Poise (unit)

The poise (symbol P; /pɔɪz, pwɑːz/) is the unit of dynamic viscosity (absolute viscosity) in the centimetre–gram–second system of units (CGS).^[1] It is named after Jean Léonard Marie Poiseuille (see Hagen–Poiseuille equation). The centipoise (1 cP = 0.01 P) is more commonly used than the poise itself.

Dynamic viscosity has dimensions of [math]\displaystyle{ \mathrm{force \times time/area} }[/math], that is, [math]\displaystyle{ [\mathsf{M}^1 \mathsf{L}^{-1} \mathsf{T}^{-1}] }[/math].

[math]\displaystyle{ 1~\text{P} = 0.1~\text{m}^{-1} {\cdot} \text{kg} {\cdot} \text{s}^{-1} = 1~\text{cm}^{-1} {\cdot} \text{g} {\cdot} \text{s}^{-1} = 1~\text{dyn} {\cdot} \text{s} {\cdot} \text{cm}^{-2}. }[/math]

The analogous unit in the International System of Units is the pascal-second (Pa⋅s):^[2]

[math]\displaystyle{ 1~\text{Pa} {\cdot} \text{s} = 1~\text{N} {\cdot} \text{s} {\cdot} \text{m}^{-2} = 1~\text{m}^{-1} {\cdot} \text{kg} {\cdot} \text{s}^{-1} = 10~\text{P}. }[/math]

The poise is often used with the metric prefix centi- because the viscosity of water at 20 °C (standard conditions for temperature and pressure) is almost exactly 1 centipoise.^[3] A centipoise is one hundredth of a poise, or one millipascal-second (mPa⋅s) in SI units (1 cP = 10⁻³ Pa⋅s = 1 mPa⋅s).^[4]

The CGS symbol for the centipoise is cP. The abbreviations cps, cp, and cPs are sometimes seen.

Liquid water has a viscosity of 0.00890 P at 25 °C at a pressure of 1 atmosphere (0.00890 P = 0.890 cP = 0.890 mPa⋅s).^[5]

See also

• Poiseuille
• Viscosity

References

et:Poise

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