GB2578581A - Improved aerosol sensor testing - Google Patents

Abstract

An aerosol detector test system (Fig. 2) comprises a vibrating mesh nebuliser 1 to generate an aerosol. The nebuliser may introduce aerosol into an aerosol line (7, Fig. 2) to which an aerosol sensor instrument may be connected. The nebuliser is used in testing and calibrating aerosol sensors/detectors; the sensors/detectors may be for detecting hazardous chemical aerosols, e.g. chemical warfare agents. The nebuliser comprises a reservoir for liquid, a mesh/plate 2 comprising multiple perforations, and a driver 4 for vibrating the mesh/plate. The driver can provide a variable drive frequency, perhaps between 1 Hz and 100 kHz, to enable the mesh/plate to be vibrated at variable duty cycles, so that the nebuliser can deliver variable concentrations of aerosol.

Description

Improved Aerosol Sensor Testing The present invention is concerned with improved systems and methods for testing and calibrating aerosol sensors or detectors, and especially sensors/detectors concerned with detecting/sensing hazardous chemicals, such as chemical warfare agents.

In order to test aerosol sensors, a well characterised aerosol challenge must be generated and supplied to the sensor. Since aerosol sensors, especially toxic chemical aerosol sensors, must be able to detect aerosols across a wide dynamic range, the ability to tune the aerosol concentration of the challenge across this range is critical. Numerous systems and aerosol generation devices have been investigated over the years for their ability to consistently and reproducibly test aerosol sensors/detectors, but there remains the need for more consistent and more reproducible systems and methods.

The present invention is generally concerned with providing improved systems and methods for testing aerosol sensors/detectors, and preferably also calibration of such sensors/detectors, which especially are capable of generating variable but reproducible and consistent concentrations of aerosol for the testing.

Thus, in a first aspect, the present invention provides for use of a vibrating mesh nebuliser in the testing, and preferably calibration, of aerosol sensors/detectors, thus testing the ability of sensors or detectors for their ability to detect and/or quantify the amount of an aerosol. The aerosol sensors/detectors are preferably for detecting hazardous chemical aerosols, such as chemical warfare agents.

The Applicant has devised a new approach for testing aerosol sensors, which utilises a vibrating mesh nebuliser for generating the aerosol to be used in the testing. Until now such nebulisers have solely been used in the field of medicine for generating medicinal aerosols. In particular, aerosol delivery utilising nebulisers is an important mode of delivery of drugs, particularly bronchodilators, for the treatment of respiratory diseases, notably asthma and chronic obstructive pulmonary disease.

The use of vibrating mesh nebulisers to provide aerosol for use in an aerosol sensor test system has never previously been considered or indeed successfully implemented.

Nebulisers are widely used for drug delivery, converting liquid medication into aerosol droplets which can then be inhaled and deposited in the lungs. Vibrating mesh nebulisers have been developed recently, and are now available from many different companies, providing aerosols in the respirable range for drug delivery. The vibrating mesh nebulisers are based on the vibration of a mesh or plate containing multiple perforations. As the plate vibrates up and down, liquid passes through the apertures forming liquid jets that break up to form droplets with sizes that are characteristic of the aperture sizes. The vibrating mesh nebuliser has a very different design to conventional jet nebulisers. The key component is the central aperture mesh/plate that is perforated with precisely formed holes. The mesh/plate may be vibrated by a piezo ring which acts as a micro-pump drawing liquid through the holes to generate consistently sized fine droplets.

This approach to producing an aerosol is advantageous for developing aerosol sensor test and evaluation systems for a number of reasons, such as it enables both solids (as a solution) and liquids to be aerosolised, and because the concentration of aerosol is set by controlling the amount of aerosol generated only the minimum quantity of test chemicals need be used. This is advantageous in terms of reducing the level of waste and reducing safety concerns when hazardous chemicals are being aerosolised.

By choosing vibrating mesh plates with apertures/holes of different dimensions/diameters, a variation in droplet/particle size can be produced. Also by changing the level of dilution of a chemical with solvent, a variation in droplet/particle size can be produced. However, once a dilution regime has been fixed, and the aperture dimension/diameter has been set, then critically the droplet/particle size is then independent of the aerosol concentration. This is in contrast to the jet nebulisers, where increasing the flow rate of the compressed air to increase droplet/particulate concentration reduces the droplet/particle size of the aerosol produced.

