Abstract:
A measuring system and process for measuring the size distribution of particles in an aerosol with a time-varying flow utilizing a measuring instrument that utilizes a time-invariant flow rate is provided. The process includes employing a vacuum source to establish a constant flow through the measuring instrument. The vacuum generated by the vacuum source is used to draw an aerosol stream through an inlet incorporating a second stream of relatively particle free gas. The first and/or the second stream may be adjusted so that at any instant of time, the sum of the two streams is a constant value equal to the flow rate through the measuring instrument.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a completion application of U.S. Provisional Application Ser. No. 60/058,682 filed Sep. 12, 1997 and claims priority therefrom. U.S. Provisional Application Ser. No. 60/058,682 is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an apparatus and process for the measurement of the size distribution of particles in aerosols. More particularly, the present invention relates to an apparatus and process that overcome the measurement limitations of instruments used to measure the size distribution of particles in aerosols, due to the instrument requirements for constant flow rate. 
     BACKGROUND OF THE INVENTION 
     Pharmaceutical aerosol delivery devices are often used to generate aerosols for the delivery of drugs to the respiratory tract. Generally, pharmaceutical aerosol delivery devices are of two types; self-propelled, and breath-propelled. Self-propelled aerosol delivery devices, exemplified by most pressurized Metered Dose Inhalers, generate an aerosol cloud by using energy supplied by the delivery device itself, i.e., liquefied gas propellants, battery operated fans, or compressed air. In contrast, breath-propelled aerosol delivery devices depend on externally-supplied negative pressure to generate the aerosol, such as the negative pressure created by the act of inhaling. For both of these types of delivery devices, the size distribution of the particles in the generated aerosol in the airway and lungs depends on the characteristics of the air stream into which the aerosol is dispensed. In addition, the efficiency of the delivery of these types of devices depends on the flow rate of gas which is drawn through the device. 
     In general, methods for measuring the size distribution of aerosol particles require that the aerosol particles are within a well-defined gas flow field. For example, inertial classifiers, one type of such instruments, operate by suspending aerosol particles in a moving gas stream. The stream is then caused to change directions, which in turn, causes the aerosol particles to cross the flow streamlines. In this manner, the aerosol particles are removed from the gas stream. In order for these types of instruments to operate properly, the flow rate of the gas stream must be constant. 
     The cascade impactor, which is a type of inertial classifier, is especially well-suited to the measurement of the size of aerosol particles. For example, U.S. Pat. No. 5,343,767 describes one example of a particular kind of a cascade impactor. Generally, cascade impactors comprise a plurality of collection stages arranged in series, with each stage designed to collect particles of successively smaller sizes. Within each stage, the gas stream is caused to flow through one or more nozzles to give the stream a relatively high velocity. The nozzles are directed perpendicular to a flat collection surface that is relatively close (less than about ten times the nozzle diameter) to the nozzle outlet. Depending primarily upon the speed of the gas stream through the nozzle, particles greater than a certain size strike the collection surface where they are retained for subsequent quantitation in a separate step. Successive stages have either fewer nozzles or smaller nozzles, so that the gas stream undergoes higher speeds and hence deposits smaller particles on the collection surface. The total gas flow rate through the cascade impactor must remain at a known and constant value during the course of a measurement because the size of particles deposited on a particular stage depends largely on the speed of gas flow. 
     However, both types of aerosol delivery devices are intended ultimately to produce aerosols into the varying flow rates which characterize the human breathing cycle. The size measurement of the particles in an aerosol in an air flow stream which is representative of the end-use situation thus presents a challenge. That is, although most measurement devices require constant gas flow, both types of delivery devices (i.e., self-propelled and breath propelled), are ultimately used in a situation with a variable flow rate. Thus, the requirement of constant flow through a measurement instrument cannot be satisfied when measuring the particle size distribution generated by a device operated in a variable flow rate. Thus, prior art methods for measuring the particle size distribution of aerosols are inadequate, in that the flow rate through the delivery device is maintained at a constant flow rate which is not representative of the end-use situation. 
