= Oxygen consumption calorimetry
<Oxygen consumption calorimetry> estimates combustion <heat release rate> from the rate at which a fire consumes <oxygen>. Its practical usefulness comes from an approximate regularity: many common combustible materials release roughly $13.1\,\mathrm{MJ}$ per <kilogram> of consumed <oxygen>. The coefficient is an empirical energy equivalent for an applicable class of fuels, not a universal physical constant.<Standard Practice for Full-Scale Oxygen Consumption Calorimetry Fire Tests (ASTM E2067-20) by ASTM International>[Public Significance and Use, §4.1: approximate energy release per unit mass of consumed oxygen]{ref}
Let $\dot m_{\mathrm{O_2,cons}}$ be the consumed <oxygen> <mass flow rate>, obtained from a consistent incoming/outgoing gas balance. A simplified model is
$$
\dot Q\simeq E_{\mathrm{O_2}}\dot m_{\mathrm{O_2,cons}},\qquad
E_{\mathrm{O_2}}\simeq13.1\times10^6\,\mathrm{J\,kg^{-1}}.
$$
For an illustrative measured consumption rate of $0.001\,\mathrm{kg\,s^{-1}}$, this model gives
$$
\dot Q\simeq13.1\times10^3\,\mathrm{J\,s^{-1}}=13.1\,\mathrm{kW}.
$$
The observable must include gas throughput: an <oxygen> fraction by itself cannot determine a consumed <mass> per unit time.
In an exhaust-collection apparatus, a hood captures combustion products together with entrained air. Measurements of exhaust flow and gas composition allow the consumed <oxygen> rate to be inferred despite dilution. Gas sampling, mixing and the flow measurement must represent the same stream; gas amounts also need consistent wet/dry conventions and reference <temperature> and <pressure>. The NIST National Fire Research Laboratory uses exhaust systems to measure large-fire <heat release rate> by this principle.<National Fire Research Laboratory by NIST>[Facility description, exhaust collection and oxygen-consumption calorimetry]{ref}<Standard Practice for Full-Scale Oxygen Consumption Calorimetry Fire Tests (ASTM E2067-20) by ASTM International>[Public Significance and Use, §4.3: oxygen concentration and exhaust-flow measurements]{ref}
A useful experimental check is an independently metered reference burner. Its fuel-consumption-based energy input can be compared with the <heat release rate> inferred from <oxygen> depletion; NIST describes natural-gas burners for this mutual confirmation. This tests the measurement chain rather than merely exposing a gas analyzer to a known mixture.<Natural Gas Confirmation Burner by NIST>[Research Overview, comparison of fuel-consumption and oxygen-consumption calorimetry]{ref}
Incomplete capture, changing fuel composition, incomplete combustion and analyzer transport delays can all affect the inferred curve. They concern different parts of the inference: lost exhaust biases the gas balance, an inappropriate energy equivalent changes the conversion, and delayed sampling shifts the response in time. A reported <heat release rate> therefore needs both its conversion assumptions and its measurement conditions.
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