The Stabinger Viscometer

In 1995, we set ourselves the task of developing a handy viscometer that offered the convenience of a density meter of our type of construction. It was clear that it needed to be a small compact measuring cell that could be easily filled and cleaned due to the necessary precise temperature control. Thus every type of capillary viscometer had to be ruled out. The self-imposed requirements seemed to be most easily fulfilled by means of a miniature rotational viscometer. We experimented with a Searle-type with a magnetically mounted rotor. Due to miniaturizing, every mounting of the measuring element affected by friction had to be ruled out. Intensive studies and experiments with the Searle principle showed us that this direction was not viable. So we tried to create a miniaturized version following the Couette principle. Thereby, the frictionless bearing of the inner cylinder is one of the main problems. In the Couette viscometer, the outer cylinder rotates faster relative to the inner cylinder (rotor) whereby the difference in radius makes up the measuring gap. A thought experiment showed that the rotor must centre in the axis of the outer cylinder so that its density is lower than the density of the liquid to be measured. Just like air bubbles concentrate in the axis of a rotating bottle. Thus the problem of frictionless bearing was solved too. A magnetic field that slows down the rotor in a predefined way without touching it completes this device. A simple formalism enables you to calculate viscosity from the speed ratio of the outer cylinder and the rotor. By digitally measuring the speeds in a highly accurate way, it is possible to capture the measuring value precisely.

Even though we already had a lot of experience in thermostatting and temperature measurement we had gained when building density meters, everything else that had to do with viscosimetry was new territory for us. The production of an extremely light rotor went beyond the boundaries of what we were able to realise. Working over a period of several months, we had to develop among other things a production method for precise titanium tubes with wafer-thin walls (0.05mm), bearings for the outer cylinder, miniaturised slide ring seals, and a special powertrain by means of a synchronous motor. Ultimately, we successfully built a stable measuring device. In parallel, we studied the classic method of capillary viscosimetry according to Ubbelohde in order to have opportunities to compare. That our product is successfully sold worldwide by our licensees shows that we have done everything right with our development. Today, almost 14 years later, we master more or less the entire range of problems in viscosimetry; we have spotted the flaws of the Stabinger Viscometer, and for the most part fixed them. The short measuring time achieved with this viscosimeter principle, the extremely large measuring range, and the high resolution facilitate the successful handling of problems that have hardly been solved so far.