So then at a minimum, what would you consider the proper training and proper equipment to be able to safely do this dive?
If we were to take the 5 Rules of Accident Analysis from the cave diving community and modify them for technical or deep divers, we may develop a strategy based upon the following points:
Cave Diving Rule #1: Be trained and never exceed your limits of training.
In this case, a 220 foot dive is beyond the scope of most entry level Triox and Trimix courses. It is an "Advanced Trimix" dive by most agency standards. Therefore, minimum training in most cases would be Advanced Trimix or equivalent.
Cave Diving Rule #2: Never dive deeper than 130 feet on air.
Some cave divers may scale back 130 feet to even 100 feet or less. Reducing narcosis through the use of helium gases means that a trimix blend should be used that will keep the narcosis at 130 or even better 100 feet or less. Because of the risk of oxygen toxicity, the ppO2's should be kept below 1.3 ATA or less. Of course, this is related to Rule #1 in that the diver needs training to dive trimix.
Cave Diving Rule #3: Always reserve at least 2/3 of your gas supply for exit.
This rule will allow 1/3 of the gas to be used for penetration. In the event of a catastrophic loss of gas for one of the divers at the maximum point of penetration, the team may each have 1/3 of gas per each diver in a single set of tanks for the exit. In deep diving, we must reserve at least enough gas to get two divers from maximum depth to either the surface or to the first deco gas switch while ascending at the correct rate of 30 feet per minute and stopping for all decompression stops until the surface or the gas switch. For a 220 foot dive, this minimum gas may have to get divers from 220 to 70 feet on a bounce without a bottom stage or a 3rd deco gas to include all deep decompression stops beginning at 75% of depth.
Cave Diving Rule #4: Always carry at least 3 lights.
We can apply this to deep diving by always carrying adequate back up equipment. This may require everything from back up gauges and timers, to spare masks, to uplines and DSMB's and redudant buoyancy. This could mean wearing a drysuit to balance the diver's rig to having a liftbag in case of wing and drysuit failure. If entanglement is an issue, 3 cutting tools may be appropriate. Lights are not just for cave divers and night divers, but are also a useful signaling device in open water diving. Having 3 lights such as a primary and 2 back up's may help keep a team together in the low light conditions at depth.
Cave Diving Rule #5: Always maintain a continuous guideline to open water.
In technical deep diving, maintaining a safe supported ascent plan is important. This could range from tying additional deco cylinders not planned for use during the dive to a line, to having support divers in the water, to having a boat, chase boat, or surface crew ready to render assistance to the dive team. Deploying a DSMB signaling HELP may provide much needed support for a low on gas, out of gas, sick, or injured diver. Being able to be picked up if needing or planning a drifting deco is important as is a good deco plan and gas management for decompression gases. All this and more becomes a lifeline to the surface.
While not listed as one of the 5 Rules of Accident Analysis, having a robust and well-maintained gear configuration may be considered Rule #7 and no solo diving may be considered Rule #8. Therefore, a buddy is an indispensable tool for such dives as is having divers equipped with streamlined, modern gear with up to date service and maintenance records.
For me, the minimum gear to safely make this dive would be a backplate and wing, twin AL80's filled with Trimix 18/45, an AL80 bottom stage, 2 deco bottles of 50% and 100% oxygen and an experienced similarly equipped buddy.
But ... I would have been okay with a low volume mask, freediving fins, 3 cutting tools and a spotter if pressed... in my younger days!
