Reference Manual
00809-0100-4024, Rev BA
September 2005
4-13
Rosemount 5600 Series
Figure 4-4. Noise threshold
Customized
Noise
Threshold
Table
(ATP-table)
You can filter out weak disturbing echoes by creating a noise threshold table.
This technique should only be use in special situations, for example at the
bottom of tanks with weak disturbing echoes. In such tanks the transmitter
may lock at disturbances close to the bottom when the tank is empty. Setting
up a noise threshold in the region will guarantee that the transmitter starts
following the surface when the tank is filled again. Make sure the surface
echo amplitude in the bottom region is always stronger than the noise
threshold. (See Figure 4-4)
Also, this function can be used in areas where occasionally there are strong
echoes present. For those large areas registering a False Echo may not be
sufficient.
Furthermore, the ATP-table can be used to remove influence for the tank
nozzle or a still-pipe inlet at the top of the tank. The Hold Off Distance (UNZ)
can also be used to manage such cases.
Do not create noise thresholds around echoes which are already registered
as interfering echoes. The general amplitude threshold is the lower limit of
the noise threshold table. (See Figure 4-4).
Registration of
False Echoes
The False Echo function allows you to let the transmitter register disturbing
echoes caused by objects in the tank. This makes it possible to detect a
product surface close to a disturbance echo even if the surface echo is
weaker than the disturbing echo. (See Figure 4-5).
When should I register?
See the following recommendations before you register new interfering
echoes:
• Make sure that a correct amplitude threshold is set before you register
any disturbance echoes. See description of the SpectraThreshold
window.
• Keep the number of registered echoes to a minimum.
• Compare the list of interfering echoes with the tank drawing or by visible
inspection of the tank. Note if there are objects like beams, heating coils,
agitators, etc. which correspond to the found echoes. Only register
echoes which can be clearly identified as objects in the tank.
• Make sure that the level is stable before you register a disturbance echo.
A fluctuating level may indicate a temporary disturbance which is not due
to an interfering object.
• Do not register a disturbance echo if the amplitude is significantly smaller
than the amplitude of the surface echo when the surface is at the same
level as the disturbance. (In some cases weak disturbance echoes can
be filtered out by creating a noise threshold table.
• It may be necessary to register new disturbance echoes at a later stage
when objects have become visible due to surface movement.
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