1. n. [Drilling Fluids]

An instrument for measuring the viscosity of a fluid by passing the fluid at a known pressure gradient or velocity through a length of tubing of known diameter. The viscosity of base oils for oil muds, which are Newtonian fluids, is measured using a glass capillary tube in a thermostatic bath, when performed according to API procedures.

See: Newtonian fluid,  oil mud,  rheology