Cathodic Protection Coupons
IR drop errors are a significant source of inaccuracy in potential measurements on underground structures. There are a number of sources of them including the structure’s own cathodic protection (CP) current and stray currents from other sources. In theory, IR drop free potential measurements can be made by interrupting all current sources simultaneously and making an “instant off” potential measurement. In practice, this is very difficult so cathodic protection coupons are used instead. CP coupons simulate an uncoated part of the structure to which they are electrically bonded. Measurements are made by momentarily disconnecting the coupon and recording the “instant disconnect” potential. This reading approximates an IR-drop free potential on the structure at the location of the coupon. EDI’s Instant-off sensors are rod coupons fitted to our LongLife™ electrode (Model ULI) or SlimLine™ electrode (Model USI) at time of manufacture. Our Magnetic Switch System (Model SM) can be used to simplify instant-disconnect measurements with all types of CP coupons.
EDI also offers a CP coupon (Model UC) which is designed to make potential measurements that are substantially free of IR-drop error without either interrupting current or disconnecting the coupon. It is known as a Concentric CP coupon because the potential measuring port is located in the center of the coupon, well away from IR drop fields in the environment. This design is preferred for locations where interrupting currents or disconnecting the coupon to make measurements is not practical. It is particularly well suited for use with remote monitoring units. EDI’s concentric CP coupon is typically installed at the base of a test station since access through a soil tube is needed for making potential measurements. This product may also be ordered with a contained packaged zinc reference electrode which eliminates the need for access through a soil tube and a 1 sq. cm. coupon for measuring AC current density.
Instant-off rod sensors are installed according to the installation procedures for the reference electrode to which they are attached. The only exception is that the probe should be pressed into undisturbed soil within 15 cm (6 inches) of the reference electrode. The potential of the electrode should be checked against a portable electrode of known accuracy before the hole is backfilled. The coupon can be connected to the structure at the test station through our magnetically operated switch (Model SM). To disconnect the coupon for measurements, hold the activating magnet next to the switch. Detailed installation instructions for the concentric CP coupon can be downloaded by clicking the link below.
Potential Measurement Errors
Current flowing through an electrolyte creates a voltage drop proportional to the magnitude of both current and electrolyte resistance. The portion of this voltage drop which occurs between the reference electrode and the structure becomes incorporated into the potential measurement as an error. This error, commonly called IR Drop error, can be several hundred millivolts or more. IR drop errors are only one source of errors in potential measurements. To learn more about potential measurement errors and how to minimize them, down load our Corrosion 2001 paper on Measurement and Instrumentation Errors.
Our concentric CP coupon was thoroughly tested in both lab and field tests. The results of these tests were presented at Corrosion 2005; download a copy of this paper entitled Field Tests on an Advanced Cathodic Protection Coupon to see just how effective this design is.
For More Information
Complete Product Information and Installation Instructions are available for downloading as pdf files. Click on the documents you wish to download.
Concentric cathodic protection coupon (Model UC).
Installation Instructions for concentric CP coupon.
Instant-off rod sensor for bagged reference electrodes (Model ULI)
Instant-off rod sensor for reduced diameter reference electrodes (Model USI)
Magnetic Switch System for Instant-disconnect measurements (Model SM)
EDI products may be purchased from any corrosion control products distributor or directly from EDI.