EANx tables question... help!

[NorthO]

Okay, here's the deal...

I first got certified (OW and AOW) with CMAS and I learned to use the DCIEM tables, which I've used for the last few years.

Then 3 months ago I got my nitrox cert with PADI and of course they gave me their recreational dive planner, but I'd like to continue using the DCIEM tables, so I tried to see if they match. Of course regarding depth everything's the same, but I'm not sure I understand how they calculate the O2 exposure...

Here's an example:

Let's say I'm doing a dive on 1.4 ata for 50 min. On the PADI tables I end up with 35% allowable exposure, and I'm not supposed to go over a total of 100% per 24h.

However, if I use the CMAS tables, I also end up with 35% (33.5% exactly) allowable exposure, but they say that when I reach 100% I only need a 2h surface interval. After that, the limit for a 24h period is 750 OTU for the first day and 350 OTU for the next, and when I do the maths I have only 81.5 OTU for my 30 min dive, which means I still have more than 650 OTU available for the rest of the day instead of PADI's 65%???

Does that mean the 100% limit isn't that big a deal or do I get it all wrong? I know my computer takes care of all the maths, and anyway I'll probably never reach that 100% limit for a long time in my diving career, but I'd really like to understand...

Thanks

 

[Belce]

I would say they are different organizations with different standards on this subject.

You say you prefer to use the DCIEM tables, you will have learned to use formulas to convert depth with your enriched air blend to equivalent air depth, just use that EAD with the DCIEM table for the EAD. The formula for EAD is an absolute formula that doesn't depend on your dive cert authority.

Personally I would suggest using a dive computer for nitrox with the % dialed in there.

 

[Karl_H]

Okay, here's the deal...

I first got certified (OW and AOW) with CMAS and I learned to use the DCIEM tables, which I've used for the last few years.

Then 3 months ago I got my nitrox cert with PADI and of course they gave me their recreational dive planner, but I'd like to continue using the DCIEM tables, so I tried to see if they match. Of course regarding depth everything's the same, but I'm not sure I understand how they calculate the O2 exposure...

Here's an example:

Let's say I'm doing a dive on 1.4 ata for 50 min. On the PADI tables I end up with 35% allowable exposure, and I'm not supposed to go over a total of 100% per 24h.

However, if I use the CMAS tables, I also end up with 35% (33.5% exactly) allowable exposure, but they say that when I reach 100% I only need a 2h surface interval. After that, the limit for a 24h period is 750 OTU for the first day and 350 OTU for the next, and when I do the maths I have only 81.5 OTU for my 30 min dive, which means I still have more than 650 OTU available for the rest of the day instead of PADI's 65%???

Does that mean the 100% limit isn't that big a deal or do I get it all wrong? I know my computer takes care of all the maths, and anyway I'll probably never reach that 100% limit for a long time in my diving career, but I'd really like to understand...

Thanks

Hi NorthO,

The percentage reduces with surface interval, normally CNS oxygen percentages half for every 90 minutes of surface interval, just like nitrogen saturation reduces with SI, but at a different rate.

I'm guessing the 1.4atm is actually 1.4 PPO? That difficult to achieve for 50 minutes unless you're using a very rich oxygen mixture or a rebreather set-up? This may well be a case where multilevel planning should be considered or a different nitrox mix.

There's two types of oxygen toxicity, pulmonary and CNS - you normally only consider the CNS issues when diving as the pulmonary kind is more related to the long term effects of oxygen at low pressures, where as CNS is related to (relatively) short exposures at higher pressures. As I recall OTUs are normally related to pulmonary toxicity while the percentages are related to CNS, in diving terms if you have an issue with pulmonary toxicity you normally have a much bigger issue with CNS toxicity

Maybe that sheds some light? I'm sure there will be some tech instructors out there who can add too (or correct ) the above.

Karl

 

[rstofer]

So CMAS has a different point of view on O2 off-gassing than PADI. PADI believes in a 24 hour period and CMAS believes something else. Take your pick! Just don't try to mix and match.

