Viscous Force
The velocity of a calibrated sphere descending through a liquid-filled tube constitutes a primary measure for determining dynamic viscosity. This measurement, known as the fall rate, is governed by the balance between gravitational forces and viscous drag. Metrologists use the time of descent between two fixed marks to calculate the absolute viscosity of the medium.
The accuracy depends on the precision of the tube’s internal bore.
Terminal Velocity
Steady motion is achieved almost instantly after the sphere is released into the fluid. At this stage, the fall rate becomes constant because the net force on the object is zero. This terminal velocity must be maintained for the calibration to remain valid.
External vibration can disrupt this equilibrium.
Fluid Resistance
Variations in temperature alter the resistance of the liquid and directly affect the movement of the sphere. A higher temperature lowers the fluid resistance, causing an increased fall rate during the test. For this reason, the test tube is enclosed in a thermostatic bath.
Calibration Variable
Standard reference materials with certified viscosities are used to calibrate the viscometer tube and sphere combination. The fall rate of the certified sphere provides the calibration constant when compared against the reference value. This constant is verified at specific temperature points, which are typically controlled to within a few thousandths of a degree Celsius.
Any scratch on the sphere alters the fall rate and invalidates the certificate.
