Nitrogen loading on second dive

[pengwe]

A hypothetical question (with dive tables):

Two dives in a day:
First dive to 35m (115ft). Bottom time of 14 minutes. That leaves me in group K.
One hour surface interval puts me in Group C
Second dive to 25m (80 ft), starting in Group C, gives me 9 minutes on the bottom.

The next week I do it differently:
First dive to 20m (65ft), for 26min. I'm in Group K again.
1 hour surface interval, so I'm in Group C.
Second dive to 25m (80ft), 9 minute bottom time.

All dives have 9m/min ascent rates, and safety stops.

I know the tables say not to do this. Then there was a workshop that said maybe it's OK. My question is, physiologically, is my body in the same state at the beginning of the second dive in each case? If I'm in Group C, does it matter how I got there?

 

[emttim]

A hypothetical question (with dive tables):

Two dives in a day:
First dive to 35m (115ft). Bottom time of 14 minutes. That leaves me in group K.
One hour surface interval puts me in Group C
Second dive to 25m (80 ft), starting in Group C, gives me 9 minutes on the bottom.

The next week I do it differently:
First dive to 20m (65ft), for 26min. I'm in Group K again.
1 hour surface interval, so I'm in Group C.
Second dive to 25m (80ft), 9 minute bottom time.

All dives have 9m/min ascent rates, and safety stops.

I know the tables say not to do this. Then there was a workshop that said maybe it's OK. My question is, physiologically, is my body in the same state at the beginning of the second dive in each case? If I'm in Group C, does it matter how I got there?

Hypothetically, your body should have the same amount of nitrogen at the end of each time. However, what your computer or your table or whatever is telling you is your theoretical nitrogen loading at that point in time. There is no guarantee that your body has the same amount or level of nitrogen in it regardless of being in the same pressure group. Part of the reason that we are taught to make progressively shallower dives is due to the possibility of a tissue compartment being saturated beyond the capacity that it can withstand without having symptomatic bubbles.

Diving a deeper depth than the first one you did may or may not be ok, people do it all the time and I've done it too. I think the main thing that the tables are trying to stress is to not make an even deeper dive if you've already done a deep dive. For instance, maybe it's ok to do a 65' dive after a 40' dive but maybe not a 100' dive after an 80' dive. The important thing is to always do your safety stops...they are far more important than is usually stressed in Open Water class.

Personally I would do both of those profiles w/safety stops and safe ascent rates. Would I assume I have the same amount of nitrogen in my body? Probably not, but I'd just be using the pressure group or what my dive computer is telling me to plan the next dive anyway.

 

[maged_mmh]

looking at the RGBM: these two scenarios differ; a slower tissue will retain/gain nitrogen in the deeper dive more; if you did these with a DC; you'll notice difference in the NDL - i.e. it'll not remain 9mins. for the 2nd dive in both.

 

[ucfdiver]

What tables are you using? Sounds like a deco dive (115ft/14min)?

 

[tschmitt]

Diving deeper on a repetitive dive is discouraged because you can easily end up passing an NDL and find yourself with a deco obligation - which as a recreational diver one is not trained for. The general rule is to error on the side of conservatism. We round up on depths, round up on times, dive shallow each repetitive dive, etc.