Far end blanking is a design function that ignores the zone below the zero or empty
level where false echoes may 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 (P-87) may be used to
extend the range into the far end blanking. Range extension is entered as a percent of
P-3. As range extension reduces the protection afforded by the far end blanking,
it should be used judiciously. Avoid excessive range extension as this may
reduce the measurement’s reliability and accuracy. As range extension is factory
set for 20% of P-3, if it is found that false echoes are appearing ahead of the blanking
zone, P-87 should be reduced accordingly.
Blanking is automatically corrected for sound velocity change where temperature
and/or velocity compensation are used, keeping the blanking at the distance at which
it was entered.
Agitator Discrimination
In applications where there is an agitator operating in the vessel, the blades may
interfere with level readings when the material level is lower than the blades. In such a
case, the agitator discriminator (P-73) can be turned on (factory setting).
With the agitator turned on, the reading will not change unless the echo is closer for at
least 5 consecutive measurements nor will it change unless the echo is farther for at
least 2 consecutive measurements.
This feature allows the HydroRanger I to remain locked on the true echo, even if there
are occasional false echoes due to the agitator blades, electrical noise or crosstalk from
other ultrasonic units.
ringing
true echo
(level)
false echo
typical receiver signal
P-5
near blanking
far end
blanking
typical processed signal
0
level
end of
transmit
range extension (P-87)
as % P-3
empty distance to transducer
P -3
range
PL-399 5 – 4
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