DTI tips are often threaded for interchange (like drop indicator tips), with small flats to accept a spanner; but the intent regarding user-serviceable tip change is limited only to the tips that originally came with the indicator, because of the above-mentioned importance of the length. Typically a DTI comes with only a few tips, such as a small-ball tip and a large-ball tip. Neither of the above considerations (cosine error or lever length error) matters if the dial reading is being used only comparatively (rather than absolutely). But the avoidance of mistakes of the comparative-versus-absolute-confounding type rests with the knowledge and attention of the user, rather than with the instrument itself, and thus repairers of DTIs usually will not certify the accuracy of a DTI that cannot offer an accurate absolute measurement—even if it is perfectly good for comparative use alone. Such a DTI could still be certified (and labeled) for comparative use only, but because risk of user error is involved, gauge calibration rules in machine shops either demand a "comparative use only" label (if the users can be trusted to understand and follow it) or demand that the indicator be removed from service (if not). See also Indicator diagram, a pressure-volume diagram measured on a piston engine References Further reading External links Practical metal and woodworking applications Simulator - dial indicator in millimeter with graduation of 0.01mm Dimensional instruments Metalworking measuring instruments