milliammeter, a low-resistance voltmeter and
millivoltmeter, and a potentiometer voltmeter.
The multicombination meter maybe used to
measure galvanic anode current between an anode
and structure, galvanic current between structures,
and potentials as with other types of voltmeters
Resistivity measuring instruments are units
used to test the corrosive action of a soil. Tests
regarding soil corrosivity are necessary when
designing cathodic protection systems. Informa-
tion from these tests is used to locate the most
corrosive areas where a pipeline is to be laid and
the most corrosive areas of an existing pipeline.
It is also used to decide the location for anode
One of the simplest methods for making a
resistivity test is to use a single probe resistivity
meter. It consists of a probe with two electrodes,
an indicating instrument, switches, and the re-
quired wiring. To use this instrument, the probe
is inserted into the ground and current is applied
to it. The indicating instrument gives a reading
that indicates the corrosiveness of the soil.
Buried Pipe Locator
In the field of cathodic protection work, it is
necessary to locate pipes in order to locate
interferences in the cathodic protection system.
An electronic pipe locator is used for this purpose.
The main components of the locator are the direc-
tional transmitter and the directional receiver.
Each one of these units is carried by an operator.
The operators are usually about 30 feet apart.
During actual operation the transmitter sends out
signals which travel along the pipeline. The
receiver, in turn, picks up these signals in
varying intensities, depending on the distance the
operators are from the pipe. When both operators
are directly over the pipe, a maximum response
is obtained in the phones and on the visual meter
of the receiver. Most pipe can be located easily
and accurately in this manner.
Protective Coating Leak Detector
A protective coating leak detector (referred to
as a holiday detector) is used to detect the
imperfections (holidays) in pipe coatings. The
holiday leak detector is an instrument that
operates on an electric current. When it is being
Figure 7-15.Impressed current method of cathodic protection.