The Ubbelohde viscometer is the most common type of viscometer used for the determination of the intrinsic viscosity. It was originally introduced in 1937 [33] and is shown in Figure 17.6. For the operation of the viscometer, a polymer solution of known concentration is put in the reservoir and aspirated to the upper bulb, usually by creating some vacuum in that chamber; then air is admitted so the solution flows down the capillary by gravity. The time for the liquid to flow between the two marks is recorded. This operation is repeated for increasingly dilute solutions of the same polymer/solvent. A duct parallel to the capillary allows pressure equilibration, so the flow of the fluid is only due to the hydrostatic head. Notice that the Cannon–Fenske viscometer [34] does not have the pressure equilibration duct, so it is not appropriate for accurate measurements of the intrinsic viscosity.