No non-military/commercial training agencies offer 'certification' to 500 ft.

The 'certified' depth for any given course is based on the maximum TRAINING depths allowed by that course's standards (since instructors will be teaching 'novice' divers)--it has nothing to do with the ACTUAL depths to which an individual might dive after completing the course (this applies not only to tech-dive training, but also right down to entry-level dive certificates). After being qualified as a diver--at ANY level--one implicitly agrees to accept responsibility for one's own decisions regarding dive risk, in line with an honest assessment of one's abilities.

For the depth of diving that you are proposing, you would need certification in trimix diving; once learned, the principles of trimix-diving are applicable all the way down. Qualification in closed-circuit rebreather (CCR) use may also be desirable (see below regarding gas consumption at extreme depth).

Those relatively few tech-divers who wish to dive to extreme depths have generally built up their experience over (100s of) progressively deeper dives, over the course of their entire diving careers (often measured in decades). The ones who survive tend to be those who only push boundaries once any/all problems encountered on previous shallower dives have been thoroughly analysed, and subsequent dive plans modified accordingly, and tested successfully.

Since there is an effectively inifinite number of combinations of possible dive profiles and gas mixes to dive them, traditional 'look-up' dive tables do not exist for tech- and trimix-diving. Instead, dive-planning software is used to generate 'custom' dive tables for any given dive. hese programs implement customisable algorithms based on known gas physics and diving physiology, and most are generally reliable for 'shallow' tech-dives--although some seem to have a better 'safety record' than others. However, such software is under continual development and using it, especially for more extreme dives, effectively means that the diver volunteers to act as guinea pig for the developers.

As well as training, experience, and planning tools, physical gear requirements are immense. A general rule for all tech-dive planning is that of kit redundancy: 'If you need one to successfully complete the dive, take two.' Hence tech-divers take e.g. two masks, two depth/timing devices, AT LEAST two complete independant/isolatable gas supplies (and usually three or more for trimix diving), and at least twice as much gas as they expect to need to complete the dive and all decompression stops.

Gas consumption is proportional to depth. At 500 ft (154 m), ambient pressure is >16 times that at the surface, i.e. the diver will require >16 times as much gas to fill his lungs at this depth, therefore he will use his gas >16 times faster than he would at the surface. Using a 'standard' surface consumption rate (SCR) of 25 litres per minute (lpm), this becomes ~400 lpm at 150 m. A standard 'resort' dive-cylinder charged to 200 bar holds around 2200 litres; at 150 m, the diver would drain this tank in about 5 minutes flat. (In practice, most experienced divers have much lower SCRs than 25 lpm--but even halving this would only give a 5 min increase in 'gas-time' at 150 m)

Combining the principle of redundancy with the huge gas volume required for open-circuit diving means that deep tech-divers therefore require much larger, high pressure cylinders than recreational divers. Gas mixtures optimised for extreme depths will also be totally inappropriate for the descent/ascent phases, requiring the diver to carry (multiple) 'travel'/decompression gas(es) as well, sufficient for what may be very long decompression stops (to paraphrase the Matrix: 'He's gonna need tanks. Lots of tanks').

All this weight will necessitate the use of a high-volume, almost invariably wing-style BCD (usually with a redundant bladder/inflator) to ensure that the diver can adjust his buoyancy sufficiently to cope with the weight (i.e. buoyancy) changes during the dive as he consumes his breathing gas.

This is where CCRs can be very useful for tech-diving, since they recirculate the breathing gas, removing CO2 and replenishing the oxygen content, rather than exhausting each breath to the surrounding water. More importantly, their on-board electronics and computer software continually monitor and optimise the breathing mix for the depth, based on an oxygen partial pressure 'set-point'. This means that, for any given dive profile, a CCR diver can carry much smaller/fewer cylinders than would be required by an open-circuit diver, yet still have more than sufficient gas to complete the dive safely. However, CCRs themselves, and the requisite training to use one, are not cheap, and becoming a CCR diver basically means giving up open circuit diving thereafter.