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Evaporative water loss is the loss of body water as water vapour, including evaporation from the skin. The water carries away latent heat of vaporization: if $L_v$ is the energy required per unit mass and ${\dot{m}}_e$ is the actual evaporated mass flow rate, the corresponding cooling power isFor an illustrative $L_v=2.4\times 10^6\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}$, evaporation of $1\mathrm{g}\mathrm{mi}{\mathrm{n}}^{-1}$ removes $40\mathrm{W}$. Producing that amount of sweat does not guarantee that cooling: if half drips away before evaporation, the evaporative contribution is only $20\mathrm{W}$. The relative humidity and movement of the surrounding air affect how readily water vapour leaves the skin.

An experiment must therefore measure evaporation, rather than infer it from sweating alone. A ventilated chamber can compare inlet and outlet humidity ratios $w$, defined as water vapour mass per dry-air mass. At steady state, with no condensation, storage or other moisture source, conservation of mass gives ${\dot{m}}_e={\dot{m}}_d(w_{\mathrm{out}}-w_{\mathrm{in}})$, where ${\dot{m}}_d$ is the dry-air mass flow rate. In 1978, M. J. Dauncey and colleagues described a human calorimeter that used inlet/outlet wet-and-dry-bulb temperatures and airflow for the evaporative contribution, alongside a water-cooled heat exchanger for sensible heat transfer. This separated two routes by which energy leaves the same subject.^[ref]

M. J. Dauncey is a researcher who developed methods for measuring human energy use with calorimetry. A 1978 study with collaborators described an instrument combining direct calorimetry with an open-circuit indirect calorimetry system for continuous observations lasting at least 24 hours.^[ref]

The direct system removed heat through a water-cooled heat exchanger. Control of the wall temperature minimized the mean temperature difference across the chamber walls. Evaporative heat loss was inferred from incoming and outgoing wet-and-dry-bulb temperatures together with air volumetric flow rates.^[ref]

For a dimensional example, a steady heat-loss rate of $100\mathrm{W}$ over one hour corresponds to $100\times 3600=360000\mathrm{J}$, or $360\mathrm{kJ}$. These are illustrative values. Heat loss and metabolic heat production need not match instantaneously if the body is warming or cooling, which motivates measuring complementary quantities over a defined interval.