Previous studies on high temperature viscosity measurements have demonstrated that accurate viscosity values can only be obtained if great care and time is taken with the measurements. Consequently, several laboratories have attempted to develop a simple test method, suitable for the measurement of viscosities at high temperatures for routine quality control situations, that can be carried out on-plant by unskilled staff in the industry. The test, known as the inclined plane test, consists of heating a known mass of material to the required temperature in a furnace, then pouring the liquid down an inclined plane and measuring the length (L) of the ribbon formed.
The objective of the project was to gain a better understanding of the theory underlying the inclined plane test for measuring viscosities at high temperatures, to determine the limitations of the test and to determine the reproducibility and the accuracy of viscosity values derived from the test.
It was found that some practice was necessary to obtain consistent ribbon lengths with the inclined plane test. However, for an experienced operator the test was capable of giving very consistent ribbon lengths for a specific material. The measured ribbon lengths were found to be functions of (i) the sample viscosity, (ii) the sine of the angle of inclination to the horizontal and (iii) the temperature. They were unaffected by the solidification temperature of the material and by the nature of the metal used for the inclined plane.
Ribbon lengths (L) reported by the six laboratories for the reference materials were in good agreement with one another and with the L-viscosity relation obtained in preliminary measurements. Viscosity values could be obtained with an uncertainty of +/- 15 % for materials with viscosities in the range 0.15 -1.5 Pas. The reference materials are not recommended for use above 1400 °C.
Four slag reference samples, covering a wide range of viscosities, solidification temperature ranges and emissivities, were selected with which to investigate the test method. A mathematical model was developed in order to identify the key factors affecting the process of slag flowing down an inclined plane. On the basis of the results, a protocol for measurements using the inclined plane test was drafted and an intercomparison carried out involving six laboratories.