PPO2 maximum safe value: 1.4, 1.6

[tbone1004]

tables are difficult because of the saturation that they tend to hang out in. PADI RDP hangs out in the 95% loading range when compared to buhlmann, which is not something I would be comfortable pushing in those types of conditions, especially on repetitive dives....

 

[KenGordon]

They give a no stop time for a dive beyond the MOD? Are you sure?

Ps. Have you considered taking the advice about learning from the mistakes of others contained in your signature line?

 

[Jim Lapenta]

In cold water there are other factors to consider. Deco is not an exact science. Individual physiology has a large part to play in it. People have toxed at less than 1.4.

A diver I had been on boats with in Erie toxed at 1.3 in a cave. He felt it coming on and even tried to get higher in the cave to ward it off.

Steve Lewis taught me about risk factors to take into account. Assign values to them based on my comfort level and adjust my level to them. Cold water, dark, high current, my age, fitness level, deep, etc. All of those need to be considered. Depending on the dive and the task I may be ok with 1.4 in Bonaire. Inside a wreck in the Great Lakes? 1.3 or even 1.2. Depends also on how many dives I'll be doing and how much deco.

 

[2airishuman]

They give a no stop time for a dive beyond the MOD? Are you sure?

The 120 and 130 foot values are marked "for contingency planning only." You can check yourself. A google search will turn up the RDP. I won't post a link because of copyright considerations.

Ps. Have you considered taking the advice about learning from the mistakes of others contained in your signature line?

What are you trying to say? I don't have my mind made up. I asked questions, I'm contemplating the replies.

 

[stuartv]

One of the reasons for marking the tanks permanently as to contents is to be sure dive buddies are aware of the mix.

I wouldn't assume that my buddy paid any attention to the markings on my tanks. If it were important to me for my buddy to know that I am diving where I may hit a ppO2 of 2.6, I would explicitly tell them. Rendering permanent tank markings moot.

And what if you get a fill that turns out to be 30% and you decide to use it? Will you remove or cover those stickers?

I think we're in vehement agreement. What I expect to do is use, as you call them, "permanent" stickers to mark the tanks. My name will be painted on the tanks. I will analyze the contents of the tanks at the time I pick them up from the shop where they're filled. If the results of the analysis indicate a substantial departure from 32% FO2 then there's at least one of a number of problems at work and I'm not going to dive the tank until I am satisfied that all the problems are identified and corrected. Otherwise I'll write the exact FO2 as analyzed on a hang tag with the date, location, and my initials. I'm not going to re-mark the tank and re-plan the dive because the handheld analyzer says FO2 is at 31.7% instead of 32.0%.

I have gotten banked 32 that I tested, first calibrating from a tank of O2-compatible air, that tested at 30 percent. With two different analyzers.

Is that a reason that I wouldn't dive the tank? No. At least, not if it's from a shop that I have some confidence in. Would I label the tank as 30% and dive it as 30%, yes.

If it tested at 31.4%, I would label it and dive it as 31%.

In other words, I think you may be expecting a little too much precision. Or I've had some bad luck.

Regardless, I wouldn't put "permanent" stickers on my tanks. I don't see any actual benefit and I would probably be taking them off soon after I put them on anyway.

It's about removing a complication and possible risk factor from gas management by standardizing cylinder contents, not about bottom time.

I definitely think you are putting too much emphasis on "standardizing cylinder contents." You're talking about getting fills well ahead of time, before you even know for sure where you're going. In that case, you can get whatever blend you want. I have done that before when going down to the Outer Banks, for example. When I do that, I get 28% and know that I'll be good for whatever site they decide to go to. If I get a few less minutes of NDL, that's just not that big a deal. If I'm going somewhere where getting the max NDL is really important, I'll get my tanks re-filled with Best Mix before I go.

You could always just keep an eye out for good deals on used tanks and get yourself a couple of extras. Keep 2 on hand with 28% and 2 with 32 or whatever. I bought my first tanks last summer. I got a good deal on used ones. I have kept my eyes open and repeated that maneuver. I now have 4 HP120s and 2 HP100s. All for what I believe were really good prices (thus, affordable, when purchased over time like that). I don't do it, because my diving logistics are not like yours, but I could now just keep 3 different mixes on hand to be pretty close to optimal on any random dive day that came up.

For cold, dark water, taking the advice of these experienced folks on here and not pushing your O2 limits seems like a very prudent course.

 

[Jim Lapenta]

The only tank I have with a permanent sticker on is my O2 bottle. 20 ft MOD and oxygen in big letters.