     Generally, prior art apparatus&#39; for the measurement of the size distribution of particles in an aerosol consist of a measuring instrument, such as an inertial classifier, coupled to an aerosol generating device, such as the inhaler devices described hereinabove. A vacuum source is coupled to the measuring instrument and is utilized to generate a relatively constant gas flow through the measuring instrument. Because the measuring instrument is coupled to the aerosol generating device this relatively constant gas flow must also pass through the aerosol generating device. Inasmuch as the measuring instrument is intended to measure the size distribution of the particles in an aerosol as distributed during a breathing pattern, this is an undesirable result. That is, it would be preferable if the flow from the aerosol generating device were comparable to the flow and pattern of a breathing cycle. More preferably, the flow through the aerosol generating device would be adjustable so as to correspond to different breathing flow rates, i.e., as for a child as opposed to an adult. 
     Thus, there is a need for an improved system for the measurement of the particle size distribution in an aerosol that includes an instrument utilizing a constant gas flow rate, yet capable of measuring an aerosol of varying flow rate. 
     SUMMARY OF THE INVENTION 
     The aforementioned shortcomings and disadvantages of the prior art are overcome by the apparatus and process provided by the present invention. Specifically, the present invention relates to an apparatus and process for measuring the size distribution of particles in an aerosol stream utilizing a measuring instrument that utilizes a generally constant gas flow rate. 
     Specifically, the apparatus of the present invention is a measuring system comprising a measuring instrument capable of measuring the size distribution of particles in an aerosol. Preferably, the measuring instrument will be one generally utilizing a constant gas flow rate. The measuring system further comprises a vacuum source, sealably coupled to the measuring instrument, that is utilized to force a constant flow rate of gas through the measuring instrument. Also provided is an inlet tube sealably coupled to the measuring instrument at a first end and an aerosol generating device connected to the inlet tube at a second end. 
     It is preferred that the connection between the inlet tube and the aerosol generating device is gas tight. That is, it is preferred that the connection between the inlet tube and the aerosol generating device is sufficiently leak-free that essentially all of the gas which enters the inlet tube is drawn in through the aerosol generating device. In this manner, the inlet tube functions to receive a first flow of a gas, as well as an amount of an aerosol generated by the aerosol generating device. 
     It is further preferred that the measuring system of the present invention comprises a mixing chamber that functions to couple the inlet tube to the measuring instrument. Preferably the mixing chamber comprises an inlet, hereinafter referred to as the mixing chamber inlet, capable of receiving a second flow of gas. In operation of the measuring system of the present invention, the first flow and/or the second flow of gas are preferably adjustable so that the combination of the first flow and the second flow are equal to the constant flow rate generated by the vacuum source. 
     Also provided by the present invention is an improved process for the measurement of the size distribution of particles in an aerosol stream. The process comprises employing a vacuum source to establish a constant flow through a measuring instrument. The vacuum generated by the vacuum source is used to draw an aerosol stream through an inlet incorporating a second stream of a gas. Preferably, the second flow of a gas is relatively particle free gas. The first flow and/or the second flow are adjustable so that at any instant of time, the sum of the two streams is a value equal to the constant flow rate through the measuring instrument. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above mentioned and other advantages of the present invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of the embodiments of the invention taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a schematic side view of one embodiment of the improved apparatus in accordance with the present invention; 
     FIG. 2 is a schematic side view of a second embodiment of the improved apparatus in accordance with the present invention; 
     FIG. 3 is a schematic side view of a third embodiment of the improved apparatus in accordance with the present invention; 
     FIG. 4 is a detailed, cross-sectional view of the connection region between the inlet tube and the mixing chamber as may exist in the embodiments of the present invention illustrated in FIGS. 1,  2  and  3 . 
     FIG. 5 is a detailed, cross-sectional view of the connection region between the inlet tube and the mixing chamber as it exists in a second embodiment of the improved apparatus of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As used herein, the term “aerosol” is meant to indicate a suspension of fine solid or liquid particles in gas. Thus, the measuring system and process of the present invention may be used to measure the particle size of atomized particles of liquid as well as being useful to measure the particle size of atomized particles of solids. 
     FIG. 1 is a schematic side view of one embodiment of improved aerosol measuring system  10 , in accordance with the present invention. Aerosol measuring system  10  includes a measuring instrument  12  and a gas regulating and aerosol conduit assembly  28  for coupling an aerosol generating device  30  to the measuring instrument  12 . Measuring instrument  12  can be a commercially available or otherwise conventional cascade impactor, and has a vacuum port  14  and an aerosol receiving inlet port  16 . Measuring instruments  12  of the type described above are commercially available from Graesby Corp., Smyrna, Ga. or MSP Corp, Minneapolis, Minn., and are described in USP publication  Pharmaceutical Forum , 22, 3084 (1996). A vacuum source  18  is connected to vacuum port  14  to generate a flow of air or other gas into measuring instrument  12  through aerosol receiving inlet port  16 . Typically, vacuum source  18  is adjusted to generate a gas flow of a predetermined rate into measuring instrument  12 . 
