Humidity measurements in passive heat and moisture exchangers applications: a critical issue

A reliable, quantitative assessment of humidification performances of passi ve heat and moisture exchangers in mechanically-ventilated patients is stil l to be achieved, although relevant efforts have been made to date. One of the major problems to tackle consists in the difficulty of humidity measure ments, both in vivo (during either anaesthesia or intensive care unit treat ments) and in vitro set-ups. In this paper a review of the basic operation principles of humidity sensors as well as an analysis of their usage within in vivo and in vitro tests are presented. Particular attention is devoted to the limitations arising from the specific measurement set-up, as they ma y affect the results notably.