conservatism - SF2 on the computer vs nitrox

[DrGonzo]

Apart from the depth limits with nitrox and increased bottom time, is there much difference in safety for multi day dives between
Diving on oxygen with the computer set to SF2 OR
Diving on nitrox worth the computer set to SF0

You see a lot of multi day divers using nitrox to be conservative on saturation levels.

 

[tbone1004]

on which computer?
I'm going to assume you mean diving on Air instead of Oxygen because on O2 you have a 20ft/7m depth limit and a bunch of other problems.

So assuming you mean air not oxygen, assuming you are actually diving air vs actually diving nitrox of some sort, and assuming the dives are the same depth, then the Air at a higher safety factor is more conservative because the computer is basically stopping your dive earlier. You have a double edge sword here though because you have shorter dives due to the safety factor, but also shorter dives due to diving air.

The safety factors set on your computer have nothing to do with the gas mix that you are diving. You set the safety factors based on your level of conservatism because that determines the tissue loading. What some people do to add extra conservatism is they will dive nitrox, but tell their computer they are diving air, but that is stupid imho. Tell the computer what you are actually diving, and then determine the level of conservatism that you want to dive. This is why I don't believe in computers having arbitrary safety factors because they don't tell you what they are actually doing to pad that conservatism. A computer with gradient factors where you get to control your actual saturation levels are much better.

 

[uncfnp]

Hey DrGonzo,

Nitrox is not going to be a conservative factor unless you dive it on air settings. It is more often used to give more botton time.

 

[KeithG]

in general the nitrox dives are no safer than the air dives. many (most?) divers ride the NDL limits. it just takes longer to get there on nitrox.

 

[DevonDiver]

Let's assume we conduct a dive of 56 minutes to 60'/18m.
Let's also assume that 56 minutes is the NDL at 60'/18m (SF0)

1) Diving on air will give you a saturation of X.

2) Diving on nitrox 32% will give you a lower saturation of X-a
(a is the difference in saturation rate between 79% nitrogen and 68% nitrogen)

3) Diving on SF2 would make your NDL shorter, lets say 48 minutes.
That gives a lower saturation of X-b.
(b is the difference between 56 and 48 minutes worth of saturation)​

This does not account for nitrox accelerating decompression on ascent (accelerated desaturation), which it does. It only accounts for nitrox reducing saturation (speed/volume absorption of gas into tissues).

To compare the effect of nitrox versus a more conservative safety factor, we can simply make use of the dive computers' planning function. A very simplistic comparison of max NDL - giving a guide as to which option allows greater saturation. Program a dive with nitrox and note the NDL. Program the same dive with air on SF2 and note the NDL given. The shorter NDL guides on which method is most efficient.

In reality, there's many other factors when you're considering multi-day diving and different computer algorithms. Some algorithms automatically compensate (add conservatism) for multi-day diving.... for instance, Suunto RGBM.

Nitrox is more than just a tool to increase NDL. It also has benefits in respect to off-gassing on ascent. That makes it a good idea in just about every scenario. However, in the real world, we have to consider cost. It's a personal decision if cost reflects the safety value - especially when recreational dives are exceptionally safe to start with.

Safety factors, in my opinion are under-utilized by most divers. That's because they limit dive times. Nobody like their dives cut shorter than necessary.

Personally, I apply safety factors/algorithm conservatism based on the existence and/or severity of known DCS predisposing factors. It's a decision I make before each dive, based on my body, vitality and health at that time.

If I am moderately tired, a little dehydrated, a small injury etc... I will go to SF1. If I'm really tired, noticeably dehydrated, a significant injury... then I would go to SF2. And so on... Obviously, as a dive instructor I often have to dive; otherwise I'd skip the dives if my condition was unreasonably short of optimum

.

 

[DrGonzo]

in general the nitrox dives are no safer than the air dives. many (most?) divers ride the NDL limits. it just takes longer to get there on nitrox.

True. However, the times I have seen divers using nitrox for "saturation safety" was when they were limited by the group diving on air and they took nitrox.

 

[DevonDiver]

in general the nitrox dives are no safer than the air dives. many (most?) divers ride the NDL limits. it just takes longer to get there on nitrox.

You can't really say that nitrox isn't safer because many/most divers use it in a particular way. That's a behavior of people ,not of a gas.

You can say that riding nitrox to the limits isn't safer than riding air to the limits.

Or with more clarity, you can say that riding a gas to it's limits isn't safer than ascending before you reach a limit.

You can use nitrox on an air profile, as means to ascend before the air limit (without reducing time).

Or you can use safety factors on an air profile, as means to ascend before the original air limit (reducing time)

Or.... you can just make a mental decision to ascend at a prudent time before a limit is reached.

Same result... nitrox just costs money, but doesn't cut your time.

 

[Storker]

Nitrox is not going to be a conservative factor unless you dive it on air settings.

Based on my experience I disagree completely.

My local diving is generally on EAN32 since my club can provide that very cheaply. Sure, I can get a little more bottom time on nitrox than on air, but not very much since I'm generally gas limited (or, in the winter, temperature limited. I don't appreciate staying down more than three quarters of an hour to an hour when the water is 4-5 degrees C), not saturation limited. After some 20 minutes at 30m/100', I would probably ascend a bit no matter what gas I'm breathing since I'm approaching a conservative minimum gas limit. On air, those 20 minutes would put me pretty close to my NDL, on nitrox I've got a comfortable margin to my NDL.

In this scenario I start the shallower part of my dive with a lower nitrogen saturation, and I will offgas faster in the shallows compared to if I was on air. My total bottom time is still limited by gas and temperature, however not by nitrogen saturation anymore. If that isn't "more conservative", then what is?

And my computer is always set to the gas I'm breathing. Even if the oxygen clock isn't particularly relevant for occasional rec diving, I like to have my computer work for me with the correct set of data.

 

[uncfnp]

After some 20 minutes at 30m/100', I would probably ascend a bit no matter what gas I'm breathing since I'm approaching a conservative minimum gas limit. On air, those 20 minutes would put me pretty close to my NDL, on nitrox I've got a comfortable margin to my NDL

Agreed. I think we are saying much the same thing but you said it better.

Assume a dive to a given depth with 20 minutes NDL on air and 30 minutes NDL on nitrox. The nitrox gives more NDL but if you dive it like air and only dive the 20 minutes then yes, the dive on nitrox is more conservative since it used less of your given NDL.

So yes, if you dive the same or similar dive profiles on nitrox vs air, the nitrox dive will be more conservative. Thanks for clarifying my post.

But the op referenced multiday diving and "saturation" so I assumed the dives he is referencing are approaching NDL times on their nitrox dives.

 

[Storker]

The nitrox gives more NDL but if you dive it like air and only dive the 20 minutes then yes, the dive on nitrox is more conservative since it used less of your given NDL.

It's also possible to split the difference A longer bottom time on nitrox than on air, but not necessarily all the way to the NDL. That way you get longer bottom times and more conservatism. Win-win!

Sorry for taking this out on you, I'm just a little peeved at the oversimplification that you have to choose between increased conservatism and longer bottom times. I've seen it a couple of times too often...