One of the fastest ways to determine corrosion rate in pipes and tanks is electrical resistance probe. These probes measure corrosion rate based on variations of electrical resistance of element (exposed to the fluid) in respect to reference element (mounted inside the probe and with no contact to the fluid). ER probes can be used to measure corrosion rate and also to determine efficiency of inhibitors regardless of environment and fluid type. This company provides probes in different models and lengths (From 2 to 36 inches pipe). The reduction in cross-sectional area of an element of metal or alloy as it corrodes is accompanied by a corresponding increase in the electrical resistance of the element.

The relationship is given by
R=ρ L/A
Where:
R element resistance
ρ is the intrinsic electrical resistivity of the Material
L is the length of the section
A is the cross-sectional area of the section

Consequently, periodic resistance measurements of a metal/alloy element that is continuously In practice, resistance is also a function of temperature. Therefore electrical resistance corrosion sensors need some means to compensate for the effect of temperature change. This is achieved by constructing the sensors such that the sensing element has an exposed portion that freely corrodes and a protected portion that is within the body of the sensor and not subjected to corrosion. Such a construction is illustrated in below figure. The resistances of both the exposed (RE) and protected (RT) portions of the sensors are equally influenced by temperature. Consequently, the resistance ratio of the exposed to protected elements is a function only of the loss in metal thickness on the exposed element, and therefore of corrosion.
This resistance ratio (RE/RT) is measured either periodically with a portable instrument, or continuously with a variety of continuous, automatic recording/data logging instruments and read as percentage thickness loss. Thus, total metal loss versus time is directly available, andcorro. sion rate is indirectly available as the slope of the line for metal loss versus time