Emerson Rosemount 3308A Reference Manual page 109

Reference Manual
00809-0100-4308, Rev CA
If there are disturbing objects in the tank, the Surface Threshold must be carefully set to avoid locking on
the wrong amplitude peak. In
has locked on a peak above the actual product surface. A disturbance was interpreted as the product
surface, since this was the first amplitude peak closest to device that went above Surface Threshold. The
actual product surface was interpreted as the interface or the probe end.
Figure 6-5. Surface Threshold Is Too Low
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0
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-60
A. Disturbing echo misinterpreted as product surface
B. Actual product surface
By adjusting the Surface Threshold the product surface is properly detected as illustrated in
Figure 6-6. Echo Curve Plot after Surface Threshold Was Adjusted
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80
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0
-20
-40
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A. After Surface Threshold is adjusted the product surface is correctly detected.
Example 2: Interface peak not found
In interface applications where the bottom product has a relatively low dielectric constant (<40), or if the
signal is attenuated in the upper product, the amplitude of the reflected signal from the interface is
relatively low and difficult for the transmitter to detect. In such a case it may be possible to detect the
reflected signal from the interface if the Interface Threshold is adjusted.
Figure 6-7
the interface between the upper and lower products is not detected in this case.
Service and Troubleshooting
PRELIMINARY
Figure 6-5, the Surface Threshold is too low, and as a result the transmitter
B
A
DISTANCE
A
DISTANCE
illustrates a situation where the Interface Threshold is too high. The signal amplitude peak at
Service and Troubleshooting
August 2017
250
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0
Figure
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6-6.
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