  A transition to Kraichnan ultimate regime of convection has been reported in
very high Rayleigh numbers experiments, but not in all of them. These
apparently contradictory results can be explained by a recent phenomenological
model which accounts for the non-ideality of the plate thermal properties
[Chill\`{a} et al, Physics of Fluids 16:2452 (2004)]. In this paper, we present
a direct test of this model, using a low conductivity plate. We found an
unaltered transition, not compatible with the model's predictions.
