Maximum Operating Depth (M.O.D.) S.C.U.B.A. Diving On Air.

[stepfen]

There is this video discussing a German submarine wreck (U-133) which sunk near Athens/Greece during WW2.
The video is in Greek, but I fount it very interesting. The white-haired guy in the lower right is the one who fount the wreck in 1986. He was a professional sponge collector and according to him dives around 100m (300ft?) were back then their routine. They used surface supplied air (through a hookah system). It has been few months since I watched the full video and don't remember all the details (it is almost 3hours long) but here are some things I remember or fount while I went fast through it:
- In order for somebody to be able to work at these depths, he would need to go through a "warm up" period. I don't remember details but he would need to build up his narcosis tolerance by diving day after day to greater depths. According to him, once somebody was "primed" by this procedure he would then become tolerant to the effect and be able to keep working to such depths daily for long periods (years).
- If for some reason one had to stop diving for few days/weeks his tolerance would decrease again and he would then have to "prime" himself again.
- Their dive plan was very fast descents, work/collect sponges at around 90m/300ft for I think typically 20 minutes or so and then ascent using a deco plan
Other interesting things:
- For several years they had no back up air supply plans other than CESA!!!
- Towards the end of his career they would occasionally use a 3 lt tank as a back up (spare air for 30m/100ft anyone??)!!!
- Only after 1995 or so other mixes became known/available but still at the beginning they were not used frequently
- He got hit by the bends 3 times. First time in 1990, when his hookah was broken by a passing boat and he had to CESA (!!!).
Second time in 1995 when one of the lift bags he was using got somehow attached to him(!!!!). Luckily for him this time there was a deco chamber on the boat and he was treated on the spot. Third and final time was in 1999. No details are given for this, other than that it was the only time not hit as a result of an accident - he followed the plan but got hit nevertheless (no wonder with such background). Now he said that he can walk but with great difficulties.

By they way the guys who organize this presentation are with GUE and clearly state that these practices are not safe and should not be followed by anyone any more.

I wish there was a way to translate this video somehow - apart from the story of this guy the history behind the u boat and how it was fount is also amazing.

 

[100days-a-year]

The uh, erm, uh MOD of 32% @ 1.4 P02 will be shallower than the same P02 on air and consequently be less narcotic. Phrasing seems pretty clear to me. The difference in potential narcosis between 187' and 111' is pretty significant . Unless I missed something and am entirely incompetent at maths.

 

[wetb4igetinthewater]

Enriched Air Misconceptions

From the article:


Nitrox reduces the risk of inert gas narcosis.
This is also not true. Oxygen is just as narcotic as nitrogen under pressure. Use extreme care when diving enriched air at deeper depths. In some cases, divers have more problems with narcosis when using enriched air. Always stay vigilant for signs of narcosis when making any deep dive.

Not long ago, I saw a link to an article about how using some "device" to measure narcosis, they found that people on nitrox were less narced than those on air. I've been trying to find that for a while as I regretfully did not save the link. That surprised me. Could the fact that oxygen being metabolized be the cause? That's a question for medical professionals/scientists to address.

 

[wetb4igetinthewater]

There is this video discussing a German submarine wreck (U-133) which sunk near Athens/Greece during WW2.
The video is in Greek, but I fount it very interesting. The white-haired guy in the lower right is the one who fount the wreck in 1986. He was a professional sponge collector and according to him dives around 100m (300ft?) were back then their routine. They used surface supplied air (through a hookah system). It has been few months since I watched the full video and don't remember all the details (it is almost 3hours long) but here are some things I remember or fount while I went fast through it:
- In order for somebody to be able to work at these depths, he would need to go through a "warm up" period. I don't remember details but he would need to build up his narcosis tolerance by diving day after day to greater depths. According to him, once somebody was "primed" by this procedure he would then become tolerant to the effect and be able to keep working to such depths daily for long periods (years).
- If for some reason one had to stop diving for few days/weeks his tolerance would decrease again and he would then have to "prime" himself again.
- Their dive plan was very fast descents, work/collect sponges at around 90m/300ft for I think typically 20 minutes or so and then ascent using a deco plan
Other interesting things:
- For several years they had no back up air supply plans other than CESA!!!
- Towards the end of his career they would occasionally use a 3 lt tank as a back up (spare air for 30m/100ft anyone??)!!!
- Only after 1995 or so other mixes became known/available but still at the beginning they were not used frequently
- He got hit by the bends 3 times. First time in 1990, when his hookah was broken by a passing boat and he had to CESA (!!!).
Second time in 1995 when one of the lift bags he was using got somehow attached to him(!!!!). Luckily for him this time there was a deco chamber on the boat and he was treated on the spot. Third and final time was in 1999. No details are given for this, other than that it was the only time not hit as a result of an accident - he followed the plan but got hit nevertheless (no wonder with such background). Now he said that he can walk but with great difficulties.

