Quantified Scientist Test Finds Apple Watch Series 12 Heart Sensor Closely Tracks ECG Chest Strap

Apple rebuilt the heart sensor for the Series 12, which now measures heart rate every five seconds even when the wearer is not active. In 9to5Mac's own review of the watch, the added data was found to give a more detailed picture of how ordinary moments affect heart rate, including carrying groceries, doing yard work, drinking an afternoon cup of coffee or getting through a stressful stretch at work.

In the tests published by ter Horst and reported by MacMagazine, the Series 12's performance was strongest during indoor cycling, where its readings were nearly identical to those of the Polar H10, producing a correlation of 1.0. During running, the watch recorded a correlation of 0.98 with the chest strap, with ter Horst noting minor deviations at higher heart rates.

Weightlifting produced the lowest correlation of the three activities, at 0.97. Ter Horst described the result as "quite good" relative to competing devices, saying it places the Series 12 above 85% of the devices he has tested for that activity, which he considers one of the hardest for wrist-based heart rate sensors.

Against his longer-term testing of the Apple Watch Series 11, ter Horst said the Series 12 delivered broadly similar heart rate accuracy overall, and that the older model actually performed slightly better during running in his tests. He raised the possibility that Apple is still adjusting its algorithms for the new sensor.

Ter Horst also tested the Series 12's step counter. After manually counting roughly 5,000 steps, he found the watch was generally accurate or only slightly overcounted.

The full video is available on The Quantified Scientist channel.