Yep, that's what happens when you use your limited knowledge to confuse people who have even less knowledge.
The PADI tables before the RDP were indeed the same as the US Navy tables. The problem for recreational diving was that the US Navy tables were designed for divers doing one dive a day, and they created extremely long surface intervals. This made the typical 2-tank dive we all know today close to impossible. PADI did extensive research that resulted in 3 significant changes, not just the one you know about, which made the modern 2-tank dive possible.
- The surface intervals with the old tables were very long because they were governed by the 120-minute compartment, the slowest in that table's system. PADI's research showed that for the overwhelming majority of the dives done by most recreational divers, the 40-minute compartment would work, but they eventually decided to use the 60-minute compartment. This dramatically lowered the surface intervals.
- The old tables required significant rounding because of the big jumps in pressure groups based on those surface intervals. Since rounding always goes to the more conservative, that meant even longer surface intervals. PADI nearly doubled the pressure groups, significantly lowering the amount of rounding being done.
- In order to lower the surface intervals even more, PADI decreased the bottom times for the first dives.
In summary, the first two actions above, about which you seem to be unaware, made the PADI tables considerably LESS conservative because they so dramatically reduced the surface intervals. If you were to use the old table first dive limits and then dive with the same surface intervals as the PADI tables, you would find your bottom time for the second dive extremely short. So the PADI tables are only more conservative for the first dive bottom times. Overall, though, they are significantly
less conservative.
I would be willing to bet that you are only following the old tables for first dive limits. Are you also following them for those long surface intervals, or are you getting in the water when everyone else is on a second dive?
Thank you for such an informative response.
If I am diving on tables, I use the same table set for the entire series of dives, until I hit a 24 hour surface interval. Then, when I start a new series of dives, I may use tables again, or I may use a computer, depending on where I am going & what I plan to do. I mostly use tables for a singe dive to 90' or less in a given day, or repetitive dives to 40' or less. If I am in a hard bottom location that is positively known to be 30' or less, I generally don't bother to look at the tables & I may not even bring a SPG, as hitting a NDL is pretty much impossible in that case. If I am going deeper or doing repetitive dives to more than 40' or diving mixed gasses, I normally use a computer. For dives that are planned to be close to NDL or PPO2 limits, I often carry 2 computers, as I value redundancy when risks begin to increase.
My comment was made primarily in reference to the fact that the standards have changed & many instructors, who are not long timers, are unaware of this. The fact that people giving instruction have a limited scope of knowledge causes me concern. My personal opinion is that, industry wide, this is an issue that does not receive the sunlight it deserves. In many young divers & many young instructors, confidence exceeds knowledge & bravado exceeds experience. I do not fall as deeply into this category as I once did, but I do still try to remind myself that it can still happen to me too. That is a necessary self check, even as each of us becomes better informed through life's continued experiences.
Also the old tables do still work when used as intended. They worked fine for me before the new tables came out. They continue to work the same for me today. If something works, I tend not to change it unless I have a reason to do so. The increased bottom time allowed by the use of mixed gasses & modern computers prompted me to make a change to sometimes using those options several years ago when I began to desire diving those profiles. Prior to that, I saw no need to use something other than the old tables that had worked well for me for 30+ years. When doing simple dives, I still use the old ways, the old tables, the old gas (air), & the old equipment. Sometimes I don't even use a BC. There are some situations where they can be more of a hindrance than a help.
Thank you for pointing out the shorter surface intervals on the new tables. I had not looked at those tables closely enough to notice that difference, as I have never actually used them to plan a dive for myself. I was unaware of that advantage. That is good information for me to know. I never had any intention of mixing & matching tables, but knowing that the surface intervals are also different, will now make me 100% certain to avoid that possibility.
By the way, 2-tank dives were quite common in the early 1980's, when the old tables were still the best thing available. Two tank dives were not "close to impossible" back then. It was important to do your deep dive first in order to maximize your bottom time. Square dives gave you your most efficient use of the constraints. The cognitive work load was a little higher back then, but we still got the job done on a regular basis.
