TM0101-2001
NACE International 5
5.4 The voltage difference between a reference electrode
and a metal tank can be measured with a voltmeter. The
reference electrode potential is normally positive with
respect to the ferrous tank; conversely, the ferrous tank is
negative with respect to the reference electrode.
5.5 A tank-to-electrolyte potential is measured using a DC
voltmeter having an appropriate input impedance (or
internal resistance, for an analog instrument), voltage
range(s), test leads, and a stable reference electrode, such
as a saturated copper/copper sulfate electrode (CSE),
silver/silver chloride (Ag/AgCl) electrode, or saturated
potassium chloride (KCl) calomel reference electrode. The
reference electrode can be portable or one designed for
permanent installation.
5.5.1 CSEs are usually used for measurements if the
electrolyte is soil or fresh water; they are sometimes
used for measurements in salt water. If a CSE is used
in a high-chloride environment, the stability (lack of
contamination) of the CSE must be determined before
the readings may be considered valid.
5.5.2 The Ag/AgCl reference electrode is usually used
in seawater environments.
5.5.3 The saturated KCl calomel electrode is used
mainly for laboratory work. However, more rugged,
polymer-body, gel-filled, saturated KCl calomel
electrodes that are suitable for field work are available,
though modifications may be necessary to increase
their contact area with the environment.
5.6 Meter Polarity
5.6.1 Tank-to-electrolyte potentials are often
measured by connecting the negative terminal of the
measuring instrument to the tank and the positive
terminal to the reference electrode placed in the
electrolyte, which is in contact with the tank. With this
connection, the instrument indicates that the reference
electrode is positive with respect to the tank.
Therefore, the tank voltage is negative with respect to
the reference electrode (see Figure 1a).
5.6.2 Tank-to-electrolyte potential measurements can
be made with the reference electrode connected to the
instrument negative terminal and the tank connected to
the positive terminal. This produces a negative voltage
display on digital meters (see Figure 1b).
5.7 The tank-to-electrolyte potential measurement of a
buried tank should be made with the reference electrode
placed close to the tank-to-electrolyte interface. The most
common practice, however, is to place the reference
electrode as close to the tank as practicable, which is
usually at the surface of the earth above the centerline of
the tank (See Figure 1). This measurement includes a
combination of the voltage drops associated with the:
(a) Voltmeter;
(b) Test leads;
(c) Reference electrode;
(d) Electrolyte;
(e) Coating, if applied;
(f) Tank-to-electrolyte interface; and
(g) Electrode-to-electrolyte interface.
Note: A high-input impedance (>10 megohm) voltmeter or
potentiometer voltmeter should be used to eliminate the
effects of Paragraph 5.7, (a), (b), (c), (f), and (g) on the
potential measurement
.
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