Volume fluctuations of a small fluid parcel ($⟨\delta V^2⟩=k_{B}T{\kappa }_T\bar{V}$)

For a material parcel of fixed amount inside a much larger thermal and mechanical environment, availability is $A(V)=F(V,T)+p_{\mathrm{bath}}V$. Since $F_V=-p$, its curvature is $A_{VV}=-p_V=1/(V{\kappa }_T)$. The Gaussian fluctuation approximation gives $⟨\delta V^2⟩=k_{B}T{\kappa }_T\bar{V}$. 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.