= Evaporative water loss
\b[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 is
$$
\boxed{\dot Q_e=L_v\dot m_e.}
$$
For an illustrative $L_v=2.4\times10^6\,\mathrm{J\,kg^{-1}}$, evaporation of $1\,\mathrm{g\,min^{-1}}$ 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 $\dot m_e=\dot m_d(w_{\rm out}-w_{\rm in})$, 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.<A human calorimeter for the direct and indirect measurement of 24 h energy expenditure by M. J. Dauncey and others>[abstract, measurement of evaporative and sensible heat loss]{ref}
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