Until now vibrating mesh nebulisers have had restricted control of the concentration of aerosol that can be delivered, however in order to test an aerosol sensor/detector for its ability to sense or detect an aerosol variable concentrations are required, with concentrations often needing to differ by several orders of magnitude. The Applicant has now devised a vibrating mesh nebuliser that is capable of generating variable concentrations, especially concentrations differing by several orders of magnitude.

Thus, in a second aspect, the present invention provides a vibrating mesh nebuliser comprising a reservoir for a liquid, a mesh/plate comprising multiple perforations, and a driver for vibrating the mesh/plate, wherein the driver is capable of providing a variable drive frequency to enable the mesh/plate to be vibrated at variable duty cycles, thereby providing the vibrating mesh nebuliser with the ability of delivering variable concentrations of aerosol.

The effect of varying the nebuliser drive frequency is a nebuliser with a rapidly tuneable concentration range, which is also capable of being consistent and reproducible.

The vibrating mesh nebuliser preferably comprises an electronic control system for providing the variable drive frequency.

The driver and/or electronic control system is preferably capable of providing pulses of variable drive frequency, which may be pulses of variable repetition frequency, or a pulse train of variable repetition frequency, which may be of fixed pulse width.

The drive frequency of the vibrating mesh nebuliser may be varied between at least 1 kHz to 100 kHz, or between 100 Hz to 100 kHz, providing at least up to three orders of magnitude difference in the possible concentrations of aerosol that can be generated, or may be varied between at least 1 Hz to 100 kHz, providing at least up to five orders of magnitude difference.

A novel approach of rapidly switching between different aerosol concentrations across up to 5 orders of magnitude has thus been developed. This approach has now also been incorporated into an aerosol detector test system, which system can be used for evaluating chemical aerosol sensors/detectors.

Thus, in a third aspect, the present invention provides an aerosol detector test system comprising a vibrating mesh nebuliser to generate an aerosol. The vibrating mesh nebuliser is preferably a nebuliser according to the second aspect of the present invention.

The present invention shall now be discussed with respect to the following non-limiting examples and figures in which Figure 1 is a schematic showing the nebuliser and nebuliser housing fitted to the main aerosol line of the aerosol sensor test system; Figure 2 is a schematic of the aerosol sensor test system; and Figure 2 is a circuit diagram of the driver electronics used to control the frequency of the nebuliser.

Examples

A novel approach of rapidly switching between different aerosol concentrations across 5 orders of magnitude has been conceived and developed. This approach has been incorporated into an aerosol sensor/detector test system, which system can be used for evaluating chemical aerosol sensors/detectors.

In one example, the aerosol generation was achieved through modifying, through electronic means, a commercially available medical vibrating mesh nebuliser, modified to enable varying the drive frequency across 5 orders of magnitude to provide a range of aerosol concentration levels, from the commercially available nebuliser which was only capable of providing a single drive frequency of 128 kHz.

Nebuliser control An Aeroneb® Lab vibrating mesh aerosol nebuliser was acquired from Aerogen® (Galway, Northern Ireland). The adult (4.0-6.0 pm volume median diameter (VMD)) and the paediatric nebuliser (2.5-4 VMD) were both used in the development of the aerosol test system. This Aerogen ®Vibronic nebuliser is widely used as an aerosol drug delivery system, using vibrations across a 5 mm palladium aperture plate containing 1000 perforations, at a frequency of 128 kHz, to produce a controlled drug dose.

In order to obtain a variable level/concentration of aerosol, a drive electronic circuit for the nebuliser was created that could change the frequency of vibration from 1 Hz up to 128 kHz.

Having regard to Figure 1, the nebuliser 1 having a mesh plate 2, a lid 3, and modified to comprise an electronic pulse controller (driver) 4 was fitted into a purpose-built holder/housing 5 that enables an air flow 6 of up to 0.5 L/min underneath the perforated vibrating mesh 2 to sweep the aerosols inwards and down the inlet tube to the main aerosol line 7 of the test system, designed to use a flow of about 40 l/min.

This nebuliser set-up was found to occasionally produce droplets on the underside of the vibrating mesh. In order to ensure that these large droplets did not drip onto the inner surface of the main aerosol line, a drip wire 8 was attached to the tube between the mesh 2 and the main aerosol line 7, which entrained any liquid that had not been aerosolised, and fed it to a waste pot lined with charcoal cloth 9. This waste pot was flushed with a low flow of air (0.2 1/min) through a filter 10 to ensure any vapour from the waste did not diffuse back into the main aerosol line.

