Measuring Resistivity

Particle resistivity is determined by measuring the leakage current through a dust layer
to which a high voltage is applied using conductivity cells. A number of conductivity
cells have been used in particle-resistivity measurements. For a good review of the
different kinds of cells employed, see White (1974). Resistivity can be measured by a
number of methods in either the laboratory or the field. In the lab method, dust samples
are first extracted from the flue gas leaving the industrial process and collected on
a filter as described in EPA Reference Method 5. The samples are then taken back to
the laboratory and analyzed.
Resistivity measurements are made in the field using an in-situ resistivity probe. The
probe is inserted into the duct leaving the industrial process and a dust sample is
extracted into the probe. High voltage is applied across a point and plate electrode system
inside the probe. Particles are charged and then collected on the plate. After a sufficiently
thick layer of dust has collected on the plate, the power to the point is turned
off and a disc is lowered onto the collected dust sample. The thickness of the dust
layer is first measured. Increasing voltages are then applied to the disc, and the corresponding
current is recorded until the dust layer breaks down and sparkover occurs.
The resistivity is calculated from the last set of voltage and current readings obtained
before sparkover occurs. Since these resistivity measurements are made at the industrial
process conditions, these data are generally more useful than data obtained from
the laboratory methods. A good review of in-situ resistivity measuring techniques is
given by White (1974) and Gallaer (1983).