Viscometer, capillary tube instrument

Inside film heat transfer coefficient 496 INSINGER, T. H. 486, 492, 564 Institution of Chemic al Engineers 516 Instruments, capillary tube viscometer 196... 

Orr and DallaValle (05) have designed a capillary-tube viscometer which is especially suited for use with suspensions which tend to settle. This instrument, shown in Fig. 13, consists of the following four components ... 

It may be concluded that well-designed rotational and capillary-tube viscometers are generally more useful than any other type. Accordingly, rheological equations for interpretation of data have been presented for these two types of instruments. 

The capillary tube viscometer is not suitable for settling suspensions. This instrument suffers from the fact that the shear rate is not constant across the tube radius and the fluid cannot be sheared as long as desired. It is... 

Viscoelasticity in mixing 293 Viscometer, capillary tube instrument 196... 

The basic design is that of the Ostwald viscometer a U-tube with two reservoir bulbs separated by a capillary, as shown in Figure 24a. The liquid is added to the viscometer, pulled into the upper reservoir by suction, and then allowed to drain by gravity back into the lower reservoir. The time that it takes for the liquid to pass between two etched marks, one above and one below the upper reservoir, is a measure of the viscosity. In U-tube viscometers, the effective pressure head and therefore the flow time depend on the volume of liquid in the instrument. Hence, the conditions must be the same for each measurement. 

Since fluid shear rates vary enormously across the radius of a capillary tube, this type of instrument is perhaps not well suited to the quantitative study of thixotropy. For this purpose, rotational instruments with a very small clearance between the cup and bob are usually excellent. They enable the determination of hysteresis loops on a shear-stress-shear-rate diagram, the shapes of which may be taken as quantitative measures of the degree of thixotropy (G3). Since the applicability of such loops to equipment design has not yet been shown, and since even their theoretical value is disputed by other rheologists (L4), they are not discussed here. These factors tend to indicate that the experimental study of flow of thixotropic materials in pipes might constitute the most direct approach to this problem, since theoretical work on thixotropy appears to be reasonably far from application. Preliminary estimates of the experimental approach may be taken from the one paper available on flow of thixotropic fluids in pipes (A4). In addition, a recent contribution by Schultz-Grunow (S6) has presented an empirical procedure for correlation of unsteady state flow phenomena in rotational viscometers which can perhaps be extended to this problem in pipe lines. 

In axial-flow viscometers, the sample is made to flow through a duct of regular cross-section. Capillary (circular cross-section) and slit (rectangular cross-section) viscometers are controlled stress instruments a known pressure difference (which causes shear stress in the sample) is applied over the duct length, and the resulting volumetric flow rate measured. In the extrusion viscometer, a controlled shear rate instrument, the sample is extruded through a capillary tube by the action of a constant speed piston, acting on the sample in a cylindrical reservoir to which the capillary is attached. The pressure difference between the ends of the capillary is measured. 

Another parameter that can be related to molecular weight is the relative viscosity, defined as the ratio of the viscosity of a polymer solution rj to the viscosity of the solvent rjo (see Table 3.3). Reliable for molecular weights >10 g/mol, the viscosities can be determined by measuring flow times through capillary tubes (diameters 1 mm), usually with gravity as the driving force for the flow. Automated instrumentation is widely available. Rotational and oscillatory type viscometers are used where a uniform, well-defined, or low shear rate is required. The ratio of the two viscosities, is called the relative... 

Viscometers n. Instruments for measuring viscosity including mechanical probe and torque types as the Brookfield viscometer, capillary tube types as the Cannon-Fenske or Ostwald-Fenske, and flow through orifice types as the Ford cup. 

In routine work, relative instruments are often more convenient to use. An instrument widely used for mobile liquids is the Ostwald viscometer, in which the time taken for a given volume of liquid to pass through a capillary tube under the effect of gravity is measured. This time increases with viscosity and decreases... 

In this experiment, a Tubing Shear History Simulator was coupled with a Reciprocating Capillary Viscometer to simulate the above conditions. Results from the experiment are given in Tables I and II and Figure 3, and were retrieved directly from the project data base. Total Instrument use time for this experiment was 17 hr, of which 16.5 hr were completely unattended operation.. Data analysis, including plotting of figures, required less than five minutes. 

Solution Equation (16) shows that the velocity gradient is not uniform in a capillary viscometer any more than it is in a concentric-cylinder instrument. The rate of shear dvldr is directly proportional to the radial distance from the axis of the cylinder. At the wall it has its maximum value, which is proportional to Rc] at the center of the tube it equals zero. Some intermediate value, say, the average, might be used to characterize the gradient in a given instrument. This quantity will be different for capillaries of different radii. All of this is similar to the situation in concentric-cylinder viscometers. 

The Saybolt viscometer works on a similar principle (ASTM D88)—the fluid is loaded to a tube calibrated to 60 ml. A cork is removed from the bottom of a narrow capillary and a timer is initiated. When 60 ml of the fluid is drained, the timer is halted. The time to drain the 60 ml volume is known as Saybolt seconds. The instrument operates at temperatures from ambient to as much as 99 °C. Because of the high sensitivity of viscosity to temperature, substantial efforts are devoted to maintaining isothermal conditions quoted uncertainties around the temperature are on the order of 0.03 °C. The accuracy of Saybolt viscometers is better than 0.2%. In order to ensure accuracy, they should be calibrated at regular intervals. For fully developed laminar flows, the drain time through the capillary at the base is directly proportional to viscosity. However, since the capillary section of the tube is short, a correction factor to the time is... 

Capillary viscometers are the most extensively used instruments for the measurement of viscosity of liquids because of their advantages of simphcity of construction and operation. Both absolute and relative instruments were constracted. The theory of these viscometers is based on the Hagen-Poiseuille equation that expresses the viscosity of a fluid flowing through a circular tube of radius r and length L in dependence of the pressure drop AP and volumetric flow rate Q, corrected by terms for the so-called kinetic-energy and end corrections ... 

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