A large number of applications for Hydramotion process viscometers require the continuous on-line measurement of fluids showing a high degree of non-Newtonian or shear-dependent behaviour. To explain how the instruments derive their performance in such applications, it is useful to discuss fluid behaviour first.
10.1 Shear thinning/thickening
Non-Newtonian fluids have a viscosity that is dependent on shear rate. These fluids are loosely classified as “shear-thinning” and “shear thickening”.
“Shear thinning” is used when there is a reduction in viscosity with increasing rate of shear. It is also called “pseudoplasticity” and, when time-dependent, “thixotropy”. Conversely, “shear thickening” is used when there is an increase in viscosity with increasing rate of shear. It is also called “dilatant” and, when time-dependent, “rheopexy”.
Both phenomena characterise fluids which are called “non-Newtonian”. The majority of non-Newtonian fluids are shear thinning. It is inaccurate to talk of a viscosity figure for non-Newtonian fluids — all that can be said is that such fluids have an “apparent viscosity” at a particular shear rate. The complex analysis of such behaviour is called “rheology”.
The traditional problem with measuring the viscosity of non-Newtonians in a process is that any change in the shear rate (possibly caused by a change in flow rate) will alter the viscosity. However, the XL7 exhibits very high repeatability, even in highly non-Newtonian fluids. The explanation for this comes from a closer examination of the classical shear-thinning model fluid.
A shear-thinning fluid actually exhibits two “Newtonian” regions. The first or lower Newtonian region is observed at low shear rates, whilst the second or upper region occurs at high shear rates. The viscosities at which this behaviour occurs are written as 0 and
∞ respectively. It can easily be seen that the region between upper and lower Newtonian regions is highly variable with shear.
10.2 Shear rate
The XL7 operates at very high shear rates, whereas most other commercially available viscometers are of the low-shear type. At high shear rates we can access the second Newtonian region and thus return the high in-process repeatability normally achieved in Newtonian fluids.
10.2.1 Effect on measurement
Naturally, the high shear conditions of the vibrational viscometer result in a lower reading of viscosity for a shear-thinning fluid than that obtained using a low-shear device, but the reading is usually highly repeatable and forms an excellent representation of viscosity at other shear conditions.
10.2.2 Effect on fluid
Apart from the measurement of viscosity of the fluid at different shear rates, it is also important to consider the effect that shear energy can have on the fluid itself. Many fluids are thixotropic and endurance of shear forces over time will cause a decrease in viscosity (e.g. thinning paint by stirring).
Some low-shear devices (including rotational viscometers) can impart sufficient shear energy to alter the effective viscosity of the fluid. The energy involved with the Hydramotion vibrational technique is so low that any such changes would be negligible. However, it is important to know that another device such as a pump or stirring arm in a thixotropic fluid could affect its viscosity. In such a case there is a danger that the alteration of measured viscosity is wrongly interpreted as a characteristic of the transducer, whereas it is actually a feature of the fluid and the shearing energy of the flow. The instrument does, in fact, measure the true change in viscosity.
10.3 Independence of environment
With steady flow devices such as rotational viscometers, the shear rate depends on proximity of surfaces to the sensing element. Therefore, in order to maintain a predictable shear rate, the sensing bob must be enclosed in a defined space, which can present problems of impedance to fluid flow as well as other associated issues such as debris entrapment and the need for a complex installation.
In the case of the vibrational sensor, the shear rate is defined by the “skin depth”, i.e. the depth to which the vibration permeates the fluid (typically <2 mm), not the proximity of vessel walls. The technique allows readings to be made simply by insertion of the sensor element into the fluid, with no additional apparatus, which yields high repeatability between different locations. In other words, the instrument is not conscious of its surroundings, and can be freely relocated to other measurement sites without need for recalibration or any adjustment made for installation.
10.4 Equating on-line and lab results
The relationship between the on-line process measurement and the off-line lab viscometer is often a simple one and, in most cases, the two readings will vary proportionately. Process changes are thus sensitively tracked with high repeatability, even in fluids showing exceptionally high degree of non-Newtonian behaviour such as gels and polymers.