KROHNE OPTIWAVE 7300 C Handbook page 76

Tank bottom following
TBF (Tank Bottom Following) mode is used when an insufficient amount of the RADAR signal is reflected
from the product surface. It utilizes the apparent shift of the tank bottom signal (h becomes hv - see
diagram) resulting from different microwave propagation rates in the air and in the tank product.
This mode uses the principle that while not much of the pulse is reflected, the speed of the signal changes.
This is because the speed of light in a product like oil is lower than the speed of light in a gas such as air.
The time it takes for the signal to reflect back from the tank bottom when the tank is empty is known from
the initial autocalibration. The level may be calculated from the difference between the pulse reflection time
from the tank bottom of the full or partially filled tank versus the reflection time off the empty tank bottom.
The difference in time is proportional to product level.
The tank bottom must be able to reflect back a strong signal. For this reason TBF is best used in flat-
bottomed tanks - tanks with conical bottoms are unsuitable installations.
The dielectric constant of the tank product must be known as exactly as possible. (This parameter can be
determined by a KROHNE service technician from the recording of a tank filling or draining operation with
the user program. The level in the tank should in this case change over the largest possible range.).
When measurements are taken with activated TBF mode, an estimated range for the measured value is
determined on the basis of the shifted tank bottom signal, and inaccuracy of the dielectric constant 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 TBF mode even allows
measurement in tanks without a direct useful signal, provided the tank bottom remains visible to the RADAR
device through the product.
Since the exact position of the tank bottom must be known for this process, it is advisable when using the
TBF to determine the tank height automatically with a completely empty tank, using the empty tank
spectrum quick setup mode.
Note that when measuring with the substitute measured value determined from the tank bottom signal, the
measuring accuracy will decrease by the factor √ε
i.e. the possible deviation will double; if er is only 1.56, accuracy decreases to one-quarter.
When the (Partial) TBF mode is used, the maximum measuring range is approx. 10% smaller, i.e. max. tank
height = 18 m. The measuring mode is selected in the advanced setup menu C.1.13.0
• Partial: when this option has been selected, the TBF mode 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). At higher levels, the
normal measuring method is used (reflection from the product surface). This method is sufficient for
most applications with poorly reflecting products. However, as mentioned before, Product ε
C1.14.0) must be as exact as possible.
• Full: When this option is selected, the TBF mode is active over the total measuring range (tank height).
For tank heights above 16 m, however, measuring accuracy is additionally affected. Product er (function
C1.14.0) must be as exact as possible.
- 1). For an er of 2.25, accuracy decreases by one-half,
r
OPTIWAVE 7300 C
Handbook OPTIWAVE 7300 C
(function
r
76