Nace CP 1 - Diode Bias

Field Measurements 5:27
CP 1 – Cathodic Protection Tester Course Manual
© NACE International, 2000
02/01/05
electrode is placed at a fixed location and the positive lead from the
multimeter is placed in contact with various parts of the structure. If
essentially the same structure-to-electrolyte potential is read from each
contact, continuity is indicated. It is possible that an isolated portion of the
structure could have the same potential as the rest of the structure. If you
suspect this is the case, other testing, beyond the scope of this course, will
be required.
Continuity of pipelines, cables, and similar structures can also be tested by
using a pipe or cable locator. These instruments are discussed later in this
chapter.
Diode Bias
This is a multimeter operated in the diode bias mode. A functioning diode
will typically display a meter value from 0.3 V to 0.9 V in the forward bias;
positive lead to anode, negative lead to cathode. In the reverse bias
condition, positive lead to cathode and negative lead to anode, a
functioning diode will display “OL” (overload or out of limits).
For shorted diodes, the meter will display some low voltage value in both
forward and reverse bias configuration. In the case of an open circuit diode,
the meter will display “OL” in both forward and reverse bias.
To correctly verify diode operation, at least one lead must be disconnected
from the circuit. Diodes cannot be properly checked while in the circuit or
with power on.