EAN, Air and a propensity towards bubble formation

[BubbleBubble7]

I have about 200 dives, most on Nitrox. On two occasions, when diving air, I have had DCS symptoms. The first time was skin bends, which cleared within about 15 minutes of breathing 100% oxygen (no chamber). The second incident was a few years later when I did two deep dives on air…both within my computer limits (a conservative Zoop). This incident resulted in DCSII (doc says it was probably an AGE based on symptoms but I did not do a rapid ascent or hold breath...we suspect PFO). I ended up in a recompression chamber but I was ultimately cleared (by two specialists) to begin diving again.

My question is- on my air dives when I had symptoms, my no deco time never dropped below 10 minutes. I’ve done Nitrox dives (usually around 32-33%) where I’ve dropped to 6 or 7 minutes and had no problems whatsoever.

While I understand that Nitrox extends your bottom time for all the reasons we know/understand. Is there something about enriched air that makes it inherently safer even when diving on comparatively aggressive profiles?

Also, with air as the common denominator, I'm only EAN moving forward.

 

[MaxBottomtime]

If you came within 6 or 7 minutes of your Nitrox ndl you were pushing beyond what you did on air. I've seen divers swear they made a slow ascent even when they passed me on the line. Your doc was likely correct. There is nothing in nitrox that makes it any safer than air. It's a tool that allows for longer ndls, shorter surface intervals and often more dives per day. It's not a magic potion but you still should dive conservatively.

 

[DevonDiver]

In addition to reducing nitrogen absorption, nitrox also creates a higher differential on ascent to promote bubble collapse and dissolved inert gas removal.

In short, it accelerates decompression in the same way that technical 'deco gasses' do... but at a lower efficency because the %O2 is lower (but still higher than air/21%).

Nonetheless, aggressive diving is aggressive diving... whatever gas you breathe.

If you were physiologically more prone to DCS (which your medical history seems to illustrate), then it's probably a strong (Darwinian) indication that you should make your dives very conservative.

You might also consider educating yourself on more refined methods of improving off-gassing efficiency on ascent, beyond the barest basic strategies taught on entry-level dive programmes.

Your NDL isn't a magic safeguard against DCS. Actually, your NDL can indicate very different situations, depending on your immediate diving history (repetitive and multi-day dives).

Having an awareness of how computers/decompression models calculate NDL definitely empowers more intelligent consideration of your actual state of inert gas saturation.

A 6-minute NDL on an ascent from your second deep dive of the day is fundamentally different from the same NDL following a single, moderate depth dive. Different compartment saturations...likely a different controlling compartment (slower).

An NDL that 'follows you up' is also indicative of slower compartment supersaturation... and very inefficient off-gassing strategy. It shows there's at least one compartment that isn't at a differential to effectively release inert gas.

In those cases, nitrox will help... but you should also consider doing longer safety stops, potentially split over several levels.

You might also consider your ascent speed. 'Slower is better' is simplistic advice for novice divers doing shortish, shallow dives... but deep recreational diving would greatly benefit from an optimal ascent rate.

Slower is not necessarily better when you have to consider slower compartments over repetitive deeper dives.

 

[BubbleBubble7]

Thanks, this is helpful. My current strategy (since the chamber) is to dive only Nitrox, set my computer to P1 (the "conservative" fudge factor), and do a five minute safety stop at 15ft/5m. In addition to this I stay shallower than 85ft/25m and no longer allow my no deco to drop below 10 even on the conservative settings. I've done about 40 dives with this approach and have had no issues. That said, I'm always looking for additional ways to hedge my bets further. If you have other suggestions or suggested reading I'd love to hear/see it.

In addition to reducing nitrogen absorption, nitrox also creates a higher differential on ascent to promote bubble collapse and dissolved inert gas removal.

In short, it accelerates decompression in the same way that technical 'deco gasses' do... but at a lower efficency because the %O2 is lower (but still higher than air/21%).

Nonetheless, aggressive diving is aggressive diving... whatever gas you breathe.

If you were physiologically more prone to DCS (which your medical history seems to illustrate), then it's probably a strong (Darwinian) indication that you should make your dives very conservative.

You might also consider educating yourself on more refined methods of improving off-gassing efficiency on ascent, beyond the barest basic strategies taught on entry-level dive programmes.

Your NDL isn't a magic safeguard against DCS. Actually, your NDL can indicate very different situations, depending on your immediate diving history (repetitive and multi-day dives).

Having an awareness of how computers/decompression models calculate NDL definitely empowers more intelligent consideration of your actual state of inert gas saturation.

A 6-minute NDL on an ascent from your second deep dive of the day is fundamentally different from the same NDL following a single, moderate depth dive. Different compartment saturations...likely a different controlling compartment (slower).

An NDL that 'follows you up' is also indicative of slower compartment supersaturation... and very inefficient off-gassing strategy. It shows there's at least one compartment that isn't at a differential to effectively release inert gas.

In those cases, nitrox will help... but you should also consider doing longer safety stops, potentially split over several levels.

You might also consider your ascent speed. 'Slower is better' is simplistic advice for novice divers doing shortish, shallow dives... but deep recreational diving would greatly benefit from an optimal ascent rate.

