There is a good trick you can use. The Earths magnetic field is about 0.5 Gauss. Move to a position away from any iron or steel or magnets, move the probe around in the air until it displays its maximum reading in the North i.e. positive direction and make a note of the reading and then turn the probe round 180 degrees along the axis of the probe by just rolling the probe between your finger and thumb, then the reading should decrease by about 1 Gauss. Make a note of this number and take the first reading from the second reading and then divide by 2, this will give you the value of the true zero. For example the probe may go +10.5 AND +9.6 SO ADDING 10.5 + 9.6 = 20.1 DIVIDE BY 2 = 10.5 Move the probe in the air until the display shows this number, hold it still and then press Zero and the gauge will be zeroed. The probe should then read the Earths magnetic field and by rotating the probe should show as much value North as South of about 0.5 Gauss in each direction.
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