Rule of 120 question

[Raven C]

If you're using the Rule of 120, do you ever use a mathematical equation to compensate for a less than totally square profile?

For example instead of having 40 minutes at an 80ft max depth dive do you compensate for say 1/3 the time at 60 and 3/4 the time at 80 or do you only dive the 120 rule like you would the P tables. Even if you aren't at 80ft the entire time.

Do you lenghten your time by say 10 minutes to compensate for the split difference allowing 50 minutes as long as you aren't at 80 the entire time?

 

[Kendall Raine]

Depth average. By that I mean you need to approximate your time weighted average depth and use that in your calulation of 120. While table instructions will tell you NOT to do this, it is exactly what a dive computer is doing.

Recognize that you are removing the inherent conservatism of using a square table method on a multi-level profile.

 

[Raven C]

Kendall Raine once bubbled...
Depth average. By that I mean you need to approximate your time weighted average depth and use that in your calulation of 120. While table instructions will tell you NOT to do this, it is exactly what a dive computer is doing.

Recognize that you are removing the inherent conservatism of using a square table method on a multi-level profile.

I've been working that out in my head but staying with the square profile up until now. I am not interested in taking unnecessary risks, it's just that if I dive a wreck that is one max depth but I only go there say five minutes to check something out and then I swim through at a much shallower depth I really could stay longer than what I have practiced thus far. I am just trying to understand the safest allowable dive time for calculating an appropriate dive plan. It's such a shame to only have 40 minutes if I am not at 80 feet for 40 minutes. Thanks for your response. R

 

[Uncle Pug]

RavenC once bubbled...
1) 1/3 the time at 60 and 3/4 the time at 80

2) if I dive a wreck that is one max depth but I only go there say five minutes to check something out and then I swim through at a much shallower depth

In the first example you are spending the majority of time at max depth while in the second you are only bouncing down to max depth for 5 minutes and spending the rest of the time shallower.

That makes a difference in how you weight the profile.

 

[Raven C]

Uncle Pug once bubbled...

In the first example you are spending the majority of time at max depth while in the second you are only bouncing down to max depth for 5 minutes and spending the rest of the time shallower.

That makes a difference in how you weight the profile.

Exactly, for different circumstances that need to be planned in advance so you know how long to stay during your dive, change with the max depth and time.

Maybe I would limit myself to checking the bottom out on the first dive and spend most of the time at a lessor depth going through the wreck, and then the second dive I could spend 1/3 of the time finishing what I checked out on the first dive at the lessor depth and go back to the max depth for the last 3/4 of the dive to check out anything of interest on my first look. This would also allow me to plan for the compensation that if I don't want to explore the max depth I have a plan in place to allow more time to check out the wreck at the lessor depth. R

 

[Uncle Pug]

I wouldn't do that.

 

[Walter]

There is no "Rule of 120." The depth added to the NDL equals 120 on the US Navy tables for 60 ft, 70 ft, 80 ft and 90 ft only. This accident of math does not equal a rule. Dive tables (US Navy and others) are designed to be used with square profiles only. They are not designed for multilevel diving. While it is possible to use tables for multilevel dives, I do not know of any organization that recommends doing so. If you are interested in multilevel diving, you should get a computer.

 

[Walter]

One other point, multilevel diving on tables (before computers were readily available) was once fairly common in certain circles, the techniques required you to be fairly proficient at math even when narced. 1/3 + 3/4 = 13/12 or more than a whole. I'd strongly suggest not getting involved in calculations at depth.

 

[Uncle Pug]

Walter once bubbled...
The depth added to the NDL equals 120 on the US Navy tables for 60 ft, 70 ft, 80 ft and 90 ft only.

And for 100' it is 125 and for 110' it is 130 and for 120' it is 135 and for 130' it is 140....

And for 50' it is 150 and for 40' it is 240

So really the 120 *rule of thumb* was the most conservative figure of the non-conservative USN tables.

 

[Blue Space]

Walter once bubbled...
There is no "Rule of 120." The depth added to the NDL equals 120 on the US Navy tables for 60 ft, 70 ft, 80 ft and 90 ft only. This accident of math does not equal a rule. Dive tables (US Navy and others) are designed to be used with square profiles only. They are not designed for multilevel diving. While it is possible to use tables for multilevel dives, I do not know of any organization that recommends doing so. If you are interested in multilevel diving, you should get a computer.

Hmmm....This should be interesting