JANUARY 31 2003
to those locations. The polarity of the voltage difference between the two reference
cells indicates the direction of current flow.
Accuracy. The accuracy of the reference electrodes (half cells) used to take
cell-to-cell measurements must be determined. Perfect matching of the two reference
cells is not essential, but the error must be accounted for in all measurements taken.
The accuracy of the two half-cells is determined by measuring the difference in potential
between the two half cells being used for the test. Use a suitable voltmeter on the
millivolt scale and place the two cells cone-to-cone, and measure the potential
difference. The potential difference should not be in excess of 5 mV. If no "reference"
reference electrode is available, follow paragraph 7-2.1.1 on a new or used half-cell to
get a reliable reference electrode, then test the potential difference (in mV) to other half
cells. When a reference electrode is first initiated, time must be allowed for the cone to
become saturated. This process takes up to two hours, but can be speeded up by
placing the half-cell, cone end down, in a container of copper sulfate solution. See
paragraph 7-2.1 for the procedures for checking the reference electrodes.
Figure 7-10. Positive Reading for Cell-To-Cell Survey
V oltm et er
E lect rod e
E lect rod e
C urrent Flo w in th e E a rth
D irect io n of A no dic A rea on Pipeline