Measuring Principle
The Electromagnetic Spinning Sphere Viscometer is equipped with two magnets attached to a rotor which create a rotating magnetic field. This field is represented with orange arrows in the drawing. The sample (3) to be measured is in a small test tube (2) with a smooth concave bottom. The test tube is closed to prevent evaporation of the sample. Inside the tube is an aluminium sphere (4). The tube is located in a temperature controlled chamber (1) and set such that the sphere is situated is the centre of the two magnets.
The rotating magnetic field induces eddy currents in the sphere. The resulting Lorentz interaction between the magnetic field and these eddy currents generate torque that rotates the sphere. The rotational speed of the sphere depends on the rotational velocity of the magnetic field, the magnitude of the magnetic field and the viscosity of the sample around the sphere. The motion of the sphere is monitored by a video camera (5) located below the cell. The rotational speed of the sphere is calculated by two dimensional autocorrelation of the images captured by the camera. The rotational speed of the sphere can reliably be determined this way even if dark or turbid samples are measured, as the sample layer between the cell bottom and the sphere is very thin.
The torque applied to the sphere is proportional to the difference in the angular velocity of the magnetic field ΩB and the one of the sphere ΩS. There is thus a linear relationship between (ΩB − ΩS)/ΩS and the viscosity of the liquid.
It is known the that the resistant torque applied to a rotating sphere on a flat surface in a liquid is proportional to the sphere volume and the viscosity of the liquid plus the torque caused by the friction between the sphere and the surface. It is thus possible to calculate the sample's viscosity from the two angular velocities as long as
-
the torque caused by friction is small compared to the one caused by the sample's viscosity
-
the torque caused by the sample's viscosity is still sufficiently low to allow a certain rotating speed of the sphere.
These two criteria define the measuring range of the EMS viscometer which is 0.1 to 1,000,000 mPa·s.
