Viscosity Sensors

Viscosity is similar to the density of a product except viscosity will change continually as the temperature changes. For instance, you may be familiar with motor oil. When motor oil is warm, it will flow like water, and when the temperature is cold such as in the winter, the motor oil will be so thick, it will barely flow. In fact, if thinning additives are not added to motor oil, it will have the consistency of grease if the temperature is lowered sufficiently. In large pipeline operations and in applications such as filling tomato paste into containers, the viscosity must be controlled so the amount of product flowing can be calculated accurately. Viscosity is controlled by adjusting the temperature or density of the material.

The instrument that measures the viscosity of a product is called a viscosimeter, also called "viscometer", (see ill. 1 and ill. 2). ill. 2a shows a rotating disk viscosimeter, ill. 2b shows a differential pressure type, and ill. 2c shows a vibrating element to measure the viscosity of a product. The rotating disk viscosimeter has an element or disk that is rotated at a specific speed. The amount of torque the motor requires to turn the disk will increase as the product gets thicker. The differential pressure viscosimeter uses a venturi to measure the pressure differential of product flowing through it. When the viscosity of the product changes, the pressure drop across the venturi will also change. The vibrating element viscosimeter uses an element that is vibrated at a specific frequency. The amplitude of the vibration will increase when the viscosity decreases.

Above: ill. 1: A viscosity measurement probe -- viscometer -- that utilizes a vibrating rod immersed in the process media. This unit is made by Dynatrol.

Above: ill. 2 (a) A rotating disk viscosimeter. (b) A differential pressure viscosimeter. (c) A vibrating element viscosimeter.

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