Technical Manual
Issue: A
Revision: 1
Date:21/11/16
Ref:100246
11
3. Principles of Operation
When a particle moving in a duct collides with (or passes near) a probe that is grounded to earth, a
transfer of charge takes place from the particle to the conductor. This is known as tribo-electricity or
frictional electrification and results in a charge being imparted to the probe.
It can be shown that the following empirical relationship exists:
Where
I
=
frictional current in Amps
K
a
=
a material constant
M
=
mass flow rate of particles
V
=
particle velocity
b
=
a constant derived from whether the particle
collision was elastic or plastic (a value typically between 1.4 and 1.9)
d
=
particle diameter.
It can be seen that the resultant current is dependent on a number of factors such as the nature of the
material (
K
a
), mass flow (
M
) and velocity (
V
) and any significant changes in these would result in a
change in current output.
In practice, the material will often be of a ‘monotype’ with a particle size distribution that will vary little
given the characteristics of the dust collection method. Bag filters, cyclones and electrostatic
precipitators constantly produce a small leakage of particles that are all approximately the same size,
and if the velocity of the particles remains fairly constant (say
this has a very small effect on
the frictional charge
So it can be seen that although tribo-electric probes are affected by changes in velocity and particle
size distribution, in practice, the amount of drift this induces may be very small and inconsequential
for indicative systems. If your application is subject to massive swings in velocity, consult Codel for
advice.
The main advantages of tribo-electric instruments are as follows:
They are
extremely sensitive to fine dusts
and are sensitive over a range of
<1mg/m
3
up to
2500mg/m
3
.
They are
able to detect particulates
in a size range of
0.1 to
m
.
They are
easy to install
and
do not require critical alignment.
They have
no optical surfaces
to keep clean.
Energy Tech 301
is
easy to remove and clean
and has
very low maintenance
requirements.
There are
no moving parts
and consequently
servicing requirements are very low.
d
V
M
K
I
b
a
.
.
