11.11 Function And System Design - Endress+Hauser Fieldbus Levelflex M FMP41C Operating Instructions Manual

Appendix
108

11.11 Function and system design

11.11.1 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 → ä 13) 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
Dielectric constant
The dielectric constant (DK) of the medium has a direct impact on the degree of reflection of the
high-frequency pulses. In the case of large DK values, such as for water or ammonia, there is strong
pulse reflection while, with low DK values, such as for hydrocarbons, weak pulse reflection is
experienced.
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 more than 30
years of experience with pulse time-of-flight procedures that have been integrated into the
development of the PulseMaster
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 chapter "basic setup".
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 FMP41C with FOUNDATION Fieldbus
flange:
reference point of
measurement
D
L
®
Software.
20 mA
100%
E
F
4 mA
0%
L00-FMP41Cxx-15-00-00-en-001
Endress + Hauser