Abstract: The level of liquid in a tank or a boiler is determined initially by enclosing the liquid in a column and then sensing the liquid surface in the column by causing a plunger to pass through the non-liquid medium in the column above the liquid to the liquid surface and noting the abrupt change in descent rate of the plunger when it hits the liquid surface due to the great difference between the viscosities of the non-liquid and liquid mediums. The level of liquid in the boiler is reflected on a continuing basis by permitting the plunger to float or levitate in or on the liquid as it is supported by an entrapped gas bubble. The difference in the position of the plunger when it encounters the liquid surface and the position of the plunger thereafter, as it is supported on the liquid is the correction factor that is maintained steadily until pressure or temperature changes necessitate a new determination of that difference with a fresh gas bubble at the current temperature and pressure.