The gravimetric sampling of dust of an airborne nature is not only the internationally acceptable method of determining dust concentrations but is in general use in South Africa.
While the Department of labour advocates the sampling method, they do not have their own sampling guidelines or established method and use that devised by the Department of Minerals and Energy whenever dust of an ore or coal is involved.
As an alternative the NIOSH guidelines are used.
The method and procedure outlined below thus relates directly to the guidelines for gravimetric sampling published under the auspices of the Department of Minerals and Energy – GUIDELINES FOR THE COMPILATION OF A MANDATORY CODE OF PRACTICE – No. 1 PERSONAL EXPOSURE TO AIRBORNE POLLUTANTS.
Equipment and Measurement Criteria Used
All sampling is undertaken using sampling pumps having the Government Mining Engineer Approval certificate GO8 together with one of two standard sampling trains.
Depending on the requirement for either total dust or respirable dust monitoring or both, the sampling train will differ as follows:
As respirable dust constitutes dust of a particle size below about 7,0µm an approved make fixed duty separating cyclone is used in circuit to separate particulate of a size greater than 7,0µm.
This particulate is collected in a grit pot below the cyclone and can be discarded or retained for bulk sample purposes. The cyclone design permits peak efficiency at the pump setting of 1,9 litres/second, a rate which closely equates the metabolic breathing rate of a normal healthy adult human being. Should the rate vary by more than 5% at the conclusion, the sample is discarded.
Total dust constitutes all dust that can be collected as an airborne particulate and as no respirable dust has to be separated, an impinger section replaces the cyclone to allow a uniform distribution of dust on the capture media filter. Calibration is undertaken in the same way as outlined above.
Calibration is undertaken before and after each sample.
Filter Media for use in the Equipment.
The advocated filter used for the sampling is the cellulose filter media approved in terms of the same guidelines outlined.
The weighing of filters is undertaken in our own laboratory using weighing equipment, a digital micro-balance capable of reading to the nearest 10 micrograms in terms of the guidelines, with our balance reading to 0,1 microgram.
During the weighing, determinations are made to correct the filter for moisture content by permitting it to stabilise in a compartment together with reference filters, which are always kept in the laboratory clean room. The minimum stabilisation period is 12 hours.
The analyst wears laundered cover clothing including gloves, mask and hair cap in non-static materials. The analyst is tethered using a static line to the clean room earth connection.
All mass determinations are undertaken using statistical averages of masses. Our facilities are constantly inspected by the Department Inspectorate, approved and accredited, and a periodic cross-laboratory assessment is undertaken with other laboratories.
The following things are considered on site
Positioning of Filters
Filter/pump trains are located on personnel as required by the code of practice.
The trains are secured at a height consistent with the normal breathing zone of an adult worker.
Where our assessment procedure differs from the guidelines, is the assessment of working areas where the duration of the operation does not equate an 8 hour shift.
Under such conditions we move the sampling train from time to time, remaining well within the positions normally taken by operators to achieve a more balanced and representative result as long as the operation occurs.
Results are time weighted to an eight hour shift.
Respirable or Total Dust?
Should we suspect the existence of a toxic dust with a Threshold Limit Value (TLV) in respect of respirable dust we sample for respirables.
If we consider that there is a necessity to establish a total spectrum, we sample for both. Normally we only sample for total dust for dust-control design purposes.
Further analysis of dust filters can be undertaken to establish the elemental composition of the collected samples. To achieve this as a representative sample we combine various filters from an area into one composite sample for analysis.
Depending on the requirements of the original specification, or Act/guideline the compliance can be accurately determined.
Many specifications call for a nominal “no visible dust requirement”, which becomes a very subjective requirement as not even the internationally accepted standards call for “no dust”.
The stipulation is further complicated by the presence of dust in any building, plant or even offices, as only the best of clean rooms are totally devoid of particulate. To then assess which of the dust measured has resulted from the operation is an impossibility.
TLV – Threshold Limit Value
TWA – Time Weighted Average
PNOC – Particles not otherwise classified