame  25  are designated with the cup number followed by the letter “E”. The seals on the peripheral flanges of the cups follow the shape of the cup openings in frame  25  shown in FIG. 3A, and as shown in dotted lines in FIG.  3 . 
     The crossover or interstage passageway  40  leads to a nozzle passageway or opening in seal plate  30  (FIGS. 3 and 6) having a nozzle  44  that has openings  44 A of desired size, and desired number. Particles will discharge into a second stage impactor surface of a cup  46  held in an opening  46 E of base  25 , under nozzle  44 . The tear drop shaped cup  46  has a wide end under the nozzle  44  and a narrow opposite end. The cup  46  has a flange  46 B for support and defines a passageway  46 A. The small end of the cup  46  aligns with a passageway or port  50  through the seal plate and opens to a tear drop shaped passageway  54  in the cover  24 . 
     The large end  54 B of passageway  54  overlies an opening in seal plate  30  which holds a nozzle  56  that has openings  56 A. Nozzle  56  overlies a tear drop shaped cup  58 . The openings  56 A are smaller and greater in number than the openings  44 A, and the nozzle openings decrease in size in the impactor stages to the outlet. The third stage impactor cup  58  has a flange  58 B and forms a passageway  58 A (see FIGS. 3,  3 A and  5 ) that opens to a vertical passageway  60  in seal plate  30  and to a passageway  62  in the cover  24 . That in turn connects to a nozzle  64  that discharges into a cup  66  that fits in an opening  66 E in base frame  25 . 
     A passageway  64 A that extends laterally opens through a port  68  in seal plate  30  and connects to a tear drop shaped passageway  70  in the cover  24  which directs flow through a nozzle  72 . 
     A cup  74  with a flange  74 B provides a fifth stage impactor and underlies the nozzle  72  and receives particles discharged through the nozzle  72 . The cup  74  also forms a passageway  74 A leading to an opening  76  and to a passageway  78  in the underside of cover  24 . Cup  74  fits in opening  74 E in the base, shown in FIG.  3 A. Cups  66  and  74  are also shown in FIG. 1, where the impactor cover and seal plate are broken away. 
     The crossover passageway  78  carries flow to a nozzle  80 , with openings  80 A, so flow goes downwardly into an underlying sixth stage impactor cup  82  supported with a flange  82 B. The cup  82  forms an impaction plate and provides a passageway  82 A. Passageway  82 A leads to an opening  84  and then to a passageway  86  in the underside of cover  24 . 
     The passageway  86  leads to a nozzle  88  that has openings  88 A that open to an underlying cup  90  forming a seventh impaction stage. The cup  90  is supported on a flange  90 B in an opening  90 E in the base frame  25  and forms a passageway  90 A that leads through an opening  92  to a passageway  94  in the cover  24 . 
     The passageway  94  opens to a final stage micro orifice filter nozzle  96 . The micro orifice filter nozzle  96  discharges the flow into an underlying cup  98  with a support flange  98 B that opens through bore  98 G to a fluid flow outlet passage  98 F in the cover  24 . The passage  98 F is a short cross over passageway that opens downwardly through a bore  98 H to the outlet bore  22  in the base  25 . A fitting  22 B connects to a suitable flow line, the passageway  98 F is sealed with an oval “O” ring  98 J, as shown in FIG.  3 . The seal plate  30 , as shown, and as was explained, has “O” ring type seals on one side to seal the passageways in the cover  24  and on the other side to seal on the impactor cup flanges. The seal on the bore  98 H on the bottom of seal plate  30 , shown at  98 K is sealed on a raised boss  25 K on the base which raises the surface of the base to be level with the top of the cup flanges. 
     The passageways in the cover that connect between nozzles are all sealed with tear drop shaped O-ring seals. Passageways  40 ,  54 ,  62 ,  70 ,  78 ,  86 , and  94  are sealed with seals  40 A,  54 A,  62 A,  70 A,  78 A,  86 A, and  94 A, respectively. The flanges on the impactor cups are also sealed with tear drop shaped seals. In addition to the seal  34 D, the cups  46 ,  58 ,  66 ,  74 ,  82 ,  90 , and  98 , are sealed with seals  46 D,  58 D,  66 D,  74 D,  82 D,  90 D, and  98 D, respectively. 
