Mailing List Archive Mailing List Archive
From http://www.cisatlantic.com/trimix/diving.htm Common Gas Diving Topics by George Irvine- We need to look at some topics in gas diving that are being misapplied due to misunderstanding, and then used to justify deep air diving. #1 - "air tables" are wrong. We all learned some simple things first, like PADI tables, and then Navy tables. PADI tables are Navy no-decompression tables. Navy tables are nonsense - they bend you by not doing the deep stops and then treat you by extending the shallow stops. #2 - Gas tables are more correct: unlike the more arcane Haldanean models in the air tables, gas tables were mostly developed using Bulhmann's theories which started deco deeper, but not deep enough. #3 - In real life, stops start deep, helium is your friend, it makes you fell better after the dive and keeps you from narcosis, it is EASIER to decompress from. The result is that people were first taught to believe that gas needs deeper deco than air, when in fact air needs deeper deco than is in the "air" tables. The term "decompress from gas on an air table" is an oxymoron - there is no "correct" "air" table as we know it, the gas tables are what is needed for air. Having said that, let's look at a diving situation: I want to do the "Doria", but my captain, Janet Beiser, has to limit my gear baggage since there are others on the boat. I can be a dope and dive air ( if Janet would let me), or a convolute and put air in my back tanks and take gas stages, losing redundancy in an emergency, and being forced to air at the worst possible time ( like what happened in the death of Rob Parker), or sharing air with an out of gas diver, who is now hammered , scared, and on my long hose, or I can make the exact mix for one dive and then blow it into oblivion with air, or, I can "Do It Right": I can lose the abject fear of helium and low oxygen mixtures, and make up two sets of doubles with high helium, like 50%, and low oxygen , like 14 percent, and take stages of the exact mix for the depth , probably something like 18/33, and some stages of 50/50 and my oxygen bottle. I dive the stages and try to save the back gas, but let's say I want to use the back gas. I can blow it back three times and still be ok on the oxygen, and probably pretty good on the helium, but what happens to the deco? My first dive for 25 minutes is probably a good hour of deco, my second with the diluted mix is more like 52, and the third more like 45 ( relatively 60,50,40 or padded ratios like that). In other words, for a few extra minutes in the water, I get to do it safely. I then do the same with my other set, and/or my stages were dived first, and then I do a couple of back gas dives . I keep the dives to reasonable bottom times, and end up making the deco gas last longer, and as the deco gas gets diluted or lower, the deco using the higher oxygen "reblows" is getting shorter and shorter anyway, assuming I am giving myself a decent interval between dives. Most of the deco time is on oxygen anyway. POINT here: mixes that are too low in oxygen and too high in helium are not a bad thing - this is ok. The opposite is not. The former means a tad more deco, the later means a lot more risk. For a shallower dive, lets say 130-160, I can take my doubles with something like a 16/40 and blow that up a couple of times for back gas diving in that range with the boat's compressor. The deco pickup over a higher oxygen mix is not enough to warrant the air, especially at the more insidious depths, like 150, that have enough impairment to cause an accident, but not enough to "ring your bell" and make you aware of the impairment. Keep in mind I am talking about trips where you have limited gear space and want to maximize your gas . Technical diving is fun, but it is getting a bad name due to the accidents.The accidents are due to impairment from narcosis. The accidents need to stop. I repeatedly do dives that were not even thought possible by my original dive partner , Bill Gavin, and I do them safely, and I do them all the time, and they are fun to do. If I can do 300 for three hours and then go out to dinner with my pals, then you can dive the Doria or anything else without the self-imposed risk. I know what I am talking about-learn this stuff right and stop the nonsense. If you need information to keep yourselves alive, ask me or any of the WKPP guys, and examine your own misconceptions - there is no such thing as an "air table", and the real risk is death. - G On Tue, 11 Jun 2002, Dr. Jeffrey Mark wrote: > Has anyone saved the posts from a couple of years ago on gas management > for dives on the Doria in order to minimize the number of tanks to take > but not compromising gas mixtures. > > Thanks in advance. > > Jeff > > -- > Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. > Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. > -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
Navigate by Author:
[Previous]
[Next]
[Author Search Index]
Navigate by Subject:
[Previous]
[Next]
[Subject Search Index]
[Send Reply] [Send Message with New Topic]
[Search Selection] [Mailing List Home] [Home]