This level switch uses a metal tuning fork structure to detect the presence of a liquid or solid (powder or granules) in a vessel:
An electronic circuit continuously excites the tuning fork, causing it to mechanically vibrate. When the prongs of the fork contact anything with substantial mass, the resonant frequency of the fork decreases. The circuit detects this frequency change and indicates the presence of mass contacting the fork. The forks’ vibrating motion tends to shake off any accumulated material, such that this style of level switch tends to be resistant to fouling.
It should be noted that the previous photograph of the tuning-fork style level switch is complete: the fork “paddles” are only a couple of inches long and require no physical extensions in order to properly detect liquid or solid material at that point.