Magnetic Level Indicator- Instrument bad Actor

Why will a Magnetic Level indicator give False readings randomly?

Quench system in the Coker units main purpose is to take Overhead Vapors from the off-line Coke drum. It is used during cooling off, de-pressurizing, and quenching stage of getting a drum (700 degrees, down to 200 degrees) before we can take the head off the drum, and De-coke the drum.

The overhead vapor line is tied to the Quench Tower, and goes in close to the middle/bottom of tower. The oil that is in the Quench tower, that accumulates, is sent from tower to the Quench circulation pump. This oil travels from the pump, to a cooler, then to the manual circulation valve, and back in at the Top of the Tower.

The oil falls down the tower cooling off the vapors coming from the Coke Drum. Off that same line from the pump, it splits and goes to a Level control Valve. Lets say the valve is in automatic and the set value is 80% on the valve.

When the level on tower shows 80% this valve will automatically open. If the tower is reading wrong, lets say it says its 100%, sometimes it lies and its really bone dry. The valve opens up and cavitates the pump