     Gas regulating and aerosol conduit assembly  28  includes a mixing chamber  20 , inlet tube  26  and valve  60 . Valve  60  is positioned on mixing chamber inlet  22  and is actuated to regulate the flow of gas from gas source  24  into mixing chamber  20  through mixing chamber inlet  22 . Mixing chamber  20  includes a first inlet port  19  and a second inlet port  21 . A high pressure gas source  24  is coupled to the second inlet port  21  by mixing chamber inlet  22 . Valve  60  can, for example, be a manually operated valve, a throttling valve, an automatic valve, or a computer controlled valve. In other embodiments (not shown), the gas entering mixing chamber inlet  22  can be drawn from ambient air. 
     A first end  23  of inlet tube  26  is coupled by a gas tight seal to first inlet port  19  of mixing chamber  20 . A second end  27  of inlet tube  26  is adapted to receive an aerosol to be measured from aerosol generating device  30 . Ambient air, or gas from another source (not shown) can also be received by inlet tube  26  through its second end  27 . 
     During operation of aerosol measuring system  10 , gas flow through measuring instrument  12  is set to a desired constant level by adjusting vacuum source  18 . When vacuum source  18  is operating, gas will flow at a known rate through measuring instrument  12 , for example, vacuum source  18  is preferably capable of generating a gas flow from about 15 to about 120 liters per minute. Gas flowing through the measuring instrument  12  as a result of the vacuum created by vacuum source  18  enters the gas regulating and aerosol conduit assembly  28  and thereafter measuring instrument  12 , through the second end  27  of inlet tube  26  and through mixing chamber inlet  22 . 
     Valve  60  will be adjusted to establish a desired flow of ambient air mixing chamber inlet  22 . The remaining gas flow needed to make up the difference between that established by vacuum source  18  and that provided by mixing chamber inlet  22  is drawn through first inlet port  19  of mixing chamber  20 . The flow into mixing chamber inlet  22  will typically be adjusted so that the flow rate through second end  27  of inlet tube  26  will be at the desired flow rate. 
     Specifically, the size distribution of an aerosol generated by a metered dose inhaler (MDI), i.e., aerosol generating device  30 , can be measured using aerosol measuring system  10  shown in FIG. 1 as follows. The measuring instrument  12  is calibrated for the desired flow, e.g., 100 liters/minute (l/m). The vacuum source  18  connected to the measuring instrument  12  is turned on, and flow rate through the measuring instrument  12  adjusted to 100 l/m. A high pressure gas source  24  is connected to the mixing chamber inlet  22 . The aerosol generating device  30  is attached to the inlet tube  26 . The amount of gas traveling into the mixing chamber  20  is controlled by valve  60  connected to the high pressure gas source  24 . If the aerosol is to be measured in a flow of 55 l/m through the inlet tube  26 , the flow rate through the second inlet port  21  to the mixing chamber  20  is set at 45 l/m by adjusting the valve  60 . In this manner, the gas flow rate through the inlet tube  26  is set to 55 l/m (100−45=55 l/m). The aerosol generating device  30  is then manually actuated and the aerosol travels through the inlet tube  26  at a rate of 55 l/m. The aerosol stream is then diluted by the additional gas stream introduced to the mixing chamber  20  by the mixing chamber inlet  22 . Sizing inside the measuring instrument  12  thus takes place at a constant flow rate of 100 l/m. 
     Referring now to FIG. 2, there is illustrated a schematic side view of a second embodiment of the invention, aerosol measuring system  210 . Components which are functionally and/or structurally similar to those of system  10  are identified the same number incremented by 200. Aerosol measuring system  210  includes a measuring instrument  212  and a gas regulating and aerosol conduit assembly  244  for coupling an aerosol generating device  230  to the measuring instrument  212 . Measuring instrument  212  can be a commercially available or otherwise conventional cascade impactor, and has a vacuum port  214  and an aerosol receiving inlet port  216 . A vacuum source  218  is connected to vacuum port  214  to generate a flow of air or other gas into measuring instrument  212  through aerosol receiving inlet port  216 . Typically, vacuum source  218  is adjusted to generate a gas flow of a predetermined rate into measuring instrument  212 . 
