The blog

Long-form on how the watch reads water, what each chart in the Analysis tab is actually computing, and what the sauna and cold-water research found in the people it studied. New post every other day.

• What the Analysis tab does, and what it refuses to doSix sections, fourteen documented metrics, forty citations, and no single score. Every number carries its method, its limits and a label for its evidence.
• When a session ends itself, and why the clock runs backwardsWalk off without tapping End and Contrast closes the session after five minutes of rest — back-dated to when you actually stopped, not when the timer noticed.
• Logging a sauna session without an Apple WatchTime a session live on the iPhone, or log one after the fact with its rounds and temperatures. Manual logging is free and always will be.
• What Contrast writes to Apple Health, and what it keepsEvery session is a Cooldown workout with heart rate and water temperature. A second copy stays on your iPhone, so deleting the workout loses nothing.
• Six water temperature bands, and the gap between themIcy, Cold, Cool, Warm, Hot, Very hot — and nothing at all between 20 and 33 °C, because nobody soaks at 26.
• Sauna, bath, plunge, shower: why only two can be detectedContrast has four modalities and a water sensor that can only see two of them. A sauna is dry and a shower never submerges the wrist.
• How an Apple Watch knows you got into cold waterThree gates stand between a temperature reading and a round starting: the sensor settling, a threshold at 20 °C or 33 °C, and three samples agreeing.