Analyzing your own nitrox tanks

[BRT]

Easily quantifiable from the chart below:
View attachment 914974
The problem is moisture in a nice, warm atmosphere, like much tropical diving. Errors of 1% are likely and 2% is quite possible.
If you "calibrate" by waving the analyzer in the air on a tropical dive boat then you set the analyzer on (say) 20.9 but it may actually be only 19.9, or even 18.9. So you will overestimate your tank's O2 percentage, by that 1 or 2%. That can make a difference if you are diving near your NDL....you don't actaully have as much NDL as you thought.
I just spent 10 days diving in the Solomon Islands. Tanks routinely measured 31.7-32.4%, but the analyzers were calibrated to air on the dive deck. Super warm and moist. I routinely set my computer at 31% AND stayed away from my NDL. In retrospect, I should have used 30%.

I suspect you have never seen a day in the tropics with the temperature over 90 and the humidity over 90%. those conditions would give you an error of less than 1%. When temperature goes up, relative humidity goes down. When temperature goes down, relative humidity goes up.

 

[tursiops]

I'm surprised that NAUI would offer implicit endorsement of "Kentucky windage".

Has that carried over in later editions of the same manual?

It is still in the 2012 edition. Old thinking dies hard.

 

[Norwegian Cave Diver]

Normalization of Deviance can be a poor choice.
There was a cave diver in Florida that passed away in the last couple years. He chose not to analyze a large tank that he owned and it turned out to be full of 100% and he took it to depth and died.
Always analyze.
Set your computer to the correct value and let it keep you informed and safe.

 

[elgringoperdido]

Or, maybe you don't know what you mean or do.

I am fairly sure about what I mean. And do. I am concerned about all risks. Not only decompression illness and oxygen toxicity, but nitrogen can also become narcotic at increased partial pressures. That starts to happen at around 5 atmospheres according to TDI guide to advanced nitrox (2009).

What I said was that in the few occasions where I tried multiple analyzers offered by the resort and got different readings, I limit my profile on the higher ppO2. Here's a specific example:

Let's say we want to limit our oxygen partial pressure to 1.4 atmospheres. (Does that statement imply to you that I am more concerned about oxygen toxicity than other risks? Or that I am inordinately concerned about oxygen? If so, then your reasoning is inductive, at best.)

Using Dalton's Law and assuming ppO2=34%, we have a maximum depth of about 31.2 meters (about 102 feet). If we assume ppO2=30%, then we have a max depth of about 36.7 m (about 120 feet). If I believed that the ppO2 in my cylinder was somewhere between 30 and 34%, I'd go with a depth of 102 feet in this case.

Clearly I am considering the effects of oxygen here, but I do not see how from that alone you can conclude that I am more concerned about oxygen than about nitrogen.

Now, we are also concerned about DCS. That's when the subject of using tables or computers arises (and I made no mention of that in my original post, although in my follow-up post I certainly did). For that concern it is prudent, or "conservative" in the language of diving manuals, to use the higher estimate for ppN2, which means in this example that I set my computer at 30%.

Clearly in doing this I am considering the effects of nitrogen here, but again, that statement alone would not give rise to the conclusion that I am not concerned about any other risks of diving. It is also consistent with the advice given in the NAUI and TDI training manuals I have on the shelf behind me, and I suspect that it is consistent with manuals from other agencies as well.

I think the OP wanted to know about "rules" vs "guidelines" and my advice is to follow the advice of analyzing the gas in the tank every time. I only offer a plan based on an analysis that yields inconclusive results. If you believe your ppO2 falls within a range, then you should plan the maximum depth based on that range. The fact that I did not also mention dive time or NDL or total nitrogen time in my original post does not mean that I am not concerned with DCS, or that I am less concerned with DCS than with other risk factors. I humbly and respectfully submit that your conclusion is illogical.

 

[lostsheep]

I am fairly sure about what I mean. And do. I am concerned about all risks. Not only decompression illness and oxygen toxicity, but nitrogen can also become narcotic at increased partial pressures. That starts to happen at around 5 atmospheres according to TDI guide to advanced nitrox (2009).

