This document applies to glass capillary viscometers without external overpressure or external pressure up to 1 bar, where the viscosity is determined from the flow time of a liquid volume limited by measuring marks and whose instrumental constants are determined with Newtonian standard viscosity samples. This document deals with causes and corrections of measurement deviations that occur when measuring Newtonian fluids. This document applies to Ubbelohde viscometers, but is also applicable to other types of glass capillary viscometers.