Sensors are used for corrosion measurements
In the use of sensors for direct corrosion measurements, it is important that the sensors are actually representative of the structure or component that is being monitored.

In the use of corrosion coupons (representing simulated coating defects) for monitoring the performance of CP systems on buried pipelines, the following details may be important:

• Surface condition (presence of scale and corrosion products, surface finish)
• Metallurgical condition (chemical composition and microstructure)
• Integrity of the electrical connections to the coupon and the pipewall, over many years of buried service
• Position and orientation of coupon relative to pipeline and CP current flow
• Perhaps, even stress levels when monitoring stress corrosion cracking
• Degree of soil disturbance
• Temperature
• Coupon size

Attention to sensor details is mostly essential for successful corrosion monitoring programs, where many failures can be retraced to shortcomings in this area. If the corrosion sensor is fundamentally flawed, investments in sophisticated monitoring hardware, computing systems, data transfer and software can be largely wasted. While sensor design, manufacture and installation can clearly be complex,  formal guidelines or standards are, unfortunately, rarely available. Therefore, at times, sensor designs and positioning strategies simulating worst case conditions may be deemed prudent.

References:

• C.D. Stears, O.C. Moghissi and L. Bone III: "The Use of Coupons to Monitor Cathodic Protection of an Underground Pipeline", Materials Performance, February 1998, p.23-31.
• R.A. Gummow: "Using Coupons and Probes to Determine Cathodic Protection Levels", Materials Performance, August 1998, p.24-29.

Information from: www.corrosion-club.com