A WIND-TUNNEL STUDY OF THE PARTICLE COLLECTION EFFICIENCY OF AN INVERTED FRISBEE USED AS A DUST DEPOSITION GAUGE

An overall figure for the collection efficiency of the Frisbee is around 50%. This is significantly better than for other gauges which are commonly in use for dustfall measurement. The collection efficiency fell with increasing wind speed and also with decreasing particle diameter down to a minimum at around 80-100 mu m before rising again. This behavior seems to be connected with a change in the main particle collection mechanism from deposition to turbulent diffusion. Some tests were performed to verify this finding by increasing the vertical components of turbulence over the gauge; this increased the collection efficiency and may be a useful method of improving performance in the field. A thin coating of liquid paraffin gives greater collection efficiencies at higher wind speeds due to a reduction in the blow-out of collected dust. The interference from the bottle mounted under the Frisbee for collecting excess rainfall was also investigated and found to be significant.