What I said was that in the few occasions where I tried multiple analyzers offered by the resort and got different readings, I limit my profile on the higher ppO2. Here's a specific example:

Let's say we want to limit our oxygen partial pressure to 1.4 atmospheres. (Does that statement imply to you that I am more concerned about oxygen toxicity than other risks? Or that I am inordinately concerned about oxygen? If so, then your reasoning is inductive, at best.)

Using Dalton's Law and assuming ppO2=34%, we have a maximum depth of about 31.2 meters (about 102 feet). If we assume ppO2=30%, then we have a max depth of about 36.7 m (about 120 feet). If I believed that the ppO2 in my cylinder was somewhere between 30 and 34%, I'd go with a depth of 102 feet in this case.

Clearly I am considering the effects of oxygen here, but I do not see how from that alone you can conclude that I am more concerned about oxygen than about nitrogen.

Now, we are also concerned about DCS. That's when the subject of using tables or computers arises (and I made no mention of that in my original post, although in my follow-up post I certainly did). For that concern it is prudent, or "conservative" in the language of diving manuals, to use the higher estimate for ppN2, which means in this example that I set my computer at 30%.

Clearly in doing this I am considering the effects of nitrogen here, but again, that statement alone would not give rise to the conclusion that I am not concerned about any other risks of diving. It is also consistent with the advice given in the NAUI and TDI training manuals I have on the shelf behind me, and I suspect that it is consistent with manuals from other agencies as well.

I think the OP wanted to know about "rules" vs "guidelines" and my advice is to follow the advice of analyzing the gas in the tank every time. I only offer a plan based on an analysis that yields inconclusive results. If you believe your ppO2 falls within a range, then you should plan the maximum depth based on that range. The fact that I did not also mention dive time or NDL or total nitrogen time in my original post does not mean that I am not concerned with DCS, or that I am less concerned with DCS than with other risk factors. I humbly and respectfully submit that your conclusion is illogical.

You need to stick with the convention of using FO2 and ppO2 , it would clarify you’re intent.

 

[elgringoperdido]

You need to stick with the convention of using FO2 and ppO2 , it would clarify you’re intent.

Well, fair enough. I surmised based on the comments by others that I was unclear, although it seemed clear to me when I posted it. I'm looking at the NAUI manual (same edition mentioned earlier) now and in chapter 3 it uses the notation FO₂ instead of the greek letter chi (χ). I probably should have remembered that because I did read every page of the manual and practiced all their problems before I took the test.

I sometimes get into arguments with other divers over minutiae. Part of the problem is the notation ingrained in me long before I started diving. I didn't get my first dive cert till I was 33 years old. By then I had a BS in mathematics, an MS in theoretical physical chemistry, and a PhD in experimental physical chemistry and was working as a post-doctoral researcher for the Department of Energy.

For example, Dalton's Law in the textbooks was always presented as Pᵢ = χᵢ Pₜ where Pᵢ is the partial pressure of the iᵗʰ gas in a mixture, χᵢ is the mole fraction of the iᵗʰ gas, and Pₜ is the total pressure of the mixture. (I have worked with many gas mixtures and used this relationship often in designing experiments.) The dive manuals don't disagree with that, they just don't use the phrase "mole fraction" and "the partial pressure of the iᵗʰ gas" and the symbols they use are slightly different. Same goes for a bunch of other physical relationships. But the notation with which I was already intimately familiar worked for me, so I never bothered to try to memorize the terms and symbols used by NAUI, PADI, etc.

NAUI writes: "The partial pressure of any component gas in a mixture is the fraction of that gas times the total pressure" (The omission of the word "mole" in front of fraction would be a glaring oversight in a physics or chemistry paper and a reviewer would immediately ask about it, because although volumes and amounts are proportional for ideal gases, the mass fractions are very different than the mole fractions used in Dalton's Law. For example, a mixture of N2 and O2 that is 20.9% oxygen by moles would be 23.3% oxygen by mass, assuming molar masses of 28 g/mol for nitrogen and 32 g/mol for oxygen.) You could also argue that diatomics such as O2 and N2 are not very ideal at high pressures so the simple gas laws don't work perfectly (3% error at 200 bar, 10% error at 300 bar), but that's getting into the weeds.

