Nace CP 1 - Page 168

Field Measurements 5:21
CP 1 – Cathodic Protection Tester Course Manual
© NACE International, 2000
02/01/05
3. Calculate current flow by multiplying the calibration factor by the
voltage drop measured above:
I (A)= K (A/mV) x mV drop
4. Note the sign of the voltage drop to determine the direction of current
flow. If the voltage drop reading is positive, then the direction of current
flow is from the positive to the negative terminal of the voltmeter. If the
reading is negative, then the direction of current flow is from the
negative to the positive terminal.
Based on Figure 5.10 if 10 A of battery current passes from the east to the
west outside leads, resulting in voltage drop change (∆E) of :
On = 5.08 mV
Off = 0.17
mV
∆E = 4.91 mV
Then the calibration factor will be:
mV/A04.2
mV17.008.5
A10
or
m91.4
A10
K =
−
=
Without test current applied, the voltage drop across the span between the
inner leads is 0.17 mV. Therefore, current flow is:
mA347A347.0
mV
mV17.xA04.2
dropmVxKI ====
Referring again to Figure 5.10, note that the positive lead of the digital
meter is attached to the east inner lead. The meter indicates a positive
display, showing that current is entering the meter on the positive terminal.
Again, the meter is in parallel with the pipe span, so current flow on the
pipe is from east to west.
Most digital meters will not read below 0.1 mV. If readings below 0.1 mV
are anticipated, or if a zero reading is obtained during a test, a more
sensitive meter must be used.