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Apple Watch heart rate monitor explained

by Todd Haselton | April 21, 2015April 21, 2015 8:00 am PST

Plenty of wearables on the market feature heart rate monitors. Some gadgets are more fitness focused, while others like the Moto 360, also serve as smartwatches. A lot of them feature a green light at the bottom that can be used to determine your heart rate at various intervals, and new documents published by Apple show how that light works on the Apple Watch.

According to the documentation, the Apple Watch will grab your heart rate once every 10 minutes, or more frequently when you’re exercising, but the Apple Watch needs to be tight enough on your wrist for this to work. If the sensors aren’t making contact with your skin, for example, it won’t be able to provide an accurate reading.

“The heart rate sensor in Apple Watch uses what is known as photoplethysmography,” Apple explained. This technology, while difficult to pronounce, is based on a very simple fact: Blood is red because it reflects red light and absorbs green light. Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist — and the green light absorption — is greater. Between beats, it’s less.”

The Apple Watch flashes the green LED “hundreds of times per second,” Apple said, noting that this is just one method used by the Watch. There’s also an infrared light that will activate once every 10 minutes to grab a snapshot of your heart rate, though Apple admits this method isn’t as accurate as the green LEDs. There are other boundaries, besides a loose strap, that can make it hard for the watch to grab a reading. That includes vigorous exercise during exercise activities such as boxing, or out in cold weather. It should work just fine during a jog, however.

Apple says it tested the Apple Watch with thousands of people already, but we’ll get a much better feeling for how all of these features, like the heart rate monitor, work when it officially starts shipping this week. We should have our unit on April 24, so stay tuned for additional coverage.

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Todd Haselton

Todd Haselton has been writing professionally since 2006 during his undergraduate days at Lehigh University. He started out as an intern with...