Ascent Rate at Depth

[g2]

A while back I read something that suggested when ascending from depth (say, 130fsw) your ascent rate should be fairly quick (albeit controlled), until you reach a shallower depth (60fsw), at which point you should slow the ascent to the accepted 30ft/min or slower. The intent was to reduce nitrogen loading at depth.

What is the current thinking in this regard? A reference would be appreciated.

Cheers,

g2

 

[BILLB]

g2,

60 ft per minute used to be the NAVY and all other agencies maximun ascent rate. That was then. Now it is 30 ft. per minute. I make no distinction when I am teaching OW students on this rate. For recreational divers, 30 ft per minute at any depth is actually a rolling decompression and from my experiences and others who know decompression theory, this is the approved ascent rate for avoiding an undeserved case of DCS. RGBM tables for decompression indicate that deep stops and slower ascents ARE in fact the way to ward off DCS (technical and staged decompression). This translates to what I have already said. 30 ft. per minute is the maximum ascent rate (non altitude diving). Nitrox is a REALLY big help. Get Nitrox certified.

 

[Drew Sailbum]

I don't have a reference, and I'm not sure the speed difference we're talking about would make much difference when it comes to nitrogen loading at recreational scuba depths.

But the argument goes something like this:

At 130ft (~40m) you are under a total of 5atm pressure (4 from the water + 1 from the air). The partial pressure of the nitrogen you are breathing (which corrolates somewhat with the rate at which you absorb the nitrogen) is 5 times the P.P. N2 of the gas mix in your tank (P.P. N2=3.95atm for air).

As I ascend to 99ft (~30m), I reduce the total pressure to 4atm. (The P.P. N2=3.16atm for air.) But as a percentage of the total pressure, I have reduced the total pressure by "only" 20%.

I continue to reduce the P.P. N2 at a constant rate of .79atm per 33ft (~10m) I ascend. But as a percentage of total pressure, that becomes greater and greater.

So at 66ft, I am under 3atm total pressure. As I ascend to 33ft, I reduce the pressure to 2 atm (reduce by 33%).

So in theory, any small bubbles, trapped air pockets, etc... would expand at a greater percentage rate as I near the surface - doubling in volume in the last 33ft(~10m).

So to the extent that slowing the percentage rate of change over time (that sounds like calculus - makes my head hurt!) of total pressure is relevant to reducing risk of DCI, then you can get away with a faster ascent rate at depth than you can nearer the surface.


All of that said, take it slow. It's not a race to the surface.

 

[Rick Murchison]

All the DCS theories acknowledge that bubble formation and growth are related to the ratio rather than the absolute pressure difference in the dissolved gas tension to the surrounding pressure... unpack that and it means that the thing that's important is "delta P over P" - and that a more rapid ascent is warranted when deep.
Let's look at air. With 79% nitrogen, then at 100 feet (4 ATM) a saturated liquid would hold 3.16 ATM dissolved nitrogen. Even if anything over a 1:1 ratio could cause bubbles, (Haldane theorized 2:1 - but that's been proven way too liberal) one could theoretically (assuming the body's gonna metabolize all the oxygen it needs to to make oxygen bubbling a non-factor) ascend immediately to 3.16 ATM, or 71.3 feet, where a stop would then be required before the next "unlimited" ascent. This supports both more rapid ascents *and* deep stops. It also supports *very slow* ascents close to the surface.
My personal rule (developed independently decades ago) follows:
120 FPM max below 120 FSW; 60 FPM max below 60 FSW; 30 FPM from 60 to 30 FSW; 15 FPM from 30 FSW. Stops for one minute at half depths (on a 130 FSW dive, stop for a minute at 65 FSW and at 33 FSW) plus a 3 minute stop at 15 FSW *in addition to* any deco stops required by the tables. And most important of all, a full minute from your last stop (be it a "safety" or a deco stop) to the surface.
This is a little different from what I see recommended today, but I think it will stand up under most any analysis.
Rick

 

[100days-a-year]

Close to what Rick does except no stops unless mandated by plan or computer.Most dive software supports this incremental approach to ascent rates .It is also dependent on time at depth .If your tissues aren't very saturated,there is a reduced threat of DCS.

 

[Lost Yooper]

I use 30FPM max for all depths (except for the last 20' which is a virtual crawl) and use deep stops to ensure this slow ascent. I believe (and I could be wrong) that the faster deep ascents are an old deep air procedure or something.

Ascents should be slow, especially the last 20'.

