KROHNE BM 100A Handbook - Manual (Page 59)

Handbook BM 100 A 59
7 Measuring Principle
The BM100 A has been developed from a tried and tested technology called “Time Domain
Reflectrometry” (T.D.R.). Other modern applications include checking for and locating damage along
telecommunication cable lines.
The BM100 A sends low-power electromagnetic pulses of one nanosecond width along a rod or
cable conductor. This pulse travels at a known speed : the speed of light. Upon reaching the surface
of the product to be measured, the pulses are reflected back with an intensity that is dependent on
the dielectric constant,
ε
r
, of the product.
A product’s dielectric constant, ε
r
, is an electrical property. The strength of pulse reflection from the
surface of the product being measured is registered by the instrument as a signal amplitude in volts.
The higher the dielectric constant,
ε
r
, the stronger the reflection: e.g. up to 80% of the pulse strength
is reflected from the surface of water.
The instrument measures the time between the emission and the reception of the signal : half of this
time corresponds to the distance from the instrument reference point (the flange facing) to the
product surface. This value of time is converted into an analogue output current between 3.8 and
20.5 mA or a digital signal corresponding to a calibrated distance which can then be displayed
digitally in a variety of forms from a list of choices available in the gauge’s user menu.
Readings taken using this technology have the advantage of being uninfluenced by dust, foam,
vapour and agitated and boiling surfaces. Pressure, temperature and density variations also have no
affect.
When I < 3.6 mA : the current loop is open
I = 3.6 mA : error status (displayed when fct. 1.3.2 is set to “4-20 mA, E=3.6mA”)
3.8 ≤ I ≤ 20.5 mA : measuring range
I = 22 mA : error status (displayed when fct. 1.3.2 is set to “4-20 mA, E=22mA”)