UNCERTAINTY ANALYSIS FOR THE DROPLET METHOD OF WATER VAPOUR FLUX CALIBRATION

Transepidermal water loss (TEWL) is the rate of water loss through skin, and is an indicator of skin barrier function. This is important in medical care, in occupational monitoring against skin damage, and in dermatological research. TEWL is evaluated by measuring the water vapour flux from the skin surface using instruments, which incorporate relative humidity sensors. However, TEWL instruments of different types can give results differing by as much as 50 percent of reading, due to the lack of a harmonised and traceable method of calibration. An alternative calibration method for TEWL instruments has been developed using evaporation of a droplet of water of known mass. A variety of water vapour flux rates can be generated, so that TEWL instruments can be calibrated at several values in the range of operation. This paper details the sources of uncertainty in TEWL measurement and calibration, which vary slightly with instrument type. An uncertainty analysis is presented for the TEWL calibration process. Expanded uncertainties for calibration can reduced to 10 % of reading at 10 g m -2 h -1 , and 4 % of reading at 50 g m -2 h -1 , if suitable measurement traceability is employed.