Dan, for the sake of educational accuracy for the OP: Let's clarify that PADI is
far from the
only agency that doesn't mandate a strict prohibition against using steel cylinders in conjunction with a wetsuit. In fact, the number of agencies with specific prohibition against that choice of configurations is
hugely outnumbered by those that do not feel the need to spell that out in their equipment-related standards.
For the sake of the OP, and to clarify what is/isn't taught/endorsed... here's what the Tec40 course says specifically about the issue:
The tec harness is recommended because you will use it when you move on to the Tec 45 course, and because you can use a double bladder BCD (BCD with two independent bladders and inflation/deflation systems) so you have backup buoyancy control.
• In a decompression situation, simply dropping weights to restore buoyancy may not be an option because you would have too much buoyancy to maintain a decompression stop.
• Planning for BCD failure must be part of planning any technical dive. The double bladder BCD is the simplest, most reliable option.
• The Tec 40 rig (single cylinder) is not as negatively buoyant as higher level tec rigs, so redundant buoyancy is not mandatory at this level.
.....Dry suits offer the longest durations and coldest water protection.
a. They may provide ample backup buoyancy.
And here is how the philosophy is amplified on the Tec45 course (where the student must be in full technical rig):
Weighting
1. Proper weighting and adequate backup buoyancy remain two areas commonly addressed inadequately in open circuit technical divers.
2. Perhaps the most common weighting error in tec diving is under weighting.
a. Proper weighting means you’re able to maintain your final stop depth with nearly empty back cylinders and either no or near empty deco cylinders – this is what would happen if you had a major problem forcing you into a long deco using your gas reserve, and/or decoing on back gas.
b. If you were not weighted for this, you face a high DCS risk, because you would not be able to remain at stops.
c. As an example, a properly weighted tec diver wearing high capacity doubles and two deco cylinders will be about 14 kg/30 lbs negatively buoyant at the start of a dive, and 4.5 kg/10 lbs or more negative at the end if dive goes as planned.
d. In this example, inadequate weighting would mean that in an emergency situation, besides the original problem, you also have to deal with between 4.5 kg/10lbs and 14 kg/30 lbs positive buoyancy while trying to decompress.
Backup buoyancy is critical in most open water, open circuit technical diving because a diver is substantially negatively buoyant throughout the dive.
1. Failure of the primary BCD without a backup leaves no alternative but to drop equipment (deco cylinders, weights, etc.). This can make the situation worse if the diver must discard deco gases to attain buoyancy.
2. Discarding gear may result in too much buoyancy. If the diver is already in deco, the ability to decompress effectively becomes compromised, growing worse as the diver consumes gas.
3. There is a high likelihood of surfacing with omitted decompression if the diver cannot maintain stop depths, or lacks the required decompression gases, or both.
4. A dry suit may work as a backup buoyancy device.
a. This is primarily an option when the dive will be relatively short and shallow, with short decompression – the gas requirement is low, so the diver is not substantially negatively buoyant (such as when using aluminum cylinders).
b. Limited option – most dry suits will not hold more than small amount of excess gas. Beyond a certain point, it escapes through neck/wrist seals.
c. Several manufacturers caution against inflating their dry suits to gain large amounts of buoyancy because of zipper failure issues.
d. A large volume of expanding gas is harder to control in a dry suit.
e. With deeper/longer tec dives, backup buoyancy control other than the dry suit is generally necessary.
5. Some have advocated using a lift bag or DSMB as a backup buoyancy device. This has several problems:
a. DSMBs and lift bags are not designed as buoyancy devices and are difficult to control in that role.
• They are even more difficult to control while trying to perform gas switches, handle a gas shutdown, etc.
• Even if learned and practiced, it is not a skill one would expect a diver to perform reliably in a real failed BCD emergency over the course of a real decompression. If it has not been practiced at all, it would be especially difficult.
• DSMBs/lift bags do not provide a realistic buoyancy system for positive buoyancy at the surface after completing decompression.
• Using a DSMB/lift bag as back up buoyancy would require the diver to hold on to the bag while dealing with other tasks, or it would have to be clipped to the harness. Either would compromise safety.
b. If the DSMB/lift bag is used for backup buoyancy, then it is not available to send to the surface.
c. Sending the DSMB/bag to the surface and hanging on the line for buoyancy is not a good option either.
• In all but flat seas, this will cause the diver to rise and fall, compromising the quality of the decompression.
• Once sent up, there is no way to adjust the bag’s buoyancy.
• It is not a technique that transfers well to other environments.
• Stress on the line and reel is a major issue. For this to be reliable, the diver would need to carry much heavier line and a larger reel than most tec divers prefer.
d. Trying to use a lift bag or DSMB as a backup buoyancy system unnecessarily complicates an emergency situation, and provides inadequate benefit.
6. It’s worth noting that no dry suit manufacturer and no lift bag manufacturer sanctions the use of their products as tec diving backup buoyancy devices. Some specifically warn against it.
7. The redundant (double bladder) BCD is the most realistic approach to providing backup buoyancy control.a. They are designed for the job and endorsed by the manufacturers.
b. They are used the same way as your primary BCD – a well practiced skill you use on every dive, exactly what you want in an emergency situation.
c. They are applicable to virtually all dive environments.
d. Other than a slightly higher investment, there are no meaningful drawbacks.
e. They are the only real option for open water tec diving in a wet suit.
PADI hasn't adopted a 'balanced rig' approach. I'd guess that was liability-based and also reflects a belief that different solutions have relevance in different areas. They merely demand redundant buoyancy for safety and don't allow lift-bag/DSMB as an option. Most agencies demand redundant buoyancy. I think that principle is why I see so many GUE divers sweating in drysuits here in the Philippines...
Requiring redundant buoyancy does not advocate grossly over-weighted diving. It'd be a huge leap (failure) of logic to make that connection. And if you did, then PADI certainly wouldn't stand alone in this..
We both (I am sure) know instructors who teach for GUE and PADI. If PADI were enforcing configurations or philosophies that were inherently bad, then I fail to understand how those GUE instructors could reconcile teaching for them. The fact is... PADI don't. Neither those instructors, nor myself, find any limitation on teaching safe, well-considered technical diving.
You've seen some PADI technical instructors teaching badly... in hugely unbalanced rigs. That remains an instructor issue. It also remains an issue across a broad spread of agencies.