Slower is not necessarily better when you have to consider slower compartments over repetitive deeper dives.

 

[DevonDiver]

In respect to ascent speeds:

Best Ascent Speed for Scuba Diving

In respect to general bubble management:

Subclinical DCS, Decompression Stress and Post-Dive Fatigue

A very valuable read is Mark Powell's "Deco for Divers". I suggest that as required reading for anyone doing repetitive deep recreational dives and/or tech diving.. or otherwise has a problem with decompression stress and/DCS.

I've also got a bunch of articles and videos I can send by email. Just pm me your email address and I'll get them to you.

 

[PatW]

Getting DCS symptoms with never getting within 10 minutes of NDL?

Unearned hits do happen. I understand that unearned hits usually happen to people who have circulatory system issues but I have not seen a study on this. That is people who are taking multiple medications for various circulatory problems.

It could be that you got closer to your limits and did not notice. I don't trust my observations 100%.

Now, I know an experienced dive master at Cozumel who insisted that we do a 5 minute safety stop. I understand that a five minute stop is more effective at reducing bubble formation.

Now, I know some people who dive who are concerned with perhaps suffering from DCS even if they adhere to the NDL protocols. A common way of dealing with this issue is to dive nitrox on an "air" setting on the dive computer. That gives the diver a built in safety margin. Of course, if you are diving nitrox and set for air, you would have to know your nitrox depth limit and be careful not to exceed it.

 

[DevonDiver]

Getting DCS symptoms with never getting within 10 minutes of NDL... Unearned hits do happen.

Simply staying within an NDL does not make an 'unearned' hit. There's lots you can do within an NDL that'd make a hit very deserved.

 

[MaxBottomtime]

Getting DCS symptoms with never getting within 10 minutes of NDL?A common way of dealing with this issue is to dive nitrox on an "air" setting on the dive computer. That gives the diver a built in safety margin. Of course, if you are diving nitrox and set for air, you would have to know your nitrox depth limit and be careful not to exceed it.

I have a friend who does this very thing. She says she leaves the bottom with ten minutes of ndl on her computer set to air but still gets skin bends on many dives. Her ascents are too fast and she doesn't make long stops although she swears she does. I've dived with her enough to know she's wrong.
Setting your computer to air doesn't increase safety as the rate of DCI on air is already so low. Why would you not want to know the exact profile of the gas you're diving? If you did have to go to a chamber they would look at your computer profile anyway. It makes no sense to purposely set your computer to an incorrect setting. The best way to avoid DCI is to dive conservatively and slow your ascent even more than you think it is.

 

[Duke Dive Medicine]

I have a friend who does this very thing. She says she leaves the bottom with ten minutes of ndl on her computer set to air but still gets skin bends on many dives. Her ascents are too fast and she doesn't make long stops although she swears she does. I've dived with her enough to know she's wrong.
Setting your computer to air doesn't increase safety as the rate of DCI on air is already so low. Why would you not want to know the exact profile of the gas you're diving? If you did have to go to a chamber they would look at your computer profile anyway. It makes no sense to purposely set your computer to an incorrect setting. The best way to avoid DCI is to dive conservatively and slow your ascent even more than you think it is.

I agree with you to a point. One has to be careful with slow ascents from deeper depths, along with blanket "deep stops" (though you didn't mention that) to avoid additional on-gassing. Also, we do advise divers in the OP's situation to dive nitrox and set their computers to the air setting. This adds an additional safety factor if the diver can make the gas last long enough to approach the computer's no-stop limit, as the OP apparently can.

Best regards,
DDM

 

[elmo]

Thanks, this is helpful. My current strategy (since the chamber) is to dive only Nitrox, set my computer to P1 (the "conservative" fudge factor), and do a five minute safety stop at 15ft/5m. In addition to this I stay shallower than 85ft/25m and no longer allow my no deco to drop below 10 even on the conservative settings. I've done about 40 dives with this approach and have had no issues. That said, I'm always looking for additional ways to hedge my bets further. If you have other suggestions or suggested reading I'd love to hear/see it.

I won't speculate on the cause of your previous hits, but if you are doing what you say, your current approach is far more conservative than many divers, including myself. That is a good thing. Of course, there are always things you can do to reduce your risk of DCS, but ultimately they fall into reducing the on-gassing (shallower, shorter dives, or nitrox with increased %O2) and/extending the off-gassing. Here are a few more suggestions that don't take away the fun of diving.
Repetitive diving carries a greater risk of DCS than single no-decompression dives, so if you plan on doing a second (or third) dive in a day, make it shallow. Say max depth 12m/40ft on nitrox. This suits some dive sites much better than others, so plan ahead.
Do you final ascent after your 5min safety stop as a 5-up ascent. That is, ascend at 1m/3ft per minute, or more realistically, ascend 1m in ~20s then wait for the minute on your dive computer to tick over before ascending the next minute.
If you really are doing a max depth of 25m, consider diving with 36% nitrox if you have access to it (you may not), without increasing your bottom time. Just be careful as you will be very close to your MOD.