Many functional fluids, such as cement slurries or drilling muds, contain high-density solids, typically grains of 10–100 micron in diameter, suspended in a liquid of much lower density. Coaxial cylinder rheometers, conventionally used for oilwell fluids and dense suspensions generally, are shown to give large errors owing to gravitational or centrifugal migration of the high-density solids. Further errors can arise from slippage between the fluid and the smooth walls of the rheometer.

A modification is described to a coaxial cylinder rheometer, in which the fluid is circulated, using an external pump, through the sample cup and around the rotor/stator assembly. This circulation mixes and homogenises the fluid, prevents solids build-up in both the annular gap and the cup bottom, and provides an independently-controllable shear history. Thus stable and accurate data can be obtained for fluids which give measurement problems using standard rheometers.

Results are given for various power-law/yield stress granular fluids.