Factors affecting deep scuba diving

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Please forgive my ignorance, I've been a divemaster for only a couple of years, but I have a question (that will reveal my limited knowledge). I calculate that the ambient pressure at 800' is 3500 psi, near the maximum a scuba cylinder can be filled to. Our regulators are designed to provide gas at the ambient pressure, but if that pressure equals or exceeds the pressure in the tank then would it not be impossible to suck the air out of the tank? At 1200' the pressure would be 5300psi, far beyond what a scuba cylinder could provide. without an air supply to counter the ambient pressure a persons chest cavity would be crushed. Please correct me if I'm wrong in following this line of reasoning. Kindest regards, and keep it safe.

Your math is off. To a rough approximation, you can figure half a pound of pressure per foot of depth in fresh water. Seawater is a bit more, but for rough estimates like this, it's good enough.
 
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 O2 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 CO2 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.
 
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Thanks to Doppler and Akimbo!
 
It never ceases to amaze me how the technical diving community can obsess over the smallest little detail of safety equipment and procedures while ignoring that 800 Lb gorilla staring them the face -- getting bent. Very few make any practical provisions for treating DCS, let alone a forced omitted decompression.

Commercial and military divers almost always require a treatment chamber onboard when decompression is required, thought they also use it extensively for Sur-D-O2 (Surface Decompression using Oxygen). I have seen dozens of cases of DCS in my career and all were successfully treated, beginning within minutes of reported symptoms. I have no doubt that most, if not all, would have resulted in serious long term injury or death if treatment were delayed for hours of days.

I'm the first to support people making their own risk-reward calculations in life as long as they accept the results when things go bad. But I also have to ask what the hell are you thinking? The vast majority of technical diving is performed for entertainment, which of course is recreation. Short bottom times are followed by terminally boring and risk-filled decompression that exceeds their bottom times by several times. They also get to spend big bucks on equipment and breathing gas. Unless there is a reason to dive deep, like a wreck or worksite, why go? This is fun?

Know where your nearest treatment chamber is, that it is manned and ready to take emergencies, how you can get there in an emergency, and how long it will take after you make "the call". Expedition dives that are many hours or days from a chamber should be fully trained and equipped for IWR (In-Water Recompression), which sucks compared to a chamber but beats laying on deck waiting to die. OK, rant over. It is safe for children to come out now.
 
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I recall that the press time to my deepsest saturation dive strorage depth was a little over 4 hours. I further recall the compression arthralgia I experienced to be quite painful, but lessened considerably ( but not completely ) over the 29 days we were stored at 22ata.

I had the benefits of a climate-controlled living space, 24 hour room service from a dedicated topside crew, enormous gas blend reserves, hot water suit comfort, a flush toilet, interesting / challenging work to perform AND...a whopping nice paycheck after de-saturation!

Why anyone would attempt to yo-yo another 500' or so beyond the aforementioned depth of 22ata, on open circuit, leaves me gobsmackedly baffled.

Cheers,
DSD
 
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