It is possible to make a model of electrical components to
demonstrate further complications involved in DC electrical fields of this
nature.
Two chains of randomly selected resistors are soldered
together, linking two different potentials such as the poles of a dry cell
battery. The probes of the voltmeter
can be placed in any position along either of the chains and a different
voltage will be displayed.
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The number of variations in voltage between the two nodes is
equal to the square of the number of resistors.
If a grid of resistors is constructed with random values,
and the probes of the voltmeter are placed at various locations in the grid, it
would require a computer to predict the results.
Put a third dimension onto the grid, and a very powerful
computer would have to be fed with every resistance value in order to predict
the voltage resulting from the probes placed at surface nodes.
Two or more resistances of unspecified value included in the
system would make the result impossible to predict by any computer. (However it is true that given the same
source voltage, the voltage between any two nodes would be constant.)
FIELD EXPERIMENTAL WORK
It is possible to apply this principle of an 'electrical
picture' to field work by a simple experiment.
Choose a buried, insulated, cathodically protected pipeline
running through areas of soil of different resistances. Place a metal coupon in high resistance soil
and another in low resistance soil, both connected to the pipeline. Using two copper/copper-sulphate electrodes
as probes, connected to the poles of a high resistance voltmeter
plot the potentials surrounding the coupons. In the case of the high resistance soil, the
'potential gradient' will spread much further than that from the coupon in the
low resistance soil.
The area of influence of the coupon can vary from a few
millimeters in low resistance soils, to many meters in extremely high resistant
circumstances such as fresh water, sand and gravel. As a result, it is sometimes not possible to
detect a corrosion cell or even a coating fault at ground level, if the
resistance of the soil is too low, and the amount of current is small.
The effect of this can be seen in a drawing showing a single
corrosion cell, where the back-fill is homogenous.
It is clear that the exact distance between each probe and
the actual interface between the metal and the electrolyte is crucial to any
calculation of the interface potential.
The most basic measurement in cathodic protection work is
described in Procedure 1, and is known as the 'pipe to soil potential'.
The vast majority of existing cathodic protection records
are composed of these readings which are in fact voltages.
A direct metalic connection is made to the steel of the
pipeline and the other pole of the
voltmeter is connected to a copper rod suspended in a solution of copper
sulphate in a container with a porous base.
This base is placed in contact with the ground and the resulting voltage
is recorded.
It has been seen from the sand tray experiment and from the
experiment with the steel coupon in contact with the pipeline, that the actual
potential of the ground can vary. When
carrying out Procedure 1, it is often possible to obtain significantly
differing readings by altering the position of the electrode within the radius of
the connecting lead. Errors as high
as 30% or more are common for this
reason and it can be imagined the voltages obtained by a left handed engineer
would differ from those of a right handed engineer, under certain
circumstances.
One example of where this was known to have happenned was at
a pipeline cathodic protection post where the copper connecting lead to the
pipeline, had faulty insulation just below ground level. This would cause current to flow into the
copper for two reasons. Cathodic
protection current would follow this path being the least line of resistance,
and the less noble steel of the pipeline would tend to discharge current to the
copper as the cathode of a bi-metalic corrosion cell.
Current flowing into the copper caused a considerable
voltage gradient in the soil for about 1/2 meter, and this caused the meausred
voltage to drop from -.950 volts, which was considered to be protected, to
-.750 volts which was considered not to be protected.
Another example occured during an 'over the line close
interval potential survey' where a depression in the ground voltage was caused
by a galvanised fitting to a steel 'well-point tube' which had been abandonned
during the construction of the pipeline through waterlogged soil. In this case there was no connection between
the pipeline and the scrap, and certainly no influence on the corrosion control
of the pipeline, and yet the survey results showed the the pipeline was
unprotected at this exact spot and adequately protected one meter distant.
It is crucial that cathodic protection engineers and
technicians understand the nature of DC electricity and the significance of
these voltage measurements.
It is no good understanding thermo-dynamic
theory and complex formulae if their components are grossly inaccurate. Many measurements made in laboratory
conditions cannot be made in field practice and it follows that an immeasurable
criteria is about as useful as an elastic tape measure.
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