This article refers to the Go Direct^® 3-Axis Magnetic Field Sensor (GDX-3MG)

The GDX-3MG sensor uses two three-axis magnetic-field sensor ICs which are placed at the end of the plastic rod. The precise location of each chip is:
+/- 5 mT – At the location of the dots impressed into the plastic at the end of the wand
+/- 130 mT – 5.5 mm toward the handle from the dots in the end of the wand.

Although we zero the sensor in all three axes using a special rig we have to eliminate all magnetic fields, we do not calibrate the unit. We use the calibration information provided by the chip manufacturer, read the voltage output, do the math and report the field intensities.

A precise method to calibrate this sensor would be to use a solenoid with a known number of turns per cm and of sufficient length that it will provide a uniform magnetic field near the middle. Use a good ammeter and calculate the expected magnetic field at the center of the solenoid using the well-known physics equation. Zero the sensor, with the current off, before doing this measurement.

Note that the earth’s field and magnetic objects (table legs, ring stands, etc, near the sensor can produce fields.