= Gas-globe density measurement
{title2=$\rho\simeq\frac{m_{\rm filled}-m_{\rm evacuated}}{V}$}
In an idealized rigid-globe weighing, subtract the evacuated assembly's <mass> from the filled assembly's <mass>, then divide by the internal <volume (thermodynamics)>. Establish the filled sample's <pressure> and <temperature> before comparing gases. The subtraction removes the container's large contribution, making a small gas contribution accessible. A real measurement also needs corrections or bounds for residual <gas>, <adsorption>, leaks, changing vessel volume and external <buoyancy>. The <Rayleigh-Ramsay argon density experiment> shows why preparation-route comparisons add information beyond repeated weighings of one sample.
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