My watch died on my 86m SAT dive

I don't know for sure but I suspect that Descent devices were never designed or tested for saturation diving. The depth (pressure) should be fine but it's meant to be immersed in water. If you had it in a dry chamber then perhaps the smaller helium molecules leaked through the seals and caused mechanical damage from differential expansion? Regardless of the exact cause, if it won't turn on at all then you'll have to send it back to Garmin for service.

When the device detects a pressure increase it's supposed to automatically go into the last dive mode used. So if you had last used it in Apnea mode from spear fishing then I think it would go back to that mode, although I haven't tried that myself.

Are you certain the 10/90 heliox gas mix is correct? Is that what you breathed in the chamber, or during the working lock out dives? 10% oxygen at 86 m would equate to a PPO₂ of 0.96 bar which seems excessive. The usual safety guideline is to keep long term PPO₂ exposure below 0.5 bar to prevent accumulation of oxygen toxicity.

Thanks for your reply Nradov 

Yes the mix was 10/90 or at least that's what the lst said and what we wrote on our logs, making numbers that might have been the mix to decompress once back in the chamber at 50m to surface.

I had taken my watch on air decompression surdo2 dives and never had the problem with it. Noww that you say about the lightness of helium molecules makes more of a sense. Ive hear of prior divers taking their phones and tablets in and most of them never had an issue, although some other said the screen went off during the stay, and came back to life once out of the can. But my watch still doesn't react. I wonder if the helium stayed traped in it. I brought my sony cam with a housing up to 100m and also went dead for 5 days and then csme back to life.

Strange of this because in the bell we had a backup digital gauge from UWATEC , and that one has been in constant work.

Ill bring it to a garmin tech and see if he can open it and from there ill come up to something.

If your chamber storage depth was 50 m and you were making working excursions in the bell to 86 m then a 10/90 mix would be more sensible, although the oxygen level still seems high. Most sat operations try to keep storage PPO₂ around 0.40 bar. If they kept you at 0.60 bar for days at a time then I would start to worry about chronic pulmonary oxygen toxicity.

Frontiers | Commercial Divers’ Subjective Evaluation of Saturation | Psychology (frontiersin.org)

Regardless of whether there is still helium trapped in your watch or not, the damage is probably already done. If one part inside the watch expanded a little relative to other internal parts then that could have cracked a contact or something. Even if it eventually starts working again I wouldn't trust it. Send it back for replacement.

The Uwatec (Scubapro) digital gauge doesn't have moving buttons or anything penetrating the casing, plus it's filled with some kind of epoxy. So they're pretty much indestructible. Good backup devices.

You are right about the levels of oxygen toxicity on time of exposure, as i said the lst told us they used that mixed , for sure  they worked different mixes depending on our depth during lockout and decompression to surface, i have a feeling he said that number as  just one of the ones they used , on our ppo2 bell and chamber gauge we always monitored 460 to 480 mlb, of what i can remember.

Anyway, do you think Garmin will replace the devide. Or shoul i just go for the descent 2?

The other thing is i wonder how can i get all my marks from spearfishing.