DIR Answer Required

[AaronR103]

Ok, I give. What's the 120' rule?

A

 

[Charlie99]

You're at 70fsw with 1200psi in your AL80 and your computer says you have 25 minutes remaining on your NDL. You see a really cool <whatever> at about 100fsw. Assuming it will only take a minute to descend how do you figure out how much time you can safely spend at that depth.

Would someone who uses a computer regularly explain their thought process to me? Do you start the descent and just keep a close eye on your NDL and SPG?

Post the question outside of the DIR forum and I'll be glad to post about ways to intelligently use a computer in this situation.

The DIR answer is "take a class". Oops. You did take DIR-F. Well then, until you take Tech 1, the answer is "take Tech 1".

I do believe it is still within the spirit and guidelines of the DIR forum to note that one does not have to plan the entire dive in detail if one has a good plan for the ascent and return, taking into account current location, depth, and N2 loadings. Or to put it more crudely, it doesn't matter how you got where you are (loading, depth, and gas availability). It just matters how you get back.

 

[RonFrank]

so, Dennis why are women's sac rates better? lower BMR? less skeletal muscle to feed O2? bigger brains?

Not bigger brains, just smaller lungs, and less mass. I've dove with a couple guys under 18 (think skinny, sub 140lbs), and out of the shoot, they used less air than a lot of people I've dove with.

I tend to use a lot more air at altitude, or in cold water, I'm not sure which is the real factor but maybe the cold. I use a lot less air at sealevel in warm water vs. when diving the hole for example.

 

[Cacia]

Ron, does this make sense to you? I asked the question because people are more likely to have a successful rescusitaion after being in cold water due to the decreased 02 demands of the tissues. So, since we know cold drops your metabolic rate...why would cold water increase your 02 demand?

I think the fact that women have less skeletel muscle mass is the reason, not their lung size (maybe i am wrong) But lung exchange area is related to your BSA, I think. A ratio more than an absolute. I am hoping someone will sort me out on this.
Stepah Ash for example...

 

[Cacia]

Aarron, 120 rule: 120- depth =time. I think...

maybe not a DIR answer, but I got it from Pug. and if he said it in here, I am sure that nobody would complain.

 

[lakewinni]

Aarron, 120 rule: 120- depth =time. I think...

maybe not a DIR answer, but I got it from Pug. and if he said it in here, I am sure that nobody would complain.

I think thats kind of interesting actually. If you look at it, it works from 100 feet roughly up to 60 feet and it pretty much tracks the Navy NDLs. Obviously as you go past 100 feet you approach bottom times of 0.

 

[AaronR103]

Aarron, 120 rule: 120- depth =time. I think...

maybe not a DIR answer, but I got it from Pug. and if he said it in here, I am sure that nobody would complain.

That's pretty cool. If it tracks the tables it would qualify as DIR, and most DIR people aren't going to be diving air below 100 anyway when the time starts to near 0.

A

 

[Cacia]

no kidding it is pretty cool. AND I can do the math at depth! They have been holding out on us folks. It took me how many years to stumble on this gem?

 

[TSandM]

catherine, if you think about it, the successful resuscitations due to decreased O2 consumption are in people who are already frankly hypothermic.

Assuming that most functioning divers don't reach a point of significant core temperature drop, their O2 consumption wouldn't fall on that account.

It's known that gas consumption is higher in cold water -- whether that's stress, whether cold water divers MOVE more in an effort to keep warm (I know some who deliberately use that strategy), or whether there is some metabolic adjustment to raise metabolism to stay warm (I don't know of any short of shivering) is not obvious, but the increased air consumption is a fact.

 

[TSandM]

Remember that the 120 rule came from the old Navy Tables . . . and the Navy Tables accepted something like a 4.5% risk of DCS, and that was in fit, young Navy divers.

The 120 rule is a reasonable cross-check, but if you are going to push it to the limits, you are going to need to do the deco required, which is the other half of using this strategy -- time shallow for time deep, shaping the curve -- which leads to a need for significant discipline if you are doing what would otherwise be square profile dives (eg. off charter boats).