It's pretty hard to get to the 100% limit so this becomes something of a theoretical argument but it is the only reason I bothered to buy a computer. I just didn't want to deal with it. Tables were fine as long as I only had to use one of them. When I also had to fool around with the DSAT O2 exposure table, I decided to get with the program and buy a nitrox capable computer.

Richard

 

[muddiver]

I concur, you should not be mixing dive table made by different agencies. If you want to dive Nitrox on a table, use the caculations through the formulas for your END, MOD, etc. Or see if CMAS has teh same type of tables for their air diving tables.

I typically use the U.S. Navy tables to log my dives and I have not dusted off the PADI Nitrox tables since I took the class as an intoduction to Nitrox. ANDI has a much better Nitrox table anyway.

 

[Blackwood]

It seems that you are confusing (or rather using interchangeably) CNS toxicity (generally recorded as a percentage) and pulmonary toxicity (generally recorded as OTUs).

They are not the same thing, and it's very unlikely that on "recreational nitrox" gases you'll ever come up against the OTU limit.

 

[OceanEd]

Don't mix the tables from one organization to another. All of the organizations have Nitrox tables that work well, but they are not the same.

 

[NorthO]

Thanks everyone for your input!

Belce, I already have a nitrox capable computer, I'm just trying to understand the maths behind it should anything go wrong... Plus, even if I always have my computer I always try to plan my dives ahead with the tables just to remember how to use them... Maybe it's a loss of time, but I like it!

Karl_H and Blackwood, I know it's hard to get that kind of prifile, I just wanted to have some rough numbers for an example... I'm still a recreational diver and I don't plan to do deco or use a rebreather anytime soon! During my class I remember we spoke of CNS and pulmonary toxicity, but I think they only explained the % of O2 exposure, not the OTU... I only saw it on the CMAS tables... Thanks for the info!

rstofer, muddiver, OceanEd, I didn't want to mix the tables, I'm just trying to understand the CMAS ones so I can use them instead of DSAT's...

But you all did shed some light, thanks!

PS: maybe I'm opening a can of worms, but do people still look at tables sometimes or do they rely only on their computers? I always carry a plastic version clipped somewhere should something go wrong during the dive...

 

[Karl_H]

Thanks everyone for your input!

PS: maybe I'm opening a can of worms, but do people still look at tables sometimes or do they rely only on their computers? I always carry a plastic version clipped somewhere should something go wrong during the dive...

Hi NorthO,

To be honest most rec/non-deco divers tend to use just a computer - especially when using Nitrox. Unless you use multilevel profiles the bottom time tend to be very short from tables so using a computer lets you maximize bottom time.

Once you cross over to the dark side and start tech diving (it happens to most of us !) you'll start to go back to using manual calculations for dive times and using the computer only as a back-up - although most people will use a good deco diving software to calculate profiles rather than a table. With bottom time and air being less of an issue in most tech dives as you generally just need longer deco stops the advantages of using a computer to 'give more bottom time' is not as significant and most of us prefer to err on the safe side and go by tables/profiles rather than rely on a computer.

Karl

 

[SailNaked]

Thanks everyone for your input!


PS: maybe I'm opening a can of worms, but do people still look at tables sometimes or do they rely only on their computers? I always carry a plastic version clipped somewhere should something go wrong during the dive...

No worms, I use tables like you do, before I do something new I look at the tables to see if it is reasonable. most of my diving gets into real trouble on the tables, I go to a depth considered deep then I work my way up, the total time blows my tables to tiny bits and they cry foul. My computer is flying happy as a clam. I did 23 dives (24 hours BT) in Bonaire all on Nitrox about 3-5 a day and the computer never got past the first bar on the surface,(was no more than 3:5 bars at the end of the dive before SS). and it only chirped at me once when I had the PPO alarm set for 1.3 and we were past that.