Rolling ball viscosimeter

Oil viscosity is measured in a rolling-ball viscosimeter or a capillary viscosimeter, either designed to simulate differential liberation. Measurements are made at several values of pressure in a stepwise process. The liquid used in each measurement is the liquid remaining after gas has been removed at that pressure. See page 6 of Table 10-1. 

A rolling ball viscosimeter consists essentially of a cylindrical tube which is inclined at a definite angle. The tube is filled with the fluid whose viscosity is to be measured and a metal ball is allowed to roll down through the tube. The bottom of the tube is closed so that as the ball travels downward the fluid passes upward through the space between the rolling ball and the walls of the tube. The time for the descent of the ball is accurately measured. It may be shown that the velocity of the ball is given by... 

When the rolling ball viscosimeter is employed to measure gas viscosities it is necessary to use a ball which just flts the tube. Under these conditions the rate of fall is slow enough so that it can be accurately measured. Notice that for gases at low pressures equation 12 reduces to... 

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