Non-Newtonian liquids change viscosity when exposed to different conditions. For example, some of these liquids show an increase in viscosity with an increase in applied force while other non-Newtonian liquids decrease in viscosity with an increase in applied force. The rotational viscometer can adjust the turning speed of the probe as it moves in the liquid. The viscometer detects the variation in the viscosity of the sample as the speed, sometimes call shear rate, is changed.
The unit of measure for dynamic viscosity is Centipoise (cP). There are several ways to find the kinematic viscosity of a fluid, but the most common method is determining the time it takes a fluid to flow through a capillary tube. The time is converted directly to kinematic viscosity using a calibration constant provided for the specific tube. The unit of measure of kinematic viscosity is Centistokes (cSt).Ī basic difference between the dynamic and kinematic viscosity measurements is density. Kinematic (cSt) x Density = Dynamic (cP).Density actually provides a way to convert between a kinematic and a dynamic viscosity measurement. When Should You Use Dynamic Viscosity Measurements? Dynamic (cP) / Density = Kinematic (cSt)įor a given sample, with a density greater than one, dynamic viscosity will always be the higher number. You test dynamic viscosity when you want to know the internal resistance of a fluid, or the force required to move one plane of the liquid over another. The measurement of dynamic viscosity is most useful for liquids that change their apparent characteristics as force or pressure is applied. These liquids are known as non-Newtonian fluids. Non-Newtonian fluids are sensitive to changes in the amount of force exerted on them, and can sometimes even permanently change their viscosity if a constant force has been has been exerted on them over a period of time.Īn example of the importance of dynamic viscosity measurement is to indicate the proper flow characteristics of ketchup. This product needs to have lower viscosity as it flows, to get it out of the bottle, but needs to be thick (or not as inclined to flow) when sitting on the burger. Testing the viscosity of the ketchup at different speeds (equating to different levels of force) will help ensure that the ketchup is behaving as it should.Īnother application is in the design of pumping systems. Because the viscosity of non-Newtonian liquids changes with the speed of movement, pressure and pump velocity have serious impact on the specification of proper pumps, pressure and piping size.
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