Viscometer, Brookfield controlled-stress

There are generally two approaches to the measurement of viscosity namely, controlled stress and controlled strain. In controlled stress, a known stress is applied to the fluid and the resultant shear rate is measured. Conversely, in controlled strain a known strain, or more accurately rate of strain (shear rate) is applied and the stress is measured. A falling ball viscometer is a controlled stress viscometer in which the applied stress comes from the size of the ball and gravity whereas the Brookfield viscometer is an example of a controlled strain instrument where the rate of strain is related to the rotational speed applied. 

Chocolate does not behave as a tme Hquid owing to the presence of cocoa particles and the viscosity control of chocolate is quite compHcated. This non-Newtonian behavior has been described (28). When the square root of the rate of shear is plotted against the square root of shear stress for chocolate, a straight line is produced. With this Casson relationship method (29) two values are obtained, Casson viscosity and Casson yield value, which describe the flow of chocolate. The chocolate industry was slow in adopting the Casson relationship but this method now prevails over the simpler MacMichael viscometer. Instmments such as the Carri-Med Rheometer and the Brookfield and Haake Viscometers are now replacing the MacMichael. 

For the purpose of this research, brief information about cone and plate geometry viscometer will be given. As noted in Brookfield catalog, cone and plate viscometer offers absolute viscosity determination with precise shear rate and shear stress information. The sample volume is extremely small amount and temperature controlled is easily accomplished. Cone and plate geometry viscometer is particularly advanced rheological analysis of non-Newtonian fluids (Phair Kaiser, 2009). 

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