Emerson Rosemount 3308, Reference Manual

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Reference Manual
00809-XXXX-4811, Rev AA
February 2012
2-7
Rosemount 3308
PRELIMINARY
Interface
Rosemount 3308 is the ideal choice for measuring the interface of oil and
water, or other liquids with significant dielectric differences.
Figure 2-3. Interface
measurement with a
Rosemount 3308
All probes can be used for measuring interfaces. The coaxial probe is the
preferred choice for clean liquids and when the bridle is not fully immersed. In
applications with a fully immersed probe, the twin lead probes are
recommended for nozzle installations, and the rigid single lead probe is best
for bridle mounting.
For measuring the interface level, the transmitter uses the residual wave of
the first reflection. Part of the wave, which was not reflected at the upper
product surface, continues until it is reflected at the lower product surface.
The speed of this wave depends fully on the dielectric constant of the upper
product.
If interface is to be measured, the following criteria have to be fulfilled:
• The dielectric constant of the upper product must be known. The Radar
Configuration Tools software has a built-in dielectric constant calculator
to assist users in determining the dielectric constant of the upper
product.
• The dielectric constant of the upper product must have a lower
dielectric constant than the lower product in order to have a distinct
reflection.
• The difference between the dielectric constants for the two products
must be larger than 10.
• Maximum dielectric constant for the upper product is 10 for the coaxial
probe and 5 for twin lead probes.
• The upper product thickness must be larger than 8 inches (0.2 m) for
the flexible twin lead probe and 4 inches (0.1 m) for the rigid twin lead
and coaxial probes in order to distinguish the echoes of the two liquids.
The maximum allowable upper product thickness/measuring range is
primarily determined by the dielectric constants of the two liquids.
Target applications include interfaces between oil/oil-like and water/water-like
liquids. For such applications the upper product dielectric constant is low (<3)
and the lower product dielectric constant is high (>20), and the maximum
measuring range is only limited by the length of the coaxial and rigid twin lead
probes.
Level
Interface Level
Level = Interface Level