  The mixing efficiency of a plume in a filling box and an emptying-filling box
is calculated for both transient and steady states. The mixing efficiency of a
plume in a filling box in an asymptotic steady state is 1/2, independent of the
details of this state or how the plume is modelled. The mixing efficiency of a
plume in an emptying-filling box in steady state is 1 - xi, where xi = h/H, the
depth of the ambient layer h non-dimensionalised by the height of the box H. A
deeper mixed layer therefore corresponds to a higher mixing efficiency.
