Decompression Dives

[mccabejc]

I was doinking around with me dive computer and simulator software last night, and realized I've been ignoring the workings of the decompression dive indications and functions on my computer. It raises the question: how many folks unintentionally have drifted past the NDL's during their dives? Seems to me that, at least in my case, air time would be the limiting factor and not NDL times, especially since I tend to dive no deeper than about 50 ft, and get progressively shallower during the second half of the dive while heading back to the beach.

 

[lamont]

I was doinking around with me dive computer and simulator software last night, and realized I've been ignoring the workings of the decompression dive indications and functions on my computer. It raises the question: how many folks unintentionally have drifted past the NDL's during their dives? Seems to me that, at least in my case, air time would be the limiting factor and not NDL times, especially since I tend to dive no deeper than about 50 ft, and get progressively shallower during the second half of the dive while heading back to the beach.

I didn't really do it unintentionally. i did a 2nd dive on my vyper and got punished by the computer for doing a reverse profile and went 2 mins into deco at around 100 fsw. the computer cleared before i got to 50' (was i deco diving or are suuntos rediculously conservative?).

you are correctly, though, that at 50 ft you're going to have a difficult time pushing your computer into deco. NDL for air at 50ft is going to be around 70 mins. at 2.25 cu ft/min (.75 * 3 ata) you're going to need 157 cu ft of air plus your reserve. unless you're doing aggressive repetetive dives with short surface intervals you're probably going to be more worried about air than deco.

 

[mgmonk]

I don't find it common to reach my NDLs in SoCal, but it's not rare either. Three weeks ago, off of San Miguel Island, I was diving with 3 other guys and had to keep a close eye on my computer. It was the 2nd dive of the day and we were at 70 to 80 feet along a 100ft+ wall. Had plenty of air, but wanted to make sure we turned around and came back at 40ft and slowly higher because I was within 10 mintues of my NDL. Just in case somehting went wrong or someone had a problem and went deeper, I wanted to give myself enough leeway to help out but stay shy of my limit.

Then again, plenty of dives would require a nice solid 135 minute bottom time for me to approach my NDL, and that's just silly.

 

[mgmonk]

I was doinking around with me dive computer and simulator software last night,

What computer were you doinking with?

 

[mccabejc]

Suunto Cobra.

 

[kalvyn]

I didn't really do it unintentionally. i did a 2nd dive on my vyper and got punished by the computer for doing a reverse profile and went 2 mins into deco at around 100 fsw. the computer cleared before i got to 50' (was i deco diving or are suuntos rediculously conservative?).

The Suuntos are wicked conservative (GO SOX!). I put my mosquito over its NDL fairly often, I've really stopped paying attention to it to be honest. Most recently, I was diving Dead Man's Wall in Tacoma. We planned on doing 80 FSW for 40 minutes or turning when I hit my turn pressure, whichever came first (I was on a single E8 119, my buddy was diving doubles). Well, my Mosquito was telling my to go to 10' for 16 minutes by the time we actually turned the dive. By the time we got in the shallows I had ran all that "obligation" off and actually stayed down for another few minutes after the 3 minute safety stop had counted off. I hit the beach with 1500 PSI remaining.

I'm not telling anyone to blithely ignore their dive computers here, don't get me wrong. I just knew that I was fine by the 120 rule of thumb, and I planned on turning the dive if I hit my turn pressure before the agreed upon 40 minute run time was up. My computer was giving me garbage info with respect to my NDL and I knew it.

Jimmie

 

[lee3]

I unintentionally went past my ndl times on a 3 dive day with a vytec. really stupid on my part and tought me a BIG lesson as i pushed my tank to the max with the computer showing 2 more minutes at a 15 ft ceiling when i had to surface. I hadn't realized i pushed my limit and had to do a stop at 45 feet for a few minutes using up most of the air intended for my safety stop. i had intermittent numbness in both of my arms for about 4 days. i recall grabbing a beer with some friends about 4 hours after my last dive and at one point my left arm went numb for about an hour. vytec's are notoriously conservative computers and lesson learned, i now surface with much more air than i used to no matter how gorgeous the dive site is i'm enjoying. I'm 6'4 weighing 220 and work out 5 times a week and i was able to feel the effects of pushing my computer despite its notoriety for being conservative. it certainly isn't conservative for me.

 

[Rick Inman]

Went into deco using my Mares M1 (RGBM) on my 3rd dive of the day at the T-docs last month. I did a sim dive before the dive and the M1 gave me 13 mins at 130'. But then went down to 127', peaked at the GPO under the boat for a couple of mins, and the M1 gave me a deep stop (forgot the exact depth & time). We actually hung out longer than that anyway at the aprox depth, but I still don't understand why the simulated dive was different than the actual.

 

[kalvyn]

Here's the graph of my dive I mentioned in my other post in this thread. In looking at it, the yellow triangle is where the computer started counting a deco obligation. It got up to 13 minutes at 10 fsw and I was in the water another five minutes after all obligations cleared from the computer.

Looking at this dive in retrospect, I probably should have stayed in the shallows another 10 minutes, to lower my average depth. I had the gas to do that, but boy was I getting cold!

Jimmie

 

[lamont]

The Suuntos are wicked conservative (GO SOX!). I put my mosquito over its NDL fairly often, I've really stopped paying attention to it to be honest. Most recently, I was diving Dead Man's Wall in Tacoma. We planned on doing 80 FSW for 40 minutes or turning when I hit my turn pressure, whichever came first (I was on a single E8 119, my buddy was diving doubles). Well, my Mosquito was telling my to go to 10' for 16 minutes by the time we actually turned the dive. By the time we got in the shallows I had ran all that "obligation" off and actually stayed down for another few minutes after the 3 minute safety stop had counted off. I hit the beach with 1500 PSI remaining.

I'm not telling anyone to blithely ignore their dive computers here, don't get me wrong. I just knew that I was fine by the 120 rule of thumb, and I planned on turning the dive if I hit my turn pressure before the agreed upon 40 minute run time was up. My computer was giving me garbage info with respect to my NDL and I knew it.

Jimmie

on the other hand, rule of 120 is kind of aggressive -- at least at 70+80 feet. recreational tables generally give 40 mins at 70 and 30 mins at 80. the rule of 120 there agrees with the US Navy dive tables.

OTOH, you're diving according to DIR deco rules -- you're not doing 'square' deco by popping to 15 feet and staying there for 3 mins -- so your deco profile is much more conservative than the recreational deco which is taught.

but, you probably weren't NDL diving there. if you'd had to do a rapid ascent there you might have blown off some real deco obligation. at the same time your mosquito definitely was being overly conservative telling you that you had 16 mins of deco obligation.

and my entire point is really just that kids shouldn't try this at home... i wouldn't want to see someone diving rule of 120 and ignoring their computers without better training...