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 that accounts for the nonideality of the plate thermal properties [Chillà et al, Phys. Fluids16, 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.

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The distance between grooves previously reported in the copper rough cell was wrong by a factor of 2. This does not change the interpretation and conclusion of this previous paper, which only relies on the grooves’ thickness.

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