Apple Watch is as good a smartwatch as we’ve seen enter the market, but like all gadgetry — it’s not without its quirks. The latest head-scratcher has been reports that Apple Watch won’t give an accurate reading of your heart rate when over a tattoo. To clarify just how their heart rate monitor works, Apple has created a page dedicated to walking us through the Apple Watch sensor hardware, and best practises for getting an accurate reading. They’ve also clarified that some tattoos may interfere with Apple Watch when it comes to reading your heart rate.

First, the gritty, geeky stuff. Apple Watch heart rate monitoring is done via a method called photoplethysmography. Yeah, I’m not trying to pronounce it, either.

What that tech does is measure how much blood is moving through your veins during monitoring. Faster heart rate means more blood flowing, and red blood “is red because it reflects red light and absorbs green light”.
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https://www.slashgear.com/apple-talks-apple-watch-heart-rate-monitoring-and-tattoos-01381730/