KROHNE BM 102 Handbook - Manual (Page 72)

72 BM 102
6 Measuring principle
6.1 General principle
The BM102 MICROFLEX 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 BM 102 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 a signal strength that is
dependent on the dielectric constant,
εr, of the product (e.g. up to 80% of the pulse strength is
reflected from the surface of water).
A product’s dielectric constant,
εr, is an electrical property. The strength of pulse reflection from
the surface of the product being measured is registered by the instrument as signal amplitude in
volts.
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 4 and
20mA 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, agitated and boiling surfaces. Pressure, temperature and density variations also have no
effect.
The BM 102 determines level using a “direct” measurement method, explained below.