Stokes and poise are both units of measure relating to fluid viscosity. Viscosity is the ability of a fluid (liquid or gas) to resist flow under an applied shear stress. Air and water have low viscosity and flow easily, whereas honey and oil are more viscous and have greater resistance to flow. Viscosity is typically defined in two forms: dynamic viscosity, commonly measured in poise; and kinematic viscosity, commonly measured in stokes. Conversion from dynamic viscosity (poise) to kinematic viscosity (stokes) is straightforward, provided that the fluid's density is taken into account, and is done so using the proper units.
The factor A shown in the formula above is a correction used to account for difference in units. The value of 1000 shown above is valid only for cases where the fluid density is measured in kg per cubic meter. If other units for density (such as pounds per cubic inch) are used, the formula shown will not be valid.
Kinematic and dynamic viscosity are easily confused. When using tables or a reference book to find viscosity values, be certain to note whether the values shown are for kinematic or dynamic viscosity.
Viscosity and density for gases such as air are strongly dependent upon temperature. Be certain to consult a reputable source for these values.
Determine the fluid density by consulting a standard reference table or handbook. Density is a measure of the fluid's mass per unit volume, and can vary with temperature and pressure. For example, at standard ambient pressure and temperature, the density of air is roughly 1.229 kg per cubic meter. The density of water at standard conditions is roughly 1000 kg per cubic meter.
Use the proper units. Stokes and poise are both based on metric units of measure, using the CGS (centimeter, gram, second) system. Make sure that the value being used for fluid density is also in metric units, most commonly in the form of kg per cubic meter.
Calculate the viscosity as follows. Kinematic viscosity (as in stokes) is equal to the dynamic viscosity (poise) divided by the fluid density, then multiplied by a numeric factor, A, which is used to account for the differences in measuring units:
Stokes = A x Poise / (Fluid Density) where A = 1000, if the fluid density is measured in kg per cubic meter.
For example, at normal atmospheric pressure and 15 C temperature, the dynamic viscosity of air is roughly 0.000173 poise. The density of air at these conditions is roughly 1.229 kg per cubic meter. By the formula given above, the kinematic viscosity of air is
Kinematic viscosity = 1000 x (0.000173 poise) / (1.229 kg per cubic meter) = 0.141 stokes
That is, at standard pressure and 15 C temperature, the dynamic viscosity of air is roughly 0.000173 poise, and the kinematic viscosity of air is roughly 0.141 stokes.
Warnings
References
Warnings
- The factor A shown in the formula above is a correction used to account for difference in units. The value of 1000 shown above is valid only for cases where the fluid density is measured in kg per cubic meter. If other units for density (such as pounds per cubic inch) are used, the formula shown will not be valid.
- Kinematic and dynamic viscosity are easily confused. When using tables or a reference book to find viscosity values, be certain to note whether the values shown are for kinematic or dynamic viscosity.
- Viscosity and density for gases such as air are strongly dependent upon temperature. Be certain to consult a reputable source for these values.
About the Author
Adam Purple has been writing about science and technology since 1991. He has worked in the fields of aerospace, research and development and information technology. He most recently wrote for the energy and climate blog SeacoastNRG. Purple holds Bachelor of Science and Master of Science degrees in engineering from Syracuse University.
Photo Credits
honey image by Maria Brzostowska from Fotolia.com