KROHNE BM 70 M - Manual (Page 51)

BM 70 M 51
8.6.11 Auto tank height
FCT. 3.5.1 AUTO TANKH
With this function the height of the tank can be determined by the BM 70 M itself, provided the
following conditions are met:
•
The tank has been completely drained for at least 2 minutes.
• The tank bottom has good reflectivity (not suitable for dished and tapered bottoms without
reference plate).
• The functions Fct. 3.1.3 (antenna), Fct. 3.1.4 (extension), Fct. 3.1.5 (distance piece) and
Fct. 3.1.6 (stilling well) must previously have been correctly set.
•
If the tank is taller than 30 m, the approximate tank height must previously be set manually
with Fct. 3.1.1, as otherwise the measuring range will not suffice to locate the bottom.
After pressing the → key the display shows the currently measured value for the tank height.
Press key ↵ ; the inquiry "SURE NO" now appears. If you are not sure whether the measured
value coincides with the true tank height, reject the value by pressing key ↵ .
If the measured value seems plausible for the tank height and you wish to store it, change the
option to "SURE YES" by pressing key
↑ , and then press key ↵ .
If the message: NO MEAS.VAL. appears, this means that the quality of measurement is not
adequate (inadequate bottom reflection or too many interferences) for automatic determination of
the tank height. Press key
↵ to abort the operation. In this case you will need to enter the tank
height manually with Fct. 3.1.1.
8.6.12 Empty spectrum
What is an empty spectrum ?
• Fixed and moving internals in the tank (including perhaps the tank wall) produce interference
signals (reflections) that can result in incorrect measurements.
• To enable the BM 70 M to identify and blank out such interference signals, the tank profile
(empty spectrum) needs to be recorded once only prior to (initial) start-up.
• For this purpose, the tank should if possible be completely empty and all moving parts (e.g.
agitators) switched on. If these conditions cannot be met during start-up, the tank profile
(empty spectrum) can also be recorded at a later time. Until it is recorded, however,
measurements may be inaccurate.
• Recording of the tank profile involves 1000 measurements that are carried out by the
BM 70 M and stored in the form of a so-called "empty spectrum".
•
This allows the BM 70 M to discriminate between constant or regularly recurring interference
signals and the actually useful or wanted signals (level). Interference signals are suppressed
and only the useful signal is processed.
• If no appreciable interference from internals is to be expected, recording of the empty
spectrum can also be omitted, because a partial empty spectrum (see below) of the flange
system has been stored by the factory.