A measured potential is the sum of the voltage drops occurring in the measurement circuit and those occurring in the electrolyte. Most of the individual measurement circuit voltage drops are negligible except for the one at the structure electrolyte interface which is the potential of interest. Other components of the measurement circuit voltage drop are discussed further in EDI Technical Note TN8 Measurement Circuit IR Drop.
Voltage drops occurring in the electrolyte represent an error in the measurement. These voltage drops are due to external current flowing through the electrolyte. The current can be the structure’s own CP current as well as telluric currents, foreign structure CP systems or mass transit systems. Eliminating the voltage drop error from the structure’s own CP system can be done by interrupting that current. Other stray currents are not easily interrupted so different methods are used to eliminate their error.
The most common method is CP coupons which are small pieces of metal electrically bonded to the structure so they come to the same potential as the structure. They are placed within a few centimeters of a reference electrode. When the coupon potential is measured, the short distance between the reference and the coupon reduces, but does not eliminate, the voltage drop error in the measurement. In a concentric CP coupon, the sensing port is located in the center of the coupon which reduces the electrolyte path to about a millimeter. This extremely short distance virtually eliminates electrolyte voltage drop error.