After the initial transients have dispersed and the flow has settled, the mesh may be refined. The criterion used is based on the gradient of the pressure for deciding which elements are to be refined. The user specifies a percentage of elements which are to be refined, and a criterion
is calculated for each element. A value 
of this criterion
is found such that the given percentage of elements have a value of
greater than 
, and those elements are refined. The
criterion is not simply the gradient of the pressure, because the strongest
shock in the simulation would soak up all the
refinement leaving weaker
shocks unresolved. With the element area 
in the criterion,
regions will ``saturate'' after sufficient refinement, allowing weaker shocks
to be refined.