Tattoos Interfering with the Apple Watch?

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Now that the Apple Watch is being shipped to the general public, some users are encountering an unanticipated problem – darkly-colored tattoos may interfere with the watch’s heart rate sensor and wrist detection.

The problem was first brought to our attention on a Reddit thread. According to his post, the watch would “lock up every time the screen went dark,” prompting the wearer for a password and the wearer was unable to receive notifications. The issue cleared when the watch was placed against his un-tattooed skin. Users on Twitter and other social media sites reported similar issues, as well as the heart rate sensor wildly miscalculating the user’s heart rate.

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The problem, it seems, has to do with photoplethysmography – the method the Apple Watch sensors use to measure your heart rate. The Apple Watch support page contains an explanation as to how this technology works:

 

“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. By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute — your heart rate.”

 

Apparently, darkly-colored ink (especially red and black ink) on the skin absorbs the light from the watch, hindering its heart-rate and wrist detection capabilities. It should be noted that this is issue is limited to tattoo ink, and is unaffected by skin pigmentation.

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One temporary solution has been offered so far – turning off wrist detection allows for push notifications to reach the watch. However, with wrist detection turned off, Apple Pay will not work via the watch. Hopefully a firmware update can fix the issue. Until then, if you have tattoos on your wrist, be sure to demo the watch in store before buying.

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