By they way the guys who organize this presentation are with GUE and clearly state that these practices are not safe and should not be followed by anyone any more.

I wish there was a way to translate this video somehow - apart from the story of this guy the history behind the u boat and how it was fount is also amazing.

I know the oldest surviving sponge diver on Kalymnos. He told me that if he couldn't light a cigarette when he got back on deck, he was in trouble.

 

[boulderjohn]

From the article:


Nitrox reduces the risk of inert gas narcosis.
This is also not true. Oxygen is just as narcotic as nitrogen under pressure. Use extreme care when diving enriched air at deeper depths. In some cases, divers have more problems with narcosis when using enriched air. Always stay vigilant for signs of narcosis when making any deep dive.

Not long ago, I saw a link to an article about how using some "device" to measure narcosis, they found that people on nitrox were less narced than those on air. I've been trying to find that for a while as I regretfully did not save the link. That surprised me. Could the fact that oxygen being metabolized be the cause? That's a question for medical professionals/scientists to address.

Simon Mitchell linked to a study a year or two ago, the results of which indicated that there might indeed be a benefit to nitrox in regard to narcosis.

 

[Akimbo]

Can you control the effects of elevated partial pressures,

No, no evidence of this, other than pure testosterone and/or imagination, or maybe endless training (c.f. @Akimbo).

Just to be clear, experience and training can improve performance due to nitrogen narcosis, but not susceptibility to OxTox. Where training does count in reducing OxTox risk is indirect, which includes a high degree of symptom awareness and avoiding exacerbating factors. This improves your odds of avoiding OxTox convulsions but does NOT reduce the risk to zero. Exposure duration is also an important part of the formula, which is rarely acknowledged on ScubaBoard.

I am NOT advocating an allowable PPO2 limit of 2.0 ATA, only indicating that it was the working limit in the 1970s. Using today's standard of 1.4/1.6 has a greater safety margin but still carries some risk of OxTox.

 

[Boston Breakwater]

The variety of answers here comes from people applying a lot of different factors to the definition of MOD. If the only factor is what was in the opening sentence of the thread (PPO2 = 1.4), there is only one answer, provided we are talking about diving in the ocean. (Answers below are rounded off.)

1. Start with the equation to find the ambient pressure in atmospheres for that PPO2 on air: 1.4/0.21 = 6.67.
2. Next, convert 6.67 to depth in feet by subtracting the atmospheric pressure (1) and multiplying by the depth of water equaling an atmosphere (33): (6.67-1) X 33 = 187 feet.

That works for diving in the ocean. If you are diving in a mountain lake at 6,000 feet elevation, it is different. The 6.67 will be the same, but the depth to reach that limit will be different. The atmospheric pressure is only 0.8, and the depth of freshwater equaling an atmosphere is 34 feet. (6.67-0.8) X 34 = 200 feet.

The altitude answer will be a surprise to many people who were mistakenly using theoretical depth tables to determine MOD and thinking it would be shallower, not deeper.

Hello. @boulderjohn Awesome, post. I didn't make any reference to FSW, or FFW. Which most times I do. Nearly 98% of the time, I try to be specific, and sometimes fall short. (My apologies to all.) Thanks for pointing that out. Very keen.
Cheers.

 

[tursiops]

Enriched Air Misconceptions

Let me explain the confusion.
The MOD of 32% is shallow enough that serious narcosis due to depth is not an issue. It is due only to staying shallower than 110 ft.
it is the depth you are working at that mitigates the narcosis, not the gas mixture.
Is that better?

 

[Boston Breakwater]

I set my limit diving on air to ppO2 = 1.6, so about 220 fsw. However, I always stopped at 200 fsw (except the one time my brakes didn't work and I hit 201 fsw). Being near "Lost" Angeles, our atmosphere may have a lower percentage of O-2 (tee hee). Did many deep dives on air including repetitive dives. No significant narcosis back then at those depths (I had so much residual nitrogen in my body it wouldn't let any more in!). This was almost a decade ago. Today I generally limit my dives to 100 fsw max depth and dive air almost exclusively.

Hello @drbill How are you? Thanks for posting, and nearly bringing this thread back "On Topic."
I too nearly dive air exclusively.
Cheers.

 

[Boston Breakwater]

Hello Boston Breaker,

The responses to this point are reaching the needle red line to controversial, so I don't want to go there.

The MOD for 21% at 1.4 ATA is 186.9.

I think the technical divers call this deep air...

I also think that a lot of folks would get goofy head diving with 21% long before 187 feet is reached.

This is a subject suited to the technical diving experts, of which I am not one.

I can only hope you receive a response meeting your satisfaction.

Rose

Hello, Rose. How are you? I appreciate your response.
I wanted to let you in........on a little secret. Please, don't tell anyone.
I sometimes.........ask questions, that I already know the answer too. Shhhhhhhhhhh.
Cheers.