Honeywell SLG 700 Option Manual page 157

Field Background:
The physical components used to mount the transmitter to the tank will always cause some reflection
of the radar pulse as the pulse leaves the confines of the process connector and starts to travel through
the medium in the region near the reference plane. Depending on the configuration, these reflections
may appear very similar to the reflections from the products in the tank and therefore should be
ignored. In addition obstacles present close to the wave guide can cause reflections that mimic levels.
The SLG 700 transmitter utilizes a means of subtracting out these static background reflections before
processing the data for reflections from the product(s).
All transmitters have pre-configured backgrounds for standard probe configurations which can be
selected with the Factory option. In all but the simplest applications, these should be replaced if
possible in-situ with one of the other two options using the capture mechanism described below.
The Field option 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. Ensure that the level in the tank is below these values when
acquiring the background. The field background is stored in permanent memory.
The Obstacle option is similar to the field background but 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 the tank needs to be about 30cm 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, omitting quiet regions in between.
The active Background Type selection is independent of the capture Background Type described
below. New backgrounds are applied immediately once captured. However, it is not possible to stay in
obstacle mode after collecting a field type background because it is likely that the obstacle background
is significantly longer than the field background just collected. Normal operation is to choose a
background mode, apply, then collect a background in that mode. Backgrounds can be verified on the
Echo Curve display on the Monitor menu.
Use Field Background:
This Parameter provides an option to do different operations like starting a Field background capture
and invoking verification of status of Field background capture.
Field Background Capture Status:
Field background status provides status of Success or Failure.
Field Background Capture Progress:
This will provide in progress status in terms of percentage completion.
Field Background additional Status:
This will provide detailed status of failure due to which field background capture failed
Rev.5 FOUNDATION Fieldbus Option User's Manual Page 141