Endress+Hauser Levelflex FMP50 PROFIBUS PA Operating Instructions Manual page 36

Mounting
36
Plastic or glass tanks: Mounting the probe externally at the wall
1 Plastic or glass tank
2 Metall sheet with threaded sleeve
3 No free space between tank wall and probe!
Requirements
• The dielectric constant of the medium must be at least DC > 7.
• The tank wall must be non-conductvie.
• Maximum wall thickness (a):
– Plastic: < 15 mm (0.6")
– Glass: < 10 mm (0.4")
• There may be no metallic reinforcements fixed to the tank.
Mounting conditions:
• The probe must be mounted directly to the tank wall (no open space)
• A plastic half pipe with a diameter of approx. 200 mm (8"), or some other protective
unit, must be affixed externally to the probe to prevent any influences on the
measurement.
• If the tank diameter is less than 300 mm (12"):
A metallic grounding sheet must be installed at the opposite side of the tank. The sheet
must be conductively connected to the process connection and cover about the half of
the vessel' s circumference.
• If the tank diameter exceeds 300 mm (12"):
A metal sheet with a diameter of at least 200 mm (8") must be mounted to the probe at
the process connection. Its orientation must be perpendicular to the probe (see above).
Calibration for external probe mounting
If the probe is mounted externally at the wall of the tank, the speed of signal propagation
will be reduced. There are two possibilities to compensate for this effect.
Compensation with the gas phase compensation factor
The effect of the dielectric wall can be compared to the effect of a dielectric gas phase.
Thus it can be compensated for in the same manner. The compensation factor if given by
the quotient of the actual probe length LN and the probe length meausred when the tank
is empty.
The device looks for the end of probe signal in the subtracted curve. Thus, the value of
the measured probe length depends on the mapping. In order to obtain an exact
value, it is advisable to determine the probe length manually using the envelope curve
display in FieldCare.
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1
a
Levelflex FMP50 PROFIBUS PA
3
Endress+Hauser
A0014150