The Accuracy of Measurements of Viscosity of Liquids

Most abso lut e meas urements of viscos ity have utili zed ca pi ll a ry fl ow, and required ~" 'TIl e mpir i ca l corrections amou nting to several times th eir precis ion and esti ma ted acc uracy . The range of va lu es found from these measure ments and the poss ibility of unrecognized syste ma tic e rrors make it impos' s ib le to base a realist ic estimate of accuracy on the results of onl y one type of measu rement. The reo s uits of two ind epend ent a bsolute measu re ments involving d iffe rent types of fl ow, reported in the two accompanying pape rs, a re summa rized he re. The esti ma ted acc uracy in each case is about 0.1 percent. Th e two results diffe r by 0.5 percent. It is s ugges ted that we cont inue to base the ca li bration of re la tive vi sco mete rs on the value of 1.002 centipoise (c P) for the viscosi ty of wate r at 20 °C and one atmos· phe re. This provides a ge ne ra ll y acce pted base which limi ts compa rability on ly by the prec is ion of the meas urements. However , whenever the true va lues of vi scos ity a re required the lim its of uncerta int y inc lud ing an es timate of syste mati c e rro r should be taken as no be tt e r th an ±O.25 pe rcent.