= Volume fluctuations of a small fluid parcel
{title2=$\langle\delta V^2\rangle=k_BT\kappa_T\bar V$}
For a material parcel of fixed amount inside a much larger thermal and mechanical environment, <availability> is $A(V)=F(V,T)+p_{\rm bath}V$. Since $F_V=-p$, its curvature is $A_{VV}=-p_V=1/(V\kappa_T)$. The <Gaussian fluctuation approximation> gives $\langle\delta V^2\rangle=k_BT\kappa_T\bar V$. This describes parcel-volume fluctuations, not the geometric <volume> of a fixed observation box or a globally rigid vessel. A fixed spatial box instead has number fluctuations related to the same compressibility.
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