Hall effect based magnetometers suffer from piezoresistance effect, which means the measured magnetic field gets distorted when mechanical strain is applied to the sensor.
We recently made some studies to get an idea of the magnitude of this effect, and it turns out to be huge, take a look at the orange arrow on this video. It is showing the magnetic vector measured by one of our KCeptors.
As you can see, even a slight pressure with the fingers is enough to dramatically change the magnetic vector orientation, and in consequence the heading of the sensor.
The practical consequence of this fact is: If the PCB torsion change after the magnetometer has been calibrated, the calibration becomes invalid. Changes in torsion can be caused by mounting the KC with screws, or wrapping it in elastic straps to fix it to the body.
We encourage you to test this effect with your own sensors and fixings in order to better understand which fixing system work best.
Let us know how it goes!