How Do You Measure Viscosity?

Dropping a marble or ball bearing through a liquid is a simple method for measuring viscosity, while an Ostwald Viscometer is more complex and more accurate. The ball bearing method involves more rigorous calculations than the viscometer.

The easiest way to determine viscosity is to drop a heavy sphere through the fluid in question and record the time it takes to pass completely through the fluid. The faster the ball drops, the higher the viscosity. There are, however, rigorous calculations needed to determine an actual viscosity measurement. A useful viscosity calculation for the dropped sphere method is 4r^2g(ps - pf)/9Vs. In this equation r is the radius of the sphere, g is acceleration from gravity (981 cm/s^2), ps is the sphere's density, pf is the fluid's density and Vs is the terminal velocity of the sphere.

An Ostwald Viscometer requires more involved data collection but less rigorous calculations. The viscometer is a capillary tube with a reservoir at each end. The capillary forms a u-shape with the first reservoir on one side and lower than the reservoir on the other side. To measure viscosity with a viscometer, fill the capillary with liquid until the liquid reaches just above the first reservoir. Next, place the apparatus in a water bath to equalize temperature. Suction then pulls the liquid through the tube. The amount of time that the liquid takes to move through the second reservoir is used in the equation viscosity = K t, where t is time and K is the value of a known liquid, typically water.