Question

There are three main methods for determining viscosity: the Ostwald viscometer, the falling ball viscometer, and the cup viscometer. Briefly describe how each method works.

Answer 1

OSTWALD VISCOMETER

Ostwald viscometer, also known as U-tube viscometer or capillary viscometer is a device used to measure the viscosity of the liquid with a known density. The method of determining viscosity with this instrument consists of measuring the time for a known volume of the liquid to flow through the capillary under the influence of gravity.

Knowing the value of viscosity of one liquid, one can calculate the viscosity of other liquid.

η₁ = η₂·ρ₁t₁ρ₂t₂

where η₁ and η₂ are viscosity coefficients of the liquid and water, and ρ₁ and ρ₂ are the densities of liquid and water, respectively.

FALLING BALL VISCOMETER

The falling ball viscometer typically measures the viscosity of Newtonian liquids and gases. The method applies Newton’s law of motion under force balance on a falling sphere ball when it reaches a terminal velocity. In Newton’s law of motion for a falling ball, there exist buoyancy force, weight force, and drag force, and these three forces reach a net force of zero.

viscosity is in implicit in the above eqt. thus we can solve it by itereation scheme; this can be done by setting the parameters of material and geometry, calculating the distance of y from Eq. (9) by guessing .

CUP VISCOMETER

The Cup Viscometer is a simple gravity device that permits the timed flow of a known volume of liquid passing through an orifice located at the bottom. The rate of flow would be proportional to the kinematic viscosity that is dependent upon the specific gravity of the draining liquid. It is important when using a Ford Cup and when retesting liquids that the temperature of the cup and the liquid is maintained, as ambient temperature makes a significant difference to viscosity and thus flow rate.

v= η / ρ

v=kinematic viscosity

η= dynamic viscosity

ρ= specific gravity