Residual Nitrogen Question

[Trixxie]

Hello Medical Mods....

On the PADI RDP, on table three where you find your RNT and ANDL

Why is it that a pressure group "D" diver has more residual nitrogen in their tissues on a repetitive dive at 35' than the same "D" diver does a repetitive dive to 80'? Basically the shallower your depth, the more residual nitrogen you have than at deeper depths. At 35' the RNT is 29 minutes (for D Group) at 80' the RNT is 11 minutes (for D group).

Just wondering how they came up with that.

I have never thought to ask this question before even though I have been diving for quite some time now.

I was making up sample dive table problems to give to my O/W students to practice with and the numbers struck me as interesting....

Thanks

 

[knotical]

That’s not residual nitrogen. That’s residual nitrogen times. The letter group (D) is a representation of residual nitrogen. The residual nitrogen times you cite are the “penalties” you pay for having residual nitrogen. For the same residual nitrogen, your penalty will be different at different depths. If you were doing your first dive of the day, you could stay at 80 feet for 30 minutes, but as a D-diver, you have to reduce that by 11 minutes, or over one-third. For a 35-foot dive, you reduce your potential bottom time of 205 minutes by 29 minutes, or only about one-seventh.

What sometimes helps to understand this is to note that each pair of numbers in table 3 add up to the NDL for a first dive of the day to that depth. Add up the paired numbers in any row, and you’ll always get the same number.

 

[Walter]

At 35' the RNT is 29 minutes (for D Group) at 80' the RNT is 11 minutes (for D group).

A mental image that might help is thinking it takes 29 minutes to absorb that amount of nitrogen at 35 feet, but only 11 minutes at 80 feet.

While the numbers will differ, the concept is the same for all tables, not just the RDP.

 

[Trixxie]

Gotcha - thanks

I was looking at it from another way, trying to make it WAY too complicated but your simple explanation makes perfect sense.

 

[Trixxie]

What sometimes helps to understand this is to note that each pair of numbers in table 3 add up to the NDL for a first dive of the day to that depth. Add up the paired numbers in any row, and you’ll always get the same number.

yeah I knew that part but was thinking of the white number as actual nitrogen (for some reason) and not really as a penalty as it were...

like I said, trying to make it WAY more complicated that it really was.

Thanks again

 

[wve]

I probably have no business weighing in on this with all you Instructors, but my guess is that it has to do with the fact that when you dive at shallower depths, the longer-acting compartments become the controlling ones. The longer-acting components have lower M-values than the fast-acting compartments.

As I understand it, when you dive deep, the fast compartments load up on nitrogen faster, but they can also hold more nitrogen before becoming saturated (ie, reach their M-value). Add to that the fact that they can off-gas more quickly, and you have less of a penalty.

Conversely, in shallow dives, the longer-acting compartments take control. They load up on nitrogen more slowly, but they also have lower M-values. In other words, even though they load slowly, they fill up more quickly AND they unload more slowly. Thus, even though you are shallow where you would THINK that nitrogen would be less of a problem, that isn't necessarily the case on a repetitive dive.

The D pressure group becomes a measure of overall nitrogen loading, but also a measure of how much has UNloaded during the surface interval.

So in your example, a D diver who takes a 35' dive will have already unloaded some of his fast-acting compartments, but he may still have nitrogen in the slow compartments. If he had a longer SI, then the slower compartments would have had more time to unload, too. But as a D diver, the controlling compartments for a shallow dive are still part-full, so you have to take that into account.

On the other hand, if the diver goes to 80', the fast compartments take control. They load up faster, but they can hold more.

It gets complicated when you start trying to account for 6 or 12 compartments (or more). That's what makes computers so great! But before there were computers, the guys at DSAT did the math the hard way, made certain adjustments based upon testing and experience, and came up with the numbers that we have on the tables.

It's not entirely intuitive, and I have to remind myself that it's all just THEORY anyway!

 

[Charlie99]

What you say about fast compartments controlling deep dives is true, but the residual nitrogen times are based ONLY on the 60 minute compartment on the PADI RDP, or on the 120 minute compartment on the USN-based tables.

On the dive tables, the faster compartments set the NDLs, but are not tracked for repetitive dives.

 

[peterbj7]

WVE has a useful explanation.

I regard table 3 on the PADI RDP as unnecessary and often positively unhelpful. If you work out the effect on your next dive of excess nitrogen you have on board using table 1 you'll find it'll all make much more sense. Walter's explanation corresponds with this way of thinking.

When I first learned the PADI RDP I often found myself floundering and not knowing when and how to use table 3, because I was taught to learn it rather than understand it. When teaching any con-ed to PADI-trained divers, especially nitrox, I always go over the RDP and usually I find that my student doesn't understand it, for exactly the same reason that I didn't. The look of understanding that passes over people's faces is most rewarding!

For an organisation that prides itself on being made up of professional teachers PADI falls well short of what is required.

 

[wve]

What you say about fast compartments controlling deep dives is true, but the residual nitrogen times are based ONLY on the 60 minute compartment on the PADI RDP, or on the 120 minute compartment on the USN-based tables.

Actually, now that you mention it, that's right. I forgot that little tidbit.

The rationale for using the 60 minute compartment rather than the USN's 120 minute compartment was the observation that recreational divers tend to dive deeper for shorter periods than Navy divers who might spend longer times in shallower depths (eg, working on the bottom of ship's hull). The idea was to give more credit during the surface interval based upon the difference in "typical" dive profiles.

That sorta throws a wrench in my explanation above, but I think (I hope) that the overall concept is right and that dive computers are really tracking multiple compartments at once.