KROHNE BM 70 A_P EN - Manual (Page 52)

Installation and operating instructions BM 70 A/P Page: 52 11/00
Fct 3.5.6 BD-DETECT.
•
NO
•
YES (default)
To activate the block distance (overfill) detector:
So long as a significant signal is detected within the block
distance, the measured value is "held" at the block
distance limit. This avoids any possibility of the measured
value skipping to a multiple reflection should the level rise
into the block distance range.
The block distance can have a value that is at least equal
to that specified in Sect. 8.6.3 as a recommendation!
marker 6 in the display indicates when the block distance
detector is active. If the measured value skips (e.g.
contamination of the antenna) erroneously to the block
distance limit, the BD-DETECT should be deactivated.
8.6.15 Tank bottom tracing system
Description
•
The BM 70 A/P firmware includes an additional
function for measuring in tanks with poorly reflecting
(i.e. relative permittivity
ε
R
↑
3) yet non-absorbing
products. This function is designated "tank bottom
tracing" (abbreviated to FTB) and utilizes the apparent
shift of the tank bottom signal resulting from different
microwave propagation rates in the air and in the tank
product.
•
It is necessary that the tank bottom is reflecting.
Therefore this method is not applicable e.g. in tanks
with a conical bottom.
•
This requires that the relative permittivity
ε
R
of the tank
product be as exact as possible. (This parameter can
also be determined by a KROHNE Service technician
from the recording of a tank filling or draining operation
with the user program (see Sect. 8.12). The level in the
tank should in this case change over the largest
possible range.)
•
When measurements are taken with activated FTB, an
expectancy range for the measured value is deter-
mined on the basis of the shifted tank bottom signal,
and inaccuracy of the
ε
R
value of ± 10% is taken into
consideration. If no evaluable measuring signal is
obtained within this range, a substitute measured value
is calculated direct from the tank bottom signal. This
feature of the FTB even allows measurement in tanks
without a direct useful signal, provided the tank bottom
remains visible for the microwaves through the
product.
•
Since the exact position of the tank bottom must be
known for this process, it is advisable when using the
FTB to determine the tank height automatically with a
completely empty tank, using Fct. 3.5.1, see Sect.
8.6.11.
•
It needs to be borne in mind that, when measuring with
the substitute measured value determined by way of
the tank bottom signal, measuring accuracy will
decrease by the factor (
√ε
R
- 1)! For a
ε
R
of 2.25, accu-
racy decreases by one-half, i.e. the possible deviation
will double; if
ε
R
is only 1.56, accuracy decreases to
one-quarter.
•
With BM 70 P: when the (Partical) tank bottom tracing
system is used, the maximum measuring range is
approx. 10% smaller, i.e. max. tank height = 31.5 m !
The tank bottom tracing system is operated by way of Fct.
3.5.7 and Fct. 3.5.8.
Fct. 3.5.7 FUNCT. FTB
The tank bottom tracing system is activated under this
function:
•
OFF
The FTB is deactivated; the device operates in the
standard mode.
•
PARTIAL
When this option has been selected, the tank bottom
tracing system is activated only in the vicinity of the
tank bottom, i.e. at low levels (max. 20% of the tank
height but a minimum of 0.6 m). Given higher levels,
the normal measuring method is used (reflection from
the product surface). This method is sufficient for most
applications with poorly reflecting products.
The parameter EPSILON R (Fct. 3.5.8) must be
as
exact as possible.
•
FULL ( for BM 70 A only)
When this option is selected, the tank bottom tracing
system is activated over the total measuring range
(tank height). For tank heights above 16 m, however,
measuring accuracy is additionally affected.
The parameter EPSILON R (Fct. 3.5.8) must be as
exact as possible.
Fct. 3.5.8 EPSILON R
Use this function to enter the relative permittivity.
•
Allowable input range: 1.1000 - 8.0000
•
Default: 2.0000
Enter the figure of 2.0000 if the relative permittivity is not
known, as numerous products have a
ε
R
value
≈
2.
However, expect reduced measuring accuracy and/or
skipping of the measured value on changeover from tank
bottom tracing to the normal measuring mode (=
evaluation of reflection from the product surface), see
following diagram:
time
too low
correct
too high
EPSILON R
example of a tank filling
level