How do modern metal detectors not detect cell phones, watches, etc. but still detect weapons?

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In the past year or so, every time I’ve gone through a metal detector at concerts/events, they always say you can bring phones and watches and stuff like that through the metal detector and it won’t set it off.

How can metal detectors not be set off by all the metals in these things?

Are phones and watches made out of different metals than the ones found in weapons?

In: 3

Phones and watches (and coins and car keys and …) are made with metal, but not as much metal in as solid a chunk as a firearm or hunting knife. The metal detector is tuned to ignore small amounts of metal but not large amounts.

The sensitivity on many metal detectors can be adjusted to largely ignore small personal effects, and operators can be trained to ignore certain highlighted zones if they would correspond to a hand with a watch on it.

The goal is mainly just making sure nobody has a gun or large knife on them.

Metal detectors have a sensitivity rating that can be turned up or down.

Something like a gun will be more easily detected because they are usually made of a big chunk of dense steel vs a phone which is typically made out of thin aluminum.

Depending where you go the sensitivity of the scanner will vary. Airports in the US for example are notorious for having different sensitivity ratings depending one which line you go through to get checked.

If you look suspicious they can send you to the scanner that’s more sensitive which is guaranteed to go off if you so much as have change in your pocket and then they’ll search you.

Metal detectors create a magnetic field. When you introduce metal into a magnetic field it disturbs the field (this is just a property that metal exhibits) and the machine is simultaneously detecting and reading the field. The amount of disturbance of the field depends on the amount of metal. So small amounts of metal change the field less than large amounts. The software can be set to only alert when a certain threshold of change on the field has been reached. This can be seen in a “treasure hunter” type of metal detector with the headphones that old people are walking around on the beach with trying to find lost rings. It’s chirps quietly over small stuff and loudly squeals over larger metal stuff. That’s just a speaker connected to the sensor’s signal and amplified. If you programmed it to only squawk at a certain threshold, you’d have the same thing as the concert’s metal detector.