All of the following types of level instruments can detect the location of liquid surfaces except:

Hydrostatic head measurement relies on the principle of measuring the pressure exerted by a column of liquid. While it is effective for determining the level of a liquid in a tank or vessel, it does so indirectly by calculating the liquid's pressure at a specific depth. This means that hydrostatic head measurements provide an estimation of liquid level based on pressure, rather than directly detecting the location of the liquid surface.

In contrast, radar, float, and ultrasonic level measuring instruments can directly detect the position of a liquid's surface. Radar uses electromagnetic waves to gauge the distance to the liquid surface, floats rise and fall with the liquid surface level, and ultrasonic sensors emit sound waves that bounce off the liquid surface to determine its location. This direct measurement capability distinguishes these technologies from hydrostatic head, which measures pressure rather than directly sensing surface position.