Viscometers are used to measure viscosity. There are three types of viscometers according to their working principles: rotary, off-body and capillary viscometers.
1.6.1 Rotary Viscometer
A rotary viscometer consists of two parts filled with a liquid to be tested. One part is fixed and the other rotates. By measuring the shearing moment caused by the resistance of a liq- uid, the dynamic viscosity can be obtained. A rotary viscometer is shown in Figure 1.14a, and a cone-plate rheometer is shown in Figure 1.14b. The former is composed of two concentric cylin- ders, while the latter is composed of a plane and a conical surface. If the moving part rotates at different speeds, the relationship of the shear stress and the shear rate can be obtained, which is called the rhoelogical property. This is very useful, especially for non-Newtonian fluid.
1.6.2 Off-Body Viscometer
The most commonly used off-body viscometer is composed of a ball and a test tube filled with the fluid to be tested. In order to determine the viscosity, measure the final velocity of the falling ball. As the clearance between the ball and the tube is very small, the falling ball viscometer can be used to measure the viscosity of a gas, or of a fluid under a high pressure. Another type of off-body viscometer consists of two vertical concentric cylinders. The fluid to be tested is filled between them. The outer cylinder is fixed, while the inner tube falls so that the viscosity can be obtained by measuring the final falling velocity. An off-body viscometer is mainly used to measure high-viscosity fluids.
1.6.3 Capillary Viscometer
The principle of a capillary viscometer is that while a certain volume of liquid flows through a standard capillary, because there exists a pressure difference and liquid weight, the passing time of flowing can be used to determine the viscosity of the liquid. There are two kinds of capillary viscometers, absolute and relative. An absolute capillary viscometer measures viscosity based on the viscous fluid mechanics formula, while the relative viscometer must be calibrated by a known viscosity liquid to obtain the viscometer constant first, only then can it be used to mea- sure the viscosity of the liquid to be tested. As the scaling errors do not affect the measurement results of a relative viscometer, it is more reliable.
Figure 1.15 shows a relative capillary viscometer with a known constantc. Measure the time for the liquid surface to drop fromAtoB, the kinematic viscosity of the liquid being equal to
v=ct. (1.46)
Figure 1.14 Rotary viscometers: (a) rotational viscometer (b) cone-plate rheometer.
k k Figure 1.15 Common capillary viscometer: (1) thermometer; (2) capillary
viscometer; (3) water or oil bath; (4) blender; (5) heater.
Figure 1.16 Engler capillary viscometer: (1) thermometer; (2) wood plug;
(3) lubricating oil sample; (4) heat bath; (5) receiving bottle.
If the density of the measured liquid is𝜌, its dynamic viscosity is equal to
𝜂=𝜌v. (1.47)
Commonly, commercial relative capillary viscometers are of three kinds: Redwood, Saybolt and Engler. Although their structures are similar, the volumes of the liquid and the capillary sizes are different. An Engler viscometer is shown in Figure 1.16 and its viscosity calculation formula is as follows.
Viscosity(E∘) = Time used for 200 liters liquid flowing out
Time for the same volume of water flowing out (1.48) As different viscometers obtain different relative viscosities, some empirical formulas or charts are needed to convert them to kinematic viscosity. The conversion relationships of the three common viscometers are given in Figure 1.17.
It should be pointed out that usually a commercial viscometer under normal conditions mea- sures only the body viscosity of a liquid, which does not fully reflect the rheological properties of a lubricant film.
Therefore, a number of special measuring devices are designed, some for very high viscosi- ties and some for very low viscosities. For example, a micro-viscometer is used to measure the viscosity under high pressure and high shear rate, or it can be used to obtain the visco-elastic property of a liquid. The author used optical interference techniques to measure EHL lubricant
k k Figure 1.17 Viscosity conversion table.
film thicknesses, and took the standard liquid as benchmark to calibrate the viscosity–pressure coefficient of some oils [4]. Renyou Wang studied the influence of oil viscosity on high pressure and rheological properties [5].
References
1 Huang, P. and Wen, S.Z. (1996) Non-Newtonian effects of temperature and lubrication failure mechanism analysis. Lubrication and Sealing,2, 14–16.
2 Johnson, K.L. and Tevaarwerk, J.L. (1977) Shear behavior of elastohydrodynamic oil films.
Proceedings of the Royal Society of London,A356, 215–236.
3 Bair, S. and Winer, W.O. (1979) A rheological model for elastohydrodynamic contacts based on primary laboratory data. Transactions of the ASME Journal, Series F,101(3), 258–265.
4 Yu, X. G. and Wen, S.Z. (1984) Determination of optical interference pressure viscosity coefficient of lubricant.Lubrication and Sealing,3, 10–14.
5 Wang, R.Y. (1997) High impact technology and lubricating oil viscosity testing the rheological properties of, PhD thesis, Tsinghua University.
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