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Appendix A: Technical reference
A.2 Echo processing
SITRANS LR250 (PROFIBUS PA)
240
Operating Instructions, 08/2014, A5E32221386-AC
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.
A.2.10.2 Fail-safe Behavior
The purpose of the Fail-safe setting is to put the process into a safe mode of operation in the
event of a fault or failure. The value to be reported in the event of a fault is selected so that a
loss of power or loss of signal triggers the same response as an unsafe level.
Fail-safe mode may be triggered by a loss of echo, a bad configuration, or certain device faults.
You can select one of three possible values to be reported when a Fail-safe mode is activated.
Mode
Mode (2.6.9.1.) determines the material level to be reported when LOE Timer (2.3.6.) expires.
Mode (2.6.9.1.)
SUB VALUE Use substitute value. Value (2.6.9.2.) used as output value.
LAST VALUE * Last value (Store last valid output value).
USE BAD VALUE Use bad value (Calculated output value is incorrect).