You can call it being direct, I can call it chest-thumping and being mean. Tomato, to-mah-to.
We did a scooter-assisted plummeting descent on one of our tech dives off the Brothers in the Red Sea. I was worried about my ears, but in fact, it was actually great fun! I had some pretty busy hands, between the scooter, putting air in my dry suit, and clearing my ears, though.
"What other sport is there where a cute woman has trouble getting rid of her underwear?" Doppler
For me and the GUE/UTD teams I dive with, hypoxic dives are scooter dives. So breathing bottom mix on descent from the surface is not a problem, C02 from work is not a problem. You do need to be aware of what you're breathing on descent and ascent.
For 18/45... I'll breath that all day long on the surface.
For 15/55... not going to breath it for very long on the surface or above 10'. Fine for backgas breaks at 20', going up on 02
For 12/65... immediate decent to 25' once reg is mouth. Fine for 20' backgas breaks, Need to be on 02 for 20'--> surface.
For 10/70... immediate decent to 25' once reg in mouth. Marginal for 20' backgas breaks; most use backgas for 02 breaks, but some use another deco bottle for breaks (35/25), going up on 02.
In Socal, I can't think of any GUE/UTD teams that use travel mix for OW dives on 15/55, 12/65, or 10/70. It's reg in mouth just before decent and hit the trigger down... usually pausing at 25' for team checks, then trigger down to the bottom. So for descents, we're aware of what we're breathing, but not worried about hypoxia since we at a safe depth within a couple breaths.
Descending on a gas that is not max MOD scares me. Like others have said, there's too much going on during descent. I can think of a couple times where we breathed the 50% bottle (with 10/70 or 12/65 backgas) while clipping off bottles from a hang line at 5-10', but we then returned to the surface and descended on backgas.
If we couldn't breath on the surface without a reg, we would use the 50% and plan to meet at 30' for the team-verified switch to BG or bottom stage before final descent. Personally, I'm not doing a 15/55-10/70 dive without a scooter. If you were doing a swimming decent against a raging current on a 45degree anchor line, with low viz, breathing 12/65 or 10/70, then travel gas would be a good choice.
For me the smoothest, safest 300' dive is: Open circuit, 12/65 back, 12/65 bottom stage, deco 35/25, EAN50, 02. Stage and 120' bottles on the left, 70' and 02 on the leash. Surface checks when possible, Descend on the stage, stop at 25' for checks, then trigger down. Switch to BG as a team. Move bottles at 110' and 60'. Use backgas for 02 breaks... team verification you are on 02 for ascent from 20'!
Last edited by rogerbly; 04-30-2010 at 11:38 AM. Reason: minor typos
Iin order to clean this thread a bit, you are only allowed one post per hypoxic tmx dive you did this year.
Grumman Goose, 25@2XX', 15/55, bottom stage, 70', 20'
2x USS Burns(?), 25@270', 12/65, 120', 70', 20'
I also love the trigger-down descents, if everyone's ears can take it. On the Burns, Andy, Nick and I took about 2 mins from 25' to 250'. They make fun of me for barrel-rolling down the line, but it's a great way to keep an eye on teammates behind you... and fun too.
When doing backgas breaks on 12/65 or 10/70, I sometimes notice a slightly higher heart rate the first couple minutes after the switch. That could be due to the lower ppO2 or that shocking cold from the helium... burrr.
the increased HR could be something related to a change in the density of the breathing gas suddenly. the lungs are used to a nice thin gas and suddenly a thick one comes down the pike, and the heart has to compensate.
Interesting. I'm definitely the opposite. The more helium in the mix, the colder I get.
I also taste 02 (hydrogen peroxide taste) and swear I can taste when I'm below ppO2=1.6. I often notice a moderate heart rate increase when switching *from* O2 to hypoxic backgas. Occasionally I notice an brief increase in heart rate when first switching to 02 or 50%. This is about a minute after stowing the regs, so it's not due to muscle exertion. I've seen my heart rate change on my Galileo data.
nice post Roger. clean and simple. all in order. It's still a fast drop but if your ears can take it -- go for it
Joel Silverstein, VP COO
Tech Diving Limited
TDI Advanced Trimix Instructor Trainer 0125
Need to reach me ? Cell / Text 928-230-3680
Helium in the drysuit is a different matter, where its the thermal conductivity that matters. And there, helium is a small molecule that travels faster and collides less often and therefore heat applied on one surface is transfered through the helium gas to the outside of the container (your drysuit) much faster. It still takes less heat to warm up the gas in your drysuit, but heat will be bleeding out the outer surface of your drysuit and into the water much quicker, and more heat will be bleeding out of your body into the helium to replace the lost heat.
My first DIR dive buddy! Awesome explaination; when are we going diving again?