Viscometer coaxial cylinder type

The cationic cellulose ether used is a Union Carbide Corporation product sold under the tradename UCARE Polymer JR-400. Its molecular weight is approximately 400,000. The sodium dodecyl sulfate is a high purity specimen obtained from British Drug Houses. Viscosity measurements on the solutions were carried out with a Saybolt viscometer at 25°C. The normal density corrections were applied. Shear dependent viscosity was measured at 25 C using the Haake Rotovisco viscometer, R-V3, with the MK 500 measuring head. The sensor system used was a coaxial cylinder type fitted with the MVI bob, having a radius of 20.04 mm. The radius of the stator is 21 mm. With this attachment, the range of shear rate and of viscosity is from 0.5 to 3000 sec"l, and from 8 to 10 centipoise, respectively. 

Although the international system of units (SI vmit) for viscosity is Pa-s, cps is also used often for raw materials of adhesives and PSAs. For viscous liquids, Brookfield-type viscometers or coaxial-cylinder-type viscometers, shown in Fig. 40.8, are usually used. For higher viscous... 

The Nametre Rotary B rotational viscometer measures torque in terms of the current needed to drive the d-c motor at a given speed while a material is under test. The standard sensors are coaxial cylinders or Brookfield disk-type spindles, but a cone—plate system is also available. The viscosity range for the coaxial cylinder sensors is 5 to 5 x 1(T mPa-s, and the maximum shear rate is 200. ... 

There are two main types of viscometer rotary instruments and tubular, often capillary, viscometers. When dealing with non-Newtonian fluids, it is desirable to use a viscometer that subjects the whole of the sample to the same shear rate and two such devices, the cone and plate viscometer and the narrow gap coaxial cylinders viscometer, will be considered first. With other instruments, which impose a non-uniform shear rate, the proper analysis of the measurements is more complicated. 

Typical of this class of viscometer is the coaxial or Couette type of instrument described in Volume l, Section 3.7.4. The sample fluid is contained within the annular space between two coaxial cylinders, either of which may be rotated by a motor with the remaining cylinder suspended elastically in such a way that the torsional couple exerted on the latter can be measured. If the outer cylinder of radius r2 rotates with an angular velocity cou and the inner cylinder of radius r, is stationary, and the torque (or viscous drag) per unit length of cylinder exerted on the inner cylinder is T, then, for a Newtonian fluid(49) ... 

Simple empirical viscometers of the orifice and falling ball types, and the controlled shear rate McMichael coaxial cylinder viscometer, have been used traditionally in the chocolate industry. Sophisticated rheometers are now being used increasingly because the economic pressure to reduce the cocoa butter content of chocolate has generated a need for a greater understanding of chocolate rheology (Minifie, 1999). 

The samples were sheared using a rotational viscometer with a coaxial cylinder system, based on the Searle-type, where the inner cylinder (connected to a sensor system) rotates while the outer cylinder remains stationary. The outer cylinder surrounding the inner one was jacketed, allowing good temperature control, and the annular gap was of constant width. The sensor system used was the NV type, with a rotor with a recommended viscosity range of 2x10 mPa, a maximum recommended shear stress of 178 Pa, and a maximum recommended shear strain rate of 2700 s this rotor could work with volumes from 10-50 ml. Flow was laminar. 

A variety of laboratory instruments have been used to measure the viscosity of polymer melts and solutions. The most common types are the coaxial cylinder, cone-and-plate, and capillary viscometers. Figure 11 -28 shows a typical flow curve for a thermoplastic melt of a moderate molecular weight polymer, along with representative shear rate ranges for cone-and-plate and capillary rheometers. The last viscometer type, which bears a superficial resemblance to the orifice in an extruder or injection molder, is the most widely used and will be the only type considered in this nonspecialized text. 

Procedmes for extracting valid shear stress versus shear rate data from measurements involving wide gap coaxial cylinder systems (the Brookfield viscometer being an extreme example of wide gap devices) are therefore of considerable interest in making quantitative measurements of the flow properties of non-Newtonian process products. Most of these data-treatment procedures necessarily involve some assumption regarding the functional form of the flow curve of the material. One example is that made in the derivation of data from the Brookfield-type instrument, which assumes that the speed of rotation of the cylinder or spindle is proportional to the shear rate experienced by the fluid. This assumption implies that the flow curve is adequately described by a simple power-law (which for many shear-thinning non-Newtonian fluids may be acceptable), but this assiunption is widely taken to exclude all fluids which display an apparent yield stress and/or non-power law type behaviour. 

The two-point tests are executed on various types of coaxial cylinder viscometers (rheometers), which are basically composed of two cylinders. The space between them is filled with the fresh mix under examination (Figure 12.3). The external cylinder is rotated and the movement is transferred through the liquid to the internal cylinder, where the torque is measured. The dimension of the maximal aggregate grain is decisive on the selection of a viscometer. 

The Ubbelohde-type capillary viscometer gives viscosity at a low shear rate. To obtain the full viscosity-shear rate profile, a coaxial cylinder viscometer can be employed. The shear rate in a coaxial cylinder viscometer is calculated using the following equations ... 

Many types of rotational rheometers and viscometers have been developed. The cone and plate, couette (coaxial cylinder), torsional, and disc spindle types are the most common. 

Coaxial (Concentric Cylinder) Viscometer, The eadiest and most common type of rotational viscometer is the coaxial or concentric cylinder instmment. It consists of two cylinders, one within the other (cup and bob), keeping the specimen between them, as shown in Figure 27. The first practical rotational viscometer consisted of a rotating cup with an inner cylinder supported by a torsion wire. In variations of this design the inner cylinder rotates. Instmments of both types ate useful for a variety of apphcations. 

Once the values of T Jind co are known, q can be calculated by equation (23). The constants of the viscometer appearing in the equation can be detennined by accurate measurements. Alternatively, two liquids whose viscosities are known can be used for calibration. The type of flow obtained in this type of coaxial rotating cylinder viscometer is simpler than that in a crapUlaiy viscximeter. The only difficulty lies in controlling the temperature of the system. 

The second type of viscometer, which is fairly often used in colloid science, is the CouETTE type, which exists in many constructions and the principle of which is illustrated by Fig. 11. A hollow cylinder A is set in rotation by some mechanism D a cylinder B is suspended by a wire c (on which is a mirror d) coaxially with A. In the space between A and B is the liquid, for wMch one wishes to determine tj. Thanks to the internal friction the cylinder B experiences a rotation and the wire a torsion the angle of rotation can be determined with the aid of a beam of light oti the mirror c and a reading telescope. The couple exerted on the cylinder B is Kyfn, in which K is a constant for the apparatus and ol the ai ular velocity of cylinder A as a result the cylinder B experiences a rotation, characterized by the angle of torsion of the suspension wire. If the apparatus is first calibrated with water (subscript ly) and then the torsion angle for a sol (subscript s) is measured, the expression... 

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