Apple's new watches just arrived - meet the Series 12 and the Watch Ultra 4. All three (Series 12 comes in 42mm and 46mm) feature the new S11 chip, improved optical/electrical heart rate sensors, and an upgraded Health Sensing System.Apple boldly says that both new devices have the most accurate heart rate sensing in a wearable, something Apple verified through a study of more than 1,000 participants, comparing against industry-leading competitors.Both new smartwatches will measure your heart rate every 5 seconds, while heart rate variability is measured 24 times more frequently than...