Viscometry Lab.pdf

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FDST 8080 Lab 2011 Measure the temperature of the liquid prior to measurement. The Zahn cup is provided with a bracket to hold a thermometer. Prior to measurement, raise the bracket so the thermometer stem is out of the cup. Place a finger in the ring, lift the viscometer completely out of the liquid and start the stop watch when the top edge of the cup breaks the surface. Stop the watch when the steady flow of the liquid from the orifice breaks. Repeat until consistent results are obtained. Express viscosity in Zahn seconds. IV. Capillary Viscometry Capillary viscometers are relatively simple and inexpensive instruments for measuring fluid viscosity, and when used properly, give very accurate measurements of viscosity. In this approach, gravity causes a fluid to drop between two marks in a capillary tube. The time required for the fluid level to fall a given distance measures the viscosity. The Hagen-Poiseuille equation shows that the flow rate Q is related to the pressure drop (∆P=ρgh), the tube radius R, the tube length L, and the viscosity η: Q = !("P)R4 8#L It can be shown that the “kinematic” viscosity is just ! " = #ghR4 t = kt 8LV
FDST 8080 Lab 2011 That is, the ratio of viscosity to density is proportional to the time it takes for the liquid to drop between the marks. The absolute viscosity η can be determined by separately measuring the fluid density. In some cases it is interesting to determine the “intrinsic” viscosity of a sample [η], as this can be related to the molecular weight of a dissolved solute. Capillary viscometers are of limited use in food systems, as particulate systems can clog the capillary tube. Also, dependence of viscosity on shear rate or shear history is more difficult to study. It can be very useful, however for studying clear juices, beverages, or solutions of food macromolecules such as proteins or carbohydrates. The time in seconds for the fluid to fall between the two markers is recorded. When multiplied by the capillary constant k, this gives the kinematic viscosity η/ρ. V. Juice Viscometer A variant of the capillary viscometer is the AOAC capillary viscometer for juices (AOAC 37.1.108). It Is more appropriate for fruit nectars and juice products, which may have bits of pulp and particulates that would clog a precision capillary viscometer.
Page 1 and 2: VISCOMETERS We have defined the vis
Page 3 and 4: II. FALLING BALL VISCOMETERS FDST 8
Page 5: FDST 8080 Lab 2011 Zahn Cup# Formul

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