We must be getting close to the end of this thread. It has been interesting and thought provoking.
We've talked a lot about how you determine the pressure at which you begin your ascent. In the past it hasn't been something I've thought much about. I started with the idea you ascended with about 70 b on a deeper dive and a bit less on a shallow dive. The main thing was to hit the surface with 50 b. I modified that over time and unconsciously got a good feel for how much air I'd need to ascend to meet the 50 b rule. It was never something I had trouble achieving and I've never come close to running out of air except where it was planned on very shallow dives.
People started talking about rock bottom and minimum gas calculations at the start of the thread. I'd never heard of them before. I noticed there were many different methods and numbers being bandied around. So I started to investigate.
In simple terms I took a typical rock bottom calculation that was posted on a link and used those numbers to check what kind of 'minimum gas' pressures you'd need for an ascent from 30 m for a 12 L tank. The answer came out to be about 80 b or 1200 psi in round numbers. I then did a calculation to determine how much air I'd need to ascend from 30 m on a normal dive for a range of air consumption rates. I then added that number to 50 b and found it came to about 80 b. That meant that in normal diving conditions, providing I started my ascent to surface with adequate air to ensure I hit the surface with the required 50 b I'd be okay in an emergency ascent. From what I could gather on this thread most people considered 80 b or 1200 psi a reasonable amount with which to start an ascent from 30 m. I've seen a couple of calculations outside the thread that confirmed the numbers I'd calculated were not excessively low. Some people argued that you needed to add an extra buffer to this and that may have some justification in some circumstances. Also, I found that as you reduced the depth the 50 b rule became increasingly more conservative than the minimum gas calculations.
It seemed to me that whoever came up with the 50 b rule for recreational diving had given the matter careful though. I've never seen or heard anything different in my diving both locally and overseas in depths up to and slightly exceeding 30 m.
So, the point that I've tried to make here is that if you comply with the requirements of the 50 b rule you end up with a similar ascent pressure to what you'd if you calculated it with the minimum gas approach. More for the shallower depth. If you were really worried about the rise in OOA emergencies, why would you bother changing this time honored rule for something that is more cumbersome, more complicated and less conservative for most of the recreational depths.
Some have asked how I'd determine how much air I'd use when beginning an ascent. As I said, it's not something I've really felt a need to calculate before. If you need a value I've given some in the previous posts for 12 and 15 L tanks. Add a bit extra on if you want to make the calculation more conservative.
I noticed a similar debate regarding the size of a spare air tank in the solo forum. You got the same spread in arguments and values we've seen in this discussion.
So my conclusion on the matter is that I personally see no need to change the 50 b rule. I've not seen any credible studies or evidence of anyone who had adequate air to surface with 50 b having an incident during the ascent. I don't claim to be an expert. I've just weighed up what has been presented on the thread and concluded there is no compelling support to the claim that OW divers need to learn how to do minimum gas calculations and base their ascent on those calculations. I hope that decisions made on this matter by the major recreational agencies are based on better substance than the kinds of things surfacing in this thread.
Some people are using the surface at 35 b 500 psi rule. My OW book says that is okay up to 18 m. Beyond that I'd go with the 50 b rule.
Thanks for the discussion.
Some advice on posting on a thread to make it more enjoyable and educational:
- Stick to the topic.
- Avoid personal attacks. They mostly make you look silly.
- Read what people say carefully.
- Make sure the argument you are trying to counter is the one that is being presented.
- If you don't have anything sensible or constructive to add sit back and watch the discussion. If it makes you upset, jump onto another thread.
- Be prepared to have your pet theories challenged.
- Don't expect everyone to agree with your opinions. Be prepared to substantiate them.
- When your argument has clearly been shredded be prepared to admit you are wrong.
- Don't get too worked up. Its only an internet chat forum.