     The “O” rings that are used for the seals shown above, are all mounted in a standard NGI “O” ring groove, that is trapezoidal shape and has a narrower opening on the exposed surface of the seal plate  30 , in which the groove is formed than the base portion of the groove. Getting a cutting tool into the seal plate to form this type of an enclosed continuous groove that follows the outline of any of the seals around the openings is formed as shown in FIGS. 16 and 17 by providing a flat bottom bore  30 B in the seal plate  30  that is of desired size. The bore is of large enough size to permit the tool that is used for cutting the widest portion of the “O” ring groove to enter the seal plate. In FIG. 16, a fragmentary portion of the seal plate showing a typical “O” ring groove  30 A is illustrated. The bore  30 B is greater than the maximum width of the bottom  30 D of the groove, which is shown in FIG.  17 . The bottom surface of the groove  30 D is wider than the groove opening  30 E. However, the bore or opening  30 B is large enough to permit the tool shown in dotted lines at  30 C to be inserted into this bore  30 B and then cut the continuous groove around the passageways  40 ,  54 ,  62  and the like. The “O” ring grooves are designed to provide “O” rings that seal on the flanges of the impactor cups. 
     The bore  30 B of course, would provide a problem for sealing, but in this instance, a plug  30 F that has the “O” ring groove shown in FIG. 17 formed therein is inserted in the bore  30 B after the main groove has been cut. The opening  30 E- 1  in the plug  30 F is made to align with the openings  30 E of the “O” ring groove that has been formed, so that a continuous seal is maintained, and that a seal is adequately supported. The plug  30 F can be press fitted into place, or can be held in other suitable ways. The bore  30 B extends only partially through the seal plate  30 . 
     The ability to mill with a suitable tool that is shown in dotted lines in FIG. 17, permits rapid formation of the “O” ring grooves that are necessary. 
     The cover  24  is hinged to the base with a pins that are fixed on the base and extend upwardly therefrom. The hinge members comprise two upright hinge posts  100 , that are spaced apart and are part of the base or bottom frame  25 . The posts are adjacent the opposite ends of the base frame. The upright posts  100  carry fixed dowel pins  100 A, that are oriented on the same sides of the upright posts  100 . The cover  24  has a pair of laterally extending ears  101 , which protrude from the rear side of the cover, and these ears have bores  102  that are sized to receive the dowel pins  100 A, so that the cover can be slid laterally, and the dowel pins fitted into the bores  102 . See FIG. 7 where the cover is partially on the pins  100 A. The length of the pins  100 A, and the width of and the space of the posts  100  are selected in relation to the space between ears  101  and stop lugs  103 , so the cover can be installed only in the open position, when the ears of the cover are engaged at all with the pins  100 A. 
     The cover  25  also has projecting stop lugs  103  that are spaced from the ears a distance that is slightly greater than the lateral width of the associated upright post  100 . The posts  100  have a small projection that forms a stop lug  100 B that is positioned so that when the cover  24  is being slid on the pins  100 A, as shown in FIG. 7, the lug  103  on the cover will be engaged with this projection  100 B until the cover  24  is fully seated on the pins  100 A with the lugs  101  up against the side surface of the respective post  100 . Then the cover  24  can be closed, but until that time there is an interference, so that the cover  24  will not be closable until the recesses in the cover are aligned with the seals on the seal plate to insure that the passages and cups will be sealed. 
     In other words, the cover  24  will be maintained in its open position as shown schematically in FIGS. 7 and 8 until the lugs  103  have cleared the posts  100 , which gives the correct position for sealing on the “O” rings. 
