PPO2 maximum safe value: 1.4, 1.6

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no reason to permanently mark tanks. Banked nitrox has huge advantages over PP blending, but you still technically have to remove all stickers at VIP every year, so a small piece of painters tape with the mix, mod, and initials is good enough
 
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.

I believe you mean a ppO2 of 1.6.

Anyway, I wouldn't rely on the markings either. It's just one more means of communication.

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?

Yep. If I decided to use it.

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.

I am skeptical of handheld analyzers and see them as a safety item rather than as a primary means of identifying the gas in a tank. Maybe the calibration gas was rich. Maybe the sensors were approaching end of life. Maybe humidity was a factor during calibration. Maybe the gas from the tank was diluted before it reached the sensor.

A responsible shop dispensing banked nitrox should be producing an accurate mix. +-2% is an industry standard, but it's easy to do better than that, and should be measuring by pressure or volume in such a way that the analyzer is just a process check. Right? I'm not getting breathing gas from some guy with a beat up Bauer in his garage.

I definitely think you are putting too much emphasis on "standardizing cylinder contents."

That seems to be the consensus, although, the divers with local experience who have commented have pointed out that dives below 110 feet are rare in this area, so it may not matter.

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.

I have been unable to find the bargains on used tanks that everyone else does. There are several sets of doubles on CL locally right now however asking prices are well over 50% of the new prices including shipping from nationwide retailers. This for cash-only, as-is, out-of-hydro (or soon to be) tanks. One is a pair of LP tanks from someone who just moved here from cave country, then there are some older HP tanks that are 5 years out of hydro and not really the size I want.

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.

That also seems to be the consensus.
 
if you don't trust the O2 analyzers, how are you sure that you have the exact mix unless you dump the tanks before you fill every dive? The bank analyzers use the same sensors as the handheld ones, and same as the ones in rebreathers. If you have your own, you know how it has been treated.

to answer your concers
Does it calibrate properly? If yes, then it's calibrated, you should calibrate against an air tank, but should note that the atmospheric air should be close to 20.9
sensors ending its life, how are you sure the fill station is swapping them out regularly? If you have your own, why aren't you swapping them out regularly?
Humidy shouldn't be because you calibrate against an air tank, I have never seen a fill station that doesn't have an air tank to calibrate against. You turn the analyzer on, verify that you are "close" with the atmospheric air, then verify it against the calibration tank
Gas from the tank can't dilute before it reaches the sensor because it is at higher pressure than the atmosphere and you would hear it if it is leaking.

Another factor that you aren't 100% sure of with the bank analyzers is if the gas is fully mixed before it got to the tanks. Remember that the sensors measure the PO2 not the FO2, so they have to analyze on the intake of the compressor. There may have been insufficient blending before it got to the sensors, and may have picked up CO in the compressor if there was a bad ring in one of the stages. Rare, but people have died because of it.

In technical diving we have to rely on the handheld analyzers because any sort of PP filling, or topping off is not analyzed by the fill station, so you have to trust the analyzers and we almost never dump gas before we fill normal backgas bottles. Good enough for that use should be good enough for you.
 
I use 1.4 as a plan limit and 1.6 as a contigancy as is normally recommended. I do this because looking at a cns chart or using a formula and excel you can plot the cns exposure curve.

Here ia a chart http://wetlands.simplyaquatics.com/d/14029-1/otu_cns.pdf
look at the chart and see what happens at 1.5 and 1.6 on the right side. things skyrocket at 1.6 or 1.65. It does not take much of a mix error or sensor error to get you in doo doo thinking you had 32 and you actually had 33.5%. using the mod for 32 when actually having 33.5 would put you on the high rise of the cns curve and you would not know it. I respect the buffer provided by using 1.4 and not 1.6.. For those that use the best mix concept.. you can really get your self hurt. Here is perhaps a better chart

http://www.borrett.id.au/downloads/cns_percentage_tracking_v1-1.pdf
 
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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.

I don't doubt what you're saying, but that's new information for me. The medically-based controlled studies I've seen haven't documented a case at below 1.7. What symptoms did your acquaintance have? Was hypercapnia a possible alternative explanation for what occurred?

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.

I think that makes sense.

I am trying to say there are reasons all the agencies picked these MOD numbers.

One of my questions was whether all agencies use 1.4. I know PADI does. Do GUE, TDI, SSI, etc., also use 1.4?

The reasons are that now and again someone fits and drowns. Normally that happens with long exposures or high ppO2 but not always.

I was under the impression that the lack of studies, an abundance of caution, and wildly varying and poorly understood variation from one individual to the next (and one dive to the next) also played a role. I am unaware of any confirmed cases of ox tox at 1.6 in the literature.

if you don't trust the O2 analyzers, how are you sure that you have the exact mix unless you dump the tanks before you fill every dive?

The bank analyzers use the same sensors as the handheld ones, and same as the ones in rebreathers. If you have your own, you know how it has been treated.

Trust but verify. I believe the O2 analyzers, for the most part, I just don't consider them reliable enough indicators to take precedence over other data that should also be valid. If you have two clocks and they disagree, one of them is wrong, same principle here.

