DIN 1342, Part 1, October 1983 (Viscosity, Rheological Terms) gives this definition: "Viscosity… The property of a substance to flow and to be deformed irreversibly under the action of a stress. Flow energy is converted thereby into heat…" A distinction is made between dynamic and kinematic viscosity: Dynamic viscosity is a material constant dependent only on temperature over a wide range of pressure. The unit of dynamic viscosity h is 1 Pa · s (= 1 Ns/m²). Because the numerical value is equivalent to the now obsolete unit cP, dynamic viscosity is quoted preferably in the similarly approved unit 1 m Pa · s: 1 cP  1 mPa · s. Kinematic viscosity u is the quotient of the dynamic viscosity and density:

u = h/s; Unit: m²/s. Here again, preference is given to the likewise approved unit mm²/s because its numerical value is equivalent with the now obsolete unit cSt: 1 cSt  1 mm²/s.

The implications of viscosity for the choice of mechanical seals: The higher the viscosity of the medium to be sealed, the greater the difference between the temperature of the medium in the sealing gap and in the stuffing box. The start-up torque and the friction rate increase, as does the load on the torque transmission elements. Up to a dynamic viscosity of around 2500 mPa · s it is possible to use single mechanical seals, but starting at approximately 800 mPA · s account must also be taken of the other operating conditions (e.g. sliding velocity and the medium's boiling characteristics) as there is a risk of insufficient build-up of lubrication film. In cases of excessive viscosity, double mechanical seals are used.