Deep Stops

[Weston]

I just read a fascinating article in the recent Dive Training about the recent research into deep stops, that is taking a safety stop for one minute at half your deepest depth before slowly ascending for a 3 minute safety stop. The theory behind this seems to make sense. Basically, if you dive to 100 ft, make a one minute stop at 50 ft before the 3 minute safety stop at 15-20 ft.

Here is my question: How does one factor in multilevel diving? Often during multilevel dives, the dive will reach, say 100 ft, and then spend most of the dive time at 60 to 70. For deep stops, would one still make the 50 foot stop for make the deep stop at half the depth most time was spent?

Any thoughts?

[DRE]

Personally I start my deepstops deeper, at 80% of the profile in ATA (different from depth in feet) for all my dives. For rec dives, 50% does work and it's always based on max depth during the dive, as that is the max compression you encounter for inert gas to be absorbed into the bloodstream. If you spend most of your time at a shallower depth (like you mentioned 70ft) you're basically already doing a deepstop, but moving around does affect the offgassing, so as you ascend keeping as still as possible will be the most efficient in terms of offgassing, as well as being completely horizontal in the water column.

[Omicron]

These are sometimes called Pyle Stops as well, for the guy that first really started talking about them. Here's his writeup of how he had that "aha!!" moment:



I'm all in favor of them - I feel much better after doing deep dives when I do the deep stops. A lot less tired.

[John H. Moore]

Hi Weston,

Back in San Diego, I used to do a lot of deep canyon diving (though only to approx. rec limits). For a couple years now, I've been doing my first stop at 90 feet, with one minute stops every 10 feet and then longer stops at 20 and 10 feet. The one minute stop is really just a way to enforce an ascent rate of 10 feet per minute.

I've had some GUE folks suggest a slightly shallower first stop (70 feet?).

If particularly hammered, I'll add in a longer stop at 2.5 atmospheres.

This is just my personal way of doing it and should in no way indicate technical dive training. If you want real training on this stuff, you'll have to drink the Koolaid...

John

[Omicron]

Canyon diving???

That sounds interesting - I bet you get to see some awesome underwater walls!!

[John H. Moore]

Not like you probably imagine. The most dived canyon there is more slope than wall, athough there are walls here and there... from 1 foot to probably 50 feet at the most. Walls more clay-like than rock in most places.

Then there's the next canyon up, dived a lot less because access is tricky, but only a couple miles away -- this one has sheer vertical rock walls in excess of 100 feet. Very nice. Loads of life on these walls.

[DRE]

Here's a copy of a post by G. Irvine on the topic:


"In determining where to start your decompression, the logic is very simple: you want to let gas escape from the tissues prior to bubbling. Once it bubbles, it will not escape. Rising up rapidly from depth is a good way to trap gas in tissues by forming bubbles which will then grow when you are higher in the water column. On the other hand, gas that bubbles into the blood is generally trapped by the lungs, but those with any kind of pulmonary or cardiac shunt are at risk if this occurs. If it occurs to fast, and or the bubbles grow too large, they can block the effective lung function and will damage the capillary beds of the lungs. From depth, you want to remove gas in solution form.

The best way to do this is to begin your decompression stops at 80% of your profile in atmospheres rounded up . For a ten atmosphere dive, the first stop is 8 atmospheres, or about 240 feet. At the same time, the travelling time between 300 and 240 should be at 30 feet per minute max, so ti should take you two minutes to get to 240.

There is a fine line between getting rid of gas and adding gas at this end of the deco. All you are trying to do is buy time to get the gas coming out in solution, and there is a point of diminishing returns for stops in the lower end of the deco range. The maximum deep stop is 5 minutes, the minimum is 20 seconds ( 30 FPM ascent) . The best way to assure your ascent is at the correct speed is to physically stop every ten feet. That will get you your 20 seconds per ten feet . The range of bottom times that determine the length of the deep stops is 0-150 minutes. For 0, you still have the 30 foot rate, for 150 minutes you max out at 5 minutes per stop . Anything beyond this is effective saturation and the maximum applies.

These deep stops are equally divided at all depths up to 65 percent of the profile. At that point you begin lengthening the stops. Between 65% and 45% , the steps slightly lengthen, but max out at 10 minutes. Between 45% and 35%, the max is 20 minutes, between 35 and 25% , the max is 30 minutes, subject to certain parameters.

Going back to the deepest stop, if you switched gases, and 80% is where you need to switch gases on a long dive, you are maximizing the effect. If you use a helium based gas you further improve the results. Air is unacceptable as a deco gas as it causes damage that can not be fixed by decompressing, and further complicates the decompression due to the body's immune response to damage and the stress of rigid red cells jamming through small capillaries.

