Test Principle of Cone Plate Viscometer

The cone and plate viscometer is also a rotary viscometer, which is a commonly used instrument for measuring the viscosity of viscous dispersion system. Its structure is shown in Figure 2-14, which consists of a circular flat plate with a diameter R and a linear concentric cone. The space between the plate and the cone is filled with the measured liquid, the plate rotates at a constant speed w, and the torque M on the cone is detected. The linear velocity of the fluid at a distance from the axis r is rw, and the distance between the shear planes (that is, the thickness of the sample) h=rtan α , when the cone-plate angle α is small (usually α<4°), h= r α , the shear rate is

γ=dv/dh=v/h=rw/r α =w/ α

The obtained γ has nothing to do with r, that is, the shear rate between the cone and plate is approximately uniform. The shear stress can be obtained from the torque as:

σ=3M/2πR³

Therefore, the viscosity of the measured liquid is

η= σ/γ=3 αM/3πwR³=M/bw

In the formula, b=2 π R ³ /(3α), which is the instrument constant. The main advantages of the cone-plate viscometer are that the shear rate is uniform, the amount of sample is small, the filling and cleaning are convenient, it can be used for the determination of sticky samples, and the data processing is simple; the disadvantage is that when the rotating speed is high, the sample has overflow and Tendency to be corrupted, not to get correct data.