UFC 3-570-06
JANUARY 31 2003
accuracy is questionable and it should be replaced.
Preparing the Electrolyte Solution. The electrolyte must be a fully
saturated solution of copper sulfate. The half cell body must be
thoroughly cleaned, then rinsed out several times with distilled water
before mixing the solution in the half-cell. There should be approximately
one third the volume of copper sulfate crystals installed in the half-cell,
then the remaining volume filled with distilled water. There should not be
any copper sulfate crystals in the threaded area of the half-cell. This can
be accomplished by slowly adding the distilled water to the threaded area
while rotating the half-cell. The proper solution is a deep blue in color and
after vigorous shaking; there must still be some copper sulfate crystals
that will not go into solution (fully saturated).
If the half-cell has been previously used, additional steps are required. All
parts should be inspected for cracks or other defects. O-rings should be replaced. The
cone or plug must be replaced. To provide additional protection from subsequent
leakage of the electrolyte from the half-cell, plumber's tape can be used on the threads
of both the top cap and the end plug (new or used half cells). If the copper electrode is
removed from the end cap or replacement of the end cap is required, the threads must
be sealed with proper sealant when reinstalled, to prevent leakage of the electrolyte.
Testing. To determine the accuracy of a half-cell, use multiple reference
electrodes. Using a "reference" reference electrode, measure the
difference in potential to the half-cell under test. Use a meter on the
millivolt scale, place the two cells cone-to-cone, and measure the potential
difference. The potential difference should not be in excess of 5 mV. If no
"reference" reference electrode is available, follow the procedures in
7.2.1.1 for initiation of a reference electrode on a new or used half-cell to
get a reliable reference electrode, then test the potential difference (in mV)
to other half cells. When a reference electrode is first initiated, allow
sufficient time for the cone to become saturated (up to two hours). Placing
the half-cell, cone end down, in a container of copper sulfate solution, can
speed up the process.
7-2.2
IR Drop Error. There is an IR drop error caused by cathodic protection
current flowing through the electrolyte (a resistor). This error is greater when the
current is higher; when the resistivity is higher; when the distance from the reference
electrode to the structure is higher; and on a well-coated pipeline, when the distance to
the nearest holiday is greater. An instant-OFF or an IR free potential measurement will
remove this error. This error is in the negative direction (for example, with the error, you
may measure a -0.85 volt DC potential, and after correction for the error you may
actually have -0.75 volts DC.)
7-3
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