Page 66 - Instrumentation Reference Book 3E

References 51

frequency and damping of the oscillations of a with less restriction on temperature measurement
wire vibrating transversely in a fluid depend on and thermostating. Similar arguments apply
the viscosity and density of the fluid. References to the Couette viscometer but here, even with
to this and other work are given in Bennemann precise research instruments, an accuracy of 1
and Ketterson (1976). percent requires very careful experimentation.
To study the effect of high pressure, Abbot et al. The range of viscosities and rates of shear
(1981) and Dandridge and Jackson (1981) have attainable in any type of viscometer depend on
observed the rate of fall of a sphere in lubricants the dimensions of the viscometer, e.g., the radius
exposed to high pressures (-3 GPa). Galvin, Hutton, of the capillary in the capillary viscometer and the
and Jones (1981) used a capillary viscometer at gap in a Couette viscometer.
high pressure to study liquids over a range of temp- By way of illustration, we conclude with a table
eratures (0 to 150°C) and shear rates (0 to of values claimed by manufacturers of instru-
3 x lo5 sec-’) with pressures up to 0.2 GPa. Kamal ments within each type. but we emphasize that
and Nyun (1980) have also adapted a capillary vis- no one instrument will achieve the entire range
cometer for high-pressure work. quoted.

2.7 On-line measurements
Table 2.1
It is frequentiy necessary to monitor the viscosity of
a fluid “on line” in a number of applications. parti- Viscometer Lowest Highest Shear-rate
viscosity
viscosity
cularly when the constitution or temperature of the tJP range
fluid is likely to change. Of the viscometers described (poise) (poise) (S-’)
in this chapter, the capillary viscometer and the Capillary 2 x 10-3 10’ 1 to 1.5 x 104
concentric-cylinder viscometer are those most con- Couette 5 x 10-3 4 x 107 io-’ to 104
veniently adapted for such a purpose. For the Cone-and-plate 10‘0 10-4 to 103
former, for example, the capillary can be installed Brookfield type lo-’ 5 x 105 10-3 to 106
directly in series with the flow and the pressure Falling-ball,
difference recorded using suitably placed trans- rolling-ball 104 indeterminate
ducers and recorders. The corresponding flow rate
can be obtained from a metering pump.
Care must be taken with the on-line concentric-
cylinder apparatus as the interpretation of data 2.9 References
from the resulting helical flow is not easy.
Other on-line methods involve obstacles in the Abbott, L. H., D. H. Newhall. V. A. Zibberstein, and
J. F. Dill, A.S.L.E. Trans., 24, 125 (1981)
flow channel; for example, a float in a conical Barnes. H. A., J. F. Hutton. and K. Walters, Introduc-
tube will arrive at an equilibrium position in the tion to Rheology, Elsevier, New York (1989)
tube depending on the rate of flow and the kine- Bennemann, K. H. and J. B. Ketterson. (eds), The
matic viscosity of the fluid. The parallel-plate Physics ofLiquid and Solid Helium, Wiley, New York
viscometer has also been adapted for on-line meas- (Part 1, 1976; Part 2, 1978)
urement. These and other on-line techniques are Cheng, D. C.-H., “A comparison of 14 commercial vis-
considered in detail in The Instwnent Manual cometers and a home-made instrument,” Warren
(1975). Spring Laboratory LR 282 (MH) (1979)
Coleman, B. D.. H. Markovitz, and W. Noli. Visco-
metric Flows of A7on-,Vei~toniun Fluids, Springer-Ver-
lag, Berlin (1966)
Accuracy and range Dandridge, A. and D. A. Jackson. J. Plzqs. D, 14. 829
(1981)
The ultimate absolute accuracy obtained in any Dealy, J. M., Rheonieters for Molter? Plastics. Van Ncs-
one instrument cannot be categorically stated in a trand. New York (1982)
general way. For example, using the Ostwald Galvin. G. D., J. F. Hutton. and B. J. Jones, Nom
viscometer, reproducible measurements of time Newtonian Fluid Mechanics, 8, 11 (1981)
can be made to 0.3 percent. But to achieve this The Instruinent ,2/lilnual. United Trade Press, p. 62 (5th
absolutely. the viscometer and the fluid must be ed., 1975)
scrupulously clean and the precise amount of Kamal, M. R. and H. Nyun, Poljnier Eng. and Science.
20; 109 (1980)
fluid must be used. The temperature within the Lodge. A. S., Body Tensor Fields in Contin~~tan
viscometer must also be uniform and be known to Meclzanics, Academic Press. New Ycrk (1974)
within 0.1 K. Obviously. this can be achieved, but Massey, R. S., Mechanics of Fluids, Van Ncstrand, New
an operator might well settle for 1 to 2 percent York (1968)
accuracy and find this satisfactory for his purpose Petrie, C. J. S.. Elongational Flows, Pitman. London (1979)