1. n. [Drilling Fluids]

The viscosity of a fluid measured at a given shear rate at a fixed temperature. In order for a viscosity measurement to be meaningful, the shear rate must be stated or defined.

Alternate Form: AV

2. n. [Drilling Fluids]

The viscosity of a fluid measured at the shear rate specified by API. In the Bingham plastic rheology model, apparent viscosity (AV) is one-half of the dial reading at 600 rpm (1,022 sec^–1 shear rate) using a direct-indicating, rotational viscometer. For example, a 600-rpm dial reading is 50 and the AV is 50/2, or 25 cP.

See: API,  Bingham plastic model,  centipoise,  plastic viscosity,  rheology,  shear rate,  yield point