Such bottom detectors as this one are used for full depth measurements and are applied to wells or drill-holes. It uses a special sensor, which feels the pressure and can tell the distance to the bottom. When it goes down to the very bottom, the rim goes into the probe and activates the audio-signal and a light at the top. And the results of the measuring are then seen on the tape.
Note: the indicator of the datum point is behind the probe holder.