Endress+Hauser Levelflex M FMP40, Operating Instructions Manual

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Appendix Levelflex M FMP40 with HART/4...20 mA
84 Hauser
11.2 Description of functions
!
Note!
A detailed description of the function groups, functions and parameters is given in the
documentation BA366F - "Description of Device Functions" on the enclosed CD-ROM.
11.3 Function and system design
The Levelflex is a "downward-looking" measuring system that works on the basis of the time-of-
flight method (ToF). The distance from the reference point (process connection of the measuring
device
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16) to the product surface is measured. High-frequency pulses are propagated to a
probe and led along the probe. The pulses are reflected by the product surface, received by the
electronic evaluation unit and converted to level information. This method is also known as time
domain reflectometry (TDR).
L00-FMP4xxxx-15-00-00-en-002
Fig. 3: Reference point of the measurement, details
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16
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.
20 mA
100%
4 mA
0%
D
probe length LN
L
F
E
flange:
reference point of
measurement