DIR Pre-dive Planning?

[riguerin]

I don't know where this one came from

It was born in an airport terminal in New Mexico. One of those little gems you pick up in fundies

BOT, we were also taught the 120 rule in my fundies class, as well as other planning simplification based on 32% standard gas (some have already been pointed out here).

 

[rainman_02]

because a certain someone said "if thou are using He, thou shalt never use less than 30% He for it shall do thee no good"

25/25 has about a 10% EAD compared to 30/30 @ 20%EAD so effectively your no deco times will be a bit less, but with the He, doing some deco on it is no big deal as long as you plan for it.

Based soley on the addition of O2, you get a 10% ead.

But if you figure that EAD is more appropriately calculated with the knowledge that the elimination of N2 is what gives you more time (a shallower EAD) then you should be able to get a 20-25% EAD from 25/25 or 30/30.

So, what is the point of a recreational gas for a range in which I already have gas?

I agree with LCF. The gas does not make sense (it used to make a little more) compared to the other standard gasses.

 

[TSandM]

Well, I disagree with you a little bit, rainman02, in that I DO see the point of a recreational gas for a range where I already have gas, because I have significant issues with narcosis at 100 feet in our dark, turbid water. For dives like the lovely wrecks up in Nanaimo, which are in the 90 - 130 foot range, I'll dive 30/30, push the depth to 110, get more bottom time, and have a MUCH clearer head.

Other teams will dive 21/35 and carry a bottle and do deco, but I'm not there yet.

 

[Vayu]

JJ uses a computer. In fact, many people do. I still prefer to use a gauge and tables, but i'm old school. Decompression software uses computers... I don't see why it really matters. If someone thinks a computer is what they need thats fine by me. However, If i'm in the water with them they will know the dive plan regardless of what their computer tells them.

 

[limeyx]

Based soley on the addition of O2, you get a 10% ead.

But if you figure that EAD is more appropriately calculated with the knowledge that the elimination of N2 is what gives you more time (a shallower EAD) then you should be able to get a 20-25% EAD from 25/25 or 30/30.

So, what is the point of a recreational gas for a range in which I already have gas?

I agree with LCF. The gas does not make sense (it used to make a little more) compared to the other standard gasses.

I usually assume 0 benefit from adding He, so in this case it would be the amount of O2 only that makes the difference.

I do know some people who do -20% on 25/25 but I am not sure I'd be up for that or not.

 

[rainman_02]

Well, I disagree with you a little bit, rainman02, in that I DO see the point of a recreational gas for a range where I already have gas, because I have significant issues with narcosis at 100 feet in our dark, turbid water. For dives like the lovely wrecks up in Nanaimo, which are in the 90 - 130 foot range, I'll dive 30/30, push the depth to 110, get more bottom time, and have a MUCH clearer head.

Other teams will dive 21/35 and carry a bottle and do deco, but I'm not there yet.

Why dive 30/30 at all if you're going to push the established limits of your training organization? Especially for a recreational dive?

Why not provide yourself more range and opportunity, especially if your max available depth is 130', by using 25/25?

 

[limeyx]

Why dive 30/30 at all if you're going to push the established limits of your training organization? Especially for a recreational dive?

Why not provide yourself more range and opportunity, especially if your max available depth is 130', by using 25/25?

25/25 isn't "sanctioned" at all by her agency

honestly though, it doesn't make too much difference in the scheme of things.
I'm happier taking the 20% EAD with 30/30 and pushing the PPO2 for a bit down to 120, and possibly less happy trying to give 20%EAD on 25/25 but in the end I'd dive either

 

[TSandM]

Sitting down and doing the calculations, the incremental advantages of 21/35, 25/25, and 30/30 are just not great enough to get exercised about.

As Mike Ferrara has said any number of times, if you're going to go down and dive in the 120 - 130 foot range, you're better off planning them as true decompression dives, getting some decent bottom time, and doing a rational deco, rather than trying to push "NDLs" on whatever gas mix.

When I did the Nanaimo wrecks, my hard deck was 100 fsw. I did the dives on 32% and stayed with MDLs. I would have liked a clearer head, though.

 

[rainman_02]

I thought DIR was a holistic approach...i.e. you don't pick and choose which tenets you follow.

So, instead of 25/25 or 30/30, why not 28/30 or 25/30 (since I heard somewhere that GI3 said He% < 30 is "worthless")?

Or better, and blows huge holes in this whole stupid argument: GUE "Standardized" gas is for the team. And if the team chooses 17/37 (random mix chosen for illustrative purposes) for their dives, then it is standardized as long as the whole team is using it.

And if you ask where I get that...go read some back issues of Quest, where I remember reading an article where the mix used wasn't even on the "standard" gas list.

 

[limeyx]

I thought DIR was a holistic approach...i.e. you don't pick and choose which tenets you follow.

So, instead of 25/25 or 30/30, why not 28/30 or 25/30 (since I heard somewhere that GI3 said He% < 30 is "worthless")?

Or better, and blows huge holes in this whole stupid argument: GUE "Standardized" gas is for the team. And if the team chooses 17/37 (random mix chosen for illustrative purposes) for their dives, then it is standardized as long as the whole team is using it.

And if you ask where I get that...go read some back issues of Quest, where I remember reading an article where the mix used wasn't even on the "standard" gas list.

Not sure what parts you think anyone is picking and choosing. 30/30 is the GUE standard gas. Standard gases are useful because
- everyone uses roughly the same bottom times and ascent profiles
- the more people that use a specific gas, the more confidence you can have of the limits of the gas.

The WKPP standard gases tend to be a bit different than GUE ones (less O2 and more He for a given depth)

Generally, O2 percentages for bottom gases are max, and He percentages are minimums.

Having said that, if a team wanted to add 25/25, then I dont see a huge issue, especially as it seems to fit with most DIR principles (EAD, END, ease of mixing etc.)

It's when you get 5 people coming to a dive site with 5 "incompatible" mixes that the troubles start.