UFC 3-570-06
JANUARY 31 2003
Figure 7-2. IR Drop Error
V
(+)
(-)
REFERENCE ELECTRODE
RESISTANCE
CURRENT FLOW
CURRENT FLOW
7-2.3
Anode Gradient Error. The voltage gradient of the anode causes an error
when the anode is connected in the circuit during testing (current is on). This error is
greater when the voltage at the anode is higher and when the distance of the reference
electrode to the anode is shorter. The cathodic protection circuit resistance, the number
of anodes, and the electrolyte resistivity, affect the anode gradient field size. The anode
gradient is larger when the circuit resistance is higher. The causes of high circuit
resistance are high soil resistivity, low number of anodes, and anodes being spaced too
close together. Placement of the half-cell is a major factor in determining the true
potential of the structure. In an impressed current system, if the anodes are not truly in
remote earth, there is a mixed potential reading of the structure being tested and the
anode potential when taking a potential measurement with the anode in the circuit. An
instant-OFF potential measurement will remove any possibility of this error. This error is
in the negative direction (for example, with the error, you may measure -0.85 volts DC,
and after correction for the error you may actually have -0.75 volts DC.)
7-4
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