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View Full Version : VERY different bottom times.....why?



divingchef
11-21-2007, 18:23
Im a newbie to computer use underwater and out of too many years on boats (redundant systems) or reluctance to enter into (and trust) the modern age, I dive with two computers....a tusa iq700 in my console and an oceanic veo 250 as a back up on my wrist.

I have noticed EXTREMELY different remaining bottom times when diving with the two. Up to 30 or 40 min difference. I checked and double checked that I had both set to 21% O2 and I did. Can anyone help me as to how it can be SO different?

Thanks in advance.

awap
11-21-2007, 18:29
Different algorithms. Oceanic (pelagic ?sp?) are know to be in the most "liberal" class of computers. Yet there is no eveidence that they are any more prone to bending a diver than more consersvative models. I decided long ago that Oceanic computers were my choice.

divingchef
11-21-2007, 18:36
thanks, makes sense....naui is a bit different than padi than USN than NOAA etc....I was very surprised at how different the times were.....Its tempting to ignore the conservative computer....thanks...

p.s. cool deal with the tooth fairy thing...my dad was a dentist and he tried to make a big deal out of it for his patients....oh yeah, we have a golden too....her name dixie 1 year old.

awap
11-21-2007, 18:46
thanks, makes sense....naui is a bit different than padi than USN than NOAA etc....I was very surprised at how different the times were.....Its tempting to ignore the conservative computer....thanks...

p.s. cool deal with the tooth fairy thing...my dad was a dentist and he tried to make a big deal out of it for his patients....oh yeah, we have a golden too....her name dixie 1 year old.


After watching my wife's scubapro EDI get real close to deco while everyone else had plenty of time left, I knew we were dong something wrong. That's when I did a little research and transitioned to Oceanic computers. It doesn't matter much for local and shore diving. But when I hit the Flower Gardens or Cozumel, I'd like to have all the bottom time I can get.

My daughter is the tooth fairy. I just give her a little advertising.

quasimoto
11-21-2007, 20:22
That is one thing I like about my Cochran. I can set it to whatever conservancy I want.
Oceanic and Aeris are made by the same company just in case anyone didn't know.

in_cavediver
11-23-2007, 06:40
I just want to add one thing here. Remember, the computer is applying a model based on pressure/time to predict whether you get bent. It has zero knowledge about you, the conditions, temperature etc.

Just because a computer says you have time doesn't mean you can't/won't get bent. (Those are called undeserved hits). It's all best guess and the magic limit changes dive to dive, day to day. Which brand of computer you use has nothing to do with the nitrogen loading you have from any given dive. When changing brands/models, you are just choosing how aggressive you want to dive.

Dive safe.

Defman
11-23-2007, 07:22
Oceanic and Aeris are made by the same company just in case anyone didn't know.

And Sherwood, I'm sure there are others too...

shadragon
11-23-2007, 10:23
That is one thing I like about my Cochran. I can set it to whatever conservancy I want.
Oceanic and Aeris are made by the same company just in case anyone didn't know.
I dive with a Cochran and Oceanic VEO 200 as a backup. I set my Cochran to a middle of the road conservative figure and use it as my primary, but use both to guide my depth and dive duration within NDL. If I see one getting low on bottom time I reduce my depth.

Until the perfect DC comes along that gives you a real time blood gas analysis your DC is just a guide. If you dive within minutes of DECO, then stay at the safety stop for a few extra minutes (As long as you have sufficient air, of course). It will not hurt anything and will let you off gas a little before surfacing.

shadragon
11-23-2007, 10:29
After watching my wife's scubapro EDI get real close to deco while everyone else had plenty of time left, I knew we were dong something wrong.
Have you considered that you may have been the only ones doing it right? If Diver A comes up before Diver B (with all other factors being equal) they will have less N2 loading in their system. The other divers may have chosen to set their DC's to the most liberal setting or been using a different algorithm altogether, or had NITROX, or whatever... You dive at your comfort level and not the one of the guy beside you...

Safe diving...

divingchef
11-23-2007, 11:12
After watching my wife's scubapro EDI get real close to deco while everyone else had plenty of time left, I knew we were dong something wrong.
Have you considered that you may have been the only ones doing it right? If Diver A comes up before Diver B (with all other factors being equal) they will have less N2 loading in their system. The other divers may have chosen to set their DC's to the most liberal setting or been using a different algorithm altogether, or had NITROX, or whatever... You dive at your comfort level and not the one of the guy beside you...

Safe diving...

Very good point.....

awap
11-23-2007, 11:27
After watching my wife's scubapro EDI get real close to deco while everyone else had plenty of time left, I knew we were dong something wrong.
Have you considered that you may have been the only ones doing it right? If Diver A comes up before Diver B (with all other factors being equal) they will have less N2 loading in their system. The other divers may have chosen to set their DC's to the most liberal setting or been using a different algorithm altogether, or had NITROX, or whatever... You dive at your comfort level and not the one of the guy beside you...

Safe diving...

I'd rather know exactly where the edge of the cliff is. Then I can decide how much of a safety margin I want to add. I don't want the computer to add the safety margin and then punish me when I encroach on its margin. It might be different if one model (algorithm) were demonstratably different in the protection it provided.

One problem I have noticed with the more conservative models is divers who think nothing of taking them into deco (because everybody else still has NDL left) and then casually working it off during the ascent. The only time I ever took my Oceanic computer into deco was when diving 2 Oceanic computers, one on air and one on 32% with EAN32 in the tank. I wanted to experience the deco displays and procedures.

in_cavediver
11-23-2007, 12:11
I'd rather know exactly where the edge of the cliff is. Then I can decide how much of a safety margin I want to add. I don't want the computer to add the safety margin and then punish me when I encroach on its margin. It might be different if one model (algorithm) were demonstratably different in the protection it provided.

The only danger to be aware of is with deco theory, when you get close to the 'cliff', your computer can't tell you if you are 5 feet from the edge or 15' over it already. That varies day to day, week to week etc. I'd not recommend relying solely on this algorithm to keep you safe.

Philosophically though, I do agree with you and almost never use/pay attention to the 'NDL' feature of my computer (VR3). I pretty much reference the profile I dove and decide what stops I want to do based on it, be it well with rec limits or well past. Many dives, I already know the stops I'll have and the VR3 just serves as a bailout/confirmation for my plan.

quasimoto
11-23-2007, 20:00
I just want to add one thing here. Remember, the computer is applying a model based on pressure/time to predict whether you get bent. It has zero knowledge about you, the conditions, temperature etc.

Just because a computer says you have time doesn't mean you can't/won't get bent. (Those are called undeserved hits). It's all best guess and the magic limit changes dive to dive, day to day. Which brand of computer you use has nothing to do with the nitrogen loading you have from any given dive. When changing brands/models, you are just choosing how aggressive you want to dive.

Dive safe.
Correct me if I am wrong but there are a couple of computers out there that take into consideration the water temp. This still isn't going to be perfect but it helps.

Food for thought...ever wonder why divers get those "undeserved hits"? in-cavediver pretty much summed it up. Your body changes daily.