The Units of Dynamic Viscosity

The units of dynamic viscosity µ can be a little confusing as several different sets of units are commonly used. The most formal unit is kg/ms. However, you might come across the following units, which are equivalent to one another:

1 space k g divided by m s space equals space 1 space N s divided by m ² space equals space 1 space P a space s space equals space 10 space P o i s e space left parenthesis P right parenthesis

For most gases and water-like liquids, these units are quite large, so you’ll probably see fractions of these units used. The table below gives some typical values for some common brewing fluids.

Table: Common brewing fluid viscosities

Fluid

Dynamic Viscosity (kg/ms)

Water at 20 °C

1.0 x 10-³

Air (0 °C and sea level)

1.7 x 10-⁵

Wort (11 °P and 20 °C)

2.1 x 10-³

Aqueous sucrose solution (60 °Brix and 20 °C)

60 x 10-³

To finish off this section, let us consider an alternative viscosity parameter, called kinematic viscosity. This is defined as the ratio of the dynamic viscosity µ to the fluid density ρ and is given the Greek symbol nu (pronounced new):

nu space equals space fraction numerator space mu over denominator rho end fraction

We have dealt with the concept of viscosity and its origins in Newton’s law of viscosity and Newtonian fluids. We will now consider fluids which do not conform to Newton’s law.

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