 

[stuartv]

They give a no stop time for a dive beyond the MOD? Are you sure?

Ps. Have you considered taking the advice about learning from the mistakes of others contained in your signature line?

The NOAA Nitrox-1 and -2 tables give NDLs for depths up to a ppO2 of 1.6.

 

[PfcAJ]

I wouldn't assume that my buddy paid any attention to the markings on my tanks. If it were important to me for my buddy to know that I am diving where I may hit a ppO2 of 2.6, I would explicitly tell them. Rendering permanent tank markings moot.

And what if you get a fill that turns out to be 30% and you decide to use it? Will you remove or cover those stickers?



I have gotten banked 32 that I tested, first calibrating from a tank of O2-compatible air, that tested at 30 percent. With two different analyzers.

Is that a reason that I wouldn't dive the tank? No. At least, not if it's from a shop that I have some confidence in. Would I label the tank as 30% and dive it as 30%, yes.

If it tested at 31.4%, I would label it and dive it as 31%.

In other words, I think you may be expecting a little too much precision. Or I've had some bad luck.

Regardless, I wouldn't put "permanent" stickers on my tanks. I don't see any actual benefit and I would probably be taking them off soon after I put them on anyway.



I definitely think you are putting too much emphasis on "standardizing cylinder contents." You're talking about getting fills well ahead of time, before you even know for sure where you're going. In that case, you can get whatever blend you want. I have done that before when going down to the Outer Banks, for example. When I do that, I get 28% and know that I'll be good for whatever site they decide to go to. If I get a few less minutes of NDL, that's just not that big a deal. If I'm going somewhere where getting the max NDL is really important, I'll get my tanks re-filled with Best Mix before I go.

You could always just keep an eye out for good deals on used tanks and get yourself a couple of extras. Keep 2 on hand with 28% and 2 with 32 or whatever. I bought my first tanks last summer. I got a good deal on used ones. I have kept my eyes open and repeated that maneuver. I now have 4 HP120s and 2 HP100s. All for what I believe were really good prices (thus, affordable, when purchased over time like that). I don't do it, because my diving logistics are not like yours, but I could now just keep 3 different mixes on hand to be pretty close to optimal on any random dive day that came up.

For cold, dark water, taking the advice of these experienced folks on here and not pushing your O2 limits seems like a very prudent course.

Permanent MOD markings don't make a lot of sense on backgas tanks...you can't see it when you're wearing the tank! They do, however, make sense on stage and deco bottles where the chances of putting the wrong regulator in your mouth are relatively high. I *highly* recommend dedicating different tanks to different gases for stage/deco.

Also, post analysis, writing down the oxygen percentage to the decimal (31.4%) helps avoid confusion between a requested mix and the actual analyzed mix. Did you *want* 31% or did it analyze as 31%? Going to the decimal serves to reduce ambiguity.

 

[KenGordon]

What are you trying to say? I don't have my mind made up. I asked questions, I'm contemplating the replies.

I am trying to say there are reasons all the agencies picked these MOD numbers. The reasons are that now and again someone fits and drowns. Normally that happens with long exposures or high ppO2 but not always. By and large those numbers are ok. Plenty of people can give anecdotes bout exceeding them and living. The people with the anecdotes about exceeding them and dying are not here to tell you about it. Learn from their mistakes.

PLANNING to do a dive on 1.6 is already using all the contingency. Now if you find the tide/reported depth/analyser/mix/whatever is just a bit wrong what will you do?

If you chose 27% you'd only loose 2 minutes of NDL time at 40mbut be doing a dive thousands of others have done and have room for mistakes.

 

[uncfnp]

The only tank I permanently label is my O2. I don't see any advantage to it and honestly, if that is the primary reason to risk pushing your MOD I think it is a poor choice.

That said, I can understand wanting to have your tanks filled and ready to go. My nitrox instructor is apparently somewhat "liberal" with MOD's. I was taught that 1.6 is max but 1.4 may be preferred, especially with dives with higher risk. He said he sets his computer to 1.5 and I followed his example. The vast majority of my dives are 1.4 or less but I do occasionally and briefly hit 1.5. I use 1.6 for deco only. It wasn't until reading the many threads on scubaboard on this very subject that I learned it was so controversial.

My advice, since you asked. If the majority of your dives are above 120, fill them with 32 and be smart in your dive choices. Stuartv is right that dives rarely stay at the max depth. If your dive plan is deeper, rent or suck it up, empty and refill the tanks. Personally I would not dive past 1.4 in cold muddy water.