Field Measurements 5:18
CP 1 – Cathodic Protection Tester Course Manual
© NACE International, 2000
02/01/05
Pipe resistance/ft = 2.44 µΩ /ft
= 0.00000244 Ω/ft
Total resistance = 200 ft x 0.00000244 Ω/ft
= 0.000488 Ω
Measured voltage drop = 0.17 mV
Current (I) = E/R =0.17 mV/0.000488 Ω
= 348 mA or 0.348 A
Note the calculation yields milliamps because we used millivolts in Ohm’s
Law.
Refer again to Figure 5.9. Note the meter is showing a positive indication.
This means the current is entering the meter on the positive terminal. The
positive terminal is connected to the west end of the span. Since the meter
is in parallel with the span, current flow on the pipe is from west to east.
The accuracy of this test method depends greatly on accurate knowledge of
the dimensions of the pipe. Should there be an odd-sized joint within the
span, or some appurtenance such as a valve, the calculated resistance will
not be correct. The 4-wire test method overcomes these difficulties.
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