The definition of viscosity as the relative motion of two parallel plates is quite unambiguous, however when it comes to actually putting this test into practice many problems can arise, such as the effect of the plate edge or theorizing how the plate will actually move in the experiment. Moreover, with non-Newtonian fluids, I am guessing that as well as their particular non-linear characteristic related to the shear rate attributable to the sample material there is also a damping of the shear rate, with these effects having an influence on the measured viscosity. As a result, there will not be a uniform laminar flow in a depth wise direction, and possibly no linear shear rate applied. In other words, even if you logically apply a fixed shear rate to the area filled up with the liquid, depending on the quality of the material of the plate and the sample to be measured, or the condition of the surface of the sample, issues like "slide" or "adhesion" may occur at the interface between the plate and the sample. While it is just a personal opinion, even with rotary type viscometers with a cone-plate that geometrically has a fixed opposing surface (E type), there is no guarantee that the shear rate between opposing surfaces is maintained at a constant value. So from the known viscosity of a set liquid, such as water or a Japanese viscosity standard solution, and the "shear stress" calculated by dividing the drive force of the oscillators required to measure that viscosity by the area of the wetted surface the shear rate is obtained. Also, with the vibration type viscometer there is no precise opposite surface to define shear rate. Therefore, the displacement per unit time is expressed by the root mean square of one cycle. Results for shear rate at that time can be gained for Newtonian fluids within an approximate 10/sec to 1000/sec range.įor a vibration viscometer, similarly to a rotational-vibration rheometer, the shear rate constantly changes. Therefore the oscillation amplitude was made adjustable, rather than the frequency.Īs a result, the maximum displacement between the peaks and troughs of the oscillating sine curve motion forms a range of between 0.07mm–1.2mm, focusing around 0.4mm on the viscometer. The shear rate can be adjusted even by changing the frequency of vibration, but the vibrating tuning-fork type device increases sensitivity through sharp resonance phenomena and is inviting a reduction of sensitivity if the oscillation frequency is changed. With the rheometer developed from the vibration viscometer with adjustable shear rate, the shear rate is varied by changing the amplitude of the oscillating sensor plates. Liquids with low viscosity can often be displaced by the centrifugal force from rotations and problems with repeatability can also occur with rotary rheometers. On the other hand, a lot of energy is required to rotate the rheometer and the state of the sample may be altered from that of before rotation, skewing the measurement. The main features of rotary-type rheometers are a great level of control over the variation width of the shear rate by changing the rotation frequency, and being able to apply the shear rate uniformly to the sample through configuration of the rotor. Presently, almost all rheometers are rotary type devices. In response, we did not change the natural frequency of the 30Hz oscillators, but instead set about changing the oscillation amplitude by developing a new rheometer: our new RV-10000 model. When our viscometer was first released for sale eight years ago, we received many requests from customers who wanted to know the value of the "shear rate", or be able to adjust it in their measurements. As I have previously written about tuning-fork vibration viscometers, they were incorporated two years ago as one of the standards in measuring viscosity in the first such revision to Japanese industrial standards in 19 years (JISZ8803), and have also already been accredited as a viscometer subject to calibration by the Japan Calibration Service System (JCSS). Our tuning-fork vibration rheometer was first introduced to the market in September this year at the Japan Analytical Scientific Instruments Show (JASIS) 2012.
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