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Online viscosity measurement
in the food industry
Michael Mies, Endress+Hauser Instruments International
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. Today, an
instrument such as the Proline Promass 83I from Endress+Hauser is able
to measure mass and volume as well as the density, together with the
viscosity of a material.
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, an instrument such as the Proline Promass
83I from Endress+Hauser 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. Proline
Promass 83I monitors the process quality employing the viscosity
measurement – as spin-off value in a highly accurate mass and density
measurement.
How is viscosity measured?
PROline Promass 83I from Endress+Hauser is unique in the
generation of measuring signals. With the torsional movement, shear
forces are built up between pipe wall and the material inside of the
pipes. 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 increased or decreased energy expenditure
is an indication of the process viscosity.
Torsion Mode Balance System
The patented TMB process (Torsion Mode Balance System) compensates
the forces within the measuring instrument caused by the exciter
frequency with the pendulum. 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 Proline Promass 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 patented process used by Endress+Hauser 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.
With Proline Promass 83I, this task can be carried out in the same
measuring cycle along with the measurement of temperature, density and
mass flow.<<
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