What is Tell Tell Hole? What is the necessity of Tell Tell Hole? Where it stands?

Telltale holes are like holes to get to know if there is a leakage or to know about level of corrosion in a tank or pipe to avoid any catastrophy . In most cases in pipes, the holes are drilled from the outer diamter to outermost part of corrosion allowance periphery, the design is such that when the corrosion allowance is reached a small leakage occurs at that location to indicate the upcoming danger