11/19/2023 0 Comments Viscosity cupSince the shearing flow is opposed by friction between adjacent layers of fluid (which are in relative motion), a force is required to sustain the motion of the upper plate. Definitions Dynamic viscosity Illustration of a planar Couette flow. Viscum also referred to a viscous glue derived from mistletoe berries. The word "viscosity" is derived from the Latin viscum (" mistletoe"). A fluid that has zero viscosity (non-viscous) is called ideal or inviscid. Zero viscosity (no resistance to shear stress) is observed only at very low temperatures in superfluids otherwise, the second law of thermodynamics requires all fluids to have positive viscosity. For example, the viscosity of a Newtonian fluid does not vary significantly with the rate of deformation. However, the dependence on some of these properties is negligible in certain cases. In general, viscosity depends on a fluid's state, such as its temperature, pressure, and rate of deformation. For a tube with a constant rate of flow, the strength of the compensating force is proportional to the fluid's viscosity. This is because a force is required to overcome the friction between the layers of the fluid which are in relative motion. Experiments show that some stress (such as a pressure difference between the two ends of the tube) is needed to sustain the flow. For instance, when a viscous fluid is forced through a tube, it flows more quickly near the tube's axis than near its walls. Viscosity quantifies the internal frictional force between adjacent layers of fluid that are in relative motion. Thus its SI units are newton-seconds per square metre, or pascal-seconds. ![]() Viscosity is defined scientifically as a force multiplied by a time divided by an area. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Gardner Co in their helpful Excel spreadsheet downloadable from Viscosity Cup Equivalent Wall Chart, where K and C are provided for the most popular cups.Īccording to the excellent book The Printing Ink Manual by Robert Leach, the single-operator accuracy of these cups is in the 5-10% range and the multi-operator accuracy in the 10-30% range, with ISO cups being best and Zahn cups being worse.The viscosity of a fluid is a measure of its resistance to deformation at a given rate. One supplier said words to the effect of "We're not changing our cups to meet ASTM standards because our users would be confused between old and new cups."įor this app I used the simple formula η = K.t-C/t used by the Paul N. In preparing this app I looked at many such tables and a 20s Zahn #2 cup viscosity varies from ~20-45cP depending on the table. If you have ever measured a real viscosity and compared it to a value from one of the "standard conversion tables" you will probably be surprised at the difference. ![]() Because these cups actually measure the kinematic viscosity in cSt (a denser liquid flows out faster), enter your estimated density to allow the conversion (cP=cSt*ρ). ![]() Here you can choose from one of a few types of cup, specify your time in the 15-60s range and get an estimate of the viscosity in cP. ![]() I hope that is not the case! All I am doing is helping those, like me, who want to have some idea of what is going on when the only data available are measurements from these cups. Second, they assume Newtonian viscosity, so they tell you nothing about shear thinning.Īn expert in the field has warned me that this app might add legitimacy to the use of these cups. First they are not accurate even when done properly (see below) and even less when they aren't cleaned properly between use, etc. mPa.s or cP as a convenient unit) is being used. I find it very annoying that in the 21st century such unscientific devices are used and get frustrated that no one (including me) has any idea what approximate viscosity in real units (i.e. The size of the hole is in mm and a Zahn #4 means a Zahn cup with a 4mm hole. Different cups have different shapes and hole sizes and because times too short or too long are unsuitable, for any given viscosity there is an optimum cup with times in the 15-60s range. Many printers/coaters measure their viscosity using cups with holes in the bottom by finding how many seconds it takes to empty the full cup (or, rather, for the flow to "break").
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