MIL-HDBK-1004/10
Field Measurement of Anode Output. Calculations, as in the case of
7.4.3
impressed current systems, can only give approximations of anode-to-
electrolyte resistance under actual conditions. While these calculations can
be used for an initial system design, the actual anode output encountered is
often sufficiently different from the calculated value to require adjustment
or modification of the system. This is more of a problem for sacrificial
anode systems than for impressed current systems since the output potential is
not adjustable in sacrificial anode systems. In areas where the soil
resistivity varies with location, a field measurement must be made at each
anode location. Often the only remedy for low anode output is to add
additional anodes to the system. High anode output can be remedied by
installing current limiting resistors in the anode lead wires, but this should
be avoided where possible. The actual anode output can best be determined by
actual field measurements.
Anode output is best determined by installing an anode at the
actual site of the installation and attaching it to the structure to be
protected. The anode output is measured using a current shunt (0.01 or 0.1
ohm) installed in the anode lead wire. As a single anode is unlikely to
polarize the structure to the desired potential, correction of the anode
output for structure potential is usually required. This can be done using
the structure potential factor in the simplified equation given in para.
4.2.5, or by determining the anode-to-electrolyte resistance based upon the
actual potential difference achieved and the anode output current.
Determination of Number of Anodes Required. After the output per
7.5
anode is determined, the number of anodes required for protection is
calculated. This is done by dividing the total output by the output per
anode. In practice, approximately 10 percent more anodes are installed to
allow for inaccuracies in system design, seasonal variations in anode output,
and decreased anode output as the anodes are consumed. Installation of a
limited percentage of additional anodes is not wasteful because, if the system
is properly adjusted, additional anodes simply result in longer anode life.
Determination of Anode Life. The anode life is calculated
7.6
based upon the current flow, anode weight, and anode efficiency. The
calculation involves the number of ampere hours produced by an anode per pound
on anode material consumed. Anode consumption can be calculated using the
formula:
EQUATION:
W
=
YSI
(22)
where
W
=
anode consumption
in pounds
Y
=
number of years
S
=
anode consumption
rate in lbs/A yr
based upon actual
anode efficiency
I
=
current output in
amperes
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