Nitrox for shallow water artifact diving??

[lowwall]

Take a look at the attached file,
SHALLOW-WATER NITROX DIVING, THE NASA EXPERIENCE.

NASA uses EAN46 in its Neutral Buoyancy Lab, a huge 12m deep pool for training astronauts. The attached paper gives a summary of their dive lengths and depths and notes they have had no oxygen toxicity events.

CONCLUSIONS: The suit configuration produces a maximum equivalent air depth of 7 meters, essentially eliminating the risk of DCS. Based on average run depth and time, approximately 17% of the training runs exceeded the NOAA oxygen maximum single exposure limits, with no resulting oxygen toxicity. The NBL suited training protocols are safe and time tested. Consideration should be given to reevaluate the NOAA oxygen exposure limits for PO2 levels at or below 1 ATA.

Here's the graph of all of their dives of 6+ hours duration. Blue diamonds represent actual dives. The axes are time and average PO2 for the dive. The NOAA single exposure recommended limit is the red line.

 

[ginti]

Have you tried looking for the image? (I use DuckDuckGo which turned up the image on a Russian site!)

thanks, good suggestion! Duckduck go sent me to the GUE blog, there are two nice articles:
Understanding Oxygen Toxicity: Part 1 – Looking Back
Understanding Oxygen Toxicity Part II: Hypotheses and Hyperoxia

There is also a link to the NOAA diving manual 2001:
NOAA Diving Manual

Here you can find an explanation of the table, where I found a trace of another aspect, which is the maximum exposure on 24h. The latter is very relevant to the topic here, because of the repetitive dives that the OP @calabash digger uses to do. Here's the table:

Although the manual mention that these data appear to be very conservative, the GUE blog said that accidents happened even following these guidelines. So I would be conservative, at least for recreational diving.

However, with a nitrox course, the maximum O2% should be 40% if I remember well. With these numbers:

90 to 120 minutes on a 10 to 25 ft dive.

and given that 25ft=8m circa, the ppO2 would be 1.8*0.4=0.72, allowing for maximum exposure in 24hrs of 570minutes... more than 4 times the maximum time the OP mentioned. Well within limits...

I still do not understand how the tables were made. But there seems to be a bit of scientific information, given the two models described in the first GUE article.

 

[ginti]

Take a look at the attached file,
SHALLOW-WATER NITROX DIVING, THE NASA EXPERIENCE.

NASA uses EAN46 in its Neutral Buoyancy Lab, a huge 12m deep pool for training astronauts. The attached paper gives a summary of their dive lengths and depths and notes they have had no oxygen toxicity events.

CONCLUSIONS: The suit configuration produces a maximum equivalent air depth of 7 meters, essentially eliminating the risk of DCS. Based on average run depth and time, approximately 17% of the training runs exceeded the NOAA oxygen maximum single exposure limits, with no resulting oxygen toxicity. The NBL suited training protocols are safe and time tested. Consideration should be given to reevaluate the NOAA oxygen exposure limits for PO2 levels at or below 1 ATA.

Here's the graph of all of their dives of 6+ hours duration. Blue diamonds represent actual dives. The axes are time and average PO2 for the dive. The NOAA single exposure recommended limit is the red line.
View attachment 664660

Very interesting, thanks.

The Gue article I mentioned before said that, at low pp, it is extremely rare to get oxygen toxicity symptoms, so scientific modelling is very hard and not reliable. The good news is that getting ox toxicity is rare in this range as the paper you linked shows (max ppO2=1)

 

[Wibble]

The CNS exposure becomes a real issue with rebreathers doing long dives (which all deep dives turn into). Similarly the lungs don't like long exposures to oxygen, relying on the 'oxygen clock'.

It's also an issue on open circuit with accelerated decompression using high oxygen concentrations, particularly 100% on the final 6m/20' stop which is exactly 1.6 PP O2. If using 100% you need to take a 5 minute 'break' from the oxygen every 25 minutes to give your lungs a rest.

