Apple picked a good heart rate sensor for the Apple Watch.
After a series of tests by Mac and iOS developer Brad Larson in which raw data was extracted from both devices, the heart rate sensor on the Apple Watch was determined to be nearly on par with the Mio Alpha, a popular dedicated heart rate sensor also worn on the wrist. The Alpha produced sharper peaks, which may suggest more filtering on the Watch, but in most cases the data produced was essentially equal.