     The movable parts of the latch assembly are also maintained on the bottom frame only, and as can be seen in FIGS. 1,  2 ,  3  and  11 , the bottom frame  25  has a pair of ears  104 A and  104 B near each end, and these ears in turn are provided with a pin opening for pivotally mounting handle links  105 A on pins  105 C. The handle links  105 A are connected together with a cross handle  105 C. The handle links will move about the pin. A separate latch link  106  is mounted along the inner sides of the handle links  105 A between the ears  104 B and the respective handle link  105 A. 
     The ears  104 B each mount a fixed cam pin  106 A in openings in the ears. The cam pins  106 A protrude into the space between the ears  104 A and  104 B of each pair, but extend only a short distance so it will fit into and slide along cam slots  107  on each of the latch links  106 . The latch links  106  have bores  106 D that support fixed pins  106 B that extend laterally toward the associate handle link (see FIGS. 9 and 10) and which are rotatably mounted in a bore  105 D of the associated handle link. The handle link has a crank arm  105 F and the bore  105 D is in the crank arms so it is offset from the pivot pin  105 B. The distance between pins  105 B and bores  105 D acts as a crank arm when the handle links  105 A are pivoted on pins  105 B. The pins  106 B of the latch links then act as crank pins and will move the end sections  106 C of latch links  106 . The path of movement of the latch links  106  is defined by the cam pins  106 A traveling in cam slots  107  on one side of the latch links  106 . The cam slots  107  have a long angled section  107 B that has a low end  107 C, and a short section  107 A that extends upwardly at an angle from the low point  107 C. The low point  107 C forms a junction between cam slot section  107 A and cam slot section  107 B. The latch links  106  have hook ends  106 E that will fit over cross pins  108  that are supported on and extend between the ears  109  on the cover  24 . 
     When the latch handle assembly  105  is moved to its latching position, which is the lowered position shown in FIGS. 1 and 10, the cam pins  106 A are in the upper ends of the slots section  107 B and the pins  105 D and  106 B go slightly over center, which provides a detend position. The latch hooks  106 E are clamped against the pins  108  to securely latch the cover  24  in place, compressing the O-ring seals. 
     When unlatching the handle bar  105 C is raised or moved counterclockwise in FIGS. 1 and 10. The movement of the latch link starts to raise pin  106 B and the cam pins  106 B slide along the cam slot section  107 B. This causes the latch hooks  106 D to move up and away from the cross pins  108 , to release the cover as shown at positions  106 F and  105 F in dotted lines in FIG.  10 . Continued movement of the latch handle  105 C counterclockwise will cause the cam pins  106 A to move in the slot sections  107 B and toward low point  107 C and move the latch links  106  upwardly to the position as shown at position  106 G and  105 G. The cam pin is then in the low point  107 C, the handle  105  moves through a substantial arc and the latch link primarily raises. The last portion of movement of latch handle  105  causes the latch link to move so cam pin  106 A moves into the cam slot section  107 A, which moves the latch link rapidly away from the cover, as shown by position  106 H and  105 H in FIG.  10 . The cover  24  can be opened. 
     The cam section  107 C lifts the latch link hook section  106 E away from the pin  108 , so that the cover is released. When clamped down, a very secure, tight seal is obtained by compressing the O-ring seals on seal plate  30 . 
     As shown in FIG. 1, the base  25  has support brackets  25 F that have upright arms  25 G that will support the impactor assembly  10  with the hinge edge downwardly, letting it stand in an upright position with the handle side extended upwardly. 
     The bottoms of the impactor cups can be supported so they clear the supporting surface. This means that when the cover  24  is opened, after the test has been run, tray  36  can be lifted out of the bottom frame, manually or with a fixture. When the tray is lifted all of the impactor cups are removed as a unit. The cups may be placed either in a separate container and sealed, or otherwise processed for recovering and analyzing the particles in each impactor cup. 
     The flow paths through the impactor are shown essentially in FIG. 3, with arrows  99 . The flow path is from the inlet to the outlet through the series of impactor nozzles or orifices. 
