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The internal resistance to flow of a fluid. The unit of viscosity is the Pascal second.

Note 1 : The viscosity of a solution of a polymer is commonly expressed in one of the following ways:

(a) viscosity ratio: the ratio of the viscosity of a solution to the viscosity of the pure solvent (formerly known as relative viscosity).

(b) specific viscosity: the viscosity ratio less unity.

(c) limiting viscosity number: the value obtained by extrapolating, to zero concentration, the ratio of the specific viscosity of a solution to the concentration of the solute (formerly known as intrinsic viscosity).

Note 2 : Some fluids such as size mixings exhibit anomalous viscosity effects and cannot therefore be characterized by a single measurement. The flow behaviour of a mixing is best described flow curve relating apparent viscosity to shearing stress. If the shearing stresses operative in sizing were known, then the apparent viscosities of the mixings at these stresses could be related to their sizing behaviour. Without this knowledge, measurements at some arbitrary stress have to be used: these are of value in characterizing a particular type of size and can often be relate the take-up of size by the warp.

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