Aerosol Test Line Having regard to Figure 2, stainless steel tubes, crosses and T-pieces (40 mm OD, 36 mm ID) were joined to form an aerosol line with aerosol flight length 1.67m in length. The diameter of the tube was chosen based on calculations of Reynolds number, ensuring a laminar flow was achievable in the main aerosol line 7. A flow of compressed air is pulled through a HEPA filter 11 into the main line at a flow of 40 L/min using a vacuum pump 12. The aerosol is introduced into the line using the nebuliser described. A differential pressure meter 13 and a humidity meter 14 were fitted to the line so that the relative pressure and humidity in the line could be measured. An endoscope 15 was fitted to give a view of the aerosol inlet region. The length of the test line was chosen based on Stokes settling velocity calculations, which determined that with a length of 1.67m the majority of aerosol produced would reach the aerosol testing outlet. An outlet tube was positioned at this position, the diameter of the tube chosen based on isokinetic flow calculations to ensure sampling was non-biased with respect to particle size of the aerosol. The tube follows a 90° sweep, delivering the aerosol to a 4-way splitter, each arm of which is fitted with flow taps. The taps chosen did not require use of lubricant grease, which meant that they did not produce a background vapour in the line.

One of the arms of the 4-way splitter was sent to the Grimm optical particle counter 16 (pulling at 1.2L/min), which provided a real-time measurement of the level of aerosol being produced in the line. The exhaust from the Grimm was re-introduced to the line at a second 4-way splitter a short way downstream. A second of the 4-way splitter arms was used to take sample tubes. Glass packed DAAMS sample tubes (CAMSCO) were used to collect aerosol samples, which were then eluted and analysed by gas-chromatography (GC) or liquid chromatography (LC) mass spectrometry (MS). This sample tube analysis enabled calibration of the on-line GRIMM instrument, and ensured a quantitative measure of the aerosol being produced. The third arm of the 4-way splitter could be connected to an aerosol sensor instrument, for which this aerosol line was developed. The exhaust from the aerosol sensor was directed back to the main aerosol line via the second 4-way splitter. The fourth arm of the 4-way splitter was used as a bypass, which could be opened and closed as needed to maintain broadly the same flow off the main aerosol line.

In the system droplets/particles of 1-3 pm in diameter can be generated within the flow tube at aerosol concentrations that can be tuned from 15 mg/m3 to 0.012 mg/m3. The nebuliser is interfaced to a stainless steel flow tube to which chemical sensors can be connected and their performance against aerosolised chemicals assessed. The flow tube typically operates at a negative pressure of - 3.5 mbar relative to the laboratory and typically operates at a flow rate of 40 1/min. The aerosol concentration with the flight tube is constantly monitored using an optical particle counter and confirmed by off-line analysis of samples collected onto glass packed DAAMS tubes.

Electronic control of the vibrating mesh nebuliser The test system developed here uses the vibrating mesh nebuliser, but control electronics have been developed to provide a variable drive frequency (pulse train of variable repetition frequency) from at least 1Hz up to 100kHz. This variation in drive frequency enables a rapid switching between different concentration regimes.

A signal generator (UDB1002 Direct Digital Synthesis) that can provide a frequency output from 0.01Hz to 2MHz was used in conjunction with a voltage controlled pulse width generator (LS74121) to provide a signal consisting of a variable frequency pulse train with a set pulse width. This signal was sent to a DC motor driver module board (LMD18200T Texas Instruments) and the output was used to drive the nebuliser (Aerogen® AeroNeb® Lab). Having regard to Figure 3, the circuit diagram used is illustrated.

Characterisation of Aerosol Test Rig The system was tested using the chemicals tris (2-ethylhexy) phosphate (TEHP) and triphenyl phosphate (TPP), both simulants for chemical warfare agents.

TEHP and TPP were obtained from Sigma Aldrich Chemical Company. Methanol was obtained from Fisher Scientific. Solutions of TEHP and TPP were made up in HPLC grade methanol (99.8 % pure).

