Many of you will be aware that the 'Target ID' number given by a detector is related to the 'Target frequency'. 'High frequency' targets read low-down on the ID scale, typically because they are small, thin, or made of a metal that's a poor electrical conductor. Conversely, 'low frequency' targets tend to be physically larger, and more likely to be composed of better conducting metals. By careful measurement, it's possible to calibrate the ID scale of a given detector. I've done this for my Fi