I guess that's all stuck in my head long before I started diving. I will say that all of the instructors I have had were tolerant of my over-analyses. In fact, a couple of them have expressed admiration for it. One even invited me over for dinner with him and his wife, also a dive instructor, and we discussed diving physics at length. In his day job he was an engineer, and I know that they sometimes use different notation and terminology than scientists, so he was probably already used to translating that sort of thing in his head.

I think I also now understand the question about the relative importance of DCS vs oxygen concerns. I only mentioned using the higher number for the profile (and in my head profile refers to depth), and I didn't post anything about run time or TNT, so maybe it seemed that I was thinking of oxygen more than nitrogen. But I'd say that on an average dive I probably think more about nitrogen. Sometimes I even hang far longer than three minutes at 15 feet on those tropical recreational dives, if the dive leader will put up with it. Especially if the water is clear and there are turtles and fishes and dolphins to look at.

 

[SuitableSkies]

I need to get round to uploading my design to GitHub.

If you want simple electronics then Mark Munro's rebreather monitor design is really easy, really robust. If you have a look at his webpage and scroll down to the single display and click on the schematic and parts list. I've made dozens of these for people over the years. In fact, when I opened up my Analox analyser I was a bit shocked to see that it is exactly the same design inside. Details of PPO2 Displays

If you want fancy then I recommend some kind of Arduino-based controller, an ADS1115 board as the analog to digital converter (they have a good enough level of precision for analysers) and a display.

I use the Lilygo T-Display S3 which is an integrated ESP32 with an OLED display in my rebreather controller & ppO2 monitoring projects. Sensor connected to the ADS1115. You need to add an external EEPROM to store the calibration data (not necessary with Arduino as they have on board memory). Whole thing runs off a single 14500 battery, will do about 7-8 hours on a single charge and charges from USB.

If you find a reasonable priced CO sensor then let me know. I've been thinking about doing a CO checker for my own diving use.

Yeah for sure. The electronics looks dead simple for this kind of project. I'd probably go with an ATTiny chip and life would be great. With an amplifier circuit, even the standard 10 bit ADC is probably enough right? Though your ADC is only 5 bucks so might as well just use that. On the other hand, finding appropriate sensors at reasonable prices looks impossible ...

It would also be cool to have a setup that either threads into the din tank or connects to the bc inflation hose

 

[tursiops]

Well, fair enough. I surmised based on the comments by others that I was unclear, although it seemed clear to me when I posted it. I'm looking at the NAUI manual (same edition mentioned earlier) now and in chapter 3 it uses the notation FO₂ instead of the greek letter chi (χ). I probably should have remembered that because I did read every page of the manual and practiced all their problems before I took the test.

I sometimes get into arguments with other divers over minutiae. Part of the problem is the notation ingrained in me long before I started diving. I didn't get my first dive cert till I was 33 years old. By then I had a BS in mathematics, an MS in theoretical physical chemistry, and a PhD in experimental physical chemistry and was working as a post-doctoral researcher for the Department of Energy.

For example, Dalton's Law in the textbooks was always presented as Pᵢ = χᵢ Pₜ where Pᵢ is the partial pressure of the iᵗʰ gas in a mixture, χᵢ is the mole fraction of the iᵗʰ gas, and Pₜ is the total pressure of the mixture. (I have worked with many gas mixtures and used this relationship often in designing experiments.) The dive manuals don't disagree with that, they just don't use the phrase "mole fraction" and "the partial pressure of the iᵗʰ gas" and the symbols they use are slightly different. Same goes for a bunch of other physical relationships. But the notation with which I was already intimately familiar worked for me, so I never bothered to try to memorize the terms and symbols used by NAUI, PADI, etc.

