Description
The Moudi consists of two main parts: the cascade impactor itself and its associated housing, with hinged door access. The combined unit measures 210 x 210 x 641 mm and weighs approximately 14 kg.
Close Information
The Moudi consists of two main parts: the cascade impactor itself and its associated housing, with hinged door access. The combined unit measures 210 x 210 x 641 mm and weighs approximately 14 kg.
The non-rotating version of the NanoMoudi (Model 125A) operates as a conventional impactor and is supplied without the housing.
In the rotating versions, Models 120 and 125B, the nozzle and impaction plates rotate at 1 rpm relative to each other, to form a near-uniform particle deposit on each substrate.
This has the effect of increasing the deposited particle mass by a factor of up to 1000, thus more particle mass can be collected without the risk of overloading.
The rotational motion is generated by individual stepper motors which drive the impaction plates at each stage (see schematic). Power for the stepper motors is controlled via electronics contained with the housing of the impactor, with power being distributed to each motor by miniature flat-ribbon cables from stage to stage.
The housing also contains electronics for precisely controlling the volumetric flow rate. These provide a readout of the cabinet temperature and also the six absolute pressure readings from the transducers used to monitor pressure between the more critical micro-orifice stages of the impactor.
The flow stability data derived from these measurements provides a quick and easy means of identifying any problems emanating from leaks or blocked nozzles within the system.
If, for example, one or more of the absolute pressures on the stages begins to drop while the flow remains constant, it indicates that the nozzles in these stages are becoming dirty and should be cleaned. The micro-orifice nozzles used in the final stages are quite small and can become partially occluded due to particle deposition by impaction or diffusion (Brownian and turbulent). When this occurs, the pressure drop across the nozzle plates will rise.
The Moudi Model 120 has a flow rate of 30 L/min and cut sizes from 18 microns at the inlet to 56 nanometres at the lowest stage. The standard configuration is ten stages (eleven including the inlet) with cut sizes at 18, 10, 5.6, 3.2, 1.8, 1.0, 0.56, 0.32, 0.18, 0.10 and 0.056 microns plus final filter.
The NanoMoudi Model 125 has a sampling rate of 10 L/min and cut sizes spanning the range from 10 microns to 10 nanometres. The standard configuration is thirteen stages with cut sizes at 10, 5.6, 3.2, 1.8, 1.0, 0.56, 0.32, 0.18, 0.10, 0.056, 0.032, 0.018 and 0.010 microns plus final filter.
A suitable vacuum pump is required to generate the desired flow rate.
The impaction plates can be prepared in the laboratory and inserted into the impactor at the time of use. Covers are provided for the plates to keep them clean during transit. The impactors were designed and calibrated based on the use of a 25 nm thick substrate having a diameter of 47 mm. Aluminium foil, PTFE film or thin membrane filters are also suitable.
In some cases, it may be necessary to apply a sticky surface to the substrates to reduce particle bounce. This is normally the case if foils are used and the sampled particles are dry.
Special coating masks (with a 37 mm diameter hole punched in the centre) used in conjunction with a silicone spray are available as options and are ideal for this purpose.
If the mass size distribution is to be determined from the impactor, the substrates must be weighed before and after a run. It is important that the weight of the sticky material applied to the substrate be stable. It is essential therefore to ensure that all the solvent be evaporated completely before use.
In the case of the rotating versions, overall control of the impactor is assumed by a programmable VGA display/controller located on the front of the housing and accessed by means of a membrane keypad.
The unit can be operated in two modes: manual or timed operation. Under manual operation, the vacuum pump switches on, all of the impaction plates start to rotate and the sensor outputs begin to transmit data to the screen immediately the 'start' button is pressed. Operation will continue and data collected at 1 minute intervals until terminated by the 'stop' button.
Under timed operation, the start and end of the run is predetermined by the operator. Sampling times can be adjusted between 5 minutes and 7 days with appropriate data logging intervals of between 1 and 15 minutes. The sampling interval should be selected such as to provide sufficient mass for subsequent gravimetric or chemical analysis. Check the inlet flow rate using a suitable flow meter prior to the start of the test.
Additional controls serve to download and erase the environmental and pressure related data from the memory.
Applications
The unique aerodynamic design of the MOUDI (Micro-Orifice Uniform Deposit Impactor) family of impactors makes it possible to collect size-fractionated airborne particulate matter over the widest possible range of particle sizes - 10 nm to 18 microns.
Rigorous adherence to aerodynamic principles, unique micro-orifice stages and the ability to spread out the collected deposits ensures that the Moudi range provides unparalleled capabilities and data confidence for the collection and study of the increasingly wide range of particles of interest to industrial and academic aerosol researchers.
In common with the original Models 100 and 110 (which continue to be available on request), the new Moudi Model 120 and NanoMoudi Models 125A and 125B operate on the same basic principle as any other multiple-nozzle inertial cascade impactor.
The Moudi is designed such that as the aerosol stream passes through each stage, particles having sufficient enough inertia will impact upon that particular stage collection plate, whilst smaller particles with insufficient inertia will follow the stream lines and pass to the next impaction stage.
The stages are assembled in a stack in order of decreasing particle size (where the nozzles are smaller, the air velocity through the nozzles is higher and finer particles are collected) until the smallest particles are collected at an after-filter.
The particle size distribution is determined by weighing the particles captured on each stage. If desired, the captured particles can also be analyzed chemically by standard means.
In the case of the Moudi, the range of the impactor has been extended down to 10 and 56 nanometers respectively by the use of thousands of small micro-orifices in the lower stages. These serve to reduce jet velocity, particle bounce, pressure drop and evaporative loss from collected particle samples containing semi-volatile material.
One of the unique features of the Moudi is the ability (if desired) to rotate the impactor plates to spread out the particle deposit uniformly over the stage. This minimizes particle build-up under each nozzle and reduces possible particle blow-off by the jets. It also allows the collected particle mass to be greatly increased before over-loading occurs.
The range of Moudi impactors is noted for its aerodynamic stage design resulting in sharp cut-off characteristics and low inter-stage particle losses. These design characteristics combined with stringent quality control processes have made the Moudi the impactor of choice by researchers for whom high quality sample and data quality are essential. It is these characteristics that form the basis of the second generation of Moudi impactors.
The new generation of Moudi impactors have additional attributes that make the impactors more user friendly and assist the user in gaining ever greater measurement precision.
Three new models are available in addition to the original 100 Series:
The Moudi Model 120 is a rotating model designed to operate at 30L/min. It has 10 impactor stages with cut-point diameters from 0.056 to 10 microns (56 to 10,000 nanometres) plus a final filter.
The NanoMoudi Model 125 is similar in construction to the Moudi but operates at 10 L/min
and has 13 impactor stages from 0.010 to 10 microns (10 to 10,000 nanometres) plus a final filter.
The Model 125 is available in two versions, the Model 125A (non-rotating) and 125B (rotating).
The current range of Moudicascade impactors represents the very latest in air sampling andparticle sizing technology.
Common to all Moudi impactors, multiple nozzles at each stage provide for the desired flow conditions resulting in low inter-stage losses, low pressure drop, sharp cut efficiency curves and stable, predictable cut points. Indeed, the nozzles of corresponding stages are the same as the earlier versions such that a direct data correlation can be made between the Moudi Impactors of both generations.
Wide Range: Use of micro-orifice nozzles extends range down to 10 nm: sizes particles according to their aerodynamic diameter between 10 nm and 10 microns, in 10 or 13 logarithmically spaced stages (dependent on model).