The Aeroneb® nebuliser (4-6 lam VMD) was used and the drive frequency of the nebuliser was set to a range of values from 1Hz to 10kHz, as shown in Table 1. The aerosol line was allowed to stabilise, and the aerosol was drawn through DAAMS sample tubes, using a flow rate of 1.2L/min (switching off the bypass arm and switching on the sample tube arm). The samples were eluted from the DAAMS tubes and analysed by LC-MS (Agilent).

Table 2. Aerosol concentration measured in the line compared to the set drive frequency of the nebuliser. Data is given for TEHP aerosol generated in the droplet/particle size range 1-311m and from concentrations of approximately 0.012 to 15mg/m3, purely by varying the drive frequency of the nebuliser, and TPP aerosol generated in the particle size range 1-3)wm and from concentrations of approximately 0.05 to 0.9mg/m3.

Nebuliser frequency Chemical Concentration as measured using aerosol collection on DAAMs sample tubes and LC-MS analysis (mg/m3) Concentration of particles as measured using a laser aerosol spectrometer (mg/m3) kHz TEHP 15.4 14.7 1 kHz TEHP 3.00 2.54 0. 1 kHz TEHP 0.41 0.301 0.01 kHz TEHP 0.072 0.050 0.001 kHz TEHP [A] 0.012 1 kHz TPP 0.83 0.87 0.1 kHz TPP 0.34 0.27 0.01 kHz TPP 0.078 0.052 [A] -Unable to collect sample tubes at this concentration due to the required collection time Advantages of (modified) vibrating mesh nebuliser for use in testing aerosol sensors * Wide range of concentrations (up to 5 orders of magnitude) available with rapid tuning.

* Highly flexible in terms of setting a concentration value across wide range of values. A number of other nebuliser systems increase the concentration by increasing the number of nebulisers, which results in a system with very little flexibility in the control of aerosol concentration.

* Since control of aerosol concentration is independent of dilution flows, there is little impact on the other optimised parameters in the rig, such as pressure, overall flow rate and sampling efficiency.

* Advantageous for toxic chemicals as only the amount needed is aerosolised, thereby reducing the hazard, and the waste.

* Different perforation sizes in the mesh can be chosen in order to adjust the particle sizes.

* Generation of aerosols of solids (as a solution) or liquids can be achieved.

* Does not heat sample/aerosol or need to heat sample/aerosol (as ultrasonic nebulisers can do).

Other nebulisers/aerosol generators are not as suitable for being used to test aerosol sensors. For example, the jet nebuliser produces a high concentration of aerosol, but is not readily tuneable. The vibrating orifice aerosol generator (VOAG) typically uses a pressurised syringe (8 Bar) of approximately 30 ml which would not be suitable for hazardous chemicals, and the concentrations produced cannot be readily tuned. A recirculating atomiser uses compressed air atomisation to produce aerosol, however these tend not to be readily tuneable for concentration, and the large concentrations generated are not suitable for testing sensors for hazardous chemicals. In ultrasonic nebulisers a solution is held on a plate, which is vibrated at ultrasonic frequencies, however these tend to heat the solution which is to be aerosolised. They are also not readily tuneable for a range of particulate concentrations over a wide dynamic range.

Claims (7)

1. Claims 1. Use of a vibrating mesh nebuliser in the testing and calibration of aerosol sensors/detectors [sensors or detectors for detecting/quantifying aerosols].
2. 2. Use according to Claim 1, wherein the aerosol sensors/detectors are for detecting hazardous chemical aerosols, such as chemical warfare agents.
3. 3. A vibrating mesh nebuliser comprising a reservoir for a liquid, a mesh/plate comprising multiple perforations, and a driver for vibrating the mesh/plate, wherein the driver is capable of providing a variable drive frequency to enable the mesh/plate to be vibrated at variable duty cycles, thereby providing the vibrating mesh nebuliser with the ability of delivering variable concentrations of aerosol.
4. 4. A vibrating mesh nebuliser according to Claim 3, wherein the drive frequency can be varied between at least 100 Hz to 100 kHz.
5. 5. A vibrating mesh nebuliser according to Claim 4, wherein the drive frequency can be varied between at least 1 Hz and 100 kHz.
6. 6. An aerosol detector test system comprising a vibrating mesh nebuliser to generate an aerosol.
7. 7. An aerosol detector test system according to Claim 6, wherein the vibrating mesh nebuliser is a nebuliser according to Claims 3 to 5.