Experimental study on factors affecting the end effect in gas viscosity measurement using capillary-tube viscometer

Abstract

Because of its simple principle and high adaptability to severe operational conditions, the capillary-tube viscometer has been widely used for viscosity measurement. However, difficulties in accurately correcting the end effect induced measurement deviation will result in great uncertainty for measurement results. In order to solve this problem, in this work, we studied factors affecting the end effect by conducting the high pressure nitrogen viscosity measurement at low flow velocity with an improved capillary-tube viscometer. The experimental results indicated that the influence of the end effect became less significant with the decrease in flow velocity (v) and tube inner diameter (d) and varied inversely with the length of tube (L). We defined the ratio of measured viscosity to standard viscosity obtained from the NIST database as the viscosity deviation coefficient (Ce). From the Ce vs v, Ce vs d, and Ce vs L curves, we have observed that there existed a threshold velocity (vthreshold), a threshold diameter (dthreshold), and a threshold length (Lthreshold) at which Ce got closer to 1.0. It suggested that under certain experimental conditions, the influence of the end effect on gas viscosity measurement became negligible. Based on that, we established end effect free capillary-tube viscometry and compared the nitrogen viscosity results measured by this method with the data provided by the NIST database. The results presented a good match with error within 1.2%. These insights will contribute to improving the accuracy of a capillary-tube viscometer especially under high pressure.

Evaluating the gas-laden ability of polymer melt under atmospheric conditions using a modified torque rheometer

“…Typical types of absolute rheological measurement equipment include capillary viscometers, rotation rheometer, etc. 22–28 However, absolute rheological measurements can only generally deal with more or less homogeneous solids and liquids within the limitations of laminar flow and linear viscoelastic responses. It is not generally suitable for characterization of gas-laden melts since it requires a certain pressure to keep the gas in the melt.…”

“…Although a series of modifications were developed to allow for pressurization of the melt with the gas, either in the supercritical or subcritical state, the test results were more or less limited using the absolute rheological testing devices. 22–25 Generally, an absolute rheological measurement is mainly concerned with the single-phase gas-melt and its viscosity. Gas diffusion and gas separation/escaping are rarely studied from the perspective of rheological measurements due to the dynamic and changing rheological properties of the two-phase (melt and gas) flow.…”