Viscosity measurement accuracy

In view of experimental simplicity and accuracy, viscosity measurements are extremely useful for routine relative molecular mass determinations on a particular polymer-solvent system. K and a for the system are determined by measuring the intrinsic viscosities of polymer fractions for which the relative molecular masses have been determined independently - e.g. by osmotic pressure, sedimentation or light scattering. 

A re-examination of the finer points in the structure of condensed films certainly seems called for, however, with an accuracy if possible of 0 5 per cent, or less in the areas, and there is no doubt that Dervichian is right in expecting that the combination of pressure, potential, and viscosity measurements will probably yield important information. 

The viscosity index (ASTM D-2270, IP 226) is a widely used measure of the variation in kinematic viscosity due to changes in the temperature of petroleum between 40°C and 100°C (104°F and 212°F). For crude oils of similar kinematic viscosity, the higher the viscosity index the smaller is the effect of temperature on its kinematic viscosity. The accuracy of the calculated viscosity index is dependent only on the accuracy of the original viscosity determination. 

Viscosity measurements can be made using several different methods and attendant equipment. To help relate viscosities reported in different units, Table III reports comparative viscosities using different methods of measurement. The accuracy will lessen as non-Newtonian flow (thixotropy, pseudoplasticity, etc.) increases, but it serves as an excellent guide. 

Viscosity measurements can be carried out with greater ease and accuracy than a determination of the molecular weight of a polymer by an absolute method. A reliable relationship between intrinsic viscosity and molecular weight (preferably the weight-average value) for hardwood xylans is, therefore, of interest. This relationship usually takes the form of the Mark-Houwinck equation,... 

The wedge material determines the sensitivity and accuracy of the density and viscosity measurements. Table 5-2 lists the tested wedge materials and their acoustic properties. For the tests, transducers (longitudinal and shear) were attached to wedges with epoxy glue. They are excited by a wideband pulse and... 

Using pitot tube as a differential pressure detection device, it need to consider factors that affect measurement accuracy, including the pitot tube coefficient, air density, temperature, pressure, thermal expansion coefficient, kinematic viscosity, and installation locations. 

Recently, Veith and Cohen (3) analyzed nylon 6 in TFE using the universal calibration method and peak retention data from narrow PMMA fractions. Silanized silica columns obviate the solvent incompatibility problem of TFE with styrene-based packings and give reproducible results. The accuracy of the calculated nylon 6 molecular weights was cross-checked with an independent end-group analysis (for M ) and intrinsic viscosity measurement (forMJ. 

Continuous measurement of viscosity. The accuracy of the Mark-Houwink equation will be improved if the intrinsic viscosity of a fraction at each retention volume can be measured continuously. An automatic viscometer having a capillary of 0.5 mm in diameter and a length of 200 mm was constructed and applied to the estimation of molecular mass of di- and triblock copolymers of polystyrene-polyisoprene P(S-IP) and di-block copolymer of P(S-MMA) [32]. In the case of copolymers which have homogeneous... 

Can be used only for measuring clean, steady, medium- to high-speed flow of low-viscosity fluids, (accuracy is also affected if fluid viscosity is high or flow is laminar)... 

Equation 10.5.2 fits available data (see Figure 10.2.3 and de Kruif et al., 1985) within measurement accuracy. Higher order expansions do not seem to be usefiil because they are applicable over increasingly small concentration regions. Various approaches are being used to compute viscosities at higher concentrations. The hydrodynamics for multiple particle interactions become very involved. They have been studied mainly by simulation (e.g., Brady and Bossis, 1988 Phillips et al., 1988). Other workers have used an approach based on nonequilibrium thermodynamics (Russel and Cast, 1986). Finally, Woodcock (e.g., 1984) uses molecular dynamics simulations, ignoring the medium viscosity, to calculate the flow-induced structure and then the viscosity. 

Vortex-shedding flow meters typically provide 1% of flow rate accuracy over wide ranges on Hquid, gas, and steam service. Sizes are available from 25 to 200 mm. The advantages of no moving parts and linear digital output have resulted in wide usage in the measurement of steam, water, and other low viscosity Hquids. 

To solve a flow problem or characterize a given fluid, an instmment must be carefully selected. Many commercial viscometers are available with a variety of geometries for wide viscosity ranges and shear rates (10,21,49). Rarely is it necessary to constmct an instmment. However, in choosing a commercial viscometer a number of criteria must be considered. Of great importance is the nature of the material to be tested, its viscosity, its elasticity, the temperature dependence of its viscosity, and other variables. The degree of accuracy and precision required, and whether the measurements are for quaUty control or research, must be considered. The viscometer must be matched to the materials and processes of interest otherwise, the results may be misleading. 

Orifice. Orifice viscometers, also called efflux or cup viscometers, are commonly used to measure and control flow properties in the manufacture, processing, and appHcation of inks, paints, adhesives, and lubricating oils. Their design answered the need for simple, easy-to-operate viscometers in areas where precision and accuracy are not particularly important. In these situations knowledge of a tme viscosity is uimecessary, and the efflux time of a fixed volume of Hquid is a sufficient indication of the fluidity of the material. Examples of orifice viscometers include the Ford, Zahn, and Shell cups used for paints and inks and the Saybolt Universal and Furol instmments used for oils (Table 5). 

A constant is often determined from measurements with a Newtonian oil, particularly when the caUbrations are suppHed by the manufacturer. This constant is vaUd only for Newtonian specimens if used with non-Newtonian fluids, it gives a viscosity based on an inaccurate shear rate. However, for relative measurements this value can be useful. Employment of an instmment constant can save a great deal of time and effort and increase accuracy because end and edge effects, sHppage, turbulent interferences, etc, are included. 

Using equation 3, the viscosity of any pitch can be calculated from two measurements in the range of 10 —10 mPa-s(=cP), exhibiting a precision similar to what may be expected of direct measuremeat. By employing equatioas 3, 4 or 5, and 6, the viscosity of pitch at any temperature can be calculated, with an accuracy adequate for most engineering purposes, from the R-and-B softening poiat and the Tl content. 

PEs, as other polymers, exhibit nonlinear behavior in their viscous and elastic properties under practical processing conditions, i.e., at high-shear stresses. The MFI value is, therefore, of little importance in polymer processing as it is determined at a fixed low-shear rate and does not provide information on melt elasticity [38,39]. In order to understand the processing behavior of polymers, studies on melt viscosity are done in the high-shear rate range viz. 100-1000 s . Additionally, it is important to measure the elastic property of a polymer under similar conditions to achieve consistent product quality in terms of residual stress and/or dimensional accuracy of the processed product. 

How does yield stress depend on a filler concentration It is shown in Fig. 9 that appreciable values of Y appear beginning from a certain critical concentration cp and then increase rather sharply. Though the existence of cp seems to be quite obvious from the view point of the possibility of contacts of the filler, i.e. the beginning of a netformation in the system, practically the problem turns on the accuracy of measuring small stresses in high-viscosity media. It is quite possible to represent the Y(cp) dependence by exponential law, as follows from Fig. 10, for example, leaving aside the problem of the behavior of this function at very low concentrations of the filler, all the more the small values of are measured with a significant part of uncertainty. 

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