PPO2 maximum safe value: 1.4, 1.6

[stuartv]

32% isn't an appropriate gas for dives deeper than 100ft. Both oxygen and nitrogen contents are too high.

So you're advocating to never go deeper than 100' unless you're on Trimix?

 

[stuartv]

Huh? Offgassing is a lot less effective if you're cold. So, getting that extra margin to your NDL could be very nice, even if your dive is limited by gas or water temperature

I took his post to be a poorly worded way of saying they don't generally dive below 50'.

Not sure why that's relevant to the OP who said his dives will be 60' to 130'.

 

[PfcAJ]

So you're advocating to never go deeper than 100' unless you're on Trimix?

Pretty much, yea.

 

[2airishuman]

32% isn't an appropriate gas for dives deeper than 100ft. Both oxygen and nitrogen contents are too high.

I know that's the DIR philosophy. The belief that helium is required for anything over 100ft is not universally held. If I determine that I can't dive safely without helium to any particular depth, then I'm not going to dive at that depth. We all set our limits.

I wouldn't use 32% beyond 110, keep your PPO at 1.4 no more for the working part of your dive. As well when it comes to PPO and toxing, both the PPO and the time are the factor. 1.4 for 2 hours at 140ft depth, may have you at a higher risk to tox than 1.6 for 30 min at a 20ft stop. Its not just the PPO that is the factor. Vacation divers have toxed due to the use of 32% or 34% over multiple dives over multiple days. Start looking at the math of 34% for 5 dives a day, 7 day of the week. You would find you can actually be pushing the envelope.

Read the opening post. Your examples are textbook but are unrelated to what I'm doing.

For repeat dives I have the computer to monitor O2 exposure.

The other factor is contingency. If you are at the max PPO for the mix then you have no room for error, what if something happens that might have to take you deeper? Are you prepared to leave your buddy if he goes deeper, now increasing your PPO into dangerous areas? Just because it can be done doesn't me it should be.

I'm diving in areas that have a hard bottom, as I noted upthread.

If your truly interested in getting depth and time, then you should really take the course, learn the science and the safety factors. Trying to figure out how far you can push a 32% could end up being the first domino in the string that will lead to a very bad situation.

I will take the class because I need the C card.

If the pattern that developed during the e-learning portion of my OWD class should continue, then I'll end up learning more from SB than the class.

Just make sure your prospective buddy is competent to make an informed decision about joining you or not.

Of course.

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

Banked 32 isn't always exactly 32. I would never mark my tanks "permanently" (except for dedicated EAN80 and 100% O2 bottles, and even then it would just be "permanent" stickers). You should relabel them each time you get a fill, with your name or initials and the date. And you're going to analyze them anyway. How big a deal is it to put the FO2 and MOD on the label while you're doing it?

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%.

If the max depth to a hard bottom is 130, then you're probably not going to be actually swimming at 130. Unless you like crawling in mud...? If I were diving EAN32 and the bottom was 130, I would just swim up a bit off the bottom. Maybe just a little bit higher than I normally would.

True, but I may want to plan to be deeper than 111 feet.

 

[tbone1004]

So you're advocating to never go deeper than 100' unless you're on Trimix?

correct. at that point it is best to introduce normoxic trimix to combat narcosis if you are planning on staying that deep or doing anything while down there.

Hard bottom at 130, planned depth of 1001-110, I'm OK with 32%, it's not ideal, but I get narc'd fairly easily and in cold water like that it's not my favorite. Your gas is technically "safe" to breathe to the bottom, so if you drop something, you can pick it up. I like to keep total PO2 and PN2 under the 3.2 limit because there have been a bunch of studies indicating O2 contributing to narcosis. For that, 21/35 would be my choice. Total narcotic gas level of 3.25. During "best mix" calculations you first set your PO2 to whatever number it is you choose, 1.2, 1.3, 1.4 etc, and that's your O2 %, then nitrogen is typically set to 3.2, with the rest being helium. I subscribe to O2 being narcotic, and that means PO2+PN2=3.2 and the rest is helium.

The biggest thing I would change to your statement is not planning to do any decompression. Getting the training at least in backgas deco whether you plan to do accelerated deco or not will make those dives much safer. Many divers get spooked when they see their NDL's run out and it causes unnecessary anxiety, and you can then do your gas planning based on your total amount of gas carried to set your dive time limits and anything less than that just means less deco.


If you are planning on diving deeper than 110, especially in the dark and cold where narcosis typically onsets much faster and easier, I would HIGHLY encourage you to take a combination ART/DP class where you learn recreational trimix and accelerated deco to prevent you from doing something stupid at depth. I think Steve Lewis is teaching a bunch of those classes in the cold stuff and it would be a very good use of your money and time to take that class with him or someone comparable. Whether you decide to fill with trimix or not, at least you have the ability. I know 100% I won't dive deeper than 110 in those conditions without trimix, it's just non negotiable, I know my body too well to risk it. I can function at depth, I've done studies in chambers to prove that I can function under a heavy narc, but it is completely different in the water and it is an extraordinarily uncomfortable experience.

