Nace CP 1, Course Manual

Page: 515 / 535

TM0101-2001
4 NACE International
may require the use of recording or analog instruments
to improve measurement accuracy.
Dynamic stray
currents include those from electric railway systems
and mining equipment.
4.3 Instrument Effects on Voltage Measurements
4.3.1 To measure tank-to-electrolyte potentials
accurately, a digital voltmeter must have a high input
impedance (high internal resistance, for an analog
instrument) compared with the total resistance of the
measurement circuit.
4.3.1.1 A digital meter used to measure tank-to-
electrolyte
potential should have an input
impedance of 10 megohms or more. However, 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 effect of measurement circuit resistance
errors may be evaluated using an instrument with two
or more input impedances (internal resistance, for
analog
instruments) and comparing the values
measured using different input impedances. If the
measured values are essentially identical,
measurement circuit resistance errors are negligible.
Corrections must be made if measured values are not
essentially identical.
Digital voltmeters that have a
single input impedance cannot be used for indicating
measurement circuit resistance errors. Alternative
measurements can be made with a potentiometer or by
using
two digital voltmeters with different input
impedance values.
4.3.5 Specialized equipment that uses various
techniques to measure the impressed current
waveform and calculate a tank-to-electrolyte potential
free of voltage drop is available. This equipment may
minimize problems resulting from spiking effects,
drifting of interrupters, and current from other direct
current (DC) sources.
4.4 Instrument Accuracy
4.4.1 Instruments must be scheduled for periodic
calibration to a certified standard. 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 that has been appropriately calibrated for
accuracy.
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Section 5: Tank-to-Electrolyte Potential Measurements
5.1 Voltmeters used to measure AC voltage, DC voltage,
or other electrical functions usually have one terminal
designated “Common” (COM). This terminal is either black
in color or has a negative symbol (-). The positive terminal
is either red in color or has a positive symbol (+).
5.1.1 The positive and negative symbols in the
voltmeter display indicate the direction of the current
flow through the instrument. For example, a positive
value in the voltmeter display indicates current flowing
from the positive terminal through the voltmeter to the
negative terminal (Figure 1a).
5.1.2 Usually, one voltmeter test lead is black and the
other is red. The black test lead is connected to the
negative terminal of the voltmeter, and the red lead is
connected to the positive terminal.
5.2 The usual technique to determine the DC voltage
across
battery terminals, across a tank-to-electrolyte
interface, or from another 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,
preceded by a positive symbol or no symbol at all. In each
situation the measured voltage is positive with respect to
the instrument’s negative terminal.
(See instrument
connections in Figure 1a.)
5.3 Voltage measurements should be made using the
lowest range on the instrument. For an analog instrument,
the voltage measurement is more accurate when it is
measured in the upper two-thirds of a range selected for a
particular instrument.