Shear rate is the speed of deformation in the shear mode (which is typical of fluids and can be represented as layers sliding one onto another). It is expressed (SI units) in reciprocal seconds [1/s], since it derives from radiant per second, with radiant being dimensionless (just a number). For Newtonian fluids, shear rate is directly proportional to flow rate (for a given channel geometry), and a simple relationship is used to calculate its maximum value at the wall of the channel. When the fluid is non-Newtonian (notably the case of all plastic melts), this relationship gives only a first approximation, which is called the apparent value of wall shear rate. The true wall shear rate can be estimated by applying the Rabinowitsch correction, and is higher than the apparent value.