The components used for making potential measurements and the equivalent electrical schematic are shown below. A reference electrode located close to the structure is connected to the meter by a test lead. A second lead wire connects the structure to the meter. In this simple DC circuit, the driving voltage is the potential that exists between the reference electrode and the structure. When a measurement is being made, current will flow through the circuit as a result of this potential. The magnitude of the current flow follows Ohm’s law, I = E/R. The current is proportional to the driving voltage and inversely proportional to the sum of all resistances in the circuit. For example, if the circuit potential is one volt and the sum of the resistances is ten mega-ohms (MW), a tenth of a micro-amp will flow through the measurement circuit.
Voltage drops occur across each of the resistive elements in the measurement circuit. These voltage drops are separate and distinct from the more commonly discussed voltage drops, or IR drops, which are due to external current flowing through the electrolyte. In the figures, the external current is shown as ie. Both measurement circuit voltage drops and external voltage drops become incorporated into potential measurements causing errors. Different methods must be employed to minimize errors caused by each type. Download our paper Effect of Measurement and Instrumentation Errors on Potential Readings from the Technical section of our website to learn more.