 

[2airishuman]

The biggest thing I would change to your statement is not planning to do any decompression. Getting the training at least in backgas deco whether you plan to do accelerated deco or not will make those dives much safer. Many divers get spooked when they see their NDL's run out and it causes unnecessary anxiety, and you can then do your gas planning based on your total amount of gas carried to set your dive time limits and anything less than that just means less deco.

I can see the wisdom in what you are saying and there may come a day when I follow your advice.

I've done the planning for a bunch of imaginary deco dives with both back gas deco (air and ean) and accelerated deco. I have a spreadsheet somewhere where I calculated the max bottom time on a pair of HP-100s with air deco at various depths taken from the IANTD tables, based on the gas planning consequences of all the deco stops. Then I did the same thing with accelerated deco for the last two stops. This is my way of learning things. In the process, I have not been able to convince myself that there are any dives that require planned deco that I want to do badly enough to make it worth it. Least of all do I ever want to carry an O2 bottle with me to depth. I take your point however that incidents do occur where an unplanned deco obligation is incurred due to entanglement or some other problem. If it happens I'll keep my cool, watch my computer, do what it says, and try to make sure it doesn't happen again.

 

[claymore]

I am from Minnesota and am guessing that the OP is diving in the the old iron mines and Lake Superior. Many of the mines are 200+ feet deep with with almost vertical walls in many places. In Superior the main diving along the north shore is the Medira wreck that is 20'-110' and the Hesper which id much shallower unless you go out to Isle Royal.
Either place you are dealing with cold water at depth which can increase your O2 suceptibity and your N2 loading and if you are in some of the mines they can get really dark at 80'+ which tends to increase Narcing in some people.

When I am diving the mines on Nitrox I use 32% which gives me an MOD of 111' at 1.4 and a contingency of 130' at 1.6. I only dive a single tank and will do 1 deep dive a day on nitrox. Since OX TOX is time, depth, exertion and temperature dependent I can feel safe at 1.6 as a contingency PO2.

If a diver is doing multiple long deep dives over a short period or multiple dives per day over several days on say a 32% mix the risk factors are there to us a more conservative PO2.

 

[Patoux01]

I wouldn't hit pO2 higher than 1.4... except during deco.

I'd rather be doing a bit of deco than pushing a pO2 at 1.6 at depth. It probably doesn't change more than a few minutes anyway.


I'm a bit surprised by the NDL you give at 130ft though....

 

[2airishuman]

I am from Minnesota and am guessing that the OP is diving in the the old iron mines and Lake Superior. Many of the mines are 200+ feet deep with with almost vertical walls in many places. In Superior the main diving along the north shore is the Medira wreck that is 20'-110' and the Hesper which id much shallower unless you go out to Isle Royal.
Either place you are dealing with cold water at depth which can increase your O2 suceptibity and your N2 loading and if you are in some of the mines they can get really dark at 80'+ which tends to increase Narcing in some people.

Lake Superior, yes, Minnetonka also, don't really intend to spend much time in the deeper parts of the mines except for training. I would like to go to Isle Royal at some future point after I have the experience for it to be safe and fun.

When I am diving the mines on Nitrox I use 32% which gives me an MOD of 111' at 1.4 and a contingency of 130' at 1.6. I only dive a single tank and will do 1 deep dive a day on nitrox. Since OX TOX is time, depth, exertion and temperature dependent I can feel safe at 1.6 as a contingency PO2.

If a diver is doing multiple long deep dives over a short period or multiple dives per day over several days on say a 32% mix the risk factors are there to us a more conservative PO2.

I do not ever intend to dive to 130' unless there is a reason to dive to 130' in the sense that there is something other than the numbers on the computer that make the dive more worthwhile than a shallower dive. In fact I would hold that to be true for anything deeper than 60'. Depth for the sake of depth is not something I wish to pursue.

I don't ever plan on diving to 130' in the mines, other than for training. To me there is no reason for that. That dive will be on air or 28% because it's a PADI class and I'm going to follow PADI's rules.

I believe we cross posted, I do have a dive computer that calculates accumulated O2 CNS burden, and I intend to heed well its advice.

 

[2airishuman]

I wouldn't hit pO2 higher than 1.4... except during deco.

I'd rather be doing a bit of deco than pushing a pO2 at 1.6 at depth. It probably doesn't change more than a few minutes anyway.

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

I'm a bit surprised by the NDL you give at 130ft though....

It's from the PADI RDP for EAN32, and includes a 3 minute "safety stop." IIRC some other tables have a 3 or 4 minute deco stop at 15' for a dive of that depth, time, and gas. Or am I missing something?