3 Structure and function
Dr. Födisch
Umweltmesstechnik
AG
Page 12 of 30
Operation manual PFM 92 C
Version 3.0
3.2 Function
The filter controller PFM 92 C is a highly sensitive system for continuous, triboelectric in-
situ filter monitoring. The qualitative monitoring of the exhaust gas is done hereby.
The measuring gas is measured triboelectrically in the exhaust gas flow by means of the
probe rod of the PFM 92 C
(see 3.2.1 Measuring principle Page 12)
.
The signal resulting from the derived current is a degree for the exhaust gas’ dust content.
The micro controller integrated in the control unit produces a dust proportional signal
which is provided as 4 … 20 mA – signal.
3.2.1 Measuring
principle
Probe
Probe rod
Insulator
Fig. 3.2:
Measuring principle
Triboelectricity:
If 2 bodies get into contact with each other by
friction or touch, a charge transfer will occur. The
charge difference arises by the exchange of
electrons between atoms on the surfaces. So a
boundary layer with a positive and negative
surface charge within a very low molecular
distance from each other is formed.
This charge difference, also called charge fluctuation, is the basis for triboelectric dust
meters which use the charge exchange between measuring probe and nearby streaming
or direct impacting dust particles.
The triboelectric signal depends on the mechanical and electric properties of the dusts.
C
i.B.
= dust concentration [mg/m³]
S
= triboelectric measuring signal
.
B
.i
c
~
S
at constant velocity!
