3.4.2
Maximum
In order to mitigate disbonding of the interior coating in the tank,
potential between a copper-copper-sulphate reference electrode and the tank
at any point shall not be more negative than minus 1.07 volt measured with
the electrode located between 6 mm 1/4 inch and 13 mm 1/2 inch and away
from the steel surface but not touching it.
3.5
TESTING, ADJUSTING, AND PLACING IN SERVICE
3.5.1
Electrode Potential Measurements
Upon completion of the installation, the tank shall be filled to maximum
working level and with the entire cathodic protection system in operation;
electrode potential measurements shall be made using a copper-copper
sulphate reference electrode and a potentiometer-voltmeter, or a direct
current voltmeter having an input impedance of not less than 10 megohms per
volt and a full scale of 10 volts. The locations of these measurements
shall be identical to the locations used for baseline potentials. The
values obtained and the date, time, and locations of measurements shall be
recorded.
3.5.1.1
Tank-to-Water Potential Measurements
The following series of tank-to-water potential measurements with a
portable reference electrode placed not more than 13 mm 1/2 inch away from
but not touching the tank wall shall be performed:
a.
On a vertical line midway between two anode strings beginning at a
point 300 mm 1 foot below water level and continuing at points 900
mm 3 feet apart until the bottom of the tank is reached.
b.
On a second vertical line midway between two anode strings on the
opposite side of the tank from the first vertical line beginning
at a point 300 mm 1 foot below water level and continuing at points
900 mm 3 feet apart until the bottom of the tank is reached.
c.
Across the bottom of the tank in a line between the two vertical
lines at 900 mm 3 foot intervals.
d.
In at least four places which are closest to anodes.
3.5.1.2
Reference Electrode Calibration
The portable reference electrode used for the test shall be calibrated
against the standard electrode specified in paragraph PERMANENT REFERENCE
ELECTRODES.
3.5.1.3
Test Measurement Recording
All test measurements and their locations, as well as measurements made
with the permanent electrodes simultaneously with the test measurements,
shall be recorded.
3.5.2
Adjusting
Final adjustment of the rectifier output current shall be made so that
repeated voltage readings taken as specified for testing meet the criteria
in paragraph CRITERIA OF PROTECTION.
SECTION 13111A
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