UFC 3-570-06
JANUARY 31 2003
Figure 7-3. Anode Gradient Error
RECTIFIER
R
V
REFERENCE ELECTRODE
(-)
(+)
(+)
(-)
7-2.4
Contact Resistance Error. Poor contact of the reference electrode to the
electrolyte results in error. The contact resistance of the half-cell to earth is problematic
under rocky or dry conditions. Apply water on dry ground and/or use a very high
resistance voltmeter. Cathodic protection voltmeters have selectable input resistance
from 1 million to 200 million ohms. Normally, 10 million ohms is the selected scale;
while taking a potential measurement, switch the input resistance to the next higher
selection. If the reading does not change, the contact resistance is insignificant. If the
reading does change, select the next higher input resistance, and continue until the
reading does not change. If the highest selection still changes the reading, add water
and retest. This error is in the positive direction (for example, with the error you may
measure -0.85 volts DC, and after correction for the error you may actually have -0.95
volts DC.) The input resistance of the meter used must be much greater than the
contact resistance to ensure accuracy of the measurement.
7-5
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