referring to the figures , an embodiment of dispensing system according to the invention comprises a hand held dosimeter having a height of approximately 200 mm and a weight of approximately 200 g . the dosimeter has a mouthpiece ( 1 ) and a nebulizer ( 2 ) attached , and a body ( 3 ) which contains a control valve , electronic circuitry and a battery . these components can be accessed through base ( 4 ). the dosimeter is connected to a compressed air source via tube ( 10 ). to operate the dosimeter , the patient removes mouthpiece ( 1 ), and pours a liquid form of the medication ( which be a liquid or a powder in a fluidised form , or any other similar form ) into the nebulizer ( 2 ). then , the dosimeter is connected to the compressed air supply via tube ( 10 ). by means of a switch device to be described later , the presence of a positive pressure in the dosimeter activates the control circuitry in the dosimeter , and switches it on . in this embodiment , although the nebulizer has only one well in which the medication sits , the dosimeter is nevertheless pre - programmed to deliver the correct dose of two different nominated drugs . the dosimeter could , however , be constructed and pre - programmed so as to deliver any desired number of nominated drugs . the drug which has been loaded into the nebulizer is selected using selector buttons ( 5 ) and ( 6 ). once the drug type is selected , led &# 39 ; s ( 7 ) and ( 8 ) indicate the drug type selected by the patient . button ( 9 ) is a re - set button , so that the user can correct any errors in selection . the nebulizer used in the dispensing system according to the invention can be any suitable nebulizer design with a known calibration constant which is used to nebulize substances such as medicaments , which is suitably adapted to fit the dispensing system , and calibrated with the dosimeter . suitable nebulizers include those which use a source of compressed air to nebulize the medicament , and are , for example , described in european patent no . 672 , 266 ( medic - aid limited ), the contents of which are incorporated by reference . when the drug has been selected , the patient breaths in through mouthpiece ( 1 ). a pressure sensor ( to be further described later ) within body ( 3 ) detects a pressure drop within the mouthpiece ( 1 ) due to the patient &# 39 ; s inhalation , and then delivers a pre - programme pulse of nebulized medicament into the first 50 % of the inspiratory profile , until the dose regime programmed into the dispensing system has been delivered . the dose is calculated from a known rate of output against time for the drug selected , and the sum of all the nebulizer pulses which the dosimeter has delivered . further information on how the doses of drug may be derived and pre - programmed into the dosimeter may be obtained from gb 2 , 294 , 402 ( medic - aid limited et al ), the contents of which are incorporated herein by reference . in this instance , clinical trials have been conducted to determine the dose of drug that must be delivered to achieve the correct therapeutic effect , and this dose has been programmed into the dosimeter for the two nominated drugs . when the programmed dose has been delivered , the led adjacent the button relating to the selected drug flashes rapidly and a buzzer sounds , indicating that the treatment is finished , and thereby ensuring that a precise dosage of drug is delivered to the patient on every occasion . the nebulizer used in this embodiment is shown in more detail in fig2 and 3 . compressed air is supplied via port ( 17 ) to the nebulizer jet ( 11 ), and this works in conjunction with baffle ( 12 ) to aerosolize the liquid drug which has been placed in nebulizer bowl ( 19 ). this nebulizer is venturi nebulizer which draws in air through the spout ( 13 ) into the centre of the baffle ( 12 ), and then up and out through the outlet of the spout ( 13 ) into the mouthpiece . as the nebulizer only generates aerosol during inspiration , the patients inhaled air is drawn through valve ( 14 ), and further through mouthpiece to the patient . a small proportion of this air is drawn down through the centre of the nebulizer to operate the venturi system . the flow through the nebulizer is completely independent of the patient &# 39 ; s flow , and the nebulizer produces a constant rate of output . valve ( 14 ) creates a pressure drop within the mouthpiece , and this is monitored through port ( 18 ). on exhalation valve ( 14 ) will close , and the patient will exhale through valve ( 15 ). further information as the nebulizer on the nebulizer is contained in ep 627 , 266 ( medic - aid limited ). the complete assembly is held in place by catch ( 16 ). when the lower end of the catch is displaced the mouthpiece can be removed . the top end of the catch can then be tilted down to release the nebulizer bowl assembly from the main dosimeter case ( 3 ). this allows the whole