The plasma conversion sensor was part of the technology of a runabout.
In 2369 the runabout used by Captain Picard, Deanna Troi, Data, and Geordi La Forge collided with temporal fragments. After this the fuel of the starboard antimatter pod was completely lost.
Data checked the plasma conversion sensor and found that according to the sensor data, the starboard engine had been in continuous operation for 47 days. (TNG: "Timescape")
The Star Trek Encyclopedia (4th ed., vol. 2, p. 149) defined "plasma conversion sensor" as a, "Starfleet
instrument used to measure the consumption of matter and antimatter fuel during engine use."