On-line viscometers are known for determining the yield stress and plastic viscosity of non-Newtonian or Bingham type fluids, calculated from formulae for laminar flow. However, in general, such known on-line viscometers are not capable of handling the extremes or demands which exist in the mining and mineral processing environments. For example, rotational types often suffer from reliability problems especially when applied to slurries. They also have difficulty in handling coarse solid particles and high process pressures. The static force measurement type viscometers are not suited to coarse particle slurries and will give erroneous readings when entrained air is present. Similar comments apply to vibrating element type viscometers.
The general technique of measuring the pressure differential between two points in a laminar fluid flowing through a tube of known diameter at a known rate is well known. This is shown, for example, in U.S. Pat. Nos. 3468158, 4680957 and 4384792. A technique using a vertical tube to measure slurry viscosity is shown in Australian Patent Application No. 31949/77. U.S. Pat. No. 3520179 discloses a rheometer or viscometer for non-Newtonian fluids using a capillary U-tube type apparatus, however this apparatus does not use differential pressure measurements, and further does not appear to be suitable for on-line measurements. U.S. Pat. No. 3465573 also discloses a U-tube type arrangement, but uses a comparison between two different paths, rather than the mere U-tube, to produce a measurement.
U.S. Pat. No. 4680957 (Dodd) would appear to be the most relevant reference to the present invention in that it discloses a viscosity level conduit to measure viscosity. However one of the main problems with this type of viscometer is the tendency for solids to settle out of solution along the conduit thus adversely affecting the accuracy of readings from the viscometer. There is no suggestion of the use of a U-tube in this specification.