TECHNOLOGY

Existing instrumentation for real-time air particulate measurement is often expensive, power-hungry, and bulky (such as condensation particle counters and many light-scattering devices). Even with these compromises instruments are still unable to detect the most hazardous particles (less than 100 nm diameter; most light-scattering devices) and aresubject to serious interference from non-carbon particles often found in mine air such as oil mist, water droplets, dust, and pollen (any device with a low airflow rate and without a particle size selector).  As a result, most particulate air pollutionstudies use air samplers that trap particles on a filter that must then be sent to an analytical laboratory for subsequent analysis.

MSHA stipulates the NIOSH 5040 method for DPM measurement in mines. This method employs a pump to draw air through a particle size selector and onto a quartz filter. After a specified period of sampling (typically eight hours) the filter is sent to an analytical laboratory for thermo-optical analysis which yields TC, EC, and OC values.

The NIOSH 5040 method is accurate and reliable, but has serious drawbacks. There is the time lag involved in getting results, the non-portability of the instruments used and that it integrates readings to exposure to particulates over the entire sampling time which makes identifying worker schedules to changing particulate levels virtually impossible.

This prevents any real-time adjustment to the mine’s ventilation, personnel location, or equipment use to modify DPM exposures. Overexposure to DPM can thus be detected using the 5040 method but not prevented. Sampler methods cannot be used for active control of combustion particle generation or site ventilation to reduce human exposures to hazardous levels of particulates.

The ICx Airtec real-time DPM monitor is based on a technique developed by Respiratory Hazards Control Branch at the National Institute for Occupational Safety and Health (NIOSH) Pittsburgh Research Laboratory (Noll et al., 2007; Noll & Janisko, 2007), subsequently licensed to ICx.

Mine air is drawn by a flow controlled diaphragm pump through a particle size selector which only permits the passage of submicron particles. These particles are collected on a filter. A laser illuminates the filter and the transmittance of the laser beam is measured in real-time by the instrument. As DPM particles accumulate on the filter, the laser’s transmittance decreases. The instrument converts this decrease in transmittance into a real-time concentration of elemental carbon in the air using calibration data obtained by the NIOSH laboratory using NIOSH Method 5040.

OUTPUT

The filter on which the particulates are captured is manually replaced (which takes only a few seconds) when it is fully loaded with black carbon. Its capacity is sufficient to operate for at least a full (12 hour) shift even in a very highly contaminated environment. In the event of overloading, a filter saturation alert is given by the instrument. However the low cost of the filter cassette allows its replacement after every shift.

The unit displays real-time EC levels on an LCD display, and can also display TC levels (converted from the EC data using a user-supplied, mine-specific conversion factor as described in MSHA publications). EC can also be displayed as an eight hour time-weighted average. In addition to the realtime display, EC data is logged internally and can be downloaded to a personal computer with a USB port.

EFFECT OF REAL-TIME MONITORING ON DATA COLLECTION

The Airtec DPM monitor generates dozens, hundreds, or more values for a fraction of the cost of the NIOSH 5040 method, depending on how the mine engineer chooses to use the unit. By any measure, the cost of DPM monitoring is drastically reduced. This cost reduction should allow a corresponding increase in EC sampling.

The real-time Airtec DPM monitor described here accurately replicates the results of the NIOSH 5040 technique used for EC and TC measurements in underground mines. The monitor does so without the delay in obtaining results associated with a laboratory analysis method. The Airtec monitor provides a large amount of real-time data at a much lower cost and promises to be a useful tool for mine ventilation engineers and hygienists to allow active reductions in DPM concentrations.