Sampler types

In the selection and design of the samplers, consideration was given to use samplers that were commercially available to the mining industry. A number of practical considerations were taken into account, especially given the constraints of underground coal sampling, which presents unique challenges including the permissibility of instruments, the confounding influence of rock dust, and the often-limited number of samples possible due to mine access constraints. Given that the commonly available sampling pumps approved for underground coal mines have a limited operating range (1.7–3.0 L/min) and are more commonly set up for sampling at 2.0 L/min, a practical way to match the airstream and inlet velocities is to utilize interchangeable nozzles with a common sampler body, this was done to adapt a personal sampler for isokinetic sampling. Four mine-compliant samplers (Table 1) described in what follows were co-located and aligned on the specialized sampling rack (Figure 3).

The Institute of Occupational Medicine personal sampler, (IOM, inlet dia. of 14.99 mm) composed of conductive plastic, is the standard measure for compliance sampling of inhalable dust at surface operations and is typically hung from the shirt collar.[24] In the current study, the plastic IOM was fitted with a 25-mm glass fiber filter with a 1.0-μm nominal pore size and mounted facing and in-line with the flow. The IOM sampling cassette and filter assembly are weighed together, so that dust depositing on the walls is included in gravimetric analysis. Including wall deposits was shown to be especially important for large particles in previous studies.[24, 25]

The second sampler configuration (IOM-MOD) utilized a stainless-steel construction of the IOM along with a custom-designed and machined-interchangeable inlet configuration to allow for isokinetic sampling (Figure 4). Two stainless nozzles were designed, with inner diameters of 3.86 and 9.40 mm, for achieving isokinetic conditions under high (2.5 m/s) and low (0.5 m/s) free-stream air velocities, respectively. At least 5.6 diameters were maintained from the nozzle inlet tip to the filter media, and a 30° tapered “knife-edge” was cut onto the end of each nozzle. The tubing used to construct the nozzles was smooth bore type with a tolerance of +/− 0.15 mm. The interior of the custom 3D printer adapter was fared so that the dust was directed towards the filter. The plastic custom nozzle adapter was removable, leaving the stainless-steel filter capsule assembly to be ashed via the LTA method[7] so that coal and inerting rock dust fractions could be determined. An SKC quartz filter, type R-100 with a 1.2-μm nominal pore size, was used. Quartz was used rather than glass fiber (as in the plastic IOM), so that the sampling cassette was appropriate for LTA for the analysis of coal dust in mixtures with limestone rock dust.[7] The stainless steel cassette weighs several grams and requires a wide-resolution microbalance and controlled weighing conditions to accurately determine collected particle masses,[25] which were achieved through the weighing chamber facility and the sensitive microbalance described above.

Original (A) andmodified IOM (B,C) samplers. Sectional view (C) of the stainless IOM-MODwith customnozzle/inlet combination. Nozzle is 38 mm long and constructed from tubing-grade stainless steel, and threaded adapter is 3D-printed and sealed with O-rings. Nozzle was kept short to minimize dust loss on interior.

The Zefon coal dust sampling cassette (ZCD, inlet size of 5.05 mm) is the industry standard for respirable coal dust sampling to meet compliance requirements (when using a size-selective cyclone on the inlet). The cassette is equipped with a 5-μm pore size PVC filter with a metal foil dome crimped to the filter perimeter. Large non-respirable particles pass through the short inlet at high velocity (1.66 m/s) and either deposit near the filter center, follow the flow radially in the zone between the foil and the filter, or settle due to gravity near the foil/filter junction. Because the dome and filter are weighed together, potential wall losses would be included in the mass measurement. The Zefon cassettes were mounted in two configurations: (1) directly in-line and facing the tunnel flow and (2) 90° to the flow and facing the tunnel floor, which is similar to the MSHA nuisance dust sampling method.[26]