How Does The Apple Watch Sensor Work?

As with other smartwatches, the Apple Watch does not entirely focus on the wearer’s fitness, yet it can successfully help you keep track of your health using a set of sensors built into its backside. These sensors include visible-light and infrared LEDs, as well as photo sensors that work together to detect the wearer’s heart rate in beats per minute (usually 72 BPM on average). The Apple Watch is programmed to measure your heart rate at ten minute intervals and stores that data in the Health app, along with other information collected using its other features: step detection by the accelerometer; pulse by the photodiodes; and sleep monitoring by the gyroscope and accelerometer. Using this information, the smartwatch can help to provide a detailed analysis of your daily activity. But how does the Apple Watch sensor work?

Price when reviewed: $399


How Does The Apple Watch Sensor Work?

How Does The Apple Watch Sensor Work?The sensor uses photoplethysmography (PPG) technology, which is a rather simple concept for such a complex name: blood absorbs green light and reflects red light, hence its red appearance.

In this regard, the Apple Watch relies on green LEDs coupled with photodiodes to identify the flow rate of blood past the watch on your wrist. Every time your heart pumps blood, the flow rate in your wrist is greater, and so is the amount of green light absorbed.

The Apple Watch flashes LEDs in alternation, several hundred times per second, making it possible to measure your heartbeat in a given time interval. The green LEDs are only activated when needed to preserve the battery.

Apple claims that the smartwatch sensor also measures infrared light to monitor your heart beat at ten minute intervals when you are not working out. But in the event that the infrared system is giving incorrect readings, the smartwatch just relies on the green LED lights for heart beat data.

How accurate is Apple’s heart rate sensor?

Studies suggest that Apple’s green LEDs are more accurate than Infrared for PPG. But it is crucial that you wear your smartwatch tightly for the heart rate sensors to function properly.

Wearing the watch too loosely may interfere with the contact between the sensors at the back of the Apple Watch case and your skin, resulting in inaccurate readings. Fortunately, the Apple Watch offers a range of bands so you can easily get a good fit for you.

Other factors that may affect the accuracy of the reading include:

  • Very cold weather could make it difficult for the sensor to get a reading
  • Fast or invariable motion that moves the smartwatch around excessively, like when boxing or playing tennis could also cause an inaccurate measurement of your heart rate
  • Major tattoos in the wrist area

Apple recommends that users get a snug and comfortable fit for their watch’s wrist strap to ensure the sensors stay in place.

For motivation, the Apple Watch allows you to record your heartbeat and send it to a buddy, who will feel the measurement played out in vibrations. This is not very practical, but could serve as a great romantic feature if working out with your spouse or partner.

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