Relative viscosity and absolute viscosity are two concepts that describe fluid viscosity, and there are some differences between them.
Relative viscosity:
Relative viscosity refers to the viscosity of a fluid at a certain temperature compared to some reference fluid (usually a solvent). It is determined by measuring the flow properties of the fluid with the reference fluid. Relative viscosity is a dimensionless value that indicates how much the viscosity of the fluid increases relative to the reference fluid. Relative viscosity can be calculated by measuring the movement time or flow rate of the fluid.
Absolute viscosity:
Absolute viscosity refers to the actual viscosity of a fluid, also known as dynamic viscosity or dynamic viscosity. It is determined by measuring the volume (flow rate) of a fluid passing through per unit area per unit time. Absolute viscosity is a physical quantity with units, usually in Pascal seconds (Pa · s) or milliPa · seconds (mPa · s).
Absolute viscosity is an inherent property of fluids and can be directly used in fluid mechanics calculations and analysis. Relative viscosity is a comparison value relative to a reference fluid and is often used for the measurement of liquid mixtures and the comparison of liquid properties.
It is important to note that the relationship between relative viscosity and absolute viscosity can be expressed by a constant called the proportional constant or the conversion factor between relative viscosity and absolute viscosity. This conversion factor depends on the reference fluid and temperature conditions used. To sum up, relative viscosity is a comparative value of fluid viscosity relative to a reference fluid, while absolute viscosity is the physical quantity of the actual viscosity of the fluid.