To remain competitive, today’s refiners must employ all optimization and product control techniques available. The use of online physical property analyzers is one of the key features to reach those objectives because they measure important quality properties in the process directly. All fluids that fulfil the conditions of Newton’s friction law are referred to as Newtonian fluids. Their viscosity is a material constant, which is only dependent on pressure and temperature. The viscosity for incompressible and Newtonian fluids can be derived from the so called Hagen-Poiseuille law. The fluid is assumed to flow under laminar conditions.