User's Manual

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PL-550 29
OPERATION _
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BLANKING
Near blanking (P800) is used to ignore the zone in front of the antenna where
false echoes (e.g. ladder rung) appear as an echo during the receive cycle. This is
usually indicated by an incorrect high level reading and can be overcome by
increasing the near blanking from its factory set value.
Far end blanking is a feature that ignores the zone below the zero or empty level
where false echoes can appear at levels that interfere with the processing of the
true echo.
In applications where the zero level is above the bottom of the vessel and it is
desired to monitor the zone below the normal zero, range extension (P801) can
be used to extend the range into the far end blanking. Range extension is
entered as a percentage of P006. As range extension reduces the protection
afforded by the far end blanking, it should be used judiciously. Avoid excessive
range extension as this can reduce the measurement’s reliability and accuracy.
Range extension is factory set for 20% of P006. If it is found that false echoes are
appearing ahead of the blanking zone, P006 should be reduced accordingly.
false echo false echo
true echo (level)
0
level
end of
transmit
far end blanking
range extension
(P801) as % P006
empty distance to antenna
P006
Typical receiver signal
P800
near blanking
Typical receiver signal
ran
g
e