A common question is how the Hydramotion torsional vibration (resonant) viscometers are different from the older rotational devices. On the surface the technologies may sound similar, but they’re completely different, and in many cases you’ll get different measurements with the same fluid and conditions. That’s another question we also get a lot: why does my Brookfield lab viscometer give me viscosity readings that are so much higher than my Hydramotion inline XL7 (or ViscoJet)? Here are some of the basic answers.
The Old Way: Rotating at Low Speed
Rotational viscometry dates back to the mid-20th century, along with dial telephones and gas-guzzling cars. The most common commercial versions use a motor-driven spindle or bob immersed in the fluid in a sample cup of a specific size. The torque required to turn the spindle in the fluid is a measure of the fluid’s viscosity. The geometry of the spindle and cup and the spindle’s rotational speed must match the fluid’s properties, and the cup and spindle must be changed for different viscosity ranges. Rotational viscometers typically operate at very low shear rates.
New and Improved! Vibrating at High Speed
While vibrational viscometry technology was also conceived many years ago, the resonant torsional vibration method wasn’t perfected until Hydramotion figured it out about 20 years ago. Vibrational viscometers work by creating waves within the liquid, 
and there are many types of waves that can propagate in a liquid, but the specific type is critical. Viscosity is a shear measurement that can only be truly assessed under shear conditions, so the Hydramotion technology uses shear waves. Other types of vibrational waves are avoided since they can behave unpredictably in real-world process environments.
That may sound complex, but in operation these units are simplicity itself. In the most common configuration, the solid stainless steel cylindrical sensor is submerged in the fluid and vibrated at its natural frequency around the long axis. The dissipated energy from the fluid’s resistance is measured electronically and equated to viscosity. All the action takes place near the sensor surface, which means these instruments aren’t affected by pipe or tank sizes, flow, or any of the things that limit other techniques. This brings huge benefits: high accuracy, robustness and virtually no maintenance.
This is done at very high shear rates, which has some key advantages – especially for non-Newtonian fluids, where viscosity is a function of shear rate. One great advantage is the high in-process repeatability, normally achieved only in Newtonian fluids. Another is that true viscosity changes are measured – some low-shear devices (including rotational viscometers) can impart sufficient shear energy to alter the effective viscosity of the fluid. We’ll get back to that in a future post.