Loss Of Echo (Loe); Fail-Safe Mode; Distance Calculation - Siemens SITRANS L 7ML5400 Operating Instructions Manual

Technical reference
A.4 Distance Calculation
A.3.2

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.
If the LOE condition persists beyond the time limit set in LOE Timer (2.4.2.) (Page 179) the
LCD displays the Service Required icon, and the text region displays the fault code 0 and the
text LOE.
If two faults are present at the same time, the device status indicator and text for the highest
priority fault will display. For example, if both Loss of Echo and Broken cable faults are
present, the Broken cable fault will display.
A.3.2.1

Fail-safe Mode

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 (as displayed in
Current Output Value (2.5.8.) (Page 183)) is selected so that a loss of power or loss of signal
triggers the same response as an unsafe level.
LOE Timer (2.4.2.) (Page 179) determines the length of time a Loss of Echo (LOE) condition
will persist before a Fail-safe state is activated. The default setting is 100 seconds.
Material Level (2.4.1.) (Page 179) determines the mA value (corresponding to the selected
PV) to be reported when LOE Timer (2.4.2.) (Page 179) expires. The default setting is device
dependent (standard or NAMUR NE 43-compliant).
Upon receiving a reliable echo, the loss of echo condition is aborted, the Maintenance
Required icon and error message are cleared, and the mA output return to the current
material level. [The PV region on the LUI display will show dashes (– – – – – – –) when a fault
that causes fail-safe is present, and will return to the current reading when the fault is
cleared.]
A.4

Distance Calculation

To calculate the transducer to material level (object) distance, the transmission medium
(atmosphere) Sound Velocity (2.12.1.1.) (Page 222) is multiplied by the acoustic transmission
to reception time period. This result is divided by 2 to calculate the one way distance.
Distance = Sound Velocity x Time / 2
The Reading displayed is the result of performing any additional modification to the
calculated distance as determined by:
• Sensor Offset (2.2.3.) (Page 176),
• Units (2.1.1.) (Page 172),
• Volume conversion parameters - Volume (2.6.) (Page 184), Sensor Offset (2.2.3.)
(Page 176),
• Flow parameters - Flow (2.15.) (Page 234),
• and/or Totalizer parameters - Totalizers (2.16.) (Page 242).
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Operating Instructions, 04/2021, A5E33329501-AG
SITRANS LUT400