By William Klein, K & S Labs
Why electrode configuration is important
to the electrical testing of materials
Concentric electrodes may not be used
for determining resistances of materials which are not homogeneous
and for those with directionality in their surface conductivity
ASTM D257, AATCC 76 and ESD S11.11 may
be used to characterize materials if the technologist understands
the material being tested and its intended application
False assumptions are implicit
in most simple meters and test methods purporting to measure surface
resistivity directly. They employ electrode configurations
valid only for special cases. Surface resistivity measurements
assume a desire to know the electrical resistance from point to
point on the surface. This can be measured directly by the
use of a pair of electrodes, either circular or rectangular, on
the surface without regard to the path of flow. This is
the basis of NFPA-type resistance measurements for floors adopted
by both ASTM and UL, and for both working surfaces and walking
surfaces by the ESD Association. It is necessary to specify
electrode size, spacing, and voltage.
The disadvantage of this method
is that it does not measure an inherent property of the material,
but only of the particular test specimen in the configuration
employed. The advantage is that it usually yields desired
results without ascribing false properties which may lead to serious
The use of only concentric
electrodes does not allow valid characterization of materials
if the material has any non-uniformity in its conductivity either
in surface or volume directionality. These electrodes may
mask serious continuity problems.
ASTM D257: Most cited test
method for surface or volume resistivity, is more of a tutorial
than a method, is specified for insulating materials only, has
broad applicability and gives wide options on methodology both
in electrode configuration and applied voltage, based on intended
AATCC 76: Used by the textile
industry as a simple, direct method to measure the surface resistive
properties of fabrics. The results are meaningful for thin,
flexible, relatively homogeneous materials with conductive ability
on or near the surface; and for such materials having directional
differences in surface conductance, such as with textile materials.
The method is a special case of a surface resistivity measurement
from D257 in which parallel electrodes or concentric rings are
used, with sizes and spacing appropriate to the application, and
no guard electrode employed.
ESD S11.11: Used for electrostatic
control applications of planar materials. The method is
a specific case of ASTM D257 with the electrode configuration,
weight and applied voltage fixed in an attempt to reduce lab-to-lab
variations. The electrodes are an inner disk surrounded
by a concentric ring in a fixed three-dimensional relationship.
The applied voltage is 100 volts for materials in the dissipative
ASTM D257 users usually do not read it fully before
they use it or cite it. It is particularly important to note that
"surface resistivity" is often not definable or measurable,
even in insulative materials of the sort specifically dealt with
in D257. Surface resistivity does not exist as a basic
material property in most of the materials of interest in ESD
protection. This is because surfaces do not usually have
electrical properties which are distinct from bulk properties
and the flow of current from point to point over a surface cannot
be described as a surface phenomenon only.
Figure 1 Parallel Electrodes
of the square, thus the term "ohms/square".
Although dimensionally the same, "ohms/square"
differentiates surface resistivity measurements from simple resistance
Figure 2 Concentric Ring Electrodes
The concept of surface resistivity is unnecessary
and often erroneous in materials used for ESD control. If
the material is thin, homogeneous and volume conductive as shown
in Fig.1, then the apparent surface resistivity is equal
to the true volume resistivity divided by the thickness.
Figure 3 Conductive Backing
A is the contact area of each electrode.
If L is much greater than t and the
can be approximately shown as in Figure 3b. It is actually
volume resistivity which is being measured and surface resistivity,
in fact a nonexistent material property, acquires its apparent
value depending on volume resistivity and specimen geometry. Note
that R is independent of electrode spacing.
In materials which
are surface treated to be conductive and the great bulk of which
is much more resistive, the concept of surface resistivity makes
sense, although it is usually unnecessary. This is indicated
in Figure 4.
Figure 4 Conductive Surface Layer
If a material with
directional differences in conductivity, such as woven fabric
or some fiber-loaded materials, is being measured, then the property
of resistivity may not be defined or measured. The concentric
electrode option is not valid if there is any appreciable axial
or directional non-uniformity. As shown below, concentric
electrodes mask serious continuity problems while appropriate
use of parallel electrodes allows material characterization.
Figure 5 Directional Conductivity
Figure 6 Random Fibers