Most general purpose digital meters used today for corrosion measurements have 10 mega-ohm input resistance. While this may seem high, it may not be adequate in some circumstances. Consider a structure with a 900mV potential and a circuit resistance external to the meter of 10 kilo-ohms. Ten kilo-ohms is 0.1% of 10 mega-ohms. Thus 0.1% of the voltage will be dropped in the circuit external to the meter and 99.9% will be measured by the meter. Now let’s consider the case where the structure is in concrete, rock or dry soil. The external resistance in this situation could be 1 mega-ohm or higher. The total circuit resistance would now be 11 mega-ohm 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. Measurement circuit IR drop errors always result in a lower apparent potential reading. This can result in unnecessary and costly up-grades and/or replacements of CP systems.
A preferred way of making potential measurements in high resistance circuits is with a potentiometric-voltmeter. This type of meter was the standard field meter for corrosion personal up until the mid-1970s. An internal battery in this meter applies a voltage with opposite polarity to that being measured. The applied voltage is adjusted to exactly balance the potential being measured. The applied battery voltage is then read. With no current flowing through the circuit, there is no measurement circuit IR drop. Potentiometric-voltmeters may cost significantly more than a general purpose digital meter so it is easy to understand why they are not as widely used. It is possible to convert an ordinary voltmeter to a potentiometric-voltmeter with the addition of a simple inexpensive converter circuit. Two such approaches are shown above. The circuit using a single meter requires that the potential read on the meter be reversed to give the actual potential. When using the circuit with two meters, the potential can be read directly. These are basic conceptual circuits; it may be necessary to adjust the values of some components to suit particular circumstances.
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