     Gas regulating and aerosol conduit assembly  244  includes a mixing chamber  220 , inlet tube  226 , gas tight plenum  240  and proportioning valve  236 . Proportioning valve  236  is positioned on conduit  234  and serves to bifurcate and proportion the flow of gas drawn from ambient air between mixing chamber inlet  222  and plenum inlet  238 . Proportioning valve  236  can be a manually operated valve, a throttling valve, an automatic valve, or a computer controlled valve. Additionally, proportioning valve  236  can provide a constant or a time-varying flow. 
     Mixing chamber  220  includes a first inlet port  219  and a second inlet port  221 . A first end  223  of inlet tube  226  is coupled by a gas tight seal to first inlet port  219  of mixing chamber  220 . A second end  227  of inlet tube  226  opens into gas tight plenum  240  is adapted to receive an aerosol to be measured from aerosol generating device  230 . Gas drawn from ambient air through conduit  234  into gas tight plenum  240  can also be received by inlet tube  226  through its second end  227 . 
     Gas tight plenum  240  sealably surrounds the second end  227  of inlet tube  226 . Gas tight plenum  240  may be made of machined metal or molded plastic or other suitable material. Conduit  234 , left open to ambient air, is coupled to gas tight plenum  240  by plenum inlet  238 . 
     Optionally, actuator  242  may be providing for activating aerosol generating device  230  so as to release an amount of aerosol into aerosol measuring system  210 . Actuator  242  may be any mechanical, electrical, pneumatic, or other activation mode effective to generate an aerosol from aerosol generating device  230 , providing that the activation mode chosen allows gas tight plenum  240  to remain gas tight. 
     During operation of aerosol measuring system  210 , gas drawn from ambient air enters the system through conduit  234 , at a value equal to and determined by the constant flow rate through measuring instrument  212 , as generated by vacuum source  218 . In operation, the gas flow entering aerosol measuring system  210  through conduit  234  is split by proportioning valve  236 , so that the flow rate into gas tight plenum  240  through plenum inlet  238  matches the desired flow rate profile in which the aerosol is desired to be measured. The remaining gas entering aerosol measuring system  210  through conduit  234  is directed through mixing chamber inlet  222  to mixing chamber  220 , so that the gas flow through measuring instrument  212  remains at a constant value. 
     Specifically, the size distribution of an aerosol generated by a metered dose inhaler (MDI), i.e., aerosol generating device  230 , can be measured using aerosol measuring system  210  shown in FIG. 2 as follows. The measuring instrument  212  is calibrated for the desired flow, e.g., 100 l/m. The vacuum source  218  connected to the measuring instrument  212  is turned on, and flow rate through the measuring instrument  212  adjusted to 100 l/m. The amount of gas, drawn from ambient air, traveling into gas tight plenum  240  and into mixing chamber  220  is controlled by proportioning valve  236 . If the aerosol is to be measured in a flow of 55 l/m through the inlet tube  226 , the flow rate through the plenum inlet  238  is set at 55 l/m by adjusting the proportioning valve  236 . The flow rate through mixing chamber inlet  222  will thus be 45 l/m (100 l/mn−55 l/m). The aerosol generating device  230  is inserted into gas tight plenum  240  and attached to the inlet tube  226 , so that a leak-free seal exists between the gas tight plenum  240  and the second end  227  of inlet tube  226 . The aerosol generating device  230  is actuated via actuator  242  and the generated aerosol travels through the inlet tube  226  at a rate of 55 l/m. The aerosol stream is then diluted by the additional gas stream introduced to the mixing chamber  220  by the mixing chamber inlet  222 . Sizing inside the measuring instrument  212  thus takes place at a constant flow rate of 100 l/m. 
     FIG. 3 is a schematic side view of a third embodiment of the invention, improved aerosol measuring system  310  in accordance with the present invention. Components which are functionally and/or structurally similar to those of system  10  are identified the same number incremented by  300 . Aerosol measuring system  310  includes a measuring instrument  312  and a gas regulating and aerosol conduit assembly  346  for coupling an aerosol generating device  330  to the measuring instrument  312 . Measuring instrument  312  can be a commercially available or otherwise conventional inertial classifier or other measurement device capable of measuring the size distribution of particles in an aerosol. Measuring instrument  312  has a vacuum port  314  and an aerosol receiving inlet port  316 . A vacuum source  318  is connected to vacuum port  314  to generate a flow of air or other gas into measuring instrument  312  through aerosol receiving inlet port  316 . Typically, vacuum source  318  is adjusted to generate a gas flow of a predetermined rate into measuring instrument  312 . 
