The chrono-historical stress gauge was an instrument for measuring stochastic outcomes and timeline deviations.
The Fourth Doctor used the stress gauge to determine the impact of not delivering a Christmas card to Helen Thompson. After delivering the card, the reading did not change. Romana II performed a hard reset of the gauge, according to the instruction manual, and the gauge correctly read zero. (PROSE: The Little Things)
K9 Mark II used the gauge to detect a disturbance in the space-time continuum. (PROSE: The Clanging Chimes of Doom)
At some point, the gauge broke. (PROSE: Present Tense)