Saybolt and Redwood Viscometer: The main disadvantage of the capillary tube viscometer is the errors that arise due to the variation in the head loss and other parameters. However, the Hagen-Poiseuille formula can be still applied by designing a efflux type viscometer that works on the principle of vertical gravity flow of a viscous liquid through a capillary tube. The Saybolt viscometer has a vertical cylindrical chamber filled with liquid whose viscosity is to be measured (Fig. 7.1.4-a). It is surrounded by a constant temperature bath and a capillary tube (length 12mm and diameter 1.75mm) is attached vertically at the bottom of the chamber. For measurement of viscosity, the stopper at the bottom of the tube is removed and time for 60ml of liquid to flow is noted which is named as Saybolt seconds. So, Eq. (7.1.6) can be used for the flow rate is calculated by recording the time (Saybolt seconds) for collection of 60ml of liquid in the measuring flask. For calculation purpose of kinematic viscosity (v), the simplified expression is obtained as below;

(7.1.6)

A Redwood viscometer is another efflux type viscometer (Fig. 7.1.4-b) that works on the same principle of Saybolt viscometer. Here, the stopper is replaced with an orifice and Redwood seconds is defined for collection of 50ml of liquid to flow out of orifice. Similar expressions can be written for Redwood viscometer. In general, both the viscometers are used to compare the viscosities of different liquid. So, the value of viscosity of the liquid may be obtained by comparison with value of time for the liquid of known viscosity.

 

Fig. 7.1.4: Schematic diagram: (a) Saybolt viscometer; (b) Redwood viscometer.