If I were to always use 32% I would never have to dump the tanks, right?

Does it calibrate properly? If yes, then it's calibrated, you should calibrate against an air tank, but should note that the atmospheric air should be close to 20.9

Common failure paths. Was the shop calibrated against the same "air" tank? How do we know it was air? If it is 24% because of, say, residual O2 in the nitrox stick, then the shop's analyzer and my analyzer would both read low on the sample. For example.

Another factor that you aren't 100% sure of with the bank analyzers is if the gas is fully mixed before it got to the tanks. Remember that the sensors measure the PO2 not the FO2, so they have to analyze on the intake of the compressor. There may have been insufficient blending before it got to the sensors, and may have picked up CO in the compressor if there was a bad ring in one of the stages. Rare, but people have died because of it.

I think that we share a similar level of appropriate paranoia.

In technical diving we have to rely on the handheld analyzers because any sort of PP filling, or topping off is not analyzed by the fill station, so you have to trust the analyzers and we almost never dump gas before we fill normal backgas bottles. Good enough for that use should be good enough for you.

I'm not sure quite how to respond to that. There are plenty of other ways to control and confirm gas mixing ratios. If I were doing PP filling I would be using as accurate a pressure gauge as I could reasonably use, and would also use an accurate scale to weigh all cylinders before and after. Mass is something that can be measured cheaply and with great precision. With knowledge of the molar mass and a little math the FO2 can be determined with great precision.
 
There is no such thing as maximum safe value for PPO2.
Generally, it is 1.4 for diving and 1.6 for deco. And under certain unfavourable conditions ie. cold, depth etc. then it is advisable to lower the suggested figure.
 
Max ppO2 for virtually every agency is 1.4ppO2. The only one I know that differs is ANDI, who use 1.45 ppO2.

PPO2 is for resting deco (tech diving) and emergencies (rec diving) only.

Is the OP seeking justification for breaking this rule by asking anonymous people on the internet?

On balance risk versus reward, it seems a pretty imprudent idea, no matter what science, facts, figures or references to the US Navy etc are used.
 
Max ppO2 for virtually every agency is 1.4ppO2. The only one I know that differs is ANDI, who use 1.45 ppO2.

Thanks.

Is the OP seeking justification for breaking this rule by asking anonymous people on the internet?

My PADI instructor taught me various skills on my knees in the bottom of the pool and to always donate an octo. I'm pursuing a diving strategy of neutral buoyancy and donating a long hose. So I guess I'm breaking the rules.

I am trying to figure out whether 1.4 is in the same category or not. That doesn't mean that I need anyone's permission.
 
I was under the impression that the lack of studies, an abundance of caution, and wildly varying and poorly understood variation from one individual to the next (and one dive to the next) also played a role. I am unaware of any confirmed cases of ox tox at 1.6 in the literature.
The fact that you haven't seen studies showing O2 toxicity hits at less than 1.7 didn't matter a lot to the people who have died at less than 1.7 from oxygen toxicity. I cited a post from a physician who was involved in the postmortem of a diver who died from O2 toxicity at 1.3. There are reasons why lower limits are advocated by pretty much every agency. You don't have to follow them, there are no scuba police, but there is a risk.
 
Trust but verify. I believe the O2 analyzers, for the most part, I just don't consider them reliable enough indicators to take precedence over other data that should also be valid. If you have two clocks and they disagree, one of them is wrong, same principle here.

If I were to always use 32% I would never have to dump the tanks, right?



Common failure paths. Was the shop calibrated against the same "air" tank? How do we know it was air? If it is 24% because of, say, residual O2 in the nitrox stick, then the shop's analyzer and my analyzer would both read low on the sample. For example.



I think that we share a similar level of appropriate paranoia.



I'm not sure quite how to respond to that. There are plenty of other ways to control and confirm gas mixing ratios. If I were doing PP filling I would be using as accurate a pressure gauge as I could reasonably use, and would also use an accurate scale to weigh all cylinders before and after. Mass is something that can be measured cheaply and with great precision. With knowledge of the molar mass and a little math the FO2 can be determined with great precision.

only if it is always 32%. In one of hte shops in cave country they use a digital injection system and it still varies plus or minus a little bit. It's usually close, but that uses a solenoid and multiple O2 sensors. Who knows the last time those were calibrated? They can only be calibrated to atmospheric air because there is no way to put compressed air over them, so humidity in the environment is part of what calibrates them.

The point is this, you don't actually know that the intake o2 percentage is indicative of what was in the banks that filled your tanks. There is variation, always. The shops do not normally analyze the banks multiple times, and they don't have a way to analyze exactly what you are filling. In your nitrox class you were told to analyze your tanks before you breathe them, how do you expect to analyze them without a handheld analyzer? It's not the shops job to analyze your tanks for you and tell you what you're breathing, you have to verify what is coming out of the tank, not what went into it, and the only way to do that is a handheld analyzer. People have died by not analyzing their tanks properly, don't be another one
 
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