ascent rate and gas consumption

[MATTBIN]

OK people I need some help here cos I'm getting confused.
I have 2 questions.
1) I'm working on a gas consumption of 25l/min (at the surface) this will give me 3 times consumtion (75) at 20m and 4 times (100) consumption at 30m. Is this correct.
So a 12l 230 bar cylinder (assume 50 bar safety) means I get 180 bar x 12 (approx) gas = 2160 ltrs of gas to consume. So for the depths quoted I will have 29 mins and 21mins respectively. Assuming no descent time and an absolutely flat profile (I know thats not likely), but just for calculation purposes. Am I correct?
2)Ascent rate, I've been trained to rise at 18m/min but I've seem people posting rates as low as 3m/min, my computer works on 18m/min also (its a Suunto). Which one is correct?
Matt

[Lou]

Yep, I'd say that was right, Matt.

But if you really take 4 mins to descend, take 2 mins to get back up to 5m, do a 3 min stop and then another minute to ascend you would only have about 23mins "bottom time", but 29 mins dive time, plus a safety stop.

Maybe you need a 15 tank?! ;)

[MATTBIN]

Chers Lou.
The figures used were for maths calcs only. tis true that currently I seem to be guzzling at 25l/min surface but on a 12l that is actually getting me close to RDP limits if actually average 15m (on a 20m dive) and consequently lower consumption. So I just needed to be sure of the theory re: consumption and gas planning. The other site seems to be bashing consumption rates around that dont make sense. I needed confirmation that I'm not bonkers. They are also debating ascent rates (its on the equipment page though if interested), you know which site? Matt

[Lou]

Yep, been following the thread, but i think folks are just talking at cross purposes.  About 25l/min is witihin reason  (think about filling paper bags with each breath and how many you might fill in a minute, the visualation hepls when folks are talking about 10l/min or whatever!).  Men use more air that women *in general*, and fitness, stress and temperature all comes into it too.  

How have you worked out your SAC?  

[MATTBIN]

without my log book handy, Difference between start and finish pressure e.g. 170 bar, multiply by tank size (12l) gives me a volume of 2040 lt. Divided by total time (32 mins) = 64l/min, divide by av depth factor i.e. 15mt = 2.5 times gas consumption, therefore 55/2.5 = 25.5 l/min sac equivalent. Ignore the temp change causing press drop/rise as Stoney had same air and water temp. Av depth gained from computer.
Sound good too you?
Matt

[Lou]

It depends how your computer works out the average depth, I suppose.  I am just working this through in my head.  Obviously the best way is at a constant depth, and take the start and end pressures for that depth.  

If you are using the computer's average depth and the computer samples depth at a fairly slow rate then things like reaching downwards to look at stuff, and ascending with your arm raised could cause some skewing.  It on a deeper profile it wouldn't make so much difference, but on a 10m dive, for example, it could.

Maybe (and i'm thinking aloud here for myself as well), it is worth spending half a winter training dive just mincing at a constant depth and average workrate, noting start and end pressures and going from that.  Then averaging depth, worrying about pressure/temperature variances and variable workrates over a dive are irrelevant?

[Hobby]

Hi Matt,maybe these links will help,there again maybe not.I would'nt worry to much about it however at this stage.Work on the maxim of breathing while you can and getting out when you can't,works for me, seriously though, don't become bogged down with things,it can lead to stress. http://www.tdonline.com/articles/psicuft.htm.  there's also,  http://www.flash.net/~table/gases/sac.htm.  Regards,Hobby.

[MATTBIN]

Hi Hobby, on here as well?
No I'm not going to get bogged down with it, just curious that's all. Someone on the other site suggested when I get low start to nick my buddies air (he's 15 fit as a butchers dog and sips air, oh to be like that). Good idea, I paid for it. Good maxim, get out when you cant breath no more!! I've got enough to think about on a dive without worrying about how much I'm breathing. Its just that up till now I've drunk it at a rate that prevented me overstaying bottom time, now I'm getting a bit close so I'll have to a) plan the dive on tables or b) keep an eye on the computer, gets complicated this lark doesnt it.
Take care
Matt

ps the link didnt work, this one does though
<a href="http://www.runtime-diving.co.uk/rtcalcs.HTML

(Edited" target="_blank">http://www.runtime-diving.co.uk/rtcalcs.HTML

(Edited</a> by MATTBIN at 1:38 pm on Sep. 12, 2002)

[Hobby]

Mmm,swine computer,sorry about the links Matt.Anyway,don't worry about your consumption,there are inumerable factors that can affect it and it will settle/improve with experience,familiarity etc.I did a lot of pool work for a while and after a couple of weeks it was'nt unusual to get 4-6 hrs out of a single when working.Throw one of us in a canal in winter and you'd breathe like an alligator in a handbag factory though.Hope this link works,there's some further good links at the end of it too,(it's on Rodale's).Regards. http://www.scubadiving.com/training/...ion/23airtips/