There are a few considera- 

 tions. There should be provi- 

 sions for reading the potential 

 as soon as possible after 

 interruption of the current, 

 and, to minimize loss of polar- 

 ization, for keeping the inter- 

 ruption period as short as 

 possible. A simple way to do 

 this is by means of the circuit 

 shown in Figure 99. A single- 

 pole double-throw microswitch 

 is used to interrupt the cathod 

 ic protection current and at 

 the same time connect the 

 potentiometer voltmeter and 

 reference electrode to the 

 structure under protection. 

 The voltmeter is adjusted to 

 some expected value and the 

 switch cycled as rapidly as 

 possible. This will interrupt 

 the current for about a tenth 

 of a second. The indicating 

 meter needle will kick up or 

 down scale depending on the 

 potentiometer setting. The 

 voltmeter is adjusted in the 

 direction of needle kick and 

 the switch again cycled. 

 This is continued until a 

 narrow band, probably about 

 5 millivolts wide, is found 

 where no needle movement is 

 to the true OFF potential. 



- ON POTENTIAL 



-INSTANT OFF POTENTIAL 

 (POLARIZATION POTENTIAL) 



■METER RESPONSE 

 (HIGH RESISTANCE VOLTMETER) 



UNPROTECTED POTENTIAL 



TIME FROM CURRENT OFF 



Hr 



Figure 98. Potentials related to 



time on interruption of 

 cathodic protection current. 



observed. The center of this band is very close 

 Care should be taken that the added resistance 



o± the microswitch circuit does not appreciably reduce the rectifier output 

 current. If necessary, the microswitch circuit may be used to drive a low 

 resistance relay switching circuit. This system is not feasible for general 

 pipeline use because of length of required leads and switching of one or 

 more distant rectifiers. The system works well where the protected structure 

 is relatively compact such as subsurface tanks, exterior tank bottoms, 

 interiors of water tanks, oil or water well casings, short pipeline sections, 

 and most marine installations. 



As mentioned before, the time between interruption of current and 

 measurement of potential should be as short as possible. Actual time before 

 significant loss of polarization can vary from fractions of a second to 

 hours. Any factor that quickly removes products of the cathodic reaction 

 will accelerate depolarization. All things considered, current interruption, 

 if done properly, gives the simplest and most reasonably accurate correction 

 for IR drop. At least one supplier provides an electronic system for current 

 interruption using a very short interruption time interval. 



359 



