An Apple support document shows how Apple Watch uses colored light to measure heart rate. By flashing green LED lights hundreds of times per second, the Apple Watch monitors how much blood is flowing through a users’ veins when paired with light-sensitive photodiodes. A different method using infrared light measures users’ heart rates every 10 minutes throughout the day, storing that information in the iOS 8 Health app.
The watch will determine whether the infrared method is working properly for the regular updates and can switch to the green LED light system if it’s detecting problems. To get an accurate reading, the watch needs to be fitted tight enough to stay in place, but even under ideal conditions Apple admits the watch won’t be able to get accurate heart rate data all the time.
Strangely, Apple says a “small percentage of users” won’t be able to get the heart monitor to work at all due to “various factors.” People exercising in the cold may have trouble getting an accurate reading, as will users engaging in activity where movements are irregular, like tennis or boxing.
Rhythmic activities like running or cycling fare far better for Apple Watch readings, and other sensors like the accelerometer also contribute to a more complete workout picture. For those having trouble getting a consistent heart reading, Apple suggests wirelessly connecting Apple Watch to external heart rate monitors, such as Bluetooth chest straps. [via 9to5Mac]
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