There's a lot of excitement around the Apple Watch, particularly among fitness nuts who want to try out all the new apps (like Megan on TreeHugger) and the heartbeat monitor that's built into the back of the phone. Some of those fitness nuts have tattoos, and they're finding out that the sensors don’t work on decorated wrists. One inked Apple Watch owner posted on Reddit about how he thought he had a defective phone until he tried an experiment:

...when I decided to try holding it against my hand (my left arm is sleeved and where I wear my watch is tattooed as well) and it worked. My hand isn’t tattooed and the Watch stayed unlocked. Once I put it back on the area that is tattooed with black ink, the watch would automatically lock again. 
The sensors on the Apple Watch

Those little green lights have to get through your skin. (Photo: Apple)

The sensor works by shining green LEDs at the wrist. Just as we can pick up a pulse by feeling the changing size of an artery with each heartbeat, the sensor measures the changing reflection of the red versus the flesh around it as the artery expands and contracts. It's an optical technique called photoplethysmography.

But the light has to be able to get through your tissue, and evidently the pigments in the tattoos block the light of the LED.

The Reddit user complained that there was no information about this on the Apple website, but as of April 29 there is a support page with this information:

Permanent or temporary changes to your skin, such as some tattoos, can also impact heart rate sensor performance. The ink, pattern, and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings.
The fix for the heavily inked is to get an external Bluetooth chest strap. They are a lot more accurate too, if that is any consolation to the people who shelled out for the watch. Wrist based heart monitors are notoriously finicky; they don't work well in the cold when the body tends to send less blood to the extremities; they prefer repetitive activities like running or cycling to bouncy irregular ones like boxing or tennis. But our expectations always seem to be higher with Apple products and given the cost, they should be.

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Lloyd Alter ( @lloydalter ) writes about smart (and dumb) tech with a side of design and a dash of boomer angst.