  We consider QCD with one massless quark and one heavy quark in a finite
volume of linear extent L_0 ~ 0.2 fm. In this situation, HQET represents an
expansion in terms of 1/z=1/(m L_0), which we test by a non-perturbative
computation of quenched current matrix elements and energies, taking the
continuum limit of lattice results. These are seen to approach the
corresponding renormalization group invariant matrix elements of the static
effective theory as the quark mass becomes large. We are able to obtain
estimates of the size of the 1/m-corrections to the static theory, which are
also of practical relevance in our recent strategy to implement HQET
non-perturbatively by matching to QCD in a finite volume.
