Abstract: Here is dealt the three methods-potential, salt bridge and shutting-down, their respective principle and manner in which the IR Value in impressed chathodic protection of long buried pipelines, and the actural data whereby measured. Taking for example the date recorded on some testing stakes in Zou Xian section of Lu-Ning pipeline. The article compares the three with one another in accuracy, and claims that shutting-down is superior over the other two.