VEGA VEGAPULS 43 4 … 20 mA; HART compact sensor - Manual (Page 10)

10 VEGAPULS 43 – 4 … 20 mA
26626-EN-041227
2.2 Antenna
The antenna is the eye of the radar sensor.
The shape of the antenna, however, doesn’t
give a casual observer the slightest clue on
how carefully the antenna geometry must be
adapted to the physical properties of electro-
magnetic waves. The hygienic VEGAPULS 43
radar sensors are equipped with an antenna
that can be cleaned as easily as a smooth
vessel wall. The previously used horn and
rod antennas are gone. Only a small cone-
shaped bulge protrudes into the process
vessel. The small cone acts like a lens that
focuses the radar signals into a high-fre-
quency beam. The relative dielectric constant
of the small 140° PTFE cone represents the
calculation index of the lens. The visible part
of the antenna (small cone), however, does
not give a clue as to how precisely the geo-
metrical form of the antenna has to be
adapted to the physical properties of electro-
magnetic waves. The shape governs the
focusing of the waves and hence the sensi-
tivity, just as shape governs the sensitivity of
a unidirectional microphone. The production
of such an electromagnetic lens requires
much empirical knowledge in the areas of
high-frequency physics and materials sci-
ence.
Types and versions
Hygienic design
Beside the aforementioned geometry neces-
sary for antennas used in the food and phar-
maceutical industry, the choice of materials
for the newly developed VEGAPULS 43 sen-
sors is critical for cleaning and sterilisation.
Fully automatic cleaning (CIP) and sterilisa-
tion (SIP) of entire production facilities (with-
out disrupting production or having to
dismantle and disassemble parts of the
equipment) is, in practice, not an easy task.
Dirt and contaminants get trapped mechani-
cally in pores, fissures, scratches and re-
cesses, and even remain on smooth walls
due to electrostatic attraction.
PTFE is commonly found in hygienic applica-
tions. The small plastic cone of the sterile,
pharmaceutical VEGAPULS 43 radar sensor,
which is at the same time antenna and proc-
ess seal, consists of a TFM-PTFE material.
This is a fluorothermoplastic which has addi-
tional distinct advantages compared to PTFE,
such as e.g., reduced load deformation,
denser polymer structure as well as
smoother surface (Ra < 0.8 µm). The other
known advantages of PTFE, such as, e.g.,
higher temperature resistance (< 200°C),
high chemical resistance as well as resist-
ance to brittleness and ageing are still
present or have even been enhanced.
Perfluorelastomers and fluorthermoplasts are
resistant to virtually all chemical media, such
as e.g., amines, ketones, esters, acids (sul-
phuric acid, phosphoric acid, hydrochloric
acid, nitric acid), alkalis (caustic soda), oxi-
dants, fuels and oils. Beside their use in the
chemical industry, these materials are being
applied more and more in sterilisation and
pharmaceutical technologies. The only limits
to these materials are in applications with
fluorine under high pressure or with liquid
alkali metals (sodium or potassium), where
explosive reactions may occur.