     Gas regulating and aerosol conduit assembly  346  includes a mixing chamber  320 , inlet tube  326 , a breath profile source  332  and gas tight plenum  340 . Gas is drawn from ambient air and travels through mixing chamber inlet  322  into mixing chamber  320 . 
     Mixing chamber  320  includes a first inlet port  319  and a second inlet port  321 . A first end  323  of inlet tube  326  is coupled by a gas tight seal to first inlet port  319  of mixing chamber  320 . A second end  327  of inlet tube  326  opens into gas tight plenum  340 , and is adapted to receive an aerosol to be measured from aerosol generating device  330 . Gas from breath profile source  332  can also be received by inlet tube  326  through its second end  327  after passing through aerosol generating device  330 . 
     Gas tight plenum  340  sealably surrounds the second end  327  of inlet tube  326 . Gas tight plenum  340  may be made of machined metal or molded plastic or other suitable material. Breath profile source  332  is coupled to gas tight plenum  340  by plenum inlet  338 . Examples of commercially available breath profile source  332  suitable for use in the aerosol measuring system  310 , and in the practice of the process of the present invention, are TSI model 8091 Breathing Simulator, commercially available from TSI Inc., St. Paul, Minn., the Aero-Breather™, commercially available from Amherst Process Instruments, Hadley, Mass. or the animal respirator, commercially available from Harvard Apparatus, Holliston, Mass. Alternatively, breath profile source  332  may simply be a bellows driven by a computer-controlled stepper motor. 
     Optionally, actuator  342  may be provided for activating aerosol generating device  330  so as to release an amount of aerosol into aerosol measuring system  310 . Actuator  342  may be any mechanical, electrical, pneumatic, or other activation mode effective to generate an aerosol from aerosol generating device  330 , provided that the chosen activation mode allows gas tight plenum  340  to remain gas tight. 
     During operation of aerosol measuring system  310 , gas flow through measuring instrument  312  is set to a desired constant level by adjusting vacuum source  318 . Gas flowing through the measuring instrument  312  as a result of the vacuum created by vacuum source  318  enters the gas regulating and aerosol conduit assembly  346  and thereafter measuring instrument  312 , through the second end  327  of inlet tube  326  and through mixing chamber inlet  322 . 
     Breath profile source  332  will be adjusted to provide the desired breath profile of time-varying flow rate through gas tight plenum  340 , and thus, inlet tube  326  through second end  327 . The flow into inlet tube  326  through second end  327  will typically be adjusted to a value equal to and representative of a breath flow rate at which the aerosol is desired to be tested. The remaining gas flow needed to make up the difference between that established by vacuum source  318  and that provided by inlet tube  326  is drawn through mixing chamber inlet  322 . Mixing chamber inlet  322  may simply be left open to ambient air. 
     Specifically, the size distribution of an aerosol generated by a dry powder inhaler (DPI), i.e., aerosol generating device  330 , can be measured using aerosol measuring system  310  shown in FIG. 3 as follows. The measuring instrument  312  is calibrated for the desired flow rate, e.g., 120 l/m. The aerosol generating device  330  is inserted into the gas tight plenum  340  and connected to the inlet tube  326 , so that a leak-free seal exists between the gas tight plenum  340  and the second end  327  of inlet tube  326 . Thus, air entering second end  327  of inlet tube  326  consists solely of air tat has passed through aerosol generating device  330 . The plenum inlet  338  is attached to a breath profile source  332 . The vacuum source  318  is turned on and the flow rate through the measuring instrument  312  adjusted to 120 l/m. 
     Initially, all the gas enters aerosol measuring system  310  through mixing chamber inlet  322 , as drawn in by the vacuum source  318 . A signal is given to initiate motion of the breath profile source  332  in such a fashion that the gas flow into the gas tight plenum  340  is flowing according to a time-changing profile which approximates the incoming air flow profile that a person generates during inhalation. This time-varying flow enters the gas tight plenum  340 , causing an identical time-varying flow rate through the aerosol generating device  330 . This flow automatically causes the aerosol generating device  330  to release an amount of aerosol. The aerosol then travels through the inlet tube  326  at the same time-varying flow rate. The gas entering the mixing chamber inlet  322  automatically decreases so that the flow rate through the measuring instrument  312  remains at a constant level of 120 l/m. 
