Reference: Gummow, R. A., "Using Coupons and Probes to Determine the Level of Cathodic Protection", Unpublished paper presented at NACE TechEdge seminar; "Role of Coupons in Monitoring Cathodic Protection", Denver, November 20, 1997.

Gummow, R. A.,"Using Coupons and Probes to Determine Cathodic Protection Levels", Materials Performance, August 1998.


Review: The two papers are essentially the same. They are carefully crafted papers containing a comprehensive review of relevant background material. Extensive citations document virtually all statements in the paper.

In the Materials Performance article, Gummow expresses the following opinion about cost effectiveness.

"The savings realized by not having to add more CP current could, in itself, pay for the cost of retrofitting CP coupons."
Gummow gives the following clear explanation of accounting for IR drop:

"The polarity of IR drop when the current direction is toward the structure, as is the case with a cathodic protection current, is additive thereby making the measured potential numerically greater than the polarized potential. This makes it appear that the structure is better protected than it really is. Conversely, when the current direction is away from the structure, as might be the case with a stray current, the polarity of the IR drop subtracts from the polarized potential and makes the structure appear more poorly protected than it is. Accordingly, when the IR drop component is significant, as would be the case for large current or high soil resistivity, then substantial measurement error arises unless corrective action is taken."

Gummow lists the following attempts to account for IR drop.

Gummow points out that in some instances interruption of the current is impractical or even impossible. He further points out that, because most of the IR drop occurs very near a coating holiday, it is impractical to place the reference electrode close enough to a holiday to remove most of the IR drop.

His visualization lends credence to the use of coupons.

"However, if the holiday could be disconnected from the pipe, the reference electrode placement is not as critical."

The following are helpful quotations from Gummow's paper. Each pertains to the use of coupons to monitor cathodic protection.