Can you please tell me why ? iv been doing this for 2 years and my buddie for the past 11 years on a daily basis, and he is not even technical dive , and we never had any kind of problems. as far as i know , every dive is a deco dive, so where is the problem if im having x deco stop time and im finishing them at y ceiling level.
i admit that the only mistake that im doing is that im diving sometimes deeper than 40 M .
Hey, if you're fine doing deco diving without having an instructor sign off on you as understanding decompression theory/procedures (and I'm in no position to cast stones on that point, though I don't come to SB asking for advice on how to dive beyond my certifications -- not to be confused with my training and experience -- and expect a constructive response, either), what's with you calling going below 40M a "mistake"?
IMO, you're not even doing a very good job of planning these out based on a standard of what info you could glean from SB, Google, and Wikipedia. And gearwise, at least do the two hour PADI EAN course so you can sling an 80 of nitrox 40% to deco on. Hell, sling an extra tank of air if that's the best you can do. But single 15L deco dives to 40m for 30min? You must have the world's lowest air consumption rate
For air at 40m for 30 minutes (assuming a flat profile here, not multi-level, and that you never dip below 40m, which we know you do...), V-Planner at +2 conservatism (and I'd bump to +3 if I were on air for dive+deco) tells me that for deco on air you'll need to do stops at 70ft-1min; 60ft-3min, 50ft-3min, 40ft-6min, 30ft-8min, and 20ft-
49min. Total dive time of 1hr, 42min, total deco of 70min, and burning around 155cf of air. My Petrel would say the same as V-Planner, so let's go to the Cobalt for an easy to generate RGBM profile: same dive on air of 30 minutes at 40m, deco on air would only be 38min at the most risky settings, but at .64cfm you're burning 136cf of gas doing so.
The RGBM profile I just ran isn't so hugely different from what you say you're doing--though pushing a computer to its most risky settings for deco diving on air in order to get in the ballpark of the profile you're using says nothing good about your practice. So, while I might not dive that profile, it's perhaps not entirely insane.
But running just a15L tank for these profiles seems really stupid. Do you carry redundant air? If not, what happens when your reg free flows at 40m, 29 minutes into the dive, and you lose half the gas you had left? Your deco profile is already at the very edge of what's conceivably "safe" and now you don't even have enough gas to complete it. Ditto re: something going wrong and your breathing rate spiking. I won't even discuss what happens if your buddy has a major gas loss and the two of you are left to deco on your air only...maybe it's every man for himself at that point.
You also said you know what stops you need to do in case your computer dies and you can't follow your buddy's computer, that you know how long to stop at what depth. That's a good thing, but do you run a deco plan and take it with you for contingencies like having to stay down an extra 5, 10, or 20 minutes (you probably lack the gas to make those survivable contingencies, but I digress)? Do you have a back up timer and depth gauge, or at least an SMB and marked ascent line you could use? Otherwise, you're guestimating time and depth for the profile you memorized/wrote down.
I won't fault you for just for doing decompression diving without having an expensive c-card saying that someone thinks you're ready to do it--though that is most assuredly the best and safest way to go--but I will fault you for not thinking it out very well at all.