nebulizer unit to be removed after the treatment has been completed , and cleaned completely separate from the main dosimeter case . this is further graphically illustrated in fig7 and 8 . fig4 shows an internal section through the dosimeter , in particular showing the electronic configuration of the device . air enters the dosimeter via tube ( 10 ) and through flexible tube ( 22 ) to the manifold ( 21 ) and onwards to valve ( 20 ). to determine whether there is pressure in the tube ( 22 ), it is configured around wall ( 28 ). when there is no pressure in the tube , the tube collapses . however , under pressure it expands , and moves lever ( 29 ) against switch ( 30 ), which switches the dosimeter on . the dosimeter can be operated from various different compressed air sources ; firstly it can use either a continuous air supply of approximately 6 litres per minute , generated from a conventional air compressor , or a compressed bottled gas supply such as is used in hospitals . with these compressed air sources , when the compressed air is not being supplied to the nebulizer via tube ( 23 ) and port ( 17 ), it must be vented externally through tube ( 24 ) out of the base of the dosimeter via an orifice which matches the size of the jet ( 11 ) diameter in the nebulizer . this maintains a constant pressure in the system irrespective of whether the valve is directing air to the nebulizer or the vent . in an alternative and preferred embodiment , which can be used in conjunction with other forms of dosimeter , the dosimeter can be operated in conjunction with a low flow rate compressor system having a flow rate of 1 - 2 litres per minute , which has a fitted accumulator . this compressor system generates a low flow of , for example , 1 . 5 litres per minute in the accumulator , and this allows the dosimeter to produce pulses of compressed air to the nebulizer with an equivalent flow rate of around 6 litres per minute , where the flow from the compressor is matched to the mean flow through the nebulizer ( 1 . 5 litres per minute ). in this configuration tube ( 24 ) is sealed , and when the valve ( 20 ) is closed the pressure in the supply tube ( 10 ) is diverted into the compressor &# 39 ; s accumulator . port ( 18 ) connects the pressure sensor ( 25 ) to the mouthpiece . this is attached to the printed circuit board ( 31 ) which is powered by battery ( 26 ). when the electronic system detects the patient has inhaled , valve ( 20 ) diverts the pressure flow into tube ( 23 ) and out to a nebulizer via port ( 17 ). the battery is inserted into the dosimeter via door ( 27 ) into base ( 4 ). an example of a suitable low flow rate compressor which can be used as described above is shown in fig5 and 6 , and comprises a small air compressor with a flow of approximately 1 . 5 litres per minute , and an accumulator with a volume of about 0 . 2 litres , which may be formed , for example , in the tubing between the compressor and the dosimeter , or using a bellows and spring arrangement within the compressor casing . below is described a rubber hemisphere accumulator which works particularly well . the compressor operates until the accumulator has reached its capacity , and then switches off . the dosimeter receives pulses of air from the accumulator until the accumulator has been emptied to a specified volume ( 0 . 1 litre ). the compressor then cuts in and refills the accumulator . the compressor produces a mean output flow rate which is known to the dosimeter , and the dosimeter controls its supply of air so that it does not exceed the mean flow rate from the compressor , and any one pulse delivered by the dosimeter does not exceed accumulator capacity . this system therefore utilizes the compressor , which is approximately one - quarter of the size of a conventional compressor system , and one quarter of the energy requirement . such a compressor is also cheaper to manufacture and provide . the accumulator in this embodiment should have a linear pressure to volume relationship over the period of a pulse delivery , which is typically 1 . 5 seconds long , with a nebulizer jet flow of 6 litres per minute , the accumulator should provide a volume flow of 150 millilitres at a constant pressure ( 1 bar +/− 10 %). it is difficult to obtain a steady linear pressure to volume relationship . fig9 shows a natural rubber hemispherical accumulator . the rubber has a linear load to extension between an extension of 100 % and 500 %. a diaphragm 91 is a hemispherical component which is held trapped between two rings 92 and 93 . in fig9 the diaphragm is shown in three positions labelled a , b , c . position a is the unloaded position from which the diaphragm starts and as compressed air is supplied via a port 94 , the diaphragm expands to position c with a 200 % extension . the compressed air supply is then stopped either by the actuation of a microswitch on the diaphragm surface ( not shown ) which stops