Underground Tank #3 - It Isn't Full...But Maybe It Is?

Much of our work involves addressing issues with dry fire hydrants, standard fire hydrants, and underground water storage tanks. Many underground fire protection water storage tanks rely on the Cistern Vent and Sight Assembly to indicate water levels. These assemblies are generally reliable, with thousands in use without issue. However, we are sometimes contacted about sight assemblies that do not display "full," even when the tank is actually filled with water.

In most cases, this issue is caused by the float not moving freely along the metal guide rod inside the vent tube. To resolve this, first remove the vent cap by loosening a few screws to access the float guide rod. Upon inspection, you may notice corrosion or debris on the rod, which can restrict the movement of the float. Using steel wool and a small amount of WD40 on the rod typically restores proper function. After cleaning, reassemble the system and it should operate as intended.

It is important to note that if you must remove the entire rod assembly to carry out repairs, exercise caution to avoid bending the rods. The float will not function correctly on a bent or crooked rod.