KROHNE BM 100A Handbook - Manual (Page 60)

60 Handbook BM 100 A
The BM100 A can determine level in two ways, depending on the dielectric constant of the product:
Direct mode: Level and / or interface measurement, for products where the
dielectric constant is above1.4*.
Direct mode: Inverted interface measurement is dealt with in the Service Manual.
Tank bottom following (TBF) mode: Level measurement, for products with low
dielectric constants (below 2.1) TBF level measurement is explained in the Service
Manual.
* This is the minimum value for a BM100A with a coaxial probe. This value will vary slightly
depending on the probe type used.
7.1 Direct mode
Level and Interface Measurement
1. Flange reflection
2. Level measurement
3. Interface measurement
Level measurement of one product
The pulse is emitted by the instrument and guided along the probe. It reflects off the first product
surface it meets and returns to the instrument.
The distance from the gauge flange to the product is proportional to the time taken:
Distance = c
0
. time taken
2 where c
0
is the speed of light in air.
Level is determined by subtracting the distance to the product from the tank height. Note that the
instrument is normally delivered with level being measured from the end of the probe, where the
tank is taken to be empty once the product falls below this point.