How accurate is my phone's magnetometer?

Question

I'm trying to build a linear motor, for which purpose I bought this large permanent magnet. The website claims it has up to "13,500 Gauss", which is 1.35 tesla. However when I put my phone against the magnet I'm measuring about 3 milli tesla. That's orders of magnitude different!

I thought "Perhaps the field strength is dropping off quickly over the distance". But my phone and the magnet were literally touching. Not only that, I tried many different spots against my phone (in case perhaps the Magnetometer was in some odd corner), but the max I could ever measure was 3.5 milli tesla.

So then I thought, "Perhaps the phone's magnetometer is just not designed for this range of magnetic field strength; It's really supposed to be just the a compass, and the earth's magnetic field is around 50 micro tesla". But I don't know. Which one is it?

How accurate is my phone's magnetometer? And how wide is the range in which it is accurate?

(My phone specifically is a moto g(7) plus, but I suspect nearly all phones have the same or a very similar magnetometer)

Answer 1

So then I thought, "Perhaps the phone's magnetometer is just not designed for this range of magnetic field strength; It's really supposed to be just the a compass, and the earth's magnetic field is around 50 micro tesla".

Likely it's that. I don't know what chip is in your phone, but a Bosch BMM150 or BMM350 would be a pretty likely choice. The BMM150 datasheet says "Magnetic field range: ±1300μT (x,y-axis), ±2500μT (z-axis)", so the largest magnitude it could read is 3103μT. The BMM350 says ±2000μT all axes, which works out to 3464μT if every axis is pinned.

Within that range, it's pretty accurate — probably within 1μT.

Answer 2

How accurate is my phone's magnetometer?

In its intended range of operation? As a rule of thumb they are quite accurate, better than 8 bits of accuracy after a 2-point calibration, and more than 10 bits of resolution. That's for the cheapest sensors. There are of course much better sensors that cost more and perform accordingly.

However when I put my phone against the magnet I'm measuring about 3 milli tesla.

Putting your phone against a 1T magnet is a bad idea. Everything ferrous in that phone will probably get magnetized, and will cause permanent offset for the built-in magnetometer.

As you have learned, the built-in phone magnetometers are designed to sense Earth's magnetic field and are orders of magnitude too sensitive to measure 1T fields.

You can fairly easily measure the strength of a magnet by dropping it through the center of a coil with a couple turns of wire connected to a resistor. The height of the drop above the coil determines the velocity when crossing the coil, and the induced current will be proportional to the strength of the magnet.