tonyc:
The problem this saturday is that half of my dive time was spent on the ascent from max depth. Now, Shouldn't I be taking on nitrogen at that time??
I wish I could figure out how to post my dive profile, but I'll try to explain it. Lets take the dive on the Sue Jack, second dive of the day....
1.) for the first seven minutes we decended to a depth around 87 feet.
80 ft/7min= 11.4 ft/min.
2.) We hung out on the wreck for nine minutes (16 min total time) and
depth was between 87 and 68 foot.
3.) Then we headed back at a slow ascent which has a nice straight line
when graphed and follows a slope of about 4 foot per minute.
So my questions are....
1.) At that slow of an ascent aren't I still taking on more nitrogen on the way back? at 60 foot, 50 foot etc?
2.) Is it safe to use my bottom time as 16 minutes for this dive- SSI says
bottom time is time till you start your direct ascent at 30ft/min- not 4
foot per minute.
You're asking a question that requires a lot of deco theory to explain.
1. Simple answer: Yes.
Complex answer: yes, you're still taking on ("on-gassing") Nitrogen at 60fsw, 50fsw etc, but because you're on your ascent, you're also off-gassing, so the question is rather where are you on-gassing and where are you off-gassing, and what's important?
The DIR-malligned Buhlmann model recognized a long time ago this complexity that the body does not treat all Nitrogen equally. Say what you want about it being an old, klungy model, but it now has tens of millions of datapoints of dives done safely with it.
Its model divides the human body into theoretical "Compartments". Each Compartment is different in that it has its own rate of on/off-gassing (compartment half-time), and a different surface maximum saturation value (M-value). What happens on a dive is that your local pressure and time spent there determines how much Nitrogen goes into each Compartment, based on each individual Compartments' half-times and the pressure differentials.
At your maximum depth, all compartments are on-gassing.
Remember that when any compartments exceeds its M-value, you now have too much Nitrogen in that compartment to safely make a direct ascent to the surface, so you are now in a Decompression profile.
Also note that when you're in Deco, you can have more than one compartment exceed 100% of its M-value. To avoid (or to get out of) a Deco dive, all of the compartments must be below their M-value limits.
There is something that is referred to as the "Controlling Compartment". Generally, it is whichever Compartment that is having the greatest effect on your dive - for example, the one that first crossed into Deco, and/or the last one to cross out.
(Whew!)
Okay, so you're now ascending. With the reduction in local pressure, some Compartments start to off-gas, but others are still on-gassing.
The reason that they do this is because the compartments have different half-times.
"Fast" compartments have low half-times (like 5 minutes), so your time you spent at 80+fsw did a good job of filling them up to pretty close to an 80fsw pressure saturation equivalent. As such, when you go shallower, the Compartment's effective pressure (think of it as an "Internal Pressure") results in a reverse of flow: off-gassing.
But the "slow" compartments never had enough bottom time to fill up, so they're perhaps at only ~35fsw pressure saturation equivalent. As such, your ascent to 50fsw wasn't enough to reverse the flow yet, so they're still on-gassing. It will be slower than before, but its still on-gassing.
Suffice to say that even though the model looks very simple, the ways in which it reacts are actually very sophisticated.
Now we can finally get to your question about ascent rates.
Let's say for sake of illustration that your time at 87fsw was enough to load your fastest Compartment (let's call it #1) to an 85fsw pressure equivalent and also right up to 99.9% of its M-value, so we would say that its the Controlling Compartment, because if you stayed an instant longer, you would go into Deco, because 85 < 87 and the Compartment would thus continue to on-gas.
But let's say you didn't -- you ascended. Since yo'ure now shallower than 85fsw, that compartment exceeds the current depth and starts to off-gas.
Home free, right? Not so fast.
Even though there's only one Controlling Compartment, the others are still along for the ride. The second-fastest Compartment wasn't the Controlling Compartment, but its been busy on-gassing all this time too. Let's say that it was loaded to 49fsw and that its 100% M-value limit is 55fsw equivalent.
Now your rate of ascent becomes important, because what's really happening is that Compartment #2 is now the Controlling Compartment: you need to get shallower than 55fsw before the #2 compartment reaches 55fsw or else you'll shift into Deco, even though it is no longer due to the loading in Compartment #1.
This is a longwinded way of saying that even though you've gone shallower, you're still taking on Nitrogen and you may still hit a Deco limit...and what makes it hard to understand is that you've had multiple Compartments in a horse race, and your depth change can cause a jump between horses.
Okay, finally time to answer your 2nd question:
2. "It depends", but I'd say No.
Extra time spent due to a slower-than-perscribed ascent must be addressed exactly how the decompression model that you're using says to use it, so you cannot cross between a Suunto and SSI or whatever.
For example, the old, old USN table said that for an ascent rate slower than 60ft/min, the extra time that was spent deeper than 50fsw would be calcuated and added to the bottom time, and that extra time spent shallower would be lost (not added to the bottom time, nor credited towards any deco stops, nor added to the surface interval).
Personally these days, I consider all time spent deeper than ~30fsw to be the "Bottom Time", unless I am that deep (or deeper) on a Required Deco Stop.
I want to know how to figure out nitrogen loading on the multilevel dives without just looking at the time left on my vyper. At my job I give lots of drugs that run continously based on weight and although the pumps can be programmed and make it a "brainless" operation, I still like to do the calculations myself. I want that same understanding when it comes to diving.
With the near-continuous granularity of dive computers' depth sensors and clocks, I personally believe that this is futile to attempt. My opinion is to do what the machine says, but to be knowledgeable enough yourself to know when you are getting an obviously erronious information...eg, if you punch 210 x 45 into a handheld calculator, the expected value is a bit over 8000, so 3.141516 is wrong.
Similarly, insfoar as recovery modes for figuring out when you can dive after a dive computer failure, one low risk approach is to assume that at the time that you have ended the aborted dive that you have the worst case N2 onloading in the next Deco models. This can be done with Tables a lot easier than with a "fresh" dive computer. Others are free to disagree with this approach, but what most people fail to realize is that ALL of these are only models (non-deterministic tools), they ALL have risk, and that diving is always about risk management.
-hh
