Yearbook 2013/2014 - ehedg

European Hygienic Engineering & Design Group
Modern level detection and measurement technologies
Sticky media, foam, changing media and hygienic installation present challenges for fill level
detection and measurement. In order to improve reliability and reduce the downtime of machinery
it is crucial that a level sensor switches off when the tank, vessel or tube is empty, even if the tip
is still covered with the medium. Any medium that produces foam is especially tricky because
an overflow protection has to detect it, whereas a level measurement and an empty detection
should mask the foam. A modern level measurement and detection should also work even if the
medium changes. This article gives a guideline of technologies to solve these challenges, as
well as some information on hygienic mounting.
Daniel Walldorf, Baumer GmbH, Friedberg, Germany, dwalldorf@baumer.com, www.baumer.com
Frequency sweep technology
for level switches
Frequency sweep technology for level switches detects fill
level of a tank, vessel or tube on the basis of the DK value.
As opposed to a classical capacitive sensor, this technology
opens the possibility to distinguish different media (e.g.,
liquid and its foam) and adhesions from sticky media to a
full tank (Figure 1). Clever set-up strategies make it possible
to use the same sensor with the same set up for a large
variety of media. Advanced sensor versions usually make it
possible to do visual set up and to get a measurement output
from the sensor (e.g., for condition monitoring of a tank).
Figure 2. Setup of a sensor for sticky media (chocolate).
Figure 1. The frequency sweep technology opens the possibility to
distinguish different media and adhesions from sticky media.
The working principle involves analysing an inductorcapacitor
(LC) circuit for its resonance frequency where the
medium to detect influences the capacitor. Therefore, the
resonance frequency depends on the medium in front of the
sensor tip. At the resonance frequency, power consumption
is at its minimum.
Hydrostatic level measurement
One might think that measuring level by hydrostatic pressure
is not very modern. Nevertheless, it is with reason that
this measurement approach remains the most popular
technology. The food, beverage and pharmaceutical industry
facilities typically operate machinery using a wide temperature
range from 0°C to 140°C. Therefore, it is critical to use a
pressure sensor with good temperature compensation.
This is typically reached by sensor manufacturers with an
internal temperature measurement and an identification of
temperature compensation for each individual sensor during
the production process. In most data sheets, the temperature
error is indicated either as temperature coefficient or as a
value for a total error band available in the compensated
temperature range.