How Does Apple Watch’s Heart Rate Monitor Work?

By Marc Saltzman

Underneath the Apple Watch case — the part that touches your skin — is a ceramic cover with sapphire lenses. Beneath that is a series of small sensors, which can give you a slight vibration to tell you something (Apple’s Taptic Engine) but also a heart rate monitor to give you an idea about the intensity of your workout — or simply your resting heart rate.

The heart rate sensor uses infrared (IR) and visible-light LEDs (light-emitting diodes) and photodiodes to detect your heart rate — measured in beats per minute (BPM). The average heart rate is 72 BPM, but when you start your workout and your muscles need more oxygen, your heart beats faster to pump oxygen-filled blood throughout your body.

Every ten minutes, Apple Watch measures your heart rate and stores that information in the iPhone’s Health app. This data, along with other information it collects about your movement, goes toward estimating the number of calories you’re burning.

On how the heart rate sensor works, Apple explains on its website that it uses what is known as “photoplethysmography.” “This technology, while difficult to pronounce, is based on a very simple fact: 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. “When your heart beats, the blood flow in your wrist — and the green light absorption — is greater,” says Apple.

By flashing its LED lights — in an alternating fashion and hundreds of times per second — Apple Watch calculates the number of times the heart beats per minute. Apple says its heart rate sensor can also use infrared light, which is what it uses when it measures your heart rate every ten minutes. But if this infrared system isn’t giving a reliable reading, your smartwatch switches to the green LEDs.

If someone you know also has an Apple Watch, you can send him or her your heartbeat.

For the heart rate monitor to work effectively, Apple Watch must be worn with a snug fit. For example, if your watch band is too loose, the back of the watch case might not touch your skin enough or might move around during a reading. For all the bands available with Apple Watch, you can tighten it a bit if need be.

Other factors can also affect a reading, such as weather — readings might be off if it’s too cold out — as well as fast or constant movement that can move the watch around too much for an accurate heart rate measurement, such as an intense game of squash. And some people just don’t give a good reading at all. It happens.

If you’re not getting a good reading — some Apple Watch owners with major tattoos say the heart rate monitor won’t work, for example — don’t forget Apple Watch and iPhone can work with external heart rate monitors, such as a chest strap, using wireless Bluetooth technology.