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David, >Occasionally, I do a multi-level, bimble-around-the-place dive and dive an >air computer but deco on O2. I'm comfortable to leave the computer on the >line or bend it when I've done sufficient deco. Note: this is based on rules >of thumb which allow me to do sufficient deco. I have done many dives using mix (usually 18/40) with 50/50 deco gas and an air computer. Like you I am just wandering around and I use the air computer as a guide for the deco. I have found that the air only schedule is longer than a mix schedule with deep stops and deco gas. There are a few rules though. 1. Do the deep stops. 2. Use the deco gas (50/50) for all depths shallower than 70 ft. 3. perform the dive like a multilevel dive (no sawtooth stuff) This is only for some dives where a set of tables is not really necessary (like wnadering around on a reef where the bottom is known but is highly variable) Do not misunderstand me, when on a real mix dive (when I expect that the stops will be greater than 10-20 ft) then the dive is planned and executed on tables (and I bend my poor computer....backup depth gauge/timer. On a dive I did Sunday the computer informed me that I needed to stay at 20 ft while on my 10 ft (table) stop and will finally clear 60 hrs after getting out of the water.... I am glad it is not adaptive...<grin> It still indicated 37 min of deco left when I got out of the water) Steve David Shimell Project Manager, Sequent Computer Systems Ltd., Sandton, South Africa. Email: shimell@se*.co* <mailto:shimell@se*.co*> -----Original Message----- From: David Reinhard [SMTP:reinhard@oc*.co*.au*] Sent: Wednesday, June 09, 1999 5:51 AM To: techdiver@aquanaut.com Subject: Re Last Stop Depth Thanks to all who have contributed to my knowledge with your replies. Unfortunately there are too many replies for me to respond personally to all of you A number of you have pointed out that air is a poor choice for decompression. I fully agree, and it is not my practice to do so. It was remiss of me not to point that out in my original post. Below is my reply to David Shimell which I had intended to go to the list but I think may have only gone to his personal email address. At 01:19 07-06-99 -0700, you wrote: >David > >The reason there is a significant difference between a last stop at 6 vs. 3 >is that you are doing the deco on air which is a poor choice. In reality I don't do my deco on air. I was just running hypothetical profiles to see the effect of different stop depths. While I naturally expected a difference it was the magnitude of the difference that surprised me. While the pressure difference between 3m and 6m is not insignificant I was particularly surprised that 1m could make such a big difference. You are quite right, of course. Deco should be done on an appropriate nitrox mix and/or pure o2. But being very cautious I still deco to the air schedule and take the safety benefit rather than do accelerated deco. So by choosing 6m rather than 3m I have been spending a lot of extra time in the water. However due to the large ocean swells we often encounter where we are diving it may at times still be appropriate to go for the 6m stop. Thanks for your comments, Dave. I also received the following comment which I am particularly interested in:- (quote) "If you have done sufficient deco, then spending further time in the water is increasing your risk. OK, this risk is small but never-the-less a risk and it bites some people. Doing excessive deco is therefore not adding safety." Do others concur with this idea? As I said above I deco on nitrox but operate on an air schedule. In effect this would constitute "excessive deco" for the gas I am using. So is there a problem with this practice? Does doing extra deco increase or decrease your risk? If you are going to increase your deco at what point should you do it, eg only on the last stop? Come to think of it Pyle stops are in effect increased deco beyond that required by the tables. One of the thoughts I have had about this is what actually constitutes "excess" deco. Given the variability in deco tables what constitutes the "right amount" of deco on one table could constitute "too much" deco on another table. Given that we don't really know what the "right amount" of deco actually is for any particular individual on a particular dive profile how can we hope to know what is "too much" deco. (The only definition of "right amount" of deco I can think of is that amount which leaves you unbent after a dive - but you don't really know that until after the dive, since tables are merely a hopeful guideline which fortunately are right most of the time). In reality I would be very surprised if a small variation in stop depth made a large practical difference to the decompressing diver. If this were the case I am sure we would have a lot more bent divers (hmmm... on the other hand we do have a number of divers that get bent while apparently following the tables... I wonder...). As has been pointed out the use of nitrox/o2 for deco significantly reduces the differences I observed, so it is even less of a problem to deco diving than it would initially seem. It has also been mentioned that Zplan adjusts all stop depths if you change the last stop depth. I had noticed this and have found it inconvenient compared to other software that keeps the same standard stop depths and only adjusts the last stop. I guess I didn't consider the possible effect of this change on the total deco time. I noticed another post on this list about a new shareware deco program that does not do this, so I would be interested in having a look at that program when it is available. Thanks to all, regards, Dave. -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- Ocean Internet - "The Quality ISP" http://www.ocean.com.au/info.html -- 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'. 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