NAUI writes: "The partial pressure of any component gas in a mixture is the fraction of that gas times the total pressure" (The omission of the word "mole" in front of fraction would be a glaring oversight in a physics or chemistry paper and a reviewer would immediately ask about it, because although volumes and amounts are proportional for ideal gases, the mass fractions are very different than the mole fractions used in Dalton's Law. For example, a mixture of N2 and O2 that is 20.9% oxygen by moles would be 23.3% oxygen by mass, assuming molar masses of 28 g/mol for nitrogen and 32 g/mol for oxygen.) You could also argue that diatomics such as O2 and N2 are not very ideal at high pressures so the simple gas laws don't work perfectly (3% error at 200 bar, 10% error at 300 bar), but that's getting into the weeds.

I guess that's all stuck in my head long before I started diving. I will say that all of the instructors I have had were tolerant of my over-analyses. In fact, a couple of them have expressed admiration for it. One even invited me over for dinner with him and his wife, also a dive instructor, and we discussed diving physics at length. In his day job he was an engineer, and I know that they sometimes use different notation and terminology than scientists, so he was probably already used to translating that sort of thing in his head.

I think I also now understand the question about the relative importance of DCS vs oxygen concerns. I only mentioned using the higher number for the profile (and in my head profile refers to depth), and I didn't post anything about run time or TNT, so maybe it seemed that I was thinking of oxygen more than nitrogen. But I'd say that on an average dive I probably think more about nitrogen. Sometimes I even hang far longer than three minutes at 15 feet on those tropical recreational dives, if the dive leader will put up with it. Especially if the water is clear and there are turtles and fishes and dolphins to look at.

Yep, minutia and overthinking, and add to that TMI. Any points you are trying to make are lost in a sea of words.
Except in the extremes of the diving world, ideal gases work fine.
You know a lot, but you don't have to tell us all of it, all at once.

 

[lizardland]

Yeah for sure. The electronics looks dead simple for this kind of project. I'd probably go with an ATTiny chip and life would be great. With an amplifier circuit, even the standard 10 bit ADC is probably enough right? Though your ADC is only 5 bucks so might as well just use that. On the other hand, finding appropriate sensors at reasonable prices looks impossible ...

It would also be cool to have a setup that either threads into the din tank or connects to the bc inflation hose

Yeah the code is so simple it would probably run off an ATTiny. It might come down to what sort of display you use. The small cheap OLED displays you can find on ebay need a bit of memory for the screen buffer but I'm pretty sure I've seen projects on Youtube where they managed to make it work.

I use the TDisplay S3's because I was designing a module that would fit inside a Hammerhead rebreather handset. The form factor is perfect, the screen is lovely and the whole thing will run off an AA sized 3.7V lithium battery which is what the Hammerhead was designed for. But it's massive overkill for what I need.

The ADS1115's are great for reading cells. On the finest gain setting they will read 0-256mV at 15 bits. No amplifier needed for a really precise reading. It tracks perfectly to what I read with my voltmeter.

Sensors are always the big cost. I don't think there's really a cheap option. I dive CCR so I've generally got a used cell that doesn't have enough life left to trust in the rebreather but is fine for analysing. You can get cheap O2 sensors on Aliexpress but... a funeral is a hell of a lot more expensive than an oxygen sensor.

For testing I made an oxygen cell simulator. It's basically just 3 adjustable voltage dividers on a PCB that step a 1.5V AA battery down to the 0-200mV range that you'd expect from an O2 sensor. I can dial in whatever FO2/ppO2 I want and it will simulate the output of a typical rebreather's 3 cells. I think I paid $15 from PCB-Way for 5 boards and the parts were peanuts on ebay. Cheaper than buying cells just for testing and development.

 

[JediCheese]

You can get cheap O2 sensors on Aliexpress but... a funeral is a hell of a lot more expensive than an oxygen sensor.

I for the life of me can't find cheap O2 sensors on AliExpress. Do you have any suggestions on what to search for?

I plan to use them to verify vs a good sensor (typically on test builds). Just hard to justify spending $100 on a spare sensor that I don't really need.