TRACEABLE CALIBRATIONS FOR WATER VAPOUR FLUX INSTRUMENTS

Water vapour flux instruments are used for the determination of transepidermal water loss (TEWL). TEWL is the rate of water loss through skin and is an important indicator of skin barrier function. The comparison of data recorded by different TEWL instruments is difficult due to the lack of a clear validated calibration method. Current state of the art membrane-based calibration techniques are based on an ASTM standard E96- 00 - Standard Test Methods for Water Vapour Transmission of Materials but this method is shown to be unsuitable for the calibration of TEWL devices. Two recent UK Department of Trade and Industry projects have gone some considerable way towards developing a traceable alternative calibration method. This approach to calibration consists of a 1 µl droplet of water dispensed into the base of a specially designed calibration cap and the flux time-series curve is then logged until the droplet has fully evaporated. A calibration factor can be derived from the relationship between the mass of water dispensed and the area under the flux time-series curve. A series of calibration caps of different sizes provide different water vapour flux rates over the range of interest. The approach and detail of the calibration method, and some preliminary results are reported. By implementing a harmonized and traceable calibration, disparity between results from several different TEWL instruments was approximately halved in the range around 15 g m -2 h -1 .