Viscometry Lab.pdf

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FDST 8080 <strong>Lab</strong> 2011 Research instruments such as the Hoeppler viscometer are also available. These have precision made tubes surrounded by an outer jacket, through which constant temperature water can be circulated. The sample tube can be evacuated to remove air bubbles, then sealed. Here, the tube is tilted at a 10° angle, and the ball is only slightly smaller than the inner diameter of the tube; thus, “wall” effects are important and incorporated into the analysis. For the Hoeppler viscometer, the absolute viscosity is given by: " = T #(SG s $ SG l ) # B where T is the time interval of the falling ball and B is a ball constant. Here SG is the specific gravity (ρ/ρ water) Type Serial Number Ball Diam @ 20°C (mm) Wt of Ball (g) SGB @ 20°C Ball Constant A2 8440 15.9051 4.6958 2.2290 ---------- C33 8665 15.8059 4.6088 2.2291 0.009529 F6 8269 15.6300 4.4516 2.2266 0.077944 H8 8058 15.5512 15.5447 7.8939 0.13630 K10 8114 14.9846 13.9058 7.8933 1.2691 M12 6869 13.4933 10.1712 7.9071 10.804 Where possible, use the falling ball viscometer to measure the viscosity of the same liquids you measured in the rotational viscometer. If the liquid is too opaque, this may not be possible. Record the time it takes for the ball to pass from the first marker to the final one. We will also need to record the specific gravity of the liquid. III. Zahn Cup-Type Viscometer The Zahn cup is an easy-to-use device for assessing the viscosity of oils, paints, syrups, batters and other liquids. The cup is filled to the top and the liquid allowed to flow through an opening in the bottom. The viscosity of the liquid is measured in Zahn number, that is, the time in seconds for a known volume of liquid to flow out of the cup. For thin fluids, a cup with a small orifice is used; for more viscous mixtures, a cup with a larger whole is used. Although easy to use, the geometry and driving force for flow are difficult to describe. Thus, no exact formulas exist to convert viscosity measurements in “Zahn numbers” into absolute viscosity. However, it is common practice to report the “viscosity” in Zahn numbers. Some empirical formulas have been developed, however, to relate Zahn number to kinematic viscosity. For the Boekel brand cups:
FDST 8080 <strong>Lab</strong> 2011 Zahn Cup# Formula T, Zahn Seconds Range Specifications 1 η/ρ = 1.1(T-29) 45 - 80 2 η/ρ = 3.5(T-14) 25 - 80 3 η/ρ = 14.8(T-5) 20 - 75 4 η/ρ = 11.7(T-7.5) 20 - 80 5 η/ρ = 23(T-0) 20 –75 Zahn Cup # 1 2 3 4 5 Orifice Diameter (in) .078 0.108 0.148 0.168 0.208 Zahn Range (s) 45-80 25-80 20-75 20-80 20-75 Centistoke Range* 18-56 40-230 150-790 220-1100 460-1725+ Application Very thin oil Thin oil Medium oil Heavy mixture, Batter, syrup Very heavy mixture, Heavy syrup *Centistoke is a measure of kinematic viscosity = absolute viscosity (cP)/density (gcm -3 )
Page 1 and 2: VISCOMETERS We have defined the vis
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