potential, however, to invert the electrode two or three times, to equalize 

 internal temperature. 



All reference electrodes are subject to polarization to some degree 

 when placed in an electrical circuit with current flowing. This means that 

 any appreciable current drawn by the potential measuring circuit (voltmeter) 

 would result in lower potential readings. In general, the accuracy of 

 potential measurements will increase as the measuring circuit current 

 approaches zero. The most accurate voltmeter suitable for field or labora- 

 tory use is the standard-cell potentiometer with a suspension- type galvano- 

 meter. When balanced for reading of potential, the reference cell current 

 is zero. The most easily handled and easily operated instrument for general 

 field use, ashore or afloat, is the electronic type, preferably with an 

 input resistance .(sensitivity) of 10 megaohms or more. This ensures a 

 reference cell current of a fraction of a microampere. Conventional volt- 

 meters of 1 QQQ or 5 000 ohms per volt are not suitable for these measure- 

 ments. 



When a suitable voltmeter is connected between the substructure and the 

 reference electrode, the measured potential is a combination of the poten- 

 tial between the reference electrode and electrolyte (soil or water), and 

 the potential between the substructure and electrolyte. The substructure- 

 to-earth half -cell potential is the variable of interest. The reference 

 half-cell potential is considered constant for practical engineering 

 purposes. The copper sulfate electrode has been found to be practical for 

 field use as its half-cell potential is reasonably constant over a wide 

 range of conditions. The actual value of its half-cell potential is not 

 important. The total cell potential, as measured, is the one used in 

 engineering practice. This is the structure-to-electrolyte potential. 

 Several discussions of the copper sulfate electrode covering its theory, 

 history, and development are available in the literature (NACE, 1979). 



It has been established that the most anodic areas to be expected on a 

 freely corroding steel structure in moist soils and waters will show a 

 potential of about -0.8 volt measured to a copper sulfate electrode con- 

 tacting the electrolyte as close as possible to the anodic area. Allowing 

 for some variation in potential of the most anodic areas, the value of 

 -0.85 volt to a copper sulfate electrode contacting the electrolyte has been 

 adopted as a practical indication of satisfactory protection. Wide and 

 varied experience in many environments has indicated the accuracy of this 

 criterion for practical field use. 



While the copper sulfate electrode is by far the most common, other 

 electrodes are in use. These include calomel (usually the saturated type), 

 silver-silver chloride, and occasionally pure zinc. Other common metals 

 are not sufficiently stable for reference electrode use. The calomel 

 electrode, while very stable, is more adapted to laboratory work than field 

 use because of its largely glass construction. The silver-silver chloride 

 electrode also is quite stable and may be encountered frequently in marine 

 operations. Pure zinc (Special High Grade, 99.99 percent pure) is occasion- 

 ally used as a reference electrode but is subject to variations of as much as 

 5Q millivolts, making it suitable only for approximate values. Table 55 shows 

 the potential readings for these common reference electrodes compared to the 

 reading for copper sulfate electrodes at 25° Celsius (77° Fahrenheit): 



355 



