Metal detectors are typically very sensitive and need to be calibrated periodically. The goal of this calibration is to locate the extremely narrow sensitivity setting that will allow you to locate metal without the metal detector producing excess chatter.
Turn the metal detector on, adjust the sensitivity to the maximum setting and adjust the discrimination to the minimum setting.
Pass the piece of iron in front of the coil at a range of 3/4 inch to 3 inches. A highly intermittent or "spitty" signal indicates that the metal detector may be too sensitive. A very short chirp or no signal at all may mean that the detector is not sensitive enough.
Remove the cover on the belt unit if you need to calibrate your metal detector. This unit contains the detector's batteries and electronic circuitry. You may need to disconnect the pole to take the cover off and then reconnect the pole to the belt unit.
Locate the ground adjustment trimmer. This control adjusts the amount of signal that the metal detector produces when metal passes in front of the coil. You will typically change the ground adjustment trimmer in very small fractions of a turn.
Change the ground adjustment trimmer setting until you get a single clear signal when you pass the iron piece 1 inch to 1.5 inches away from the coil. Place the cover back on the belt unit and reassemble the metal detector.