Poise

The poise (symbol P, /ˈpɔɪz/) is the unit of dynamic viscosity in the centimetre gram second system of units. It is named after Jean Léonard Marie Poiseuille.

$1\ \mbox{P} = 0.100\ \mbox{kg}\cdot\mbox{m}^{-1}\cdot\mbox{s}^{-1} = 1\ \mbox{g}\cdot\mbox{cm}^{-1}\cdot\mbox{s}^{-1}$

The analogous unit in the International System of Units is the pascal second (Pa·s):

$1\ \mbox{Pa}\cdot\mbox{s} = 1\ \mbox{kg}\cdot \mbox{m}^{-1}\cdot\mbox{s}^{-1} = 10\ \mbox{P}$

The poise is often used with the metric prefix centi- because the viscosity of water at 20C is almost exactly 1 centipoise. A centipoise is one one-hundredth of a poise, and one millipascal-second (mPa·s) in SI units. (1 cP = 10⁻² P = 10⁻³ Pa·s = 1 mPa·s)

The centipoise is properly abbreviated cP, but the alternative abbreviations cps, cp, and cPs are also commonly seen.

Water has a viscosity of 0.00899 poise at 25 °C and a pressure of 1 atmosphere. (0.00899 P = 0.899cP = 0.899 mPa·s)

Use in laboratory

A viscometer can be used to measure viscosity. Viscometers can be calibrated in centipoise.

When determining centipoise, all other fluids are calibrated to the viscosity of distilled water at STP. (In some practices, one can use the following approximation: water at 70 °F (21 °C) is approximately one centipoise).

Thicker liquids, like honey, have higher viscosities. For example, while ethylene glycol has a viscosity of just 24 centipoise at 23 C, honey has a viscosity of 2 000 centipoise and molasses has a viscosity of 5 000 centipoise. Lard has a viscosity of 100 000 centipoise.

See also

• Viscosity