80% is so much better to deco out on with its PPO2 of 1.28 at 6m

 

[boulderjohn]

So here's a problem: A diver on air makes 5 1 hour dives to 25FSW with 3 20min SI and 1 60min SI inside of a 12 hour period. How are the repetitive dives calculated?

Is the answer different if the same dives are done on nitrox?

AIR--Use 35 foot table
Dive #1--ends in PG L; after 20 minute SI is in PG I.
Dive #2--1 hour plus 48 minutes RNT; ends PG S. After 20 minute SI is in PG N.
Dive #3--1 hour plus 73 minutes RNT; ends in PG V; After 20 minutes is in PG Q
Dive #4--1 hour plus 139 minutes RNT; ends in PG Z; After 1 hour SI is in PG J
Dive #5--1 hour plus 52 minutes RNT; ends in PG T

Using the tables, the answers will be only slightly better using nitrox, but that is due to the limitations of the table design, not physics and physiology. On air, you have to use the 35 foot table for a 25 foot dive, which means you have to round up 10 feet, creating a bit of an overestimate. With the EANx 32 table, you have to round up 20 feet to use the 45 foot table, creating a much more significant overestimate. With the EANx50 table, you have to round up 25 feet to use the 50 foot table, an even more mammoth overestimate. If the EANx tables had a 35 foot level in them, the difference would be huge.

If you are using a computer, there will be no rounding errors, so you will get significantly more bottom time using nitrox. As you can see, on air, your 25 foot dives put you right at NDL on dive #4, so it was by no means unlimited. With nitrox, you would never be close to NDL.

The tables are limited because they had to be reasonably simple. They didn't think that people would be using the tables much below the depths of the shallowest level on those tables, so they did not put them in. They also did not bother explaining the idea of the safety stop and why the depth was chosen. John Haldane, who made the first working dive tables more than 100 years ago, thought you could ascend directly to the surface if your tissue pressure was 2.0 or below. That idea was changed many years ago to 1.58. That is about 20 feet. So if your dive is shallower than 20 feet, you really can stay as long as you want. Deeper than that, you start building risk.

You get a better idea of the difference when you use the same depths for the tables. Last winter I ended up doing a bunch of reef dives in the 45-50 foot region, and these were square profile dives. Since the dive shop where I got my fills charged the same for any nitrox fill, I did those dives with EANx 36. The boat operator limited dives to 60 minutes, and he provided a 60 minute SI. If I had been diving on air, I would have been limited to 54 minutes for my second dive, and I would have ended in Pressure Group X--right at NDL. With the EANx 36, I could do 60 minutes for each dive and then end in Pressure Group P--two hours away from NDL.

 

[AfterDark]

AIR--Use 35 foot table
Dive #1--ends in PG L; after 20 minute SI is in PG I.
Dive #2--1 hour plus 48 minutes RNT; ends PG S. After 20 minute SI is in PG N.
Dive #3--1 hour plus 73 minutes RNT; ends in PG V; After 20 minutes is in PG Q
Dive #4--1 hour plus 139 minutes RNT; ends in PG Z; After 1 hour SI is in PG J
Dive #5--1 hour plus 52 minutes RNT; ends in PG T

Using the tables, the answers will be only slightly better using nitrox, but that is due to the limitations of the table design, not physics and physiology. On air, you have to use the 35 foot table for a 25 foot dive, which means you have to round up 10 feet, creating a bit of an overestimate. With the EANx 32 table, you have to round up 20 feet to use the 45 foot table, creating a much more significant overestimate. With the EANx50 table, you have to round up 25 feet to use the 50 foot table, an even more mammoth overestimate. If the EANx tables had a 35 foot level in them, the difference would be huge.

If you are using a computer, there will be no rounding errors, so you will get significantly more bottom time using nitrox. As you can see, on air, your 25 foot dives put you right at NDL on dive #4, so it was by no means unlimited. With nitrox, you would never be close to NDL.

