Nace CP 1, Course Manual

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Basic Chemistry and Basic Corrosion Theory 2:14
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
Noble
Active
.600 V
+
_
Voltage
measurement
is positive
Figure 2.13 Voltage Measurement of a Noble Metal and an Active Metal
Immersed in an Electrolyte
Sign Convention
For a positive reading on a digital display or a right hand deflection of the
needle on an analog display, conventional current must flow into the
positive (+) terminal. Since most instruments used today are digital with
automatic polarity display, a surveyor may not be as concerned with the
sign of the reading when connecting the voltmeter for the measurement. It
is still important to pay attention to the connection of the instrument
terminals and the expected sign in order to detect problems during a survey.
If a voltmeter is connected such that the positive terminal is connected to
the more noble metal and the negative terminal to the more active metal,
then the reading is positive. Current will flow from the active to the noble
metal through the electrolyte and from the noble to the active metal through
the metallic path. Therefore, the reading is positive because conventional
current is flowing into the positive terminal of the voltmeter.
Structure-to-electrolyte readings are considered negative to the reference
electrode. When using a digital voltmeter, the reference electrode is
connected to the negative terminal. Connecting the reference electrode to
the negative terminal produces a negative reading. On an analog meter, the
needle will swing to the right when current enters the positive terminal.
When using an analog meter with a center zero, if you connect the
reference electrode to the negative terminal, the needle will swing to the
left, indicating the negative reading. With a left hand zero instrument, you
can still connect the reference electrode to the negative terminal, but it is