Viscosity is a measure of a fluid's resistance to flow, or its internal friction. Therefore, the above values are just reference values and actual viscosity values may differ depending on the specific conditions. It is important to note that the viscosity of a fluid can vary with temperature, pressure, and shear rate, and may not remain constant even within a specified temperature range. The viscosity of common liquids in centistokes at room temperature (25☌ or 77☏) are as follows: It is defined as the ratio of the dynamic viscosity of a fluid to its density. Universal, Redwood Standard, Ubbelohde, Gardner Holts, Zahn 5, Zahn 3 )Ĭentistoke (cSt) is a unit of kinematic viscosity commonly used to express the viscosity of fluids. (Centistrokes reference, Mobilmeter, Engler, Ford 4, Ford 3, Saybolt Measurements on Newtonian liquids by referencing to absolute and kinematic viscosity Note : This chart is intended to be an aid in comparing viscometer The measuring ranges and accuracy listed may vary depending on the specific model and manufacturer of the viscometer. Note: This chart is provided for informational purposes only and may not be applicable to all situations. Wide range of fluids (food, cosmetics, polymers)Ībsolute viscosity, viscoelastic propertiesĬomplex fluids (suspensions, emulsions, gels, pastes) Low viscosity fluids (petroleum products, chemicals) Low viscosity fluids (petroleum products, paints, resins) Low viscosity fluids (water, oils, solvents) Industry instrumentation : Units and conversion tables for process control - Instrumentación industrial : Unidades y tablas de conversión para el control de procesosĬomparison chart for commonly used viscometers for Newtonian liquids: Viscometer Type Consequently, internal friction and viscosity increase.Viscometer Comparison Chart for Newtonian liquids. In most liquids, pressure reduces the free volume in the internal structure, and thus limits the movability of molecules. For example, lubricants in cogwheels or gears can be submitted to pressures of 1 GPa and higher.Įquation 4. Highly viscous mineral oils react with a viscosity increase of times 20000 under identical circumstances.įor synthetic oil this pressure change can even result in a viscosity increase by a factor of up to 8 million. This applies to most low-molecular liquids. In case the pressure goes up from 0.1 MPa to 200 MPa, the viscosity can rise to 3 to 7 times the original value. For comparison: This same change in viscosity is usually provoked by a minor temperature change of 1 K (1 ☌). Most liquids react to a significantly altered pressure (from 0.1 MPa to 30 MPa) with a viscosity change of about 10 %. However, fluids are not dramatically affected if the applied pressure is low or medium: liquids are almost non-compressible in this pressure range. Normally, an increase in pressure causes a fluid’s viscosity to increase, too. Three factors determine a substance’s flow behavior. Still, shear rate is not the only influencer. To learn more consult “ The Rheology Handbook”. The flow behavior of non-Newtonian liquids can be far more complex than these basic examples. yogurt), while for others the viscosity increases with increasing shear rate (curve 3 | e.g. If its viscosity changes with the shear rate, a liquid is non-Newtonian and – for exact definition – one has to specify the apparent viscosity.ĭifferent shear-dependent fluids behave differently: For some, their viscosity decreases when the shear rate increases (curve 2 | e.g. Typical Newtonian liquids are water or salad oil. For a Newtonian liquid, this function is a straight line (curve 1) see figure 6. Such fluids are named ‘Newtonian liquids’ after Sir Isaac Newton.Ī viscosity function shows the viscosity over the shear rate. the shear rate – acting upon the fluid, it is ideally viscous. If a fluid’s internal flow resistance is independent of the external force – i.e. High-viscosity fluids resist deformation. Fluids which flow easily show a low resistance to deformation. viscous fluids that contain an elastic portion. The specific field of viscometry covers ideally viscous fluids, and – considering certain restrictions – also viscoelastic liquids, i.e. sweet jelly) or as a viscoelastic liquid (like e.g. According to a material’s properties, we either classify it as a viscoelastic solid (like e.g. That is, substances which are neither completely elastic, nor entirely viscous. In everyday life, we mostly come across viscoelastic materials. Scientists specify solid materials as being elastic and liquids as being viscous. Imagine all materials as classified on a virtual scale from solid to liquid. Rheology deals with the flow behavior and deformation of materials. In order to determine a fluid’s viscosity, you have to enter the field of viscometry, a subject area of a wider science called rheology.
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