Realisation of a practical inspirable dust spectrometer. Final report on CEC Contract 7260 – 04/019/08

This project set out to develop new instrumentation for dust measurement in mines upon which to base a better understanding of the factors leading to disease and, in turn, improved standards setting and environmental conditions. The general aim has been to to bring to practical realisation the dust spectrometer whose potentialhad been established in a previous CEC-funded project. The resulting instrument(SIDS) is an eight-stage cascade impactor based on the principle of rectangular jets impinging onto rotating drums. It incorporates an entry which aspirates the inspirable fraction of the ambient airborne dust. Thus it can provide the particle aerodynamic size distribution of the inspirable fraction and so enable determinationof the particle size distributions and airborne mass concentrations of any subfraction relevant to health (e.g., thoracic, respirable). It can also provide aerodynamically-classified dust samples suitable for mineralogical analysis. As a result of the rotating-drum feature, such information may be obtained for time increments within individual working shifts. The instrument is a static sampler, intended for fixed-point operation.The project was a complex exercise in technical problem-solving. The first part involved the design of the entry system so that, at the prescribed sampling flowrate of 10 1/min, it aspirates the inspirable fraction (as defined by the Vincent-Armbruster curve of 1981, adopted in 1985 by the ACGIH). This was achieved by adapting the rotating-head concept that had been applied earlier for a 3 1/min static inspirable dust sampler (Mark et al 1985). Here, continuous rotation of the sampling head was achieved by means of an electric motor, providing an angular speed of about 2 rpm. Connection to the non-rotating cascade impactor part of the instrument was made through a rotating seal. Particle losses by deposition inside the entry section between the entry itself and the first stage of the cascade impactor were kept as low as possible by careful aerodynamic design of the transition conduit. The detailed penetration characteristics of the final design as a function of particle aerodynamic diameter were measured, and an empirical best-fit curve was obtained to describe the data.The eight-stage cascade impactor part of the instrument was designed to provide a range of particle ‘cut-sizes’ from about 1 to 20 54 fan, and with minimal losses of particles between stages. Detailed calibration of the cascade impactor gave results which were in good agreement with theory for the particular configuration chosen. Rotation of the impactor drums in the desired sequence was achieved by means of individual stepping motors, driven and controlled by pre-programmed digital circuitry.In the absence of a suitable, intrinsically-safe pump capable of delivering the required 10 1/min flowrate, a new single-action diaphragm pump was developed.In order to assess the dust collected on the impactor stages, a new, automatic beta-scanning system was developed. For this purpose, the dust collection on the impactor drums took place onto polycarbonate membranes which were subsequently (after sampling) transferred to cassettes suitable for loading into the beta-scanning system prior to scanning. The beta-scanning rig was incorporated into a microcomputer system so that fully-automatic scanning and determination of dust masses could be achieved.It was recognised that one difficulty in aerosol spectrometers concerns how to use the measured data for providing the desired particle size distribution information. A review of existing numerical methods was carried out. A new procedure was developed based on a priori fitting of log-normal distribution functions to the cascade impactor data. Thus it was possible to obtain size distributions for particle diameter up to about 50 �m. By incorporating knowledge of the dust mass collected in the entry system along with the curve for the penetration characteristic of the entry system, particle size distribution information was extended to cover the whole inspirable range.Investigations were carried out on how best to conduct mineralogical analysis of the collected dust samples. An indirect X-ray diffraction method was finally adopted. Trials with the instrument were conducted both in the laboratory and underground in a coalmine. It was demonstrated how it could provide the wide range of information about inspirable dust and the various health-related subfractions. After some initial technical difficulties, the instrument performed well. It has since featured in a CEC-funded joint dust sampling project, and the results of this exercise will be reported separately. “”