Viscometer tube flow

You are asked to measure the viscosity of an emulsion, so you use a tube flow viscometer similar to that illustrated in Fig. 3-4, with the container open to the atmosphere. The length of the tube is 10 cm, its diameter is 2mm, and the diameter of the container is 3 in. When the level of the sample is 10 cm above the bottom of the container the emulsion drains through the tube at a rate of 12cm3/min, and when the level is 20 cm the flow rate is 30 cm3/min. The emulsion density is 1.3 g/cm3. 

In Section II.B of Chapter 3, the tube flow viscometer was described in which the viscosity of any fluid with unknown viscous properties could be determined from measurements of the total pressure gradient (— A4>/L) and the volumetric flow rate (Q) in a tube of known dimensions. The viscosity is given by... 

Examples are Laser Differential Microanemometry (LMA) and Total Reflection Microscopy (TMA) (8). Both LMA and TMA measure the velocity profile of the fluid in tube flow. However, such optical techniques are generally not suitable for opaque and/or heterogeneous substances such as foods. Acoustic velocimetry seems to be more promising for determining the velocity profiles of opaque substances. Such an acoustic technique has been applied by Brunn et al (19) as an on-line viscometer for flow of mayonnaises in pipes. 

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. 

PRESSURE-DRIVEN FLOW VISCOMETERS Capillary/Tube Viscometer... 

Two common methods for measuring viscosity are the cup and bob (Couette) and the tube flow (Poiseuille) viscometers. 

Ostwald U-tube n. Viscometer in the form of a U , in which the liquid flows from a bulb at a higher level on one side of the U , through a capillary, to a receiving bulb on the other side. The time is measured for a given volume of liquid to pass from one side to the other. 

Numerous methods for measuring fluid viscosity exist, for example, capillary tube flow methods (Ostwald viscometer), Zahn cup method, falling sphere methods, vibrational methods, and rotational methods. Rotational viscometers measure the torque required to turn an object immersed or in contact with a fluid this torque is related to the fluid s viscosity. A well-known example of this type of system is the Couette viscometer. However, it should be noted that as some CMP slurries may be non-Newtonian fluids, the viscosity may be a function of the rotation rate (shear rate). An example of this is the dilatant behavior (increasing viscosity unda increasing shear) of precipitated slurries that have symmetrical particles [33]. Furthermore, the CMP polisher can be thought of as a large rotational plate viscometer where shear rates can exceed 10 s and possibly affect changes to the apparoit viscosity. The reader can refer to the comprehensive review of viscosity measurement techniques in the book by Viswanath et aL [34]. 

The study of flow and elasticity dates to antiquity. Practical rheology existed for centuries before Hooke and Newton proposed the basic laws of elastic response and simple viscous flow, respectively, in the seventeenth century. Further advances in understanding came in the mid-nineteenth century with models for viscous flow in round tubes. The introduction of the first practical rotational viscometer by Couette in 1890 (1,2) was another milestone. 

Capillary Viscometers. Capillary flow measurement is a popular method for measuring viscosity (21,145,146) it is also the oldest. A Hquid drains or is forced through a fine-bore tube, and the viscosity is determined from the measured flow, appHed pressure, and tube dimensions. The basic equation is the Hagen-Poiseuike expression (eq. 17), where Tj is the viscosity, r the radius of the capillary, /S.p the pressure drop through the capillary, IV the volume of hquid that flows in time /, and U the length of the capillary. 

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 Hquid 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 Hquid 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 Hquid in the instmment. Hence, the conditions must be the same for each measurement. 

Piston Cylinder (Extrusion). Pressure-driven piston cylinder capillary viscometers, ie, extmsion rheometers (Fig. 25), are used primarily to measure the melt viscosity of polymers and other viscous materials (21,47,49,50). A reservoir is connected to a capillary tube, and molten polymer or another material is extmded through the capillary by means of a piston to which a constant force is appHed. Viscosity can be determined from the volumetric flow rate and the pressure drop along the capillary. The basic method and test conditions for a number of thermoplastics are described in ASTM D1238. Melt viscoelasticity can influence the results (160). 