     Referring now to FIG. 4, there is illustrated a detailed cross-sectional view of a preferred configuration of the connection region between inlet tube  426  and mixing chamber  420 . Components which are functionally and/or structurally similar to those of system  10  are identified the same number incremented by  400 . As is illustrated, mixing chamber  420  is sealably connected to measuring instrument (not shown in FIG. 4) by aerosol receiving inlet port  416 . Inlet tube  426  is sealably connected to mixing chamber  420 . Mixing chamber  420  comprises a substantially tubular body with external surface  454  and internal surface  456 . Mixing chamber  420  further comprises an internal, substantially tubular member  459  with external surface  452 . The internal diameter of inlet tube  426  is substantially the same as the internal diameter of the internal tubular member  459  of mixing chamber  420 . 
     As is shown, external surface  454  of mixing chamber  420  is sealed by surface  450 . Thus, gas may enter mixing chamber  420  through inlet tube  426  or mixing chamber inlet  422 . By design, the gas that enters through mixing chamber inlet  422  is directed downward along external surface  452  of the internal tubular member  459  of mixing chamber  420 . Thus, this gas is forced to travel through the channel created by external surface  452  of the internal tubular member  459  of mixing chamber  420  and internal surface  456  of mixing chamber  420 , thereby becoming evenly distributed in an annular shape around the axis of the gas flow through inlet tube  426  before it reaches the end of internal tubular member  459 . In this manner, the deposition of aerosol particles on the internal surface  456  of mixing chamber  420  may be minimized. 
     FIG. 5 is a detailed, cross-sectional view of a second embodiment of the connection region between inlet tube  526  and mixing chamber  520 . Components which are functionally and/or structurally similar to those of system  10  are identified the same number incremented by  500 . As is illustrated, mixing chamber  520  is sealably connected to the measuring instrument (not shown) by aerosol receiving inlet port  516 . Mixing chamber  520  comprises a substantially tubular body with external surface  554  and internal surface  556 . Mixing chamber  520  further comprises an internal, substantially tubular member  559  with external surface  552 . The external surface  552  of the internal tubular member  559  and the internal surface  556  of mixing chamber  520  form gap  558 , through which ambient air may enter mixing chamber  520 . Thus, gas may enter mixing chamber  520  through inlet tube  526  or gap  558 . 
     By design, the gas that is drawn through gap  558  as a result of the vacuum created by the vacuum source (not shown in FIG. 6) is directed downward along the external surface  552  of internal tubular member  559  of mixing chamber  520 . Thus, this gas is forced to travel through the channel created by external surface  552  of internal tubular member  559  and internal surface  556  of mixing chamber  520 , thereby becoming evenly distributed in an annular shape around the axis of the gas flow through inlet tube  526  before it reaches the end of internal tubular member  559 . Thus, FIG. 5 illustrates another embodiment wherein the deposition of aerosol particles on the internal surface  556  of mixing chamber  520  is minimized. 
     Although the figures contain a single representation of the measuring instrument, the aerosol generating device and the inlet tube, the measuring system and process of the present invention may be practiced and/or utilize a wide range of types of aerosol generating devices, measuring instruments and inlet tubes. For example, the present invention provides an effective measuring system and process for measuring the particle size characteristics of aerosols generated by self-propelled aerosol delivery devices, such as metered dose inhalers, breath-propelled aerosol delivery devices, dry powder inhalers and the like. The aerosol generated by nebulizers can also be tested by the described measuring system and process. 
     Likewise, the measuring instrument can be any type of particle size measuring instrument generally requiring a constant rate of gas flow through the instrument provided that this flow rate is greater than or equal to the flow rate through the aerosol generating device. Finally, although the inlet tube shown is a commonly used inlet tube; the described measuring system and process can utilize any other inlet tube capable of effectively coupling the aerosol generating device to the measuring instrument. 
     Other embodiments of this invention will be apparent to those skilled in the art upon consideration of this specification or from practice of the invention disclosed herein. Various omissions, modifications, and changes to the principles and embodiments described herein may be made by one skilled in the art without departing from the true scope and spirit of the invention which is indicated by the following claims.