 Indirect calorimetry

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.^[ref]

For a deliberately stipulated model with $q_{{\mathrm{O}}_2}=400\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$ and measured oxygen uptake ${\dot{n}}_{{\mathrm{O}}_2}=0.003\mathrm{mol}{\mathrm{s}}^{-1}$,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.^[ref]