Mike

 

[Rick Murchison]

Originally posted by Lost Yooper
I use 30FPM max for all depths (except for the last 20' which is a virtual crawl) and use deep stops to ensure this slow ascent. I believe (and I could be wrong) that the faster deep ascents are an old deep air procedure or something.

Ascents should be slow, especially the last 20'.

Mike

More rapid ascent rates at depth are used because from a bubble forming standpoint, they are the same as slower ascent rates at shallower depths. It is the change in pressure compared to the total pressure for a given time that is the chief "bubble factor." The gas you use doesn't make any difference. (to the variable ascent rate problem - the type gas makes a huge difference as to what the safe rates are - but they'll still vary with the delta P over P ratio)
Another way of looking at it is that if you consider a 30 FPM rate of ascent the maximum safe ROA at 165 feet, you should reduce your rate of ascent to 25 FPM at 132 feet, to 20 at 99, 15 at 66, 10 at 33 and 5 for the last foot - because those rates of ascent are equivalent given equal "bubble potential" in the body. And that assumes your body is offgassing at a rate that maintains a constant disolved gas tension ratio for the whole ascent.
If you have the NOAA diving manual yet, take a look at the saturation diver deco tables - (by using the sat tables we eliminate the "ongassing" argument for faster ascents at depth)although the rates are, as you can imagine, extremely slow, you'll see that they are much greater at depth than close to the surface.
Rick

 

[Lost Yooper]

Hey Rick,

The way we do it is take 80% of the profile (in ATA), and that's where the deep stops start (some of the planners do the same thing). From there, you make 30sec to 1min (depending) stops every ten feet until the program kicks in with longer stops. What the deep stops are really doing is ensuring you are maintaining a slow ascent at the deeper depths. This is particulary important for trimix dives as the helium wants to come out faster, but also makes a big difference even for air dives (not a good enough difference though). Some of these deep stops may start as deep as 220' depending on the max depth of the profile.

Ideally, you want to off-gas while the nitrogen is in solution. This is done by the slow ascent and deep stops. Once you get into your shallower and intermediate stops, you're stops will become longer, but the ascent times in between the stops can remain 30FPM (max). The really critical part is last the 20'-30' ascent where the bubbles are really going to want to come out of solution. This is OK as long as you give your lungs time to filter them out (ie. very slow final ascent to the surface).

For you and me, we can take advantage of an oxygen window by switching to 50% at 70' and/or O2 at 20' to help speed the process up. Once you surface, you will continue to bubble for a while, but not nearly as severely as you would if you didn't do all this stuff. As a matter of example, for the saturation (or near saturation) dives the WKPP are doing, they use a one foot per minute ascent from twenty feet (they clear of detectable bubbles in less than an hour after surfacing - for the most part). We have found that a 3-5 minute final ascent does wonders for us. One of the biggest mistakes a lot divers make is they shoot up from 20' after their done with their deco or safety stop.

We might be saying similar things, but coming at it from different angles.

Later,

Mike

 

[Rick Murchison]

Originally posted by Lost Yooper
We might be saying similar things, but coming at it from different angles.

Yep.
Very similar.
I probably take right at the same time to get up from a given dive as you do, but I probably have an initial rate that's faster than yours, and you'd catch up to me at about 50' or a bit less. I spend a little more time at my deep stops, but don't make 'em as close together.
And I second your observation of the "pop-up" after the safety stop in spades! I've been running an informal survey over the past several years, observing the "average" time the "average" recreational diver takes to surface after leaving the safety stop. It is an *amazing* seven seconds! Over 120 FPM! I insist on at least a full minute from leaving my last stop (safety or deco) to the surface.
Rick

 

[dc4bs]

From some of the stuff I've been reading, it sounds like the slower ascents at depth are better because, like Yooper said, you off-gas more of the N2 directly from solution. Coming up faster at deeper depths, you are more likely to form microbubbles earlier on the ascent.

You prety much WILL form some no matter what you do unless you spend a VERY long time coming up which is not practical for sport diving.

The longer you can delay the formation of any microbubbles at all results in smaller/fewer bubbles when you do reach the surface and as I understand it, you feel better after the dive.

In theory, it helps make repetitive dives safer too.

I've got 2 computers. An Uwatec Aladin Sport that gets mad if you ascend faster than 30 FPM from any depth and a Cochran Comander that allows the faster ascent rates at deeper depths but reduces to 15 FPM as you ascend from the safety / last deco stop.

I simply follow whichever is the more conservative of the 2 at any given moment and feel great when I get out of the water.