I used to work a metal detector (courthouse) and I agree that they are all calibrated differently. They should be calibrated to alert to anything weapon sized while not going off for zippers and underwire bras.
What we used to do was have a person with bracelets or a watch go through with their hands over their head. The metal detector shows what part of the body it is alerting on so if it beeped and the only part lit up was above the head, we knew that it was the bracelet or watch (by the way, precious metals did not alert but costume jewelry would).
If a person goes through with the wrist at their sides, you don't know if the alert was for the writs or the hips and then we would have to wand them.