Nace CP 1, Course Manual

Page: 484 / 535

TM0497-2002
4 NACE International
4.3.1.1 An input impedance of 10 megaohms or
more should be sufficient for a digital meter. An
instrument with a lower input impedance may
produce valid data if circuit contact errors are
considered.
One means of making accurate
measurements is to use a potentiometer circuit in
an analog meter.
4.3.1.2 A voltmeter measures the potential across
its terminals within its design accuracy. However,
current flowing through the instrument creates
measurement errors due to voltage drops that
occur
in all resistive components of a
measurement circuit.
4.3.2 Some analog-to-digital converters used in digital
and data logging instruments operate so fast that the
instrument may indicate only a portion of the input
waveform
and thus provide incorrect voltage
indications.
4.3.3 Parallax errors on an analog instrument can be
minimized by viewing the needle perpendicular to the
face of the instrument on the centerline projected from
the needle point.
4.3.4 The accuracy of potential measurements should
be verified by using an instrument having two or more
input impedances (internal resistance, for analog
instruments) and comparing potential values measured
using different input impedances.
If the measured
values are virtually the same, the accuracy is
acceptable. Corrections need to be made if measured
values are not virtually identical. Digital voltmeters that
have a constant input impedance do not indicate a
measurement error by changing voltage ranges. An
alternative is to use a meter with a potentiometer
circuit.
4.4 Instrument Accuracy
4.4.1 Instruments shall be checked for accuracy
before use by comparing readings to a standard
voltage cell, to another acceptable voltage source, or to
another appropriate instrument known to be accurate.
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Section 5: Pipe-to-Electrolyte Potential Measurements
5.1 Instruments used to measure AC voltage, direct current
(DC) voltage, or other electrical functions usually have one
terminal designated “Common” (COM). This terminal either
is black in color or has a negative (-) symbol. The positive
terminal either is red in color or has a positive (+) symbol.
The positive and negative symbols in the meter display
indicate the current flow direction through the instrument
(Figure 1a). For example, a positive symbol in the meter
display indicates current flowing from the positive terminal
through the meter to the negative terminal. One instrument
test lead is usually black in color and the other red. The
black test lead is connected to the negative terminal of the
instrument and the red lead to the positive terminal.
5.2 Voltage measurements should be made using the
lowest practicable range on the instrument.
A voltage
measurement is more accurate when it is measured in the
upper two-thirds of a range selected for a particular
instrument.
Errors can occur, for example, when an
instrument with a 2-V range is used to measure a voltage of
15 mV. Such a value might be a voltage drop caused by
current flowing in a metal pipeline or through a calibrated
shunt.
A much more accurate measurement would be
made using an instrument having a 20-mV range.
5.3 The usual technique to determine the DC voltage
across battery terminals, pipeline metal/electrolyte interface,
or other DC system is to connect the black test lead to the
negative side of the circuit and the red test lead to the
positive side of the circuit. When connected in this manner,
an analog instrument needle moves in an upscale
(clockwise) direction indicating a positive value with relation
to the negative terminal. A digital instrument connected in
the same manner displays a digital value, usually preceded
by a positive symbol.
In each situation the measured
voltage is positive with respect to the instrument’s negative
terminal. (See instrument connections in Figure 1a.)
5.4 The voltage present between a reference electrode and
a metal pipe can be measured with a voltmeter.
The
reference electrode potential is normally positive with
respect to ferrous pipe; conversely the ferrous pipe is
negative with respect to the reference electrode.
5.5 A pipe-to-electrolyte potential is measured using a DC
voltmeter having an appropriate input impedance (or
internal resistance, for an analog instrument), voltage
range(s), test leads, and a stable reference electrode, such
as a saturated copper/copper sulfate (CSE), silver/silver
chloride (Ag/AgCl), or saturated potassium chloride (KCl)
calomel reference electrode. The CSE is usually used for
measurements when the electrolyte is soil or fresh water
and less often for salt water. When a CSE is used in a
high-chloride
environment, the stability (lack of
contamination) of the CSE must be determined before the
readings may be considered valid. The Ag/AgCl reference
electrode is usually used in seawater environments. The
saturated KCl calomel electrode is used more often for
laboratory work.
However, more-rugged, polymer-body,
gel-filled saturated KCl calomel electrodes are available,
though modifications may be necessary to increase contact
area with the environment.