UFC 3-570-06
JANUARY 31 2003
Figure 7-5. Mixed Potential Error
V
REFERENCE ELECTRODE
(+)
(-)
FOREIGNSTRUCTURE
METALLICCONTACT
7-3
PRACTICAL MEASUREMENT OF CATHODIC PROTECTION
POTENTIALS
7-3.1
Test Criteria Selection. The method used for potential testing varies widely
for different types of structures and for the different criteria used for evaluation of the
potentials taken. Sometimes different criteria may be used for different areas on the
same structure. The criteria selected depend mostly on the type of the structure,
isolation/non-isolation of the structure, structure coating type and efficiency, the type of
cathodic protection system, the soil resistivity, the amount of current supplied by the CP
system, and the instrumentation available for testing.
7-3.1.1
Sacrificial Cathodic Protection System. Generally, the criterion for
sacrificial CP systems is -0.85 ON. IR error must be compensated for, usually by
placing the reference electrode as near to the structure as possible (directly over the
pipeline or tank) and as remote as possible from any sacrificial anode. This, combined
with knowledge of the structure coating, soil resistivity, the size and spacing of the
anodes and the anode current, is usually enough to determine the adequacy of the CP
applied to the structure. If in doubt, or when potential readings are questionable,
excavate to allow placement of a temporary reference electrode or permanent reference
electrode as close as practical to the structure to further minimize any possible IR error.
Sacrificial systems are normally used in low soil resistivities (low IR error), on well
coated structures with a low current requirement (low IR error), and because of the very
small driving voltage (under one volt), have a very small amount of current flow (low IR
error). If the dielectric strength of the structure coating is not good, the soil resistivity is
7-7
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