A two-parameterrheologicalmodel widely used in the drilling fluids industry to describe flow characteristics of many types of muds. It can be described mathematically as follows:

τ = YP + PV(γ),

where
τ = shear stress
γ = shear rate
YP = yield point
PV = plastic viscosity.

Fluids obeying this model are called Bingham plastic fluids and exhibit a linear shear-stress, shear-rate behavior after an initial shear stress threshold has been reached. Plastic viscosity (PV) is the slope of the line and yield point (YP) is the threshold stress. PV should be as low as possible for fast drilling and is best achieved by minimizing colloidal solids. YP must be high enough to carry cuttings out of the hole, but not so large as to create excessive pump pressure when starting mud flow. YP is adjusted by judicious choices of mud treatments. The direct-indicating rotational rheometer was specifically designed to apply the Bingham plastic fluid model.