Appendix A: Technical reference
A.2 Echo processing
SITRANS LR250 with FOUNDATION FIELDBUS
Operating Instructions, 09/2021, A5E32221411-AH
235
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, and reliability,
amongst other characteristics.
Algorithm (2.5.7.1.)
The true echo is selected based on the setting for the Echo selection algorithm. Options are
t
rue
F
irst Echo,
L
argest Echo, or
b
est of
F
irst and
L
argest.
Position Detect (2.5.7.2.)
The echo position detection algorithm 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 [see
Propagation Factor (2.5.3.)
for values]. There are three options:
•
Center
•
Hybrid
(Center and CLEF)
•
CLEF
(Constrained Leading Edge Fit)
Center
Uses center of the echo.
Hybrid
Uses the Center algorithm for the top part of the vessel, and the CLEF algorithm for the part
nearest the vessel bottom, according to the setting for
CLEF range
.
CLEF (Constrained Leading Edge Fit)
•
Uses the leading edge of the echo.
•
Is used mainly to process the echo from materials with a low dK value.
In an almost empty flat-bottomed vessel, a low dK material may reflect an echo weaker than
the echo from the vessel bottom. The echo profile shows these echoes merging. The device
may then report a material level equal to or lower than empty.
The CLEF algorithm enables the device to report the level correctly.
Example: CLEF off: Position set to Hybrid
Vessel height: 1.5 m; CLEF range set to 0 (Center algorithm gives the same result.)