The rotary viscometer is the only instrument suitable for determining the thixotropic behavior of a paint. This continually measures shear stress under conditions of gradually increasing shear rate (normally this increase is time-proportional). From the results, the viscosity at a given point in time is calculated. Viscosity decreases as sheat becomes more intensive, but only down to a minimum figure that is characteristic for a given material.
This minimum viscosity is maintained for a time, even after the shear process has been discontinued. Only then does the paint appear to have as high a viscosity as at the , beginning of the process. A coquette-type rotary viscometer is normally used for this purpose, i.e. a viscometer with two concentric cylinders. Thixotropy can be determined as a quantity by plotting viscosity against time under conditions of increasing and then decreasing shear rat.