User's Manual

Appendix A: Technical reference
A.2 Echo processing
SITRANS LR250 (PROFIBUS PA)
210 Operating Instructions, 01/2014, A5E32221386-AB
A.2.9
Damping
Filter Time Constant (2.6.8.1)
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
.
If two faults are present at the same time, the fault code, error text, and error icon for each fault
are displayed alternately. For example, Loss of Echo and faulty power supply:
Upon receiving a reliable echo, the loss of echo condition is aborted, the Service Required icon
and error message are cleared, and the reading returns to the current level.
A.2.10.1
LOE Timer
LOE Timer (2.3.6.)
determines the length of time a Loss of Echo (LOE) condition will persist
before a Fail-safe state is activated. The default is 100 seconds. Fail-safe Mode determines the
level to be reported when the Fail-safe timer expires.