<p>We studied a pumpdown curve in the low and medium vacuum regimes for a conductance-limited pumping. Senda [H. Senda, SEI Technical Review 176, 1 (2010).] predicted that the exponential decrease of the chamber pressure, followed by the reciprocal behavior with time near the lower limit of the viscous flow. He proposed a pressure boundary between the exponential decrease and the reciprocal decrease as a "Bifurcation pressure". We experimentally found bifurcation pressures and they quantitatively compered with the theoretical estimation. The result of our experiment agreed fairly well with values that had been obtained by the theoretical calculation.</p>