Viscometer shear rate range

The Ferranti-Shidey viscometer was the first commercial general-purpose cone—plate viscometer many of the instmments stiU remain in use in the 1990s. Viscosities of 20 to 3 x 10 mPa-s can be measured over a shear rate range of 1.8-18, 000 and at up to 200°C with special ceramic cones. Its features include accurate temperature measurement and good temperature control (thermocouples are embedded in the water-jacketed plate), electrical sensing of cone—plate contact, and a means of adjusting and locking the position of the cone and the plate in such a way that these two just touch. Many of the instmments have been interfaced with computers or microprocessors. 

Haake has introduced other viscometers, including the RheoStress RSlOO, which offers controlled stress as well as controlled shear rate and oscillatory modes over a temperature range of —50 to 350°C (ambient to 500°C is also possible). This versatile viscometer covers a shear rate range of 10 ... 

The MacMichael viscometer is probably the most straightforward rotatioaal viscometer. The outer cup rotates and the inner cylinder is suspended from a torsion wire. The drag on the inner cylinder is measured as degree of twist on the wire. Wires of different stiffness are available, and the maximum viscosity is ca 10 mPa-s. The shear rate range is limited, ca 2-12, but with modification, higher shear rates can be attained. The iastmment is best... 

The Ravenfield model BS viscometer is a wide shear rate range iastmment with several possible measurement systems cone—plate, parallel plates, concentric cylinders, and taper plug. The last gives shear rates of up to 10 , and the cone—plate of up to 8 x lO". The viscosity range is... 

More recent developments in the rolling ball area include an automated micro viscometer, the Paar AMV 200, from Paar Physica. The specimen to be measured is introduced into a glass capillary down which a gold-covered steel ball roUs. The rolling time is measured automatically. The shear stress may be varied by changing the inclination angle of the capillary tube. The shear rate range is 10 1000, which makes the instmment useflil for... 

A cone and plate rotational type viscometer is used to obtain rheological data in the low-to-medium shear rate range. It gives a constant rate of shear across a gap, and therefore, equations for this instrument are simple when the angle is small (less than 3°). For this reason the cone and plate viscometer has become a standard tool... 

Two of Miller s suspensions were slightly non-Newtonian in behavior (0.8 < n < 1.00). For these he determined the differential viscosities over the range of shear rates from 5.8 to 77 sec.-1 with a MacMichael viscometer. They were constant over about the upper 70% of this shear-rate range and were found to be equal to the experimentally determined turbulent-flow viscosities in the heat exchanger. 

Viscometer type Lowest viscosity Highest viscosity Shear rate range (s 1)... 

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. 

A rheological instrument such as a viscometer can be used to evaluate t and 7 and hence obtain a value for the shear viscosity, 17. Examples of Newtonian fluids are pure gases, mixtures of gases, pure liquids of low molecular weight, dilute solutions, and dilute emulsions. In some instances, a fluid may be Newtonian at a certain shear-rate range but deviate from Newton s law of viscosity under either very low or very high shear rates (2). 

Measurement of viscosity [r ] is more complicated than estimation of the molar mass, due to the non-Newtonian behaviour of HA solutions during the flow. It is well known that the HA viscosity strongly depends on the shear rate (7) even for very dilute solutions. Unfortunately, the shear rate range of the usual viscometers used for the [rj] measurements, both off-line (i.e. Ubbelohde viscometer 1200-1500 s ) and on-line to a SEC system (i.e. DV 2500-3000 s ) are too high for measuring the HA... 

Here t, 4, and 4 2 are three important material functions of a nonnewtonian fluid in steady shear flow. Experimentally, the apparent viscosity is the best known material function. There are numerous viscometers that can be used to measure the viscosity for almost all nonnewtonian fluids. Manipulating the measuring conditions allows the viscosity to be measured over the entire shear rate range. Instruments to measure the first normal stress coefficients are commercially available and provide accurate results for polymer melts and concentrated polymer solutions. The available experimental results on polymer melts show that , is positive and that it approaches zero as y approaches zero. Studies related to the second normal stress coefficient 4 reveal that it is much smaller than 4V and, furthermore, 4 2 is negative. For 2.5 percent polyacrylamide in a 50/50 mixture of water and glycerin, -4 2/4 i is reported to be in the range of 0.0001 to 0.1 [7]. 

The capillary tube viscometer is a flow-through-restriction type. When used carefully, it is capable of accuracies of better than 2 percent over its applicable shear-rate range (300 to 4000 s 1). For laminar flow of a nonnewtonian liquid in a capillary tube, it can be shown [4] that the wall shear stress xw and the shear rate at the wall % are given by... 

For 10-/mn particles suspended in water at room temperature, fR is of the order of 10 min. Hence, it is only in dilute systems of small particles that complete relaxation of a suspension structure can take place within the shear rate range of most rotational viscometers on the market. 

Shear thinning and thixotropic properties of non-Newtonian materials at the shear rate ranging from 0.1 to 50 s can be measured by the standardized method." The standard describes three methods of measurement. Method A is used to measure apparent viscosity of material by measming torque with spindle rotating at constant speed. Apparent viscosity in centipoises (equal to mPa.s) is calculated by multiplication of scale reading of viscometer by a scale factor, which depends on the spindle number and rotation speed. If the material is Newtonian, its viscosity does not depend on shear rate and measurement at one speed is sufficient. Non-Newtonian materials require measurements at different shear rates as described in methods B and C. 

Each of the flow curves obtained by the Haak viscometer were analyzed in the low shear rate range by using the theoretical models of both Bueche and Graessley. The shear stress, shear rate data taken from each flow curve were fitted to both equation (5) and equation (12) by linear regression. Best values for molecular response time and zero shear viscosity were then calculated using equations (6), (7), (15), and (16). These results are presented in Table 7. 

The apparent viscosity of various polymer solutions as a function of the shear rate (Y) was measured using a co-axial rotational viscometer (Contraves low-shear 30) at 20°C. The apparent shear rate range that can be obtained with this viscometer is firom 0.01 to 130 s. This range encompasses the shear rate range of 0.1 - 10 s encountered in a typical reservoir away from the wellbore [7]. 

Standard laboratory viscometers have a shear rate range 0.1 -1000 s-i. At the moment these include the... 

Somehmes the position of the typical behaviour along the shear-rate axis is such that the parhcular measurement range used is too low to pick up the higher-shear-rate part of fhe curve, see for instance figures 10 -13 (nofe that the typical shear-rate range of mosf laboratory viscometers is 0.1 to ICP s-i). This is particularly true for polymer melfs and shampoos. On the other hand, if fhe shear-rate range is too high, then the... 

Fluids exhibiting Newtonian behaviour have constant viscosities as defined by Eqn. (1), and single point measurement at a convenient shear rate is sufficient to describe such a fluid. However, non-Newtonian fluids have viscosities that are shear rate-dependent, and a single point measurement is inadequate to describe the flow behaviour. Here, the relevant shear rate range in the engineering application must be assessed and used in determining the measurement conditions for the viscometer/rheometer. Shear rate-dependent viscosity is often referred to as apparent viscosity, Tja, but there is no need to make this distinction if it is accepted that viscosity, as defined in Eqn. (1), can be variable. However, it is essential that values of T are quoted with their corresponding values of shear rate (or shear stress). 

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