“Who will know if the mass changed, or the scales changed?”
Easy, you can tell if the scale has changed by using a 1 kilogram weight standard, no wait...
In aviation maintenance, the torque wrenches are calibrated so that certain fasteners are tightened to specific range e.g. “35 to 42 inch pounds”, with a high degree of accuracy.
The torque wrenches themselves, particularly if dropped on the floor, will be knocked out of accuracy/ inaccurate and need periodic checks in any case. The device THEY are compared against, ALSO need periodic adjustment and/or calibration. Presumably THIS device is standarized against yet another standard device, which also has been calibrated against even yet ANOTHER precision device or specification.
Somebody, somewhere, must have tightened a bolt by hand and said “Yeah, that’s about right...”?
I can see that you both get my drift....