48
BE 57-660 Eclipse Model 700 Guided Wave Radar Transmitter
Since the outer tube of the coaxial probe is grounded, there
are no proximity affects and there is no influence from the
nozzle. The only reflections along the length of the probe
are expected. Those being the fiducial (reference signal) and
the return signal from the process.
On the other hand, a single rod probe mounted in the exact
same nozzle will have additional (unwanted) reflections
where the probe enters and exits the nozzle. These reflec-
tions are a result of the impedance changes that occur at
those points:
• The large reflection is due to the impedance developed
between the rod and nozzle ID as compared to the imped-
ance developed between the rod and the tank ID. (The larg-
er the nozzle ID, the smaller the reflection).
One way to eliminate the reflection at the bottom of the
nozzle is to use a continuous stillwell in conjunction with a
caged GWR probe. In doing so, there will be no impedance
changes throughout the length of the probe.
Refer to the premier Model 706 GWR transmitter (I/O
Manual BE 57-606) for additional information on Caged
probes.
Obstructions
Metallic obstructions in the vicinity of a single rod probe
can also affect the performance. If the level reading repeat-
edly locks on to a specific level higher than the actual level,
it may be caused by a metallic obstruction. Obstructions in
the vessel (e.g., pipes, ladders) that are located close to the
probe may cause the instrument to show them as level.
Refer to the Probe Clearance Table for recommended clear-
ance distances. The distances shown in this table can be dra-
matically reduced by utilizing the Echo Rejection feature.
This feature is accessible within the transmitter or with the
ECLIPSE Model 700 DTM. Refer to section 4.3.
NOTE: Use caution when rejecting large positive going signals as the
amplitude of the negative going level signal can be reduced
when passing through them.
Mismatch Depends
on Mounting
Single Rod Probe in a Stillwell
Model 706 Caged Probe
(waveform is similar to that of a coaxial probe)
Obstruction
Nozzles
• 2" Diameter minimum
• Ratio of Diameter:
Length should be >1:1
• Do not use Pipe Reducers (restriction)
Distance
to Probe
Acceptable Objects
<15 cm (6")
Continuous, smooth, parallel conductive surface, for example a metal tank wall;
important that probe does not touch wall
>15 cm (6")
<25mm (1") diameter pipe and beams, ladder rungs
>30 cm (12") <75mm (3") diameter pipe and beams, concrete walls
>46 cm (18") All remaining objects
PROBE CLEARANCE TABLE
Fiducial
Mismatch Depends
on Mounting
Level
Fiducial
No Mismatch
Level
Results in the
same sensitivity as
coaxial probe.
Obstructions
Nozzles
• 2" Diameter minimum
• Ratio of Diameter:
Length should be > 1:1
• Do not use Pipe Reducers (restriction)
