Suunto question for nitrox divers

[yknot]

Before anyone starts the computers rot your brain chant I'll start with the following. I use a Suunto Vyper but because my dive profiles are usually conservative I don't spend much time watching the tach on the side, as in there isn't much need to ascend when things get in the yellow because my profiles don't tend to go there anyway. That said, I set and used my computer for a nitrox dive for the first time recently. I noticed that when set in nitrox mode, the tach style gauge doesn't function as a NDL countdown (as it does with air) but rather as an O2 exposure clock. I'm trying to figure out what I'm failing to understand about all of this but, with Suunto's rep for being very conservative and with some diver's tendency to let the computer dictate a safe profile, couldn't you encounter a deco obligation (exceed the programming NDL time limits) before entering the caution zone for O2 exposure? If so, when set in nitrox mode the NDL info would not be available in the normal manner.

 

[mrobinson]

Oh man - now that I read this, I have to sit down and firgure this out too!
I'll let you know if I get anything (... dumb brain, work...)

 

[Kim]

I'm not quite sure what you are asking. The time left to NDL is always displayed both in air and nitrox mode. As far as the OLF oxygen tracking is concerned that doesn't have to do with your NDL but your oxygen exposure. It IS possible to reach an NDL but still remain way inside the PO2 limits.

 

[ShakaZulu]

In addition to tracking diver's exposure to nitrogen, the instrument tracks the exposure to oxygen, if set to NITROX Mode. These calculations are treated as entirely separate functions. The Oxygen Limit Fraction (OLF) is a combination of two methods tracking the oxygen toxicity: the Central Nervous System Toxicity (CNS) and Oxygen Tolerance Unit (OTU). Both fractions are scaled so that the maximum exposure is expressed as 100%. Each of the 11 segments represents 10%. The fraction closest to the maximum limit is displayed. When OTU% reaches the CNS% limit the lowest segment starts to blink. Your NDL is display as digits only........I think on the Mosquito, your NDL is still displayed on the left Bar Graph.

 

[Kim]

When OTU% reaches the CNS% limit the lowest segment starts to blink.

Wow... thanks for that Shaka...that was something I didn't know. I recently had that blinking on a live aboard trip and no-one could tell me what it meant. The bar itself never went above 30% so I presumed I was OK. Now I'm going to have to reasearch this a little further. It was the first time I had used a Vyper in Nitrox mode and the manual wasn't very clear about the OLF bar graph at all.

 

[Damselfish]

I think the number display of NDL is considered the "normal" manner not the tach. The tach is an extra nicety. When you're in Nitrox mode and need to see both, there's only so much real estate so it makes sense to me they would use the numbers for one and the tach for the other. And I think it makes sense to use the numbers for NDL and the less specific tach for O2.

 

[yknot]

Without diverging into lessons about situational awareness, sometimes it's nice to use the computer's advantages over tables or other methods of computing NDL bottom times. Alot of my diving doesn't follow a square profile anyway. For example, there are dives we make to say 120'. We don't necessarily head straight to that depth. We might swim for a bit at 30' and head down after 10mins. In these cases I probably won't be tracking bottom time specifically between 30' and 120'. When I hit 120', my NDL bottom time, as expressed digitally in minutes, will likely be less than the current dive time. I suppose I could watch that time count down to zero but I have also used the tach gauge and ascended when the graph hits the yellow. In the nitrox mode there is no NDL graph. Is it correct then to say you would have to rely on the NDL digital display? As another interesting bit also, alot has been said about the conservative nature of Suunto computers compared to others. My dive buddies on my nitrox dive were using older Oceanic's and insisted that the MOD for the mix was rock hard because their computers are preset at a limiting PPO2 of 1.4. For some reason, Suunto must consider a PPO2 of 1.6 acceptable because you can adjust that on their computers and watch the MOD change.

 

[MY01SHADOW]

Before anyone starts the computers rot your brain chant I'll start with the following. I use a Suunto Vyper but because my dive profiles are usually conservative I don't spend much time watching the tach on the side, as in there isn't much need to ascend when things get in the yellow because my profiles don't tend to go there anyway. That said, I set and used my computer for a nitrox dive for the first time recently. I noticed that when set in nitrox mode, the tach style gauge doesn't function as a NDL countdown (as it does with air) but rather as an O2 exposure clock. I'm trying to figure out what I'm failing to understand about all of this but, with Suunto's rep for being very conservative and with some diver's tendency to let the computer dictate a safe profile, couldn't you encounter a deco obligation (exceed the programming NDL time limits) before entering the caution zone for O2 exposure? If so, when set in nitrox mode the NDL info would not be available in the normal manner.

It could be, that at that given depth, the computer is telling you that you are constrained by you o2 clock, not NDL times. What depth are you diving when this occurs? If it's under 35fsw that's probably your constraint.

I could very well be wrong though, let us know what you find out.

Mike

 

[yknot]

It could be, that at that given depth, the computer is telling you that you are constrained by you o2 clock, not NDL times. What depth are you diving when this occurs? If it's under 35fsw that's probably your constraint.

I could very well be wrong though, let us know what you find out.

Mike

Depth maxed out at 70', most of dive at about 40-50'. I need to pull out my nitrox tables and look but feel it's more likely, at least with a single nitrox dive, rather than 5 a day, to hit NDL limits before O2 limits. Maybe Suunto's point is that the O2 clock is a more dangerous concern than NDL limits. After all, the pain and discomfort of the bends couldn't possibly compare to passing out underwater as far as undesirable occurances go.

 

[Kim]

After all, the pain and discomfort of the bends couldn't possibly compare to passing out underwater as far as undesirable occurances go.

I'm really not sure if thats true. From what I have read (not experienced) those that know about this often say that they would prefer to simply drown than die in the throes of the bends. If you haven't read it - check out Shadow Divers.

That said we shouldn't be anywhere near these choices diving recreational profiles and that's the information we get from our computers if we understand it - and the computer is working correctly. Of course it was probably a good idea to run the tables as well to create multiple sources of information - and realize that everything is best guess to start with.