ELI5, How does a Scott air paks’ Pak-Alert work?

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I know it uses motion sensors but what tells it you’re moving?

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As you’re moving, the respirator backframe, which the firefighter wears as a backpack, is constantly shifting and moving against your back, as well as moving through space.

A gyroscope can pick up and measure this motion. If it stops measuring it, it enters a pre-alarm condition and then sounds an alarm.

Motion sensors typically use a 3D accelerometer but may also use a 3D gyro if they also want to detect rotation. A 3D accelerometer has 3 accelerometers arranged along 3D axis. 1 detects left/right acceleration, 1 detects forward/backward acceleration, 1 detects up/down acceleration. Each accelerometer contains a mass on a spring structure. When you start moving it the mass moves in the opposite direction. It’s the same effect how you are pinned to your seat when you accelerate in a car. When you stop moving accelerometer the mass moves forward along the direction of movement. The accelerometer measures the distance the mass moves from its rest position by measuring capacitance between the mass and stationary walls inside the accelerometer. Human motion looks like a spike in acceleration as we typically don’t sustain acceleration like cars. An external processor analyses the output to detect the spikes.

Virtually all smartphones on the market has a 3D accelerometer and a 3D gyro. Search your app store for a “sensors” app that plots realtime sensor output and you can see what the accelerometer and the gyro output.