Signal Interferences And Background Echoes; Field And Obstacle Background; Static And Dynamic Backgrounds - Honeywell SLG 700 User Manual
Smartline level transmitter guided wave radar
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2.4 Signal Interferences and background echoes
Interfering reflections can occur near the top and bottom of the probe. These interfering
echoes occur or when the pulse encounters a transition, such as from nozzle to tank, or when
the pulse exits the process connector for a rod or wire probe, or when the pulse is reflected
from the end of the probe. Unwanted reflections can also occur, from deposits on the probe or
from interfering structures such as an inlets, outlets, ladders and so forth, which are
positioned near the probe. If the user suspects deposits on the probe then it should be
inspected and cleaned, if necessary. The top and bottom zones in which these interferences
occur can be configured as blocking distances within which no measurement will occur.
Coaxial probes are less susceptible to these interferences and also have smaller upper
blocking distances. For all probes, the effects of interfering reflections near the process
connector can be reduced by background subtraction.
Release R102 offers two type of background echo acquisition modes and either can be
operated statically or dynamically.
2.4.1 Field and Obstacle background
The field background is meant to reduce the effect of the process connector reflection created
when the radar pulse traverses between two regions of different impedances. The preset
length varies from 1.32m (standard temperature and pressure gauge) to 2.38m (high pressure
high temperature model) from the measurement reference plane (bottom of the process
connector). The user needs to ensure that the level in the tank is below these values when
acquiring the background. The field background is stored in permanent memory and can be
displayed using the Honeywell DTM or DD.
The obstacle suppression background can be used in place of the field background and is
intended to both suppress process connector reflections as well as any false echoes generated
by obstacles in the tank (ladders, pipes, valves) in the vicinity of the probe. There is no limit
on the length that can be specified by the user. As with the field background, the level in tank
needs to be about 20cm below the end of the requested echo. One difference between the
obstacle suppression echo and the field background echo is that the sensor algorithms analyze
this echo and store only those sections of the profile that are found to contain false echoes.
For example if a ladder exists 2m down a tank and a pipe inlet 19m down the tank, the user
should obtain an obstacle echo up to approximately 20m. The sensor will automatically
detect the two objects and permanently store the relevant data.
2.4.2 Static and Dynamic backgrounds
Release R102 introduced automatically updated background profiles. The intent of this
feature is to provide enhanced immunity against measurement conditions. With dynamic
backgrounds on, the sensor periodically schedules automatic updates to the background.
Echoes are only collected if the level is outside of the transition zones and the signal is of
good quality. Data is collected up to approximately 20 cm from the level at the time, if this
distance is within the requested background echo length.
The most recently updated background is also stored in permanent memory and is applied
after a sensor reset if dynamic background is enabled. At all times the sensor maintains a
copy of the original user acquired (static) background echo and will revert to this if the
dynamic background feature is once again disabled. Re-enabling dynamic background at
that point starts the process anew. It is recommended that this feature is turned on in all
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SLG 700 SmartLine Level Transmitter User's Manual
Revision 7
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