Nace CP 1, Course Manual

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Basic Chemistry and Basic Corrosion Theory 2:12
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
Conventional Current Flow
Figures 2.10 shows the actual electrochemical current flow that exists in a
corrosion cell. In corrosion and cathodic protection work,
conventional
current flow
is used. This is a flow of current in the direction of the positive
ion transfer (so called positive current). The use of conventional current
simplifies the understanding of corrosion cells and the use of cathodic
protection.
Use of Voltmeters
An analysis of meter polarity connection and sign displayed allows the
determination of the direction of conventional current flow.
Voltmeter Hook Up
When measuring voltage across an element of a circuit, the voltmeter is
connected in parallel across the element. A parallel connection is made
when the external circuit is not broken. For example, the voltmeter in
Figure 2.11 is connected in parallel to Resistor B of the external circuit.
+
_
+
_
VOLTS
Parallel Connection
R
A
R
B
R
C
E
I
Figure 2.11 A Voltmeter Connection is a Parallel Connection
Current, I, of the external circuit does not flow through the voltmeter to
complete the circuit, an indication the voltmeter is in parallel instead of in
series with the circuit. All voltage measurements are made in parallel.