Perhaps I missed it, but the OBP (Over Bottom Pressure) required at these depths is a significant problem for gas planning. 1200' is 534 PSIG (0.445 PSI/FSW). Add 500 PSI+ (or about 100 PSI if using a larger hose like commercial divers) for the optimum demand regulator performance required at that depth and suddenly a lot of the deep mix on your back is inaccessible. Also, you can't use the same RMV that keeps your lungs adequately ventilated at 200' at 1200'.
Edit: Most commercial diving operations use an RMV of 1.5 CFM to calculate gas requirements. My best guess is that no less than 0.75 CFM at that depth on Trimix would be required and probably closer to 1.0.
To make matters worse, you need a much richer O
2 mix on ascent to optimize decompression so there is even more deep mix that is unusable. This won't help deep Scuba divers but here is a related technique that saturation divers use:
I just thought of a gas planning "trick" saturation divers can get away with and illustrates some diving physics to those who never had any reason to think about it. Remember the usable gas calculation? Pressure in tank minus bottom pressure minus first stage IP
We get to cheat on bailout bottles. There is a charging whip in the transfer chamber the bell mates to. We top our bailouts off when we reach holding or saturation depth so it is the max pressure the cylinder allows as read at depth, or over bottom pressure. The tank sees the same differential pressure as it would on the surface, but can hold 445 PSI more surface equivalent gauge pressure at 1000', for example.
Sure some teams have forgotten to bleed the pressure down before decompression. Yep, the blow-out disks pop and scare the crap out of everyone. Nothing like loud and unexpected rushing gas noises to put divers in chambers on high alert instantly.
To be honest, we also overfilled as standard procedure.
The more I have been thinking about this nonsense, the more I wonder why such dives are not done with FFM...
There are, but not the kind that recreational divers are familiar with.
BandMask 18 | Kirby Morgan
Lightweight fiberglass and stainless steel hats have almost totally replaced the use of masks in commercial diving due to better head protection and communications (dry ears). Most divers also find them more comfortable on long dives as long as they don/t have to pop their heads out of the water much.
... Even something as simple as a retention strap on the regulator will keep it in your mouth during a seizure.
Keeping the regulator in your mouth is a small part of the problem. Keeping lips sealed around the regulator is the show-stopper in a seizure or diminished or lost consciousness event.
... I believe I see a problem. I've never used a FFM so I'm not sure how quickly the gas cycles through... However, the mask itself will be holding a certain amount of gas will it not?...
Dead air space is significantly higher in a FFM, or lightweight hat used in commercial diving, than a mouthpiece. It is the price we pay for hardwire communications that is necessary for safety and performing the work. Commercial divers are trained to breathe deeply and as rapidly as they want to manage CO
2 buildup under high gas densities and workloads. However, they have unlimited gas supplies from a large number of sources, most of which are switched for us at the surface and in the bell.
The divers are only responsible for one gas switch, to their bailout bottle.