Falling-body viscometer

This method is usually employed to measure the viscosity of liquids and high-dense gases. The method is characterized by high uncertainty (3 %), but has some advantages for measurements at high pressures. The theoretical and experimental problems of this technique have been discussed by Kawata et al (1991). 

For falling bodies of various shapes (a cylinder, a cylinder with semispherical ends, or an arrow-like body), the viscosity is determined by measuring the time t of fall through a fixed distance at a constant velocity as 

For a free-fall of a sphere of radius r through a distance h, in a liquid enclosed in a tube of radius R, the viscosity equation is derived as 

In the above equationis the correction factor for die wall effect. The correction factor has been studied flieoreti-cally and experimentally (Faxen, 1923 Barr, 1931 Fidleris and Whitemore, 1961 Sutterby, 1973 and Kawata et al, 1991). Faxen (1923) expressed the factor as a function of the Reynolds number, Re, and the ratio r/R) 

Stake s law is applicable provided the Reynolds number. Re, is much less than unity. Oseen (1913) derived the following working equation for a sphere moving with a velocity V, which is applicable for a wide range of Re number 

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In the case of high pressures, different types of viscometer have been employed owing to the need to reduce the volume of fluid required. The most popular have been falling-body viscometers and torsional-crystal viscometers. Neither of these have, however, completely developed theories so that their accuracy is intrinsically limited. On the other hand, the newly developed vibrating-wire viscometer that makes use of the damping of a transverse oscillation of a thin wire enjoys a complete theory. 

In this paper we present a high pressure study of various lubricants, performed on a falling body viscometer in which the knowledge of the plunger position is continuously obtained by ultra-sonic techniques. 

In principle, there are three basic types of viscometers capillary viscometers, falling-body viscometers, and oscillating-body viscometers. 

Kawata, M., Kurase, K., Nagashima, A., Yoshida, K. Isdale, J.D. (1991b). Falling-body viscometers, in Measurement of the Transport Properties of Fluids - Experimental Thermodynamics, Volume III, eds. W.A. Wakeham, A. Nagashima J.V. Sengers, pp. 97-110. Oxford Blackwell Scientific Publications. 

Heiks and Jegart of Mound Laboratory [50,56] measured the viscosity of uranyl sulfate solutions in light water over a concentration range of 0.176 to 2.865 molal and over a temperature range of 20 to 90°C by the use of Ostwald capillary viscometers. Using the falling-body viscometer referred to above, Barnett et al. of Mound Laboratory have measured the viscosities of light- and heavy-water solutions of uranyl sulfate at temperatures up to 250°C [55]. Table 3-9 presents a comparison of viscosities... 

Falling ball viscometers are based on Stokes law, which relates the viscosity of a Newtonian fluid to the velocity of the falling sphere. If a sphere is allowed to fall freely through a fluid, it accelerates until the viscous force is exactly the same as the gravitational force. The Stokes equation relating viscosity to the fall of a soHd body through a Hquid may be written as equation 34, where ris the radius of the sphere and d are the density of the sphere and the hquid, respectively g is the gravitational force and p is the velocity of the sphere. 

In the Irvine-Park falling needle viscometer (FNV) (194), the moving body is a needle. A small-diameter glass or stainless steel needle falls vertically in a fluid. The viscous properties and density of the fluid are derived from the velocity of the needle. The technique is simple and useflil for measuring low (down to lO " ) shear viscosities. The FNV-100 is a manual instmment designed for the measurement of transparent Newtonian and non-Newtonian... 

The results of low-temperature measurements (below 473 K) are not given usually in the Appendix. Numbers separated by a slash (/) show the minimal temperatures or pressures available in the publications (before a slash) and in the Appendix tables (after a slash). Sat. means that equilibrium pressure is not shown in the publications but was near (above) the saturation vapor pressure at measured temperature, a) - CF -capillary flow OD- oscillating disk QV-quartz viscometer FB-falling-body... 

In this section the three techniques employed for the measurement of aqueous electrolyte solutions were discussed. These were the capillary-flow technique, the oscillating-disc technique and the falling-body method. From the discussion of the techniques it is apparent that measurements performed with the capillary-flow viscometer and the oscil-lating-disc viscometer enjoy a low degree of uncertainty. Hence, measurements would be expected to attain an uncertainty of better than 2%. Measurements performed with the... 

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