     The nozzles and the orifice sizes are selected to provide at least 5 cut points at all desired flow ranges that are between 0.4 μm and 6.0 μm. In addition, one stage should provide particles between 5 μm and 10 μm. A pressure drop across the impactor of less than 100 inches of water at the maximum flow rate is desired. 
     The integrity or continued accuracy of the nozzle or orifice plates of the cascade impactor can be checked by measuring the differential pressure between the impactor nozzles, as was stated, and in order to do that, a test cover indicated generally at  116  is placed onto the base shown in top view in FIG.  11  and in bottom view in FIG. 12, with the base frame in place. The O-ring seals are maintained on the seal plate, and the test cover is placed over the “O” rings and nozzles and overlies the arrangement shown in FIG.  3 . The test cover  116  has recesses that replicate the recesses in the cover  24  that is used for the regular impactor flow. These recesses are shown in FIG. 12 at  117 A through  117 G. The exhaust passageway is shown at  121  in FIG.  14 . The test cover  116  has a separate passageway or bore  118 A- 118 G open to each of the recesses  117 A- 117 G in the cover and a separate passageway  118 H opens to the exhaust passageway  117 H in the cover, which corresponds to passageway  98 F. Each of the passageways  118 A and  118 G, in turn are connected to a tube fitting shown generally at  119 A- 119 H. The passageways shown at  118 A- 118 H are thus open to the individual recesses, and the exhaust passageway  121 . The pressure in the passageways can be sensed by a series of pressure sensors in a housing. These sensors are indicated at  120 A— 120 A. The sensors provide electrical inputs to a computer  134 . The sensor output can be arranged to provide absolute pressure at each passageway, differential pressures across each adjacent pair of nozzle or orifice plates. A standard rate of flow is established, for example, 100 L/min and the measurements taken. By periodically placing the test cover  116  on the impactor base and checking the pressure differentials can be obtained to check to see if plugging or wear is occurring. 
     A schematic representation is shown in FIG. 13 of the pressure sensors. The sensors provide signals to the computer  134 . If desired differential pressure sensors can be provided for direct measurement of pressure differentials. 
     By properly using the signals from the sensors, the computer will provide differential pressure across each of the nozzle or orifice plates. The overall pressure drop can be obtained between the inlet and the outlet. This will permit determining the operational characteristics of the impactor plates and permit calculation of flow rates. Computer  134  does the calculations, and can be used to provide alarms, and to adjust flows, if desired. 
     In order to carry out the present tests, it must be certain that the test cover is sealed properly. A vacuum source  122  is connected to the inlet and since the other fittings are connected to pressure sensors, closing the outlet with a valve will close the system. Any leak down of the vacuum indicates a bad seal. Vacuum is applied to the openings, to insure that the seals are tight. Such a vacuum can be applied with a simplified fixture as shown in co-pending application Ser. No. 09/733,115, filed on Dec. 8, 2000, and entitled LOW VOLUME VACUUM LEAK TEST FIXTURE. Other sources of vacuum also can be used. 
     Making sure that all of the seals are sealed is necessary for determining the flow rate through the impactor. A flow meter can be included in the housing for the computer. The flow is a function of the pressure drop across the impactor, so the pressure sensors also can be used for calculating flow. 
     FIGS. 14 and 15 show a final filter assembly  150  that may be placed in the final impactor cup  98 , and replacing the filter plate  96  shown in FIG.  6 . The showing is with test cover  116  in place, but cover  24  would be used for classification operations. The final filter  150  is an inverted cup having a peripheral bounding wall  152 , and an inwardly turned flange  154  that defines an open space  155  that is covered with a fine screen forming a final filter  157 . The peripheral wall  152  has a series of apertures or openings therein indicated at  158 , and as can be seen, the edges of the peripheral wall  152  rest on the upper surface of the impactor cup  98 . The final filter then filters the fluid coming through the impactor. At this stage, very few particles remain, particularly in drug inhalation devices, and the filter will provide for clean airflow out through the exhaust openings as indicated by the arrows in FIG.  14 . The screen  157  can have a desired mesh, or it can be a perforated screen of very thin material, as desired. 
