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Arthur B. DuBois is first author of the 1956 primary report introducing a rapid plethysmographic measurement of thoracic gas volume and comparing it with a nitrogen-washout measurement of functional residual capacity in normal subjects.^[ref] The experiment turns a gas-compression relation into an observable pressure-volume slope.

The apparatus enclosed the subject in a sealed chamber, used a mouthpiece shutter and recorded chamber and mouth-pressure signals with capacitance manometers on a cathode-ray oscillograph. Chamber calibration related its pressure signal to changes in body volume.^[ref] This supplied simultaneous signals rather than an estimate from one uncalibrated pressure reading.

An original Boyle's law example follows the paper's derivation. For an approximately isothermal closed gas compartment,Here $\Delta V$ is the signed change in gas volume, not a raw chamber-pressure signal. If $|\Delta P|\ll P_0$, the approximation is $V_0\approx -P_0\Delta V/\Delta P$. The original article derives the same exact-to-small-change distinction.^[ref]

For stipulated $P_0=100$ kPa, compression $\Delta V=-0.030$ L and pressure rise $\Delta P=1$ kPa, the exact expression gives 3.03 L and the small-change approximation gives 3.00 L. Their 0.03 L difference is an approximation effect in the chosen gas model, not observed instrument error in the study.

The experiment also depends on the relationship between mouth and alveolar pressure. The paper explicitly discusses failures when that relationship is disrupted.^[ref] DuBois's contribution combines the physical law, pressure sensing, chamber calibration and tests of the assumptions needed to interpret the signal.

Body plethysmography uses pressure and volume changes in an enclosing chamber to measure respiratory mechanics and compressible thoracic gas volume. For lung-volume measurement, a brief closed-airway effort relates gas compression inside the lungs to the chamber signal. The gas volume belongs to the instant of occlusion; it is not automatically residual lung volume.^[ref]

In the original 1956 experiment, Arthur B. DuBois and colleagues used an airtight chamber of approximately 600 L, a mouthpiece shutter, capacitance pressure transducers and a cathode-ray oscillograph. One signal represented mouth pressure and the other chamber pressure, converted to volume change through instrument calibration.^[ref] An open communication between mouth and alveolar gas during the closed-airway effort is essential to interpreting the mouth signal.

For an ideal isothermal gas amount, Boyle's law givesWith stipulated effective absolute gas pressure $P=100$ kPa, compression $\Delta V=-0.006$ L and increase $\Delta P=0.2$ kPa, the exact expression gives 3.006 L. Neglecting the product of small changes gives the first-order result $V\simeq -P\Delta V/\Delta P=3.0$ L. Here $\Delta V$ is the inferred lung-gas change, not an unconverted box-pressure reading.

If the measurement starts at quiet end-expiration, the interpreted lung-volume reference is functional residual capacity; linked inspiratory capacity and vital capacity maneuvers then provide other capacities. The thermal behavior of gas, instrument response and mouth-to-alveolar pressure equilibration remain physical assumptions to assess. A precise algebraic fit cannot repair a pressure signal that represents a different compartment.

Unlike tracer dilution, compression can probe gas that is not freely reached by the tracer during the test. This explains why body plethysmography and nitrogen washout can give different results without being duplicate names for the same observable.^[ref]

Helium dilution measurement of lung volume estimates communicating lung-gas volume by mixing it with a known apparatus volume containing helium. The gas concentration changes as the tracer distributes through the connected space. Connection at the quiet end-expiratory level makes the result an estimate associated with functional residual capacity.^[ref]

In an ideal closed system with equal pressure and temperature, let $V_a$ be the known apparatus volume and $c_1$ its initial helium fraction. If the lung initially contains negligible helium and equilibration gives fraction $c_2$, conservation givesStipulated $V_a=2.4$ L, $c_1=0.10$ and $c_2=0.04$ give $V_L=3.6$ L. If that unknown space includes 0.2 L of external mouthpiece volume, the represented lung volume is 3.4 L. Instrument space and lung space must therefore be distinguished before naming the reported volume.

Equilibration also matters quantitatively. Stopping at an apparent fraction 0.05 instead of the modeled final 0.04 would give 2.4 L, an underestimate of 1.2 L in the uncorrected model. The sensitivity isAt the stated values, a small positive fraction error 0.001 changes the estimate by approximately $-0.15$ L. Correct concentration measurement and the stopping definition are thus parts of the volume measurement.

The technical apparatus includes a mixing circuit, helium analysis, carbon dioxide absorption and replacement of consumed oxygen.^[ref] These features keep other gas changes from masquerading as tracer dilution. Leaks, incomplete mixing and gas not communicating with the circuit violate different assumptions. Body plethysmography asks a complementary question about compressible thoracic gas volume; the two methods need not sample identical gas compartments.

Thoracic gas volume, $V_{\mathrm{TG}}$, is the compressible intrathoracic gas volume assessed by body plethysmography at the instant of airway occlusion. It is not the volume of the entire thorax, which also contains tissues and fluid, and it does not identify a respiratory phase by itself.^[ref]

The phase matters when assigning a named capacity. In a constructed record, suppose the plethysmographic gas estimate is 3.2 L at an occlusion 0.4 L above the preceding quiet end-expiratory volume. If the measured increment and gas conditions are consistent, the corresponding functional residual capacity is $3.2-0.4=2.8$ L. Calling the unadjusted 3.2 L an FRC measurement would change the reference state without changing the observation.

The original experiment linked gas volume to pressure and inferred volume changes through Boyle's law.^[ref] In the small-change model,Thus a pressure-amplitude error can mimic extra gas volume. For an original illustration, let the actual gas volume be 2.8 L but the mouth signal represent only $7/8$ of the relevant alveolar pressure change. With all other terms unchanged, the inferred volume is $2.8/(7/8)=3.2$ L.

A second model can instead contain 2.8 L readily communicating with a tracer plus 0.4 L of compressible gas not reached during the tracer test. It can yield the same apparent 3.2 versus 2.8 L comparison. The numerical difference alone does not prove which model applies.

This is why comparison with helium dilution measurement of lung volume or nitrogen washout needs both compartment definitions and measurement assumptions. The technical standard describes the different accessible gas pools and potential pressure-equilibration errors.^[ref] Thoracic gas volume is a specific physical measurement, not a direct label for trapped gas or a substitute for every lung-volume definition.