Nace CP 1, Course Manual

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Underground Corrosion Control 3:27
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
meters having input resistances of millions of ohms are used in taking
potential measurements.
Voltage drops are negligible in the measuring circuit under the following
conditions:
•
metallic paths — when lengths are short and/or area cross-sectional to
current flow is large.
•
contact between reference cell and electrolyte — when moist and/or
large area contact is made.
•
connection points — when good metal/metal contacts are made.
•
internal circuit of the meter — when a high-input resistance meter (10
megohms, for example) is used.
•
electrolyte — when resistivity and/or cathodic protection current density
are low.
All the voltage drops in the measuring circuit are controllable except for the
one through the electrolyte. The IR drop through the electrolyte can be
reduced to nearly zero, however, by placing the reference electrode near the
structure; it can be reduced to zero by interrupting the current flow.
In addition to attempting to make the IR drop negligible, another approach
is to determine the magnitude of the IR drop, either through measurement
or calculations, and adjust the measured potential by eliminating the IR
drop.
IR drop will affect the accuracy of the data collected during cathodic
protection testing. This is discussed further in Chapter 4, Field
Measurements.
In the following discussion, the criteria noted are quoted from the
referenced documents. In many cases, other criteria also apply. The 100-
millivolt polarization criterion, for example, is applicable in many cases. It
is imperative that you know the criteria specified for the structures you are
testing. As noted earlier, there is no single, universal criterion for cathodic
protection.