Electronic level sensor
LW2720
9
5 Function
5.1 Measuring principle
The measuring principle of the unit is Frequency Modulated Continuous Wave (FMCW). The unit
continuously emits electromagnetic waves in the GHz range with a constantly varying frequency
towards the medium surface. Since the transmitter continuously changes the frequency of the
transmitted signal, there will be a difference in frequency between the transmitted and the reflected
signals (
Ò
Fig. Measuring principle). The frequency of the reflected signal is subtracted from the
frequency of the signal transmitted at that moment, resulting in a low frequency signal which is
proportional to the distance to the medium surface. This signal is further processed to obtain fast,
reliable, and highly accurate level measurements.
d
f
in
f
max
f
min
f
out
f
in
f
out
t
f
A
C
D
B
Fig. 1: Measuring principle
Δf ~ d =
distance
A:
frequency (GHz)
B:
time (s)
C:
transmitted signal
D:
reflected signal
Due to the 80 GHz technology used, the unit generates a very focused signal beam (beam angle
approx. 10°). This extends the range of possible applications and enables installation even in small
tanks and confined spaces (e.g. in tanks with agitators, heating coils, etc.).
5.1.1 Reflectivity, effect of dielectric constant
The reflectivity of the medium has a substantial effect on the reflected signal (echo amplitude). The
reflectivity is directly dependent on the dielectric constant of the medium. A high dielectric constant
provides better reflectivity, thus improving the performance of the level measurement. This also
indirectly affects the maximum achievable level measuring range. Minimum dielectric constant:
Ò
Data sheet
Dielectric constant (also: relative permittivity) of some media
Air (reference quantity)
1
Water
≈ 80
Salt water
≈ 32
W
ater, demineralised
≈ 30
Beer brew
≈ 25
Vinegar
≈ 24
Maize starch
≈ 18
Coarse soya flour, 19% moisture
≈ 18
Coarse soya flour, dry (application test recommended)
≈ 3
Vegetable oil (application test recommended)
≈ 2…4
Grist (application test recommended)
≈ 2…3
Malt (application test recommended)
≈ 2…3
Skimmed milk powder (application test recommended)
≈ 2
Palm oil (not detected reliably)
≈ 1.8
Coffee beans (not detected reliably)
≈ 1.5
GB
