Viscosity Measurements on Molten Salts with an Oscillating Cup Viscometer: Viscosity of Molten KNO3 and NaCl

An oscillating cup viscometer interfaced to an automatic data acquisition and processing system (computer, interface, software) was built for viscosity measurements in molten salts below 1400 K.

In order to avoid the “meniscus effect”, the main source of errors of the method, the measurements were carried out in the so-called “full cup” condition, i.e. with the height of the liquid column (h) of 11.2 cm, the overall height of the cup being 11.9 cm. An adequate software supplied η values at each temperature, as function of R, I, δ, h, ρ and T values in air and with liquid containing cup. The viscosity of molten KNO₃ and NaCl, both of “standard salt purity”, was reinvestigated over a temperature range of 100 K above their melting points. The standard deviation of the data obtained was ±0.38%. These data were systematically lower (3% for η_NaCl and 2% for η) than those obtained earlier by the same method, but agree within ±0.9% with the recommended Janz's “calibration quality viscosity data”.

One can conclude that the method gives accurate results only if operated in “full cup” condition.