Effect of Reduced Atmospheric Pressure on a Diffusional Sampler

All diffusional samplers use the laws of gas diffusion as the basis for the assumption that there is no change in sampling efficiency due to changes in pressure. The objective of this study was to examine experimentally the effect of atmospheric pressure on the collection efficiency of a simple diffusional sampler. The results indicate that the sampler efficiency is actually dependent on atmospheric pressure, with a decrease in efficiency with each decrease in pressure. While this loss in efficiency would be very significant at great altitudes, the loss is less than 7% at 18 000 feet, which would include any altitude likely to be sampled in industrial hygienic practice. This makes a pressure correction unnecessary at or near the earth's surface, but the requirement for correction increases markedly as pressure is greatly reduced. It was also shown that, for a diffusion path of constant diameter, the shorter the path length, the greater the effect of pressure on collection efficiency.