Some meters designed for corrosion potential measurements have selectable input impedance with the highest value typically being 250 MW. However, general-purpose digital meters with a fixed 10 MW input impedance are frequently used for corrosion measurements. While this may seem high, it is not adequate in many situations. Electrolyte resistance is highly variable ranging from quite low for potential measurements in seawater to very high for potential measurements in dry soils. The proper strategy is to select a meter whose internal resistance (input impedance) is several orders of magnitude higher than any other resistance in the circuit so that voltage drop across the meter will, for practical purposes, represent the entire voltage drop in the circuit.
For example, consider a structure with a 900mV potential and a circuit resistance external to the meter of 1 MW which is typical in concrete, rock or dry soil. The total circuit resistance when using a 10 MW meter would be about 11 MW with 90% being in the meter and 10% external to the meter. The voltage drop in this case would be similarly divided with 810mV across the meter and 90mV external to it. If a 100 MW meter were used, the circuit resistance would be 101 MW so that almost all of the voltage drop would occur in the meter. Measurement circuit IR drop errors result in a more positive apparent potential reading.
When using a meter with selectable input impedance, successive readings can be made, each time increasing the input impedance. When two successive readings are the same, the measurement can be presumed to be free of measurement circuit IR drop error. If it is not possible to obtain two successive readings that are the same, there are two methods that may be used to eliminate this error. A potentiometric-voltmeter will eliminate these potential measurements errors. Alternatively, a correction factor can be calculated from measurements made with a meter that has variable input resistance. Download our paper Effect of Measurement and Instrumentation Errors on Potential Readings from the Technical section of our website to learn more.