KROHNE BM 500 EN - Manual (Page 20)

7
TECHNICAL DATA
20
BM 500
www.krohne.com 12/2013 - 4003176301 - MA BM 500 R02 en
7.1 Measuring principle
The liquid is stored in a tank, connected to ground. The immersed measuring rod is a low
resistance rod where the ends are powered by an AC generator operating in the lower kiloherz
frequency range.
Between the rod and the tank wall is an endless amount of high level resistances. Since they
connect to the same potential (the tank wall) they can be shown as two equivalent resistances,
R1 and R2 connected to an imaginary center point. A high resistance input amplifier is connected
between the generator and the tank wall.
Since the generator is supplying a high range current it will create a significant voltage drop
across the low resistance rod. The resistances R1 and R2 form a voltage divider in range of the
immersed part of the rod. Output from this divider will indicate half the level of the liquid. The
amplifier then calculate the actual level of liquid from 0 to 100%.
The formula is:
U
out
= 1/2 x media level (%) x U
gen
Media level (%) = U
out
/U
gen
x 2
The level measurement is insensitive to adhesion.
Figure 7-1: Measuring principle
1 U
out
to amplifier
2 U
gen
3 Medium
4 Tube
5 Tank
CAUTION!
It is very essential that the media conductivity is homogenious. Otherwise R1 will not be equal to
R2 and the output voltage will be influenced.