The tables are limited because they had to be reasonably simple. They didn't think that people would be using the tables much below the depths of the shallowest level on those tables, so they did not put them in. They also did not bother explaining the idea of the safety stop and why the depth was chosen. John Haldane, who made the first working dive tables more than 100 years ago, thought you could ascend directly to the surface if your tissue pressure was 2.0 or below. That idea was changed many years ago to 1.58. That is about 20 feet. So if your dive is shallower than 20 feet, you really can stay as long as you want. Deeper than that, you start building risk.

You get a better idea of the difference when you use the same depths for the tables. Last winter I ended up doing a bunch of reef dives in the 45-50 foot region, and these were square profile dives. Since the dive shop where I got my fills charged the same for any nitrox fill, I did those dives with EANx 36. The boat operator limited dives to 60 minutes, and he provided a 60 minute SI. If I had been diving on air, I would have been limited to 54 minutes for my second dive, and I would have ended in Pressure Group X--right at NDL. With the EANx 36, I could do 60 minutes for each dive and then end in Pressure Group P--two hours away from NDL.

Why use 35FSW for 25FSW dives? The fact is there is no table information for dives less than 35FSW. Nothing in the rules says to go to the next greater depth if the original depth was not exceed.

I'd be more concern with O2tox using nitrox for shallow dives of long duration N loading is less of concern in shallow water. The O2% can be reduced to avoid that but also reduces N loading "benefit".

If I had surface supplied air and could stand the cold water I'd have no qualms about a 4-5hour dive @25fsw.

 

[Centrals]

AIR--Use 35 foot table
Dive #1--ends in PG L; after 20 minute SI is in PG I.
Dive #2--1 hour plus 48 minutes RNT; ends PG S. After 20 minute SI is in PG N.
Dive #3--1 hour plus 73 minutes RNT; ends in PG V; After 20 minutes is in PG Q
Dive #4--1 hour plus 139 minutes RNT; ends in PG Z; After 1 hour SI is in PG J
Dive #5--1 hour plus 52 minutes RNT; ends in PG T

Using the tables, the answers will be only slightly better using nitrox, but that is due to the limitations of the table design, not physics and physiology. On air, you have to use the 35 foot table for a 25 foot dive, which means you have to round up 10 feet, creating a bit of an overestimate. With the EANx 32 table, you have to round up 20 feet to use the 45 foot table, creating a much more significant overestimate. With the EANx50 table, you have to round up 25 feet to use the 50 foot table, an even more mammoth overestimate. If the EANx tables had a 35 foot level in them, the difference would be huge.

What is the reason to use 35 ft table for a 25 ft dive?
Limitation of the table design? I believe the table was NOT designed.
Since when we were taught to overestimate the depth? Are you suggesting to introduce "safety factor" for any dive?

 

[boulderjohn]

Okay folks, this feels like I am talking to an OW class with more trouble than normal.

Which table level do you use for a 36 foot dive? Think it through. There is no 36 foot level on the table. Therefore you round up to 40 feet. that's the way the tables work. If you don't hit a number on the nose, you round to the more conservative level.

Do you think the 35 foot level is only for dives to exactly, precisely, on the nose 35 feet, and if you go to 34 feet then, by golly, there is no limit? No, you round off to the next conservative level, as you do for every other level on the table.

Rounding off has always been a problem with tables. That's why when PADI made the RDP, it roughly doubled the number of pressure groups compared to the Navy tables, thereby cutting the degree of rounding off in half. But it's still a problem. That's one of advantages of computers.

I realize that I am beating my head against a brick wall, but I trust that other readers are able to understand.

 

[boulderjohn]

What is the reason to use 35 ft table for a 25 ft dive?
Limitation of the table design? I believe the table was NOT designed.
Since when we were taught to overestimate the depth? Are you suggesting to introduce "safety factor" for any dive?

Never had any instruction on tables? That's OK. Nobody uses them much anymore.

 

[Centrals]

Never had any instruction on tables? That's OK. Nobody uses them much anymore.

Learnt that 25 yrs ago.
Glad it was NOT you who taught me to dive in the beginning!

What is the ndl for 24ft?
I believe for BSAC AIR table it is about 240 mins for 9m(30ft).