Curious, why does Garmin use a resin for HR sensor instead of glass?

Uh, likely cheaper / better margins - that said, would love to see something like the back of the Apple Watch which I believe is glass, is more integrated, and allows for magnetic charging

Then they can't sell the 20 dollar 1.5ft charging cables  .  Yes, magnetic charging is sorely missed on the Garmin watches.

Maybe it's so that the sensor can be encased in the resin, providing for better optical transmission or something? Just a wild guess on my part. Seems like glass would make their lives a lot easier, not having to replace everyone's watches every year or two, so there must be some reason they've decided it's better to have to replace watches than to use glass.

The resin cracked in my VA3M after about 6 months - I didn't bother getting it fixed because I knew I'd be upgrading it soon anyways, but it was concerning that it cracked so quickly, when it definitely never got hit by anything. I suspect bugspray and sunscreen probably weakened it - but glass wouldn't have that problem. In any case, I've been careful not to get bugspray or sunscreen on my Fenix, and to wash it regularly, and hopefully the sensor never cracks... time will tell.

There is a reason and it’s what makes sense from a cost perspective, factoring in the anticipated failure rate (and resulting replacement costs) during the 1y warranty. You don’t become a multi-billion dollar business without this type of modeling, which they’ve surely done and to probably a much greater level of sophistication

I’ve had several OHR Garmin watches since the FR225 and have never had the cracking or fogginess that has been reported. Even with my 735 which seemed to have the biggest problem.

But we're not allowed to say that...

I'm almost certain that the resin used in the Fenix line is a harder material than the plastic they use in the Forerunners. On the Foreowners I've owned (935, 645) the material covering the OHR would scratch really easily and you could see hundreds of little swirl marks when you held it up to a light. The stuff used in my Fenix 6S Sapphire seems much harder and I can't see any scratches on it. Which I would expect since the entire watch is higher build quality.

Both my forerunner 235 and 935 HR-sensor glass cracked. I have now a Fenix 6x Sapphire. Time will tell if Garmin is improving.

I've done a bit of reading on this subject.  It's purely because it's cheap to do from a manufacturing cost perspective.  Like almost all products, it builds in a way to force people to upgrade after a warranty period ends.  Everything is about sales, thus the need for active consumer advocates.   This isn't just Garmin, it transcends all companies.

agreed.  probably for manufacturing ease and sealing of the sensors and LEDs.