Table 56. Electrolyte IR drop in millivolts 1.5 meters 

 from bare pipe. 



Electrolyte 



Pipe 



IR Drop 



Resistivity 



Diameter 



(mV) 



(Ohm- cm) 



(in} (nominal) 





50 



1 







50 



5 



1 



50 



15 



2 



50 



30 



3 



5 000 



1 



33 



5 000 



5 



110 



5 000 



15 



225 



5 000 



30 



331 



50 000 



1 



328 



50 000 



5 



1 100 



50 000 



15 



2 250 



50 000 



30 



3 309 



Resistivity of seawater is generally about 20 ohm-cm 

 Current density = 10 milliamperes per square meter 



usually negligible if the reference cell is within 1.5 meters (4.9 feet), of 

 the substructure. 



If the cathodic protection current ceases (turned off or otherwise 

 interrupted), the IR drop ceases instantly. The potential of the protected 

 structure (polarization potential) decays at a relatively slow rate compared 

 to the IR drop, depending on several factors, and, of course, is free 

 from IR drop. If this potential can be measured while the current is 

 momentarily interrupted, it will be close enough to the true polarization 

 potential for normal engineering purposes. This potential is usually known 

 as the "OFF" or "INSTANT OFF" potential (as contrasted with the "ON" 

 potential, read while protection current is flowing). 



The relationship between the various potentials involved is shown in 

 Figure 98. Prior to time of current OFF the only potential measurable is 

 the ON potential which includes the IR drop. After that point it is that 

 of the rapidly decaying polarization potential. The meter response curve 

 shown is typical of a conventional high resistance voltmeter. 



(b) Measurement of the Critical Component . In view of the 

 preceding background on criteria, the criterion of -0.85 volt (-850 milli- 

 volts) with the current momentarily interrupted is recommended. With this 

 criterion in use, the NACE criterion in paragraph 6.3.1.1 will be met. The 

 criterion in paragraph 6.3.1.3 usually will be met also. 



358 



