FAQ

A fallout dust unit is a device used to collect and measure dust particles that settle out of the air (fall-out or precipitant dust), typically using a bucket or multiple‐bucket arrangement, in accordance with the standard method ASTM D1739.

Key points:
It consists of a bucket (or several buckets for directional sampling) placed in the open air, which captures dust falling under gravity.

The amount of dust collected (expressed in mg/m²/day) is then compared to action levels (e.g., 600 mg/m²/day residential, 1200 mg/m²/day industrial) to assess whether fallout dust is a concern.

The units can be “single bucket” (collecting from all directions) or “four‐bucket” (providing directional information to help identify source direction) depending on monitoring needs.

The speed, at which particles settle, depends on many variables. The following page will allow you to determine settling velocities for different-sized particles.

Import dust is the term used to describe dust coming from a source other than the main source being monitored by DustWatch.

Export dust is dust that is coming from the main source being monitored. It does not matter where the dust is going as the important thing is that the dust is being exported from the main source.

Nett export dust is the term used when two DustWatch units are used on opposite sides of a dust source. The import dust from the one unit is subtracted from the export dust from the unit on the opposite side. This ideally provides a nett export.

We say ideally because there are other factors that influence this, such as heat generation over a dust source, water in the area of the DustWatch and incorrect positioning of the DustWatch units, either too far or too close to the dust source.

Following international and South African trends, we recommend dropping the frequency of sampling to one sample every quarter. Sample for one month in the middle of each of the four seasons.

This will continue the sine curve trend data so that the company will be able to have an up-to-date history of their fall-out data. So long as the results remain below 300mg/m2/day, we don’t see how any inspection authority could have a problem with this.

It would even be reasonable to only sample in the dry season, but from our experience, we have found that to have a yearly trend going back a number of years is very valuable.

From our experience if the unit is positioned too close to the source then the unit will get inundated with dust and all the buckets will be loaded.

As an example, the unit should not be positioned within 50 meters of a dusty road, unless it is not possible for security or other reasons.

Much depends on what the client requires from the fallout dust monitoring programme, and in most cases, we are asked by the client to recommend positions for the units. Each unit’s position should be considered taking both the dust source and the potential problem areas, such as towns, farms, and other mines into account.

When using more than one DustWatch, all the units should be positioned in such a way that nett export values can be determined.

In short the process of Fall-out monitoring is as follows: (The mine can choose at what stage they would like to contract out the work.)

• Purchase DustWatch units. (required)
• Install DustWatch units. (required)
• Prepare and change buckets every 2 weeks. (Optional, could be contracted out, either to us or a site agent employed by us, or the company could do it themselves with some training)
• Filter the contents of the buckets onto filters using standard Lab methods. (Optional, the company could do it or they could contract it out.)
• Weigh the filters containing the dust. (Optional, normally done by us or by the company)
• Use our excel spreadsheet or make your own, for doing some simple calculations. Draw up tables and trends and write a short report every month. (Optional, usually done by us or the company)

Once the buckets have been collected, the contents are transferred to 2-litre bottles. The bottles are then crated and sent to our offices using the company’s preferred courier. The bottles take up less space than the buckets and are less likely to leak during transportation.

We receive bottles from Botswana, sent to South Africa across an international border. We then filter the water, weigh the samples and write the reports.

When there is enough demand in an area, a site agent will be appointed, and the site agent will be able to do the bucket changing and the filtering of the buckets. Leaving only the dried filters in Petri dishes to courier to us. This is a much cheaper solution as the courier costs are significantly less.

There are many factors that influence the distance travelled by particles.

Our diagram, showing how vegetation affects fall-out dust, also gives a good indication of the different particle sizes and how wind is able to blow them around.

The open area of the buckets used to catch the dust is 0.0254m2 and the time period is usually 14 days, depending on when the buckets are changed.

The dust that collects in the water in the bucket over the 14 day period is filtered through 8 micron filter paper. The filter paper is then weighed to determine the mass of dust collected in the bucket.

The mass of dust is divided by the number of days, and then divided by the area of the bucket to yield a result in units, mg/m2/day.

To determine the 28 day average the two resutls from the two 14 day bucket changes can be added together and divided by two to get the average over the full 28 day period.

Older pages for reference purposes