User's Manual

Page 104 SITRANS LR260 (PROFIBUS PA)– OPERATING INSTRUCTIONS A5E32337685
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D: Technical Reference
Higher order mathematical techniques and algorithms are used to provide intelligent
processing of microwave reflection profiles. This “knowledge based” technique
produces superior performance and reliability.
Time Varying Threshold (TVT) Curves
A Time Varying Threshold (TVT) curve hovers above the echo profile to screen out
unwanted reflections (false echoes).
In most cases the material echo is the only one which rises above the default TVT curve.
In a vessel with obstructions a large false echo may rise above the default TVT. The Auto
False Echo Suppression feature (see below) can be used to screen it out.
The device characterizes all echoes that rise above the TVT as potential good echoes.
Each peak is assigned a rating based on its strength, area, height above the TVT, amongst
other characteristics.
The true echo is selected based on the setting for the Echo selection algorithm [Algorithm
(2.2.3.1.1.)].
Echo Lock
If the echo selected by Algorithm is within the Echo Lock window, the window is centered
about the echo, which is used to derive the measurement.
• Echo Lock Off:
SITRANS LR260 responds immediately to a new selected echo (within the
restrictions set by the Maximum Fill / Empty Rate), but measurement reliability is
affected.
Echo Position Detection
The echo position algorithm ( 2.2.3.1.3. Position ) determines which point on the echo will
be used to calculate the precise time of flight, and calculates the range using the
calibrated propagation velocity (adjusted by a propagation factor, if necessary).
The options are Center or CLEF (Constrained Leading Edge Fit), Hybrid, or Rising Edge.
CLEF uses the leading edge of the echo. It can be used to compensate for materials with
High Cal. Pt.
= 0
obstruction
at 1.2 m
material level
at 2.9 m
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false
echo
true
echo
TVT
curve
sensor reference point
Low Cal. Pt.