The transition between the laminar and turbulent flow regimes is traditionally addressed using the continuum formulation of the Navier-Stokes equations and dimensionless parameters such as the Reynolds number. However, a detailed understanding of the transition mechanisms has remained elusive. Theoretical approaches based on molecular and quantum mechanical models have been proposed but have yet to be thoroughly tested experimentally. In an effort to test a quantum-based model, specific apparatus and experiments have been designed to evaluate particular features of the laminar-turbulent transition. Hysteresis plots of flow versus differential pressure are used to examine the flow transition that occurs inside a tube with a divergent entrance. The hysteresis plots generated in these tests show several notable features and quantitative trends. The primary focus of this article is on the observed dependence of the laminar-turbulent transition behavior on the absolute pressure. Whereas the continuum-based model does not predict a pressure dependence of the laminar-turbulent transition, a molecular-based model indicates a pressure effect on the transition to turbulent flow. (c) 2006 American Vacuum Society.