In a series of experiments on the flow of flocculated kaolin suspensions in laboratory and industrial scale pipelines(26-27-2Sl, measurements of pressure drop were made as a function of flowrate. Results were obtained using a laboratory capillary-tube viscometer, and pipelines of 42 mm and 205 mm diameter arranged in a recirculating loop. The rheology of all of the suspensions was described by the power-law model with a power law index less than unity, that is they were all shear-thinning. The behaviour in the laminar region can be described by the equation ... 

Pour the first liquid to be measured into the viscometer tube and place the tube in the constant temperature bath. After allowing plenty of time for the temperature to equilibrate, measure the time of flow in the manner discussed in Section 15.2.4. Using the known calibration constant, calculate the kinematic viscosity at 25°C. Repeat with each of the other alcohols. 

A Cannon-Fenske viscometer is a capillary type of viscometer. It utilizes the flow through a capillary tube as a means of measuring viscosity. 

Saybolt Furol viscosity the time, in seconds (Saybolt Furol seconds, SFS), for 60 mL of fluid to flow through a capillary tube in a Saybolt Furol viscometer at specified temperatures between 70 and 210°F the method is appropriate for high-viscosity oils such as transmission, gear, and heavy fuel oils. 

The viscometer assembly is placed in the constant temperature column compartment of the chromatograph between the column outlet and the refractometer. A combination of two Waters Associates M-45 hydraulic filters in series with a capillary tubing coil (length 10 ft., I.D. 0.01 in.) is used to dampen the line pressure fluctuations caused by the pump. With the above pressure damping modifications the overall system noise was reduced to less than 1 millibar at 1.0 ml/min flow rate in tetrahydrofuran (THF) for a set of six p-Styragel columns 10 ,... 

Successful operation of the viscometer depends on good control of possible sources of flow variations in the system which include pump pulsations, temperature variations and restrictions in the GPC columns and fractional sections of tubing. 

The low frequency baseline noise of the viscometer can be substantially reduced by careful filtration of samples and regular checking and maintenance of column end fittings and fractional sections of tubing in the system. Figure 5 shows the effect of column screen replacement on the stability of the baseline signal at a flow rate of 1.0 ml/min. 

Viscometers Devices for measuring viscosity are called viscometers. The most common viscometer consists of a Cannon-Fenske tube, which is a U-shaped glass tube (see Figure 5.6), one arm of which consists of a capillary tube through which liquids flow slowly. The more viscous the liquid, the longer it takes for a given volume to flow through the capillary. This time is related to the viscosity of the liquid in poise or centipoise, which can be calculated from the measured time, a calibration constant, and the liquid s... 

Earlier experiments involved the collection of SEC effluent aliquots to measure solution viscosity in batches with the very time consuming Ubbelohde drop-time type viscometers. A continuous capillary type viscometer was first proposed for SEC by Ouano. Basically, as shown in Figure 1, a single capillary tube with a differential pressure transducer was used to monitor the viscosity of SEC effluent at the exit of the SEC column. As liquid continuously flows through the capillary (but not through the pressure transducer), the detected pressure drop (AP) across the capillary provides the measure for the fluid viscosity (h) according to the Poiseuille s viscosity law ... 

Mod i f 1 ed Mjymbrajne Viscometer Foi- the pulsed system a coil of tubing (the injection loop) was placed after the prefilter and liefore the membrane holder as shown in Figure P. Directional valves at each end of the loop controlled the flow path. Solvent or solution could be pumped directly to the UV to establish baseline absorbance or for calibration. To make P measurements the flow was directed through the membrane and then into the differential UV spectrophotometer. The flow could also be brought to the upstream portion of tlie membrane holder and then to the UV detector in an effort to measure the concentration at the membrane surface. 

---