| 摘要 |
A drilling tool (11; 23) having a cavity (8, 21) for containing a detectable substance. A wall (13) of the cavity or a passage (29) extending from the cavity (21) is located adjacent a surface (17; 28) exposed to wear. When the surface (17; 28) is worn down to a certain level the detectable substance is released to the exterior of the tool (11; 23). The detectable substance may then be detected in pumping fluid returned to the surface to indicate that the tool has been worn to a certain extent. |
