A rhondium sensor was an ad hoc device built at least twice by the Doctor. As they were built from whatever materials were available at the time, the versions created by the third and tenth incarnations varied greatly in design. They were used to detect delta or rhondium particles. These particles indicated the presence of some kinds of space-time fissures.
The device alerted the Third Doctor to the use of the Sontaran osmic projector, itself a device that created a quantum tunnel that connected two disparate points in time. (TV: The Time Warrior, PROSE: Lords of the Storm)