Appendix A: Technical reference
A.2 Echo processing
SITRANS LR250 with FOUNDATION FIELDBUS
240
Operating Instructions, 09/2021, A5E32221411-AH
A.2.8
Measurement Range
Near Range (2.5.1.)
Near Range programs SITRANS LR250 to ignore the zone in front of the antenna. The default
blanking distance is 50 mm (1.97") from the end of the antenna.
Near Range allows you to increase the blanking value from its factory default. But
Auto False
Echo Suppression (2.5.10.1.)
is generally recommended in preference to extending the
blanking distance from factory values.
Far Range (2.5.2.)
Far Range can be used in applications where the base of the vessel is conical or parabolic. A
reliable echo may be available below the vessel empty distance, due to an indirect reflection
path.
Increasing Far Range to 30% or 40% can provide stable empty vessel readings.
A.2.9
Damping
Filter Time Constant (2.6.7.1)
smooths out the response to a sudden change in level. This is
an exponential filter and the engineering unit is always in seconds.
In 5 time constants the output rises exponentially: from 63.2% of the change in the first time
constant, to almost 100% of the change by the end of the 5th time constant.
Damping example
time constant = 2 seconds
input (level) change = 2 m
A.2.10
Loss of Echo (LOE)
A loss of echo (LOE) occurs when the calculated measurement is judged to be unreliable
because the echo confidence value has dropped below the echo confidence threshold.
Confidence (2.5.9.1.)
describes the quality of an echo. Higher values represent higher
quality.
Echo Threshold (2.5.7.3.)
defines the minimum confidence value required for an echo to be
accepted as valid and evaluated.
If the LOE condition persists beyond the time limit set in
LOE Timer (2.3.6.)
the LCD displays
the Service Required icon, and the text region displays the fault code
S: 0
and the text
LOE
.