Notes on the Fluidity of Water in the Vicinity of 20°C: Report by the Committee on Viscosity Standards

Measurements of viscosity are practically all relative, since viscometers are calibrated with a liquid whose viscosity is assumed to be known, but many different values of the viscosity of water are in use dependent upon the authority consulted, or the conditions of temperature and upon the corrections which are made in the reduction of the data, and quite as often as not the final results are expressed as specific viscosities of one kind or another. Bulletin No. 278 of the U. S. Bureau of Standards contained certain proposals. (1) It was proposed that all viscosities be expressed in absolute units even though the measurements be relative. (2) Since the viscosity of water at 20°C is close to 0.0100 c. g. s. units, it was proposed to adopt the viscosity (or fluidity) of water at this particular temperature for the fundamental rheological standard. (3) By making the one‐one hundredth part of the c. g. s. unit the practical unit of viscosity, and calling it the centipoise (cp) an advantage would be gained in that viscosities would all be specific. This would be perhaps most appreciated in studying the viscosity of electrolytic solutions, which are compared with the viscosity of pure water. (4) Different authorities were compared, the later corrections added, and an attempt made to secure the best value to be used as standard. Since the viscosity may be calculated by formula from values in the vicinity of 20°, these values were also not neglected in arriving at the most probable value, which was given as 1.005 cp.