Hey HN! First Show HN for me - niche app, but I think it nails something that hasn't been done properly before.

Furolog (フロログ — furo = bath, log = logging) is an iOS + watchOS app that uses the Apple Watch underwater temperature sensor to automatically track onsen, sento, and sauna sessions. Everything runs and is stored locally.

How it works: The core is an algorithm built on CMWaterSubmersionManager, heart rate, and motion data that detects whether you're bathing, resting, or in a sauna and segments sessions accordingly: hot bath, cold plunge, sauna, rest. No manual input. I think this is one of the few genuinely valid use cases for CMWaterSubmersionManager outside of diving.

Automatic detection isn't perfect — and probably never will be, since everyone's body responds differently to heat. There's a manual mode for that, and anything that doesn't happen in water requires confirmation before it's logged :)

I also went fairly deep into the physiology literature while working on this. HR zones in the app are adapted from sport training zones to reflect thermal stress rather than exercise effort, along with cold interval recovery metrics and a few other tidbits I found interesting while researching.

Why I built it: I've lived in Japan for quite a while now and have been obsessively tracking heart rate in saunas and sentos with generic HR apps. The data was always scattered, and Apple Health's HR visibility is basically limited to during the session. Last summer while diving in Okinawa I realized my Apple Watch 10 had a water temperature sensor and that was the missing piece. This app is built first and foremost for me, but I'm pretty sure other people would appreciate it too.

The app gives you temperature curves, HR analysis, session timelines, and full visit history. There's also an explore view for finding nearby spas, though Apple MapKit's coverage in Japan leaves something to be desired. I'm considering migrating to OSM-linked data that users could contribute to.

Built natively in SwiftUI. Core functionality and session-level stats are free. A one-time purchase (or small subscription) unlocks lifetime/monthly aggregate stats if you want to learn more.

If you're into onsen, sento, or sauna culture, I'd genuinely love feedback. - https://apps.apple.com/jp/app/furolog/id6760871897?l=en-US

Note: Apple doesn't recommend taking Watch into a sauna or hot water BUT after two years of frequent personal use, mine has been fine :)

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