Description Of Functions; Function And System Design - Endress+Hauser Fieldbus Levelflex M FMP45 Operating Instructions Manual

Appendix
!
104
11.2

Description of functions

Note!
A detailed description of the function groups, functions and parameters is given in the
documentation BA245F – "Description of the instrument functions" on the enclosed CD-ROM.
11.3

Function and system design

11.3.1 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.
Levelflex M FMP45 with FOUNDATION Fieldbus
flange:
reference point of
measurement
D
F
L
20 mA
100%
E
4 mA
0%
L00-FMP41Cxx-15-00-00-en-001
Endress + Hauser