7ML19985JB02 SITRANS Probe LU (PROFIBUS PA) – INSTRUCTION MANUAL Page 37
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A: Technical Reference
Failsafe value
Failsafe value is a user-defined value. This allows you to enter the safest output value for
your application.
For example, you may want to select a user-defined high value to prevent an overfill
(Failsafe Hi). Or you may want to select a low value to protect a pump from operating dry
(Failsafe Lo).
Near Range (Blanking)
The crystal which produces the transmit pulse has to stop vibrating before it can receive
an echo. Near Range is the space in front of the transducer face where level
measurement is not possible, because within that distance, the echo would return before
the vibration had ceased.
The Near Range parameter allows you to set the distance in front of the transducer,
within which any echoes will be ignored
1
.
False Echoes
False echoes can appear during the receive cycle. They are often created by obstructions
within the beam path, such as a ladder rung, and are usually indicated by an incorrect
high level reading. Auto False-Echo Suppression is recommended.
Auto False-Echo Suppression
The TVT adjustment parameters allow you to set a TVT (Time Varying Threshold) curve, so
that SITRANS Probe LU will ignore false echoes.
The default TVT curve hovers above the echo profile, and effectively screens out small
false echoes. But if an obstruction is causing a large echo before the material level echo,
that echo will rise above the default TVT curve. You can use Auto False-Echo Suppression
to filter it out. If possible, rotate the instrument before using Auto False-Echo
Suppression, to lower the amplitude of false echoes.
1.
The reference point for measuring Near Range is the transducer face. The minimum recom-
mended Near Range value is 0.25 m (10"), but this can be increased.
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