Automation Industry Association

Why measure viscosity?

Quality is of paramount importance in the making of creme cheese or syrup as poor quality would cause extensive downtimes in the production process. Downtimes in the process have to be avoided. In the dairy industry, products such as creme or fermented milk products, pudding and dessert are wares whose individual characteristics can be destroyed in part or entirely if the flow characteristics of the goods are not known or are interpreted incorrectly. From the viscosity, the producer recognises the products behaviour which is important for process control and decisive for the quality of the product. Viscosity is a measured value which in earlier times was determined offline and only with great effort. These offline measuring instruments determined only the viscosity itself. Today, one is able to measure mass and volume as well as the density, together with the viscosity of a material.

What is viscosity?

The viscosity of a material is based on the changing behaviour between the molecules in a liquid. These molecules are connected with one another in a certain way. As the mobility of the molecules depends on the temperature, the viscosity of the material can decrease substantially with increasing temperatures. This means that a (flowing) movement leads to internal forces i.e. internal friction. Expressed in another way, viscosity is an indication as to how “easily” a material flows.
It is not unproblematic to measure viscosity. Physical data such as temperature, pressure and density will influence the measurement of viscosity. The fact that some materials develop differing viscosities in varying process conditions makes it even more complicated. As opposed to solid matter, a liquid is pliable when subjected to force. When encountering  a shearing force, the solid material, due to the consistency of the material, is able to deform only in a limited way to be subsequently torn apart and destroyed. A liquid can deform infinitely under the influence of shear forces. This demonstrates that the product quality of a liquid material depends to a large degree on the precise  measurement of viscosity in the process. If the viscosity tolerances are under or overstepped, an entire charge can be ruined.

How is viscosity measured?

With the torsional movement, shear forces are built up between pipe wall and the material inside of the pipes (fig. 1). These shear forces are proportional to the energy required for the generation of the measuring signal.
In this process, the energy which brings the measuring pipe into the measuring frequency is measured. As the instruments are calibrated with several materials of diverse density and  viscosity, the exciter as well as the measuring frequencies of the calibration materials are known. This data is stored in the electronics to be used as reference. Example: If the energy for the exciter frequency has to be increased, it means that the material has a higher viscosity. If however, less energy is required in order to build up the measured frequency, the material has a reduced viscosity. This means an increased or decreased energy expenditure is an indication of the process viscosity.
A patented TMB process (Torsion Mode Balance System) compensates the forces within the measuring instrument caused by the exciter frequency with the pendulum (fig. 2). Due to high exciter frequencies (up to 800Hz) it is ensured that there are no oscillation overlaps between measuring instrument and environment (caused by pumps or motors). As a footnote it must be noted that certain unique instruments do not require intake and discharge sections

All measured results such as temperature, density, mass and viscosity are shown online in qualitative as well in finite values, depending on the material, to be communicated to the outputs.

The above process used in the generation of measuring signals ensures that the customer receives an intelligent and cost effective solution without increased costs. Today, the importance of viscosity measurement is still often underestimated. If used at all, it is carried out using extensive and time consuming measuring cycles. This task can be carried out in the same measuring cycle along with the measurement of temperature, density and mass flow.

—    Michael Mies