the compressors , or by a pneumatic microswitch which then vents the compressor output . the diaphragm 91 then stays in this position until the valve sends a pulse of air to the nebulizer . when the diaphragm delivers air , the pressure drop during operation is less than 5 %. the diaghragm can supply air at a stable pressure until it reaches position b which is a 120 % extension . the microswitch would then restart the compressor , or the pneumatic microswitch would be closed . this hemisphere arrangement is relatively simple to manufacture , but has great advantages over other accumulator systems which might be used . a suitable electrical configuration for such a compressor is shown in fig6 . for the dispensing system according to the invention to work effectively , the dosimeter valve ( 20 ) should be in close proximity to the nebulizer , so that when the system detects the patient &# 39 ; s inhalation , the nebulizer starts to work as quickly as possible , typically in less than 50 milliseconds . this means that the length of tube ( 23 ) between the manifold outlet and the nebulizer jet must be short , with an internal volume less than 0 . 7 ml , preferably less than 0 . 5 ml , which represents 5 % of the shortest pulse the nebulizer delivers . if a long tube is used between the valve and the nebulizer , this tube has to be pressurized before the nebulizer starts to operate . this significantly affects the performance of the system , as to control the rate of output of the nebulizer it must be supplied with a “ square wave ” of air pressure , so that the output is constant over time , and is delivered at the start of inhalation . the nebulizer delivers pulses to inspiratory patterns between 0 . 1 and 1 . 5 seconds &# 39 ; duration . one of the simplest ways of determining the dose of medicament which is received by the patient is , as described as above , to multiply the nebulizer output rate by the duration of each pulse . the doses then ascertained by summing the amount of medicament which is received during each pulse . this calculation relies on the fact that the output rate of a nebulizer should be constant regardless of the rate of inhalation of the patient . therefore , provided that the nebulizer output rate is the same for a person who inhales slowly as for a patient who inhales quickly , the calculation will be accurate . if the nebulizer output rate varies with the speed of inhalation , the precision of the dosimeter will vary . fig1 is a graph showing the variation of aerosol output rate with the speed of inspiratory flow for a typical nebulizer . as will be seen , the output is not constant . it is therefore proposed to use the pressure sensor to provide information on the patient flow rate so that the correct nebulizer calibration rate is determined during patient inhalation . this may be done in the from of a look - up table which is quite effective , and the look - up table can have two or more calibration points as required to provide the necessary accuracy . a satisfactory look - up table can be achieved by using an approximation of the look - up table which , in the case of the graph shown in fig1 can be made up of two straight lines , one line generally following the curve to the 30 lpm point and a second line which generally follows the curve above that . such an approximation works well because , in reality , the breathing pattern of a patient is not at a fixed level , but is continually changing . another way of using the correct calibration value is to use the computer which takes the inspiratory flow rate into account . one calibration value can be used above the 30 litres per minute level , and when the flow is below about 15 litres per minute , then the calibration constant is reduced to 60 % of that value . thus , the calibration may be achieved in a number of different ways not just a multi - point look - up table . while the invention has been described above with reference to specific embodiments thereof , it is apparent that many changes , modifications and variations can be made herein . accordingly , it is intended to embrace all such changes , modifications and variations that fall within the spirit and broad scope of the appended claims . all of the above - noted patents , patent applications and publications referred to in this application are incorporated herein by reference in their entireties .
a product dispensing system comprises a calibrated nebulizer having a nebulizer jet , mouthpiece , means for sourcing compressed air , a manifold for distributing the source of compressed air in at least one direction , a nebulizer accommodation means which is accessed by port , and a valve means for controlling the manifold and thereby the flow of compressed air to the port . the manifold and the port are linked by a length of tube , wherein the internal volume of the tube from the manifold outlet to the nebulizer jet is less than 0 . 7 ml .