= Indirect calorimetry
{title2=$P\simeq q_{\mathrm{O_2}}\dot n_{\mathrm{O_2}}$}
{wiki}
<Indirect calorimetry> estimates an energy-release rate from measured chemical exchanges and a model relating those exchanges to <chemical energy>. Gas-analysis implementations measure <oxygen> disappearance and often <carbon dioxide> production; <stoichiometry> and energy equivalents then supply the inference. The physical measurement chain therefore contains both observations and assumptions, rather than a direct measurement of the entire <heat transfer>.
A controlled combustion system makes this logic concrete. A gas stream passes through a reaction region, exhaust flow and composition are measured, and the difference between incoming and outgoing oxygen amounts gives an uptake rate. In <oxygen consumption calorimetry>, that rate is combined with an appropriate energy-per-oxygen factor to estimate <heat release rate>. A falling oxygen fraction alone is insufficient: dilution and changes of total gas flow must also be accounted for.<National Fire Research Laboratory by NIST>[Facility description, exhaust system and oxygen-consumption measurement of fire heat release rate]{ref}
For a deliberately stipulated model with $q_{\mathrm{O_2}}=400\,\mathrm{kJ\,mol^{-1}}$ and measured oxygen uptake $\dot n_{\mathrm{O_2}}=0.003\,\mathrm{mol\,s^{-1}}$,
$$
P\simeq q_{\mathrm{O_2}}\dot n_{\mathrm{O_2}}=1.2\,\mathrm{kW}.
$$
This factor is an assumption for the calculation, not a universal physical constant. Different reaction mixtures, incomplete oxidation or changing product composition can require a different model. Gas volumes must likewise be tied to stated <temperature> and <pressure> before they become comparable amounts.
An independent reference tests the chain. NIST describes natural-gas reference burners whose fuel-consumption-based heat input cross-checks its oxygen-consumption calorimetry system. The agreement then tests gas sampling, flow measurement and the adopted energy accounting together, rather than merely checking one sensor.<Natural Gas Confirmation Burner by NIST>[Research Overview, reference burners and mutual confirmation of fuel- and oxygen-consumption calorimetry]{ref}
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