Device for sensing gap variation

A sensing device comprising:

• a cover having a window;
• a seat assembled with the cover and comprising a bottom first opening, a peripheral wall, a recess on the wall, and a bottom channel;
• a sliding unit comprising a bottom projection fitted in the first opening, a probe extending from the projection, a hollow cylinder, and two opposite cavities on a periphery;
• a cylindrical rotational unit comprising a rim extending inward from an inner surface, a second opening on the rim, a display plate on the rim, and two opposite guide protrusions on an inner surface opposing the display plate wherein indications are printed on the display plate, and the rotational unit is rotatably put on the sliding unit with the cylinder passing through the rim, the display plate being rested on the recess, and the guide protrusions being engaged with the cavities;
• a first biasing member disposed in the cylinder;
• and
• a second biasing member put on the cylinder and having one end fastened in the second opening and the other end fastened in the channel;
• wherein a gap variation between the seat and a point being in contact with the probe causes the first spring to expand to push the sliding unit further away from the cover and move the guide protrusions along the cavities, thereby rotating the rotational unit with the indications viewed through the window being changed.