The most common method to reduce voltage drop error in potential measurements is to use a CP coupon. These are small pieces of metal electrically bonded to the structure so they come to the same potential as the structure. They are placed within a few centimeters of a reference electrode. When the coupon potential is measured, the short distance between the reference and the coupon reduces, but does not eliminate, voltage drop error in the measurement.
Voltage drop error can be further reduced by interrupting CP current flowing to the coupon and measuring its potential immediately before it depolarizes. This measurement is often referred to as instant-disconnect potential. The potential between the reference and coupon is measured as the connection between the coupon and structure is disconnected. This task is much easier if the connection is made through an EDI Model SM Magnetic Switch. This is a sealed reed switch for use in above and below ground test stations. The switch is activated by holding a magnet next to the color band. Green bands denote normally closed switches which are momentarily opened with the magnet. These are most often used for instant-disconnect cathodic protection coupon measurements. Red bands denote normally open switches which are momentarily closed with the magnet. These can be used to electrically isolate a reference electrode in test stations which may become submerged. For best results, the use of model SM-MAG magnets is recommended.
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