Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Viscosity-ratio measurements with capillary viscometers



Robert F. Berg, Eric F. May, Michael R. Moldover


Viscosity-ratio measurements made with capillary viscometers exploit the accurate values of the viscosity of helium resulting from ab initio calculations. Accurate values of the argon-to-helium viscosity ratio are now used for primary acoustic thermometry and the most accurate redeterminations of the Boltzmann constant. Accurate viscosity ratio measurements enable the calibration of laminar flow meters with surrogate gases and their use with process gases. We review ratio viscometers that comprise one, two, and four capillaries. A single capillary is a ratio viscometer when it is used to measure multiple gases, while two- and four-capillary viscometers are inherently instruments intended for ratio measurements. We consider only gases and Newtonian liquids, with a focus on the two-capillary gas viscometer and its extension to pressures up to 100 MPa. A single-capillary viscometer could measure the ratio (viscosity of liquid water)/(viscosity of gaseous helium) thereby reducing the uncertainty of the liquid-viscosity scale relative to the international system of units.
Experimental Thermodynamics Volume IX: A, Advances in Transport Properties
Publisher Info
Royal Society of Chemistry, Cambridge, -1


capillary viscometer, helium gas, two-capillary capillary viscometer, gas viscosity, viscosity ratio, liquid water


Berg, R. , May, E. and Moldover, M. (2014), Viscosity-ratio measurements with capillary viscometers, Experimental Thermodynamics Volume IX: A, Advances in Transport Properties, Royal Society of Chemistry, Cambridge, -1, [online], (Accessed May 24, 2024)


If you have any questions about this publication or are having problems accessing it, please contact

Created May 20, 2014, Updated February 19, 2017