An inexpensive portable monitor for measuring evaporative water loss.

The measurement of evaporative water loss from the skin, using a ventilated chamber, usually requires two high-accuracy devices for the measurement of the inflow and outflow relative humidity. The water-loss rate is proportional to the difference between the inflow and outflow relative humidity, and a sensor with low absolute accuracy can be used if the same sensor measures both relative humidities. The system described uses an inexpensive sensor to measure the water-loss rate with an accuracy of 10%.