11.4 Function And System Design - Endress+Hauser Fieldbus levelflex M FMP 40 Operating Instructions Manual

11 Appendix
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11.4 Function and system design

11.4.1 Measuring principle
The Levelflex is a "downward-looking" measuring system that functions according to the
ToF method (ToF = Time of Flight). The distance from the reference point (process
connection of the measuring device see page 12) to the product surface is measured.
High-frequency pulses are injected to a probe and led along the probe. The pulses are
reflected by the product surface, received by the electronic evaluation unit and
converted into level information.
This method is also known as TDR (Time Domain Reflectometry).
probe length LN
Input
The reflected pulses are transmitted from the probe to the electronics. There, a
microprocessor analyses the signals and identifies the level echo, which was generated
by the reflection of the high-frequency pulses at the product surface. This clear signal
finding benefits from the many years experience with pulse time-of-flight procedures
that have been integrated into the development of the PulseMaster® Software.
The distance D to the product surface is proportional to the time of flight t of the impulse:
D = c · t/2,
with c being the speed of light.
Based on the known empty distance E, the level L is calculated:
L = E – D
Reference point for "E" see above diagram, Details see page 57.
The Levelflex possesses functions for the interference echo suppression that can be
activated by the user. They guarantee that interference echoes from e.g. internals and
struts are not interpreted as level echoes.
Levelflex M FMP 40 with Foundation Fieldbus
flange:
reference point of
measurement
D
F
L
20 mA
100%
E
4 mA
0%
Endress+Hauser