![]() Trumpet shaped entrances diminish the magnitude of this kinetic energy effect and appear to create flow conditions more compatible with the most convincing theoretical derivations. For square-cut ends, P/ Q appears to be independent of Q at low flow rates and linear in Q at higher rates. Different calculations have yielded various values for the “constants” m and n, and experimental tests suggest that they differ appreciably for differing velocities of flow, capillary dimensions, and entrance and exit shapes. The many attempts to derive eq (2), going back to the nineteenth century, are based on assumed flow patterns not exactly realized in practice. Where ρ is the density and m and n are presumed to be constants for a particular instrument. This question is the subject of this and the two accompanying papers. There remains a question of the accuracy of the value used for the viscosity of the initial calibrating liquid, that is of our absolute measurements of viscosity. It appears that any systematic errors associated with the step-up procedure are less than 0.1 percent at least up to viscosities of 1 P, although most of the evidence on which this conclusion is based has been obtained with capillary viscometers, leaving a possibility of some unrecognized bias. It is often assumed that this precision also represents the accuracy of such measurements. It is quite common and relatively simple to make measurements to within 0.1 percent on ordinary liquids with viscosities below 1 P. The agreement between measurements using various standard and accepted types of viscometers, 1 gives us sound grounds for believing that systematic errors can be kept below this one percent level. Such liquids normally have a very high temperature coefficient of viscosity, and problems of adequate temperature control alone make it difficult to attain better than one percent agreement. ![]() The common practice is to calibrate viscometers by a step-up technique, using a series of instruments and test fluids, based on the viscosity of water.įor viscosities above 1000 poise (P) or so, the accuracy of such measurements is generally limited by their variability. The viscosity (shear viscosity) of a Newtonian liquid is normally measured by a relative technique in an instrument calibrated using a liquid of known viscosity. ![]() However, whenever the true values of viscosity are required the limits of uncertainty including an estimate of systematic error should be taken as no better than ☐.25 percent. This provides a generally accepted base which limits comparability only by the precision of the measurements. It is suggested that we continue to base the calibration of relative viscometers on the value of 1.002 centipoise (cP) for the viscosity of water at 20 ☌ and one atmosphere. The estimated accuracy in each case is about 0.1 percent. The results of two independent absolute measurements involving different types of flow, reported in the two accompanying papers, are summarized here. The range of values found from these measurements and the possibility of unrecognized systematic errors make it impossible to base a realistic estimate of accuracy on the results of only one type of measurement. Most absolute measurements of viscosity have utilized capillary flow, and required semiempirical corrections amounting to several times their precision and estimated accuracy.
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