Nace CP 1, Course Manual

Page: 519 / 535

TM0101-2001
8 NACE International
________________________________________________________________________
Section 6: Causes of Measurement Errors
6.1 The following factors may contribute to faulty potential
measurements:
6.1.1 Tank and instrument test leads
(a) Broken or frayed wire strands (may not be visible
inside insulation of the wire);
(b) Damaged or defective test lead insulation that
allows the conductor to contact wet vegetation, the
electrolyte, or other objects;
(c) Loose, broken, or faulty tank or instrument
connections; and
(d) Dirty or corroded connection points.
6.1.2 Reference electrode condition and placement
(a) Contaminated reference electrode solution or rod,
or solutions of insufficient quantity or saturation (only
laboratory-grade chemicals and distilled water, if water
is required, should be used in a reference electrode);
(b) Reference electrode plug not sufficiently porous to
provide a conductive contact to the electrolyte;
(c) Porous plug contaminated by asphalt, oil, or other
foreign materials;
(d) High-resistance
contact between reference
electrode and dry or frozen soil, rock, gravel,
vegetation, or paving material;
(e) Reference
electrode placed in the potential
gradient of an anode without consideration of the
voltage drop caused by the anode, including reference
electrode placement over the top of the tank(s)
protected by close anodes;
(f)
Reference electrode positioned in the potential
gradient of a metallic structure other than the one
whose
potential is being measured without
consideration of the voltage drop caused by the
potential gradient of the metallic structure;
(g) Electrolyte between tank and disbonded coating
causing error due to electrode placement in electrolyte
on opposite side of coating;
(h) Defective
permanently installed reference
electrode;
(i)
Temperature correction not applied when needed;
(j) Photo-sensitive measurement error (in CSE
reference electrode with a clear-view window) due to
light
striking the electrode electrolyte solution
(photovoltaic effect); and
(k) Remote reference electrode placement in which
voltage drops are not considered as part of the
measurement.
6.1.3 Unknown isolating devices, such as disbonded
tubing or pipe systems, can cause the tank to be
electrically discontinuous between the test connection
and the reference electrode location.
(Section 11
provides guidance on methods of troubleshooting that
identify continuity or discontinuity.)
6.1.4 Parallel paths can be inadvertently established
by test personnel contacting instrument terminals or
metallic parts of the test lead circuit, such as test lead
clips and reference electrodes, while a potential
measurement is being made.
6.1.5 The use of defective or inappropriate
instruments, incorrect voltage range selection,
instruments not calibrated or zeroed, or damp
instruments sitting on wet earth can cause
measurement errors.
6.1.6 Instruments that have an analog-to-digital
converter can operate at such fast speeds that the
voltage spikes produced by current interruption are
indicated as the potential measurement instead of the
actual “on” and “off” values.
6.1.7 The polarity of the measured value can be
incorrectly observed.
6.1.8 Measurement errors can occur if the cathodic
protection current-carrying conductor is used as a test
lead for a tank potential measurement.
6.1.9 Electromagnetic interference or induction
resulting from AC power lines or radio frequency
transmitters can induce test lead and instrument errors.
This condition is often indicated by a fuzzy, fluctuating,
or blurred pointer movement on an analog instrument
or erratic displays on digital voltmeters.
For this
reason, DC voltmeters must have sufficient AC
rejection capability, which can be determined by
referring to the manufacturer's specification.
6.1.10 The use of an internal tank connection via the
fill pipe in the absence of a cathodic protection test wire
when the tank has been lined can cause faulty
potential measurements.
6.2 Several methods may be used to reduce contact
resistance caused by the following factors:
6.2.1 Soil moisture: If the surface soil is so dry that
the electrical contact of the reference electrode with the
electrolyte is impaired, the soil around the electrode
may be moistened with water until the contact is
adequate.
6.2.2 Contact surface area: Contact resistance may
be reduced by using a reference electrode with a larger
contact surface area.
6.2.3 Frozen soil: Contact resistance may be reduced
by removing the frozen soil to permit electrode contact
with unfrozen soil.