The determination is carried out by using a rotary viscometer. It is applicable to liquid products, the dynamic viscosity of which lies between 10 and 80000 mN.s/m2. The principle consists in the determination at a temperature of (20 +- 0,2) °C by measuring the couple required to overcome the resistance exercised by the viscous liquid on an immersed element rotating within it at a constant selcted speed without causing a vortex. If the results obtained at different speeds of the immersed element are in agreement, the fluid is Newtonian. If the opposite is noted, the fluid is non-Newtonian.