RE Imhof, EP Berg, RP Chilcott, LI Ciortea and FC Pascut.
IFSCC Magazine, 5(4), 2002, pp297-301.
We report the development of a new instrument, the AquaFlux™, for measuring water vapour flux density from arbitrary surfaces, including in vivo measurements of trans-epidermal water loss (TEWL), skin surface water loss (SSWL) and perspiration. It uses a closed measurement chamber equipped with an electronically cooled condenser, to maintain a precisely reproducible microclimate adjacent to the skin surface under all ambient conditions. The condenser creates a diffusion vapour density gradient, from which the flux density can be measured.
We explore the properties of this new measurement principle by means of a mathematical model, which was also adapted for calculating comparable properties of open-chamber Nilsson instruments. In this way, we found that the intrinsic sensitivity of the AquaFlux™ is ~40% higher than that of a Nilsson instrument. However, in an experimental comparison of volar forearm TEWL measurements, we found a tenfold difference in coefficient of variation. We attribute the much lower than predicted sensitivity of the Nilsson instrument to extrinsic noise from diffusion zone instability in its open measurement chamber. The model was also used to calculate the relative humidity immediately above the skin surface, where we found lower than ambient values in both instrument types. The effect of this on TEWL measurements is discussed in detail. It is concluded that, during the relatively short time of a typical measurement, such microclimate changes affect mainly the transient SSWL component rather than the underlying TEWL. Finally, the model was used to estimate the flux density that would cause the relative humidity at the skin surface to reach 100%. This sets an upper limit to the flux densities that can be measured and may be a cause of instrument non-linearity as microclimate saturation is approached.
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