When you approach a gas change, you should be coming off of back gas. For the first deep switch , this is obviously the case. Having been on a low ppo2 operating gas, you can afford to spike the ppo2 with a deco gas, whereas yo do not dare do that without breaking to back gas first. You do not use a full 1.6 ppo2 fro any part of deep decompression. The risk is too high. You don't want an oxygen reaction at depth as you will not have any chance of recovering from this, or surfacing and going back down. Be smart and rely on helium and gradient more than ppo2 for these steps. Clearly, a 1.4 or less is preferred for deep stops max, whereas shallower you can do the full 1.6 because you are able to break to a lower effective ppo2 shallow by using back gas. Some people stage a full face mask starting at these stops. JJ does this.

It takes a solid two minutes for gas to make its first pass through the
body when you switch. The switch step should be the longest of the series that uses one gas. You are getting the best oxygen window for that gas at this point, and you just came from a low ppo2, and the gradient is not that severe. As you move up, the steps do not need to be longer on the same gas. In fact, you are best served to do your last step before gas switch on back gas and to make it the shortest of the steps. Here you are relying on gradient and the toggle effect.

The toggling effect is simply alternating between higher and lower ppo2's in order to prevent the onset of lung tissue damage , swelling, adding of protective layers, and constriction of the blood vessels. The reduced ppo2, especially the closer it gets to normoxic, will prevent and reverse these effects (other than the damage if it is already done).. Using the gradient at this juncture is the best way to rid gas.

As you get up into the shallower areas prior to going to oxygen, you should take a full back gas break - what I call a "cleanup break". For instance, on a sat dive to 300, I will do 20-30 minutes on back gas at 50 feet. Cleanup breaks are effectively being done on long dives prior to gas switch if you do your last step on a gas by going back to the backgas.

In the 40-30 foot range from a deep dive with a long deco, it is
unnecessary to extend the 40 and 30 foot stops at all. In fact these one can be sharply reduced if you have no shunts. You are better served by bubbling the gas into the blood stream sat these depths, a far more
efficient and rapid way to get rid of it. Bubbles trapped here can be fixed by going back down slightly, but doing it just right means that will not happen to a well perfused diver. For instance, on a sat dive to 300 that would call for 120-140 minutes at 40 feet on any deco program, I do 20 minutes and then move up.

Following each oxygen stint, you must break to back gas. If you were
breathing oxygen dry, as in a habitat or trough, you must do a ten minute break before going back into the water. The ascent rate from your oxygen stop to the surface is one foot per minute for a long dive, a scaled down version of that tor a short dive. The greatest case of bubbling offgassing occurs in the move from 10 or 20 feet to the surface. You want that to occur under some pressure and to be controlled by the slow ascent, so that when you are up, you will not get the sudden rush of bubbles that could shunt or cause other problems.

For shorter dives, the deco gases are added from the top down. In other
words, your shortest dive might have just oxygen as the only different deco gas. A longer dive of the same profile may add the 50% gas. Still longer times would add the 35% gas and so forth. You weigh the advantage of the gas to the problem of carrying it. The effective shortening of the deco is not in play here because a shorter dive hits the minimum deco rules, so you have to do the time anyway. Longer dives demand the extra gases to stay efficient. Toggling and alternating are key to decompression. There is no way you can beat this by maintaining a high ppo2."


G. Irvine

[FreediveWI]

I've dove some of those canyons, past rec limits, and thought that they were a lot of fun. A pretty easy way to get deep and a nice sand dollar bed to swim through while you off gas on your way back to the beach.

The Irvine article is a bit much for recreational diving, since were not doing 300' saturation dives, but the deep stops still make a HUGE difference in post dive fatigue. They make a big difference even if your just diving air and staying within your no-deco limits.

On a dive like the Milwaukee I would do a minute every ten feet on my way back up- if doing a no-deco dive. For deco dives I use the right gas and tables. Diving the Willy is the same- a minute stop every 10' on the way back up.

I was hoping that Gert would post his profile from diving the Vernon last month with the Dive Rite Team. He actually deco-ed out a couple of minutes faster than they did by just using tables and a stop watch- while they were using Helium deco computers. Which proves once again that dive computers are highly over rated. Of course, deep stop were used by all on those dives.

Jon

[WIdiver_Paul]

Personally, if I am diving with someone that is using a table or computer that calls for more deco time at a depth and I have the gas, I'll sit it out with them on their stop to give myself that extra margin of safety. In most cases a little extra deco can never hurt. And it's a nice thing to do

[dfreeman]

I agree with you there Paul. If I got the air and do have to pee to bad, I hang with my buddy until he is ready to come up. Most other divers I go with do the same.

Doug