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Apple Watch's heart rate sensor on par with Mio's dedicated Alpha monitor

The heart rate sensor in the Apple Watch is nearly on par with Mio's Alpha, a popular dedicated heart rate tracker also worn on the wrist, a new comparison of the two devices has found.

A graph showing a comparison of the devices, shared Wednesday by Mac and iOS developer Brad Larson, comes from raw data extracted from HealthKit after a recent run with both devices. The Alpha produced sharper peaks, which may suggest more filtering on the Watch, but in most cases the data produced was essentially equal.

The two wearables each use optical sensors that measure reflected light to gauge blood flow. Larson noted that the sensor on the Watch takes a reading every 5 seconds while in workout mode, slowing to once every 10 minutes while in regular use.

Apple recently encountered some controversy with the sensor when it confirmed that the Watch may not work properly for people with wrist tattoos, a consequence of using optical technology. The company changed a support page to state that the "ink, pattern, and saturation" of some tattoos can make it "difficult to get reliable readings."

In practice some people have found they were effectively unable to use the Watch without disabling wrist detection, which relies on the heart rate sensor. Turning the feature off also disables Apple Pay, and tattooed people may have to buy a separate tracker to get an accurate heart rate.