     The openings  158  are provided in sufficient number and in sufficient size so that the back pressure is not affected adversely. 
     FIG. 18 is a schematic diagram of a prior art cascade impactor  126  that also is shown to illustrate sensing differential pressures between impactor stages. The impactor stages  128 A- 128 G provide particle size cutoff points as shown at desired sizes, based on the nozzle or orifice size. A filter  129  is used at the outlet. A pump  130  provides the flow of aerosol through the cascade impactor  126  from an aerosol source  131 . The pressure differential between adjacent stages is measured by differential pressure sensors  132 A- 132 G, which are used to measure the pressure drop across each impactor stage. A sensor  132 A measures the pressure drop across filter  129 . Alternatively, some adjacent impactor stages can be combined and the pressure drop across a group of stages can be measured with a single sensor. The pressure sensor outputs are also provided to a computer  134 . 
     Ambient temperature can be sensed with a temperature sensor  136 . Barometer pressure is sensed with a sensor  138 ; relative humidity can be sensed with a sensor  140 . Flow rate can be sensed by calculating the flow based on measured pressure drop across an orifice plate  142  or across the entire cascade impactor, that is between the inlet and the outlet. A separate flow sensor can be used. The sensors are connected to the computer  134  for signal processing and recording. 
     The computer  134  can also generate an electrical signal based on the sensor inputs to adjust the flow by controlling the speed of pump  130  with a speed controller to one of several preset values. 
     The pressure drop across each impactor stage in both forms of the impactors shown can be sensed by the individual pressure sensors and compared with standard or reference values which may be obtained by calibration at the factory or at user&#39;s standard calibration laboratory at periodic intervals. 
     During use in the field, when particles begin to accumulate on the small nozzles or orifices in the nozzle plates, the pressure drop across the nozzle or orifice plates increases. This increase can be detected. In addition, any leak in the system, damage to the nozzle plates due to cleaning or other causes, as well as a mistake in the assembly and operation of the impactor can also be detected automatically. 
     Preset pressure limits can be established so that when the pressure difference between the measured value during use and the calibrated set point exceeds the limit, the operator will be alerted to the situation for corrective actions. The data can also be stored in the computer memory from a flow set control  134 A and a pressure limit controller  134 B. Each pressure sensor can be individually monitored by the computer. This way, changes that have occurred during sampling can be detected and the time at which these changes have taken place will also be known. This will enable the operator to determine if the data are sufficiently accurate for use or need to be discarded. 
     Change in nozzle opening dimensions due to particle accumulation and blockage is generally not an issue when the nozzle is a few millimeters or more in diameter. For smaller nozzles, especially those found in modern precision impactors, it is important. Due to the very small nozzle or orifice diameter, the nozzle plate carrying these small nozzles must also be very thin, typically a few thousandth of an inch in thickness. Such thin nozzle plates can be easily damaged during ultrasonic cleaning. Presently, there is no convenient way of detecting the small change in nozzle diameter due to particle accumulation and/or damage during cleaning. Manual inspection by microscope is slow and labor intensive. Due to the high microscope magnification needed to see the small nozzles, the field of view is quite small, meaning that only a few nozzles can be seen and examined in a given field of view. Since micro-orifice nozzle plates with as many as 2,000 orifices are routinely made, and as many as 10,000 orifices may be needed in the future, the convenient and low cost method of detecting change in nozzle dimensions by sensing differential pressure accomplishes the objective automatically. 
     One important application of cascade impactors is to measure the size distribution of aerosols produced for medicinal uses. In such applications, the specific chemical compound, i.e. drug, is aerosolized, which is then inhaled by the patient. The most widely used devices for producing medicinal aerosols for inhalation therapy are the metered dose inhaler (MDI) and the dry-powder inhaler (DPI). These devices produce a specific quantity of drug in aerosol form with each application, usually by depressing the device with a thumb or squeezing the device between fingers to release a puff of aerosol containing the required dose which the patient then inhales. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.