Comparability and reproducibility of the results of water loss measurements: a study of 4 evaporimeters.

A study was made of the comparability and reproducibility of the results of measurements of water loss, both in vitro and in vivo, using 4 ServoMed Evaporimeters (3 single-probe and 1 double-probe instrument). The optimum time for recording transepidermal water loss (TEWL) after the initial application of the probe to the skin (in vivo), and the best technique for optimizing the accuracy of measuring TEWL were determined. An evaporation device with a constant level of water loss was constructed for in vitro studies. The volar aspect of the right forearm skin of one subject was used in vivo. Both in vitro and in vivo measurements showed that there were some differences between the results of 4 of the 5 probes. The other probe was distinctly out of range. For all probes, the reproducibility of results of successive measurements was high. Stabilization of TEWL values was reached for all probes from 30-45 s after their initial application to the skin. It is recommended that TEWL be recorded for a further 30-s period, after the initial stabilization (45 s), and that this be taken as the true value. The manufacturer's recommended calibration procedure is based only on adjustments for the standard specified humidities and zero water loss. The importance of incorporating an additional calibration procedure which includes adjustments for an actual standard constant water loss is thus strongly stressed.