BM 70 M
55
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 M 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 utilises 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 determined 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, accuracy 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 M: when the (PARTIAL) 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.
Summary of Contents for BM 70 M Series
Page 75: ...BM 70 M 75 10 Certificates and approvals 10 1 Declaration of Conformity...
Page 83: ...BM 70 M 83 Annex C Spare parts...
Page 84: ...84 BM 70 M...
Page 85: ...BM 70 M 85...
Page 102: ...16 BM 70M EEx BM 70Mi EEx 11 Approvals...
Page 103: ...BM 70M EEx BM 70Mi EEx 17...
Page 104: ...18 BM 70M EEx BM 70Mi EEx...
Page 105: ...BM 70M EEx BM 70Mi EEx 19...
Page 106: ...20 BM 70M EEx BM 70Mi EEx...
Page 107: ...BM 70M EEx BM 70Mi EEx 21...
Page 108: ...22 BM 70M EEx BM 70Mi EEx...
