UFC 3-570-06
JANUARY 31 2003
CHAPTER 7
TESTING
7-1
POTENTIAL MEASUREMENT. Cathodic protection systems must be tested
to assess system function and troubleshoot inadequate performance. Potential
measurement, based on the theory of measuring an unknown potential by relating it to a
known reference electrode, is the principal test procedure used. A measurement is
taken by connecting the high resistance voltmeter negative lead to the reference
electrode (half cell), and connecting the positive lead to the metal being tested. The
reference electrode must contact the electrolyte that is in contact with the metal being
tested. In soil and freshwater, a copper/copper sulfate reference electrode should be
used; in saltwater, a silver/silver chloride reference electrode must be used. To prevent
erroneous readings, the voltmeter used must have a minimum of 10 million ohms input
resistance under normal conditions; under rocky or very dry conditions, it should have
up to 200 million ohms input resistance.
CAUTION: If the voltmeter has a polarity switch (such as the M.C. Miller Model B3
Series) and a D'Arsonval movement (needle) that deflects only in the positive direction,
select (-). If there is no polarity switch, attach connections backwards (negative lead to
structure and positive lead to the reference cell) to prevent damage to the meter; then
interpret the positive deflection as a negative reading.
7-2
SOURCES OF ERROR. There are five sources of error when taking a
potential measurement of a structure:
The accuracy of the reference electrode
An IR error present when current is flowing
An anode gradient field present when current is flowing
Contact resistance error when the reference electrode is not in good contact
with the electrolyte
Influence of foreign structures (mixed potentials)
7-2.1
Accuracy of the Reference Electrode. To prevent erroneous potential
measurements, the accuracy of the reference electrode (half cell) must be reliable. A
valid (tested) reference electrode must be used to take all potential measurements.
Proper maintenance of the half-cell is essential. If the electrolyte in the half cell is
contaminated, or the metallic electrode is contaminated or oxidized, the potential of the
cell changes.
7-1
Previous Page
