Conference Contribution Details
RE Imhof & the TEWL Calibration Consortium.
Poste-deadline Poster, Stratum Corneum IV Congress, Paris, June 2004.
Trans-epidermal water loss (TEWL) measurements are widely used in skin testing and several instruments and measurement principles are in current use. However, there are no traceable methods of calibration and this is a source of difficulty for users and manufacturers alike. For this reason, we have initiated a research programme to develop traceable calibrations in terms of water vapour flux density for various TEWL instruments in current use.
Current methods for calibrating and comparing TEWL instruments are adapted from the ASTM-96 wet cup method for membrane permeance measurement. A typical apparatus consists of a cup or Perty dish containing water or a saturated salt solution, whose opening is covered by a semi-permeable membrane. The mean flux of water vapour escaping through this membrane can be determined from measurements of cup weight loss. The TEWL instrument is calibrated by placing its measurement chamber in contact with the membrane and adjusting its reading to agree with the gravimetrically determined mean flux.
Our mathematical and numerical modelling has now revealed that the above method is fundamentally flawed, because the flux entering the TEWL measurement chamber is generally different from the flux escaping into the ambient air. Another difficulty is that diffusion resistances that can be ignored when testing low permeance membranes have a significant effect at the higher flux densities (>5 g/[sq.m h]) needed for TEWL calibration. For this reason we are now developing a different method, where a small water drop is dispensed into the base of a TEWL measurement chamber and the flux time-series curve is recorded until it has fully evaporated. The calibration constant can then be calculated from the relationship between the mass of water dispensed and the area under the flux time-series curve.
Work is now in progress to establish measurement traceability for the dispensed water drops and to design calibration adaptors for the different instruments. These adaptors need to ensure that the flux density within the measurement chamber is uniform and also give some control of the mean evaporative flux, to allow measurement non-linearities to be characterised.
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