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Thanks David for your comprehensive reply, and to all those who have responded to my questions about gradient factors. I think my understanding of GFs was on the right track but I needed to be absolutely sure before I would consider using the concept for real deco situations. I have read the Erik Baker article and will re-read it several times to make sure it has sunk in. So with all the info I have now received I feel more confident to make use of GAP or one of the other GF based programs in the near future. In the meantime I will do some more "playing" with various profiles and running some comparisons with the Pyle method. Thanks again, Dave. At 04:34 21-01-00 -0800, you wrote: >Dave > >You would probably be best to send these questions to the Decoplan mail list >decoplan@gu*.co* <mailto:decoplan@gu*.co*> . I hope you don't object to my >cross posting of my reply. > >>JJ refers to figures such as 10-30, 40-60 etc. In GAP the gradient >>factors are in terms of decimals (between 0 and 1.0). Is there a simple >>relationship between these figures (such as a factor of 100) eg 0.2 on GAP >>is equivalent to 20 on decoplanner? So when JJ recommends starting with 30 >>low and 80 high can this be translated to 0.3 low and 0.8 high on GAP? > >The GFs are percentages, so 30 on Decoplan is a GF of 30% and is the same as >0.30 on GAP. > >>Another question, while I am at it. Setting the low gradient factor to a >>smaller number (eg 0.1 on GAP) gives deeper stops, but I presume that even >>the deepest of stops given on these programs is still in the theoretical >>"deco zone"? > >Yes, this is correct. One consideration you should be aware of is that if >you *plan* a square profile dive but *execute* a multi level profile, you >would find that the first stop would be shallower had the deco software been >aware of your multi-level dive. In this case, were you to run a GFLo as low >as 10%, there is a chance that your controlling tissue could be on gassing as >a result on the first stop executed. In this case, I would run a shorter >table or use some common sense and skip some of the deeper stops, depending >on my multi-level profile. > >> Judging by the quick example I worked out last night for a 50m >>dive 0.1 gave the deepest stop at around the same point as the 'Pyle' >>method. Would this be generally true, or are some people thinking that >>'Pyle' stops may be a little too deep? > >What works, works. > >One difference between Decoplan stops with a low GFLo and Pyle stops, is that >Pyle stops do not run in 3m/10' increments as do Decoplan stops. If you >think about this, it means that Decoplan stops will generate a smoother >curve, thereby resulting is smaller variations in tissue over pressurisation. >Since this is one of the objectives of deep stops, I would sooner go with a >GF approach. > >Your point about whether a 10% GFLo equating to a Pyle first stop, and >whether it is too deep is difficult to answer, since it depends on what *you* >the diver want to do. > >However, consider my point about the lack of 3m stops in the Pyle approach. >All that is happening is that Decoplan is operating on a greater level of >granularity. If Decoplan were to operate with stops every 9m/30', then you >would see the first stop for Decoplan arriving deeper than before (unless it >happened to be on the same 9m boundary) and the amount of time required at >each stop to increase and look more like Pyle stops for the deeper stops. >The greater the granularity, the smoother the curve, and the shorter each >stop. The result is closer to the "ideal" curve and specifically the "ideal" >first stop depth, whatever that is. > >> Also why choose (ie what >>advantages/disadvantages in choosing) the deep stops to start shallower if >>you can do deeper stops that are still in the "deco zone"? > >My old model for stops was one of having mandatory stops and deep stops. I >considered these to be in some way different. The mandatory stops were those >that a piece of deco software told me about, the deeps stops were ones I >decided to execute. I *believed* in the deco software and *liked* the deep >stop approach. However, the more I learnt, the less I believed in the deco >software with all of the padding and fudge factors. Progressively, and we >are taking a couple of years, I came to *believing* the deep stop approach >and *doubting* the mandatory stops. This was symptomatic a lack of clear >thinking on my part exacerbated by not having any deco software that would >allow me to properly plan deep stops. The GF approach has unified my thinking >in respect of stops - now I use Decoplan, there is no such thing as a deep >stop. They are all stops and part of the same deco curve. > >So, on to your question, you choose your tissue over-pressurisation, based on >what you consider sensible. I do not vary GFLo for my tables from one dive >to the next. I stick with what I use. The advantage with stops that are >only just in the deco zone is that the tissue over-pressurisation is minimal. >The disadvantage is that there is no significant tissue vs. ambient gradient >to facilitate off gassing. Move the first stops higher and you'll create a >higher tissue over-pressurisation - a bad thing bubble-wise but a good thing >for off gassing due to the higher gradient. (I have not mentioned switching >to other gasses, as this is a separate discussion.) At the end of the day it >is a compromise between reward and risk. You could run a 10% GF all the way >to the surface and get out clean and a brand new pin, but you'd be underwater >for some time. Equally, you could run straight Buhlmann with 100% all the >way and get out really quick but the chances are that you would be bent as a >pretzel. > >The best answer overall is to stick with the recommendations of JJ WRT GFs, >and find out what others do and very carefully and progressively experiment >to see what works for you. > >David Shimell >Email: shimell@se*.co* <mailto:shimell@se*.co*> >Project Manager, IBM NUMA-Q, Sequent Computer Systems Limited, >Weybridge Business Park, Addlestone Road, Weybridge, Surrey, KT15 2UF, UK >registered in England and Wales under company number: 1999363, registered >office as above > >-----Original Message----- >From: David Reinhard [SMTP:reinhard@oc*.co*.au*] >Sent: Friday, January 21, 2000 8:32 PM >To: techdiver@aquanaut.com >Subject: gradient factors > >I have just been catching up with some of the interesting material in the >"Sample Deco Dive - 220 for 25" thread (I've been busy so haven't read it >all yet). I haven't yet seen the Decoplanner software but what i am >wondering is how the gradient factors in that program relate to those in >'GAP'. JJ refers to figures such as 10-30, 40-60 etc. In GAP the gradient >factors are in terms of decimals (between 0 and 1.0). Is there a simple >relationship between these figures (such as a factor of 100) eg 0.2 on GAP >is equivalent to 20 on decoplanner? So when JJ recommends starting with 30 >low and 80 high can this be translated to 0.3 low and 0.8 high on GAP? > > Forgive me if this is a stupid question, or I have got it entirely >wrong, >but the gradient factors concept is new to me. > >Another question, while I am at it. Setting the low gradient factor to a >smaller number (eg 0.1 on GAP) gives deeper stops, but I presume that even >the deepest of stops given on these programs is still in the theoretical >"deco zone"? Judging by the quick example I worked out last night for a 50m >dive 0.1 gave the deepest stop at around the same point as the 'Pyle' >method. Would this be generally true, or are some people thinking that >'Pyle' stops may be a little too deep? Also why choose (ie what >advantages/disadvantages in choosing) the deep stops to start shallower if >you can do deeper stops that are still in the "deco zone"? > >Thanks for any advice, > 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'. > > -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- 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'.
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