Nace CP 1, Course Manual

Page: 48 / 535

Basic Electricity 1:25
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
distinguish between the resistance of the fitting and the resistance
through the electrolyte. More important, however, is the fact that
there is nearly always a voltage difference between two isolated
structures. This voltage affects the total voltage of the measuring
circuit and creates appreciable errors. A high resistance may be
indicated with the leads connected one way and a low resistance
with the leads reversed.
Digital Meters
Basis of Operation
A digital meter is based on integrated microcircuit technology. The
analog input is sampled over a specific period of time. The average
input over this time is then converted to a digital value, a coded
number, using an analog-to-digital (A-to-D) converter. For a digital
multimeter (DMM), all measurements are converted to DC volts
prior to the analog-to-digital conversion. The displayed value is not
continuous but a sample of the input value measurements. However,
the measurement is taken several times per second. The measured
value is displayed as discrete digits; the digits do not require
operator interpretation as the analog display does. If a voltage is
changing, the numbers are also changing, and it may be difficult to
read a specific value from the fluctuating members. Therefore, in
areas with stray current, a digital meter may not be as useful as an
analog meter.
Meter Hook Up
When current enters the meter on the positive terminal, a positive
sign (+) is displayed. A negative sign (–) is displayed when current
enters the meter on the negative terminal. Remembering this will
enable you to determine the direction of current flow in the circuit
with which you are working.
Also, when using a digital meter, the reference electrode is
connected to the negative terminal to obtain the proper polarity
reading.
WARNING – DO NOT TOUCH THE METAL PARTS OF THE
TEST WIRES WHEN TAKING VOLTAGE READINGS.