Nitrox on boat with air refill

[JohnnyC]

The level of O2 that requires "oxygen clean" configuration seems to be a contentious issue. I don't have a dog in this race, but from what I found it looks as if the correct PPO2 level for needing to be oxygen clean for anything *outside* scuba community is in the 23%-25% range.

Quick correction. For oxygen cleaning, you are concerned with the FO2, not the PO2. PO2 is used for calculations that have an effect on the dive plan. Think about it, a PO2 of air is .21 at 1 ATA, but .63 at 3 ATA. The FO2 is still .21.

When dealing with O2, semantics become important. We knew what you meant, but it's worth repeating.

 

[Bob DBF]

I don't know which Nitrox class IVC took, but when I took SDI Computer Nitrox, the tables and how to use them were not part of the curriculum. Unless, by "should have had to begin with," you're really talking your personal disagreement with what current course standards are. Regardless, going back to the instructor may not help as I'd guess IVC understood what the instructor taught and the instructor taught the standard curriculum. So, going over it again probably wouldn't make any difference.

Come to find out it was e-learning so you are right on that one and I get to add another reason I don't like e-learning. On the other hand, the standard curriculum is what you have to teach, and listening then passing the test does not mean the student understands everything that was taught. For most folks that just plug a number in their computer it wont be an issue, but for those who want a better understanding, a visit to the instructor could help fill in the information that is wanted since there is no book to refer to.

In my Nitrox class we were all required to have read the book and completed the problems. I am sure everyone could have passed the test walking in, but we spent the next few hours going over everything, solving new problems and discussing questions such as the OP's. Included was the book, 32% tables, 36% tables, and O2 exposure table, and since I don't use Nitrox often this would come in quite handy if I needed a refresher or returned to diving without a computer. Not bad for a PADI class, but then it's the instructor, isn't it?

I don't remember if I congratulated you on Master Diver, if not, consider yourself congradulated.



Bob
------------------------------------------------
"This is scuba board, where problems are imagined or overstated......and chests get thumped about what some would do about those "problems" "- PullMyFinger

That's my point, people, by and large, are not taught that diving can be deadly, they are taught how safe it is, and they are not equipped with the skills, taught and trained to the level required to be useful in an emergency.

 

[IVC]

Quick correction. For oxygen cleaning, you are concerned with the FO2, not the PO2. PO2 is used for calculations that have an effect on the dive plan. Think about it, a PO2 of air is .21 at 1 ATA, but .63 at 3 ATA. The FO2 is still .21.

When dealing with O2, semantics become important. We knew what you meant, but it's worth repeating.

I was going to state that PPO2 expressed in percentages and without units assumes standard pressure (which it does,) but then I realized that all I would be saying is that I'm using a more complicated notation to express the concentration (or "fraction" if you prefer) and that there is no reason for it, especially since during the fill process PPO2 will be changing proportionally to the overall pressure and O2 concentration. So, I'll use the concentration/fraction from now on.

---------- Post added November 7th, 2015 at 09:29 AM ----------

In my Nitrox class we were all required to have read the book and completed the problems. I am sure everyone could have passed the test walking in, but we spent the next few hours going over everything, solving new problems and discussing questions such as the OP's.

If you still have the books, what was the protocol for calculations on multi dives with air fills?

---------- Post added November 7th, 2015 at 09:42 AM ----------

A minor issue but.. the advice to just buy an analyzer and follow it is not all that robust either.. Your analyzer should be used to CONFIRM your mix. You should have a good idea of what the mix is before you measure it and then use the analyzer to verify it.

Now, this is a pretty deep concept that really made me think.

What you are essentially saying is that the analyzer is more of a "red flag" if the reading is off, rather than the primary method of measurement. The implication is that a reputable fill station has procedural and regulatory safeguards in place to ensure correct mix and that these safeguards are the primary (not secondary) source of data, with oxygen sensor confirming rather than determining the oxygen content. This also means that if there is a discrepancy, the correct action is not to use the gas until one can figure out what's going on, instead of going off of the sensor.

With regards to the multi tank dives, this means that the calculation based on residual pressures should always be performed (with dumping of the tank as a convenient shortcut,) then confirmed with the sensor. (Of course, since we are talking about recreational dives it suffices just to get to the ballpark and confirm that the ballpark is nowhere near the limits, but the principle is there: "calculate/estimate, confirm" vs. "ignore.")

 

[DogDiver]

Wow I just don't understand all the fuss about a simple problem.

Fill the nitrox tank from the boat fill whip.
Wait 20 minutes for the gasses to equalize/mix.
Reanalyze the gas
Reset your dive computer to the new mix.

If your diving anything but air, you should have your own analyzer. [emoji41]


Sent from my iPhone using Tapatalk

 

[macado]

Now, this is a pretty deep concept that really made me think.

What you are essentially saying is that the analyzer is more of a "red flag" if the reading is off, rather than the primary method of measurement. The implication is that a reputable fill station has procedural and regulatory safeguards in place to ensure correct mix and that these safeguards are the primary (not secondary) source of data, with oxygen sensor confirming rather than determining the oxygen content. This also means that if there is a discrepancy, the correct action is not to use the gas until one can figure out what's going on, instead of going off of the sensor.

With regards to the multi tank dives, this means that the calculation based on residual pressures should always be performed (with dumping of the tank as a convenient shortcut,) then confirmed with the sensor. (Of course, since we are talking about recreational dives it suffices just to get to the ballpark and confirm that the ballpark is nowhere near the limits, but the principle is there: "calculate/estimate, confirm" vs. "ignore.")

What dumpsterDiver said is pretty much spot on what I typically do. Confirm your mix but you should hopefully already have a good idea what it should analyze out to if you knew your starting mix (known gas) and the pressure left in the tank.

A typical scenario for me. On Saturday I know I'm diving a specific wreck so I blend some Nitrox 36% in my tanks because I want a longer NDL without going into deco limits. Not getting into too much detail but since I am partial pressure blending, my tanks are O2 clean. I add ~661psi of pure oxygen and top off with air (20.9%) ~2839psi to have ~3500psi. If I filled slowly and accurate enough I already know my tanks should analyze out to ~36% but there of course will be slight variants on that based exact PSI, tanks cooling, etc. So let's say my tanks analyze out to 35.7%. Close enough for me to call it 36%. Even a typical +/- 1% variance doesn't bother me a lot.

Part of any standard in a Nitrox course should be to always analyze your own gas that you're diving. I don't care if the dive shop already did it for you. The key here is trust but verify... Almost all will make an O2 analyzer readily available. If they don't I would be concerned but I happen to own one so I always check my own mix. Always analyze a gas that you're going to be diving. Do not blindly trust the label or someone else. I see a lot of experienced divers skip this part and go by what the dive shop says (if they didn't see them actually analyze it) or what's on the label. The point here is people make mistakes and since unfortunately no one is perfect you should always do your own independent verification.

So the second part of the part of the scenario. Since I personally analyzed my tanks myself I know I was diving 36%. I am diving Sunday but the depth is relatively shallow 40-50ft so I don't really care about what Nitrox mix I have. I know there is no way in hell I can exceed my MOD for that mix unless I take a shovel with me but I do want to at least know what is in my tanks.

Not a problem, I will top off with air and do some quick math. Since I am terrible at math, I use a program called MultiDeco which does the work for me but here is a simple way to do it (already explained in another post I think). There is probably a better way to represent this but since I'm lazy here is how I might do a quick spot check. Since you have physics background, I know this must be pretty basic for you but there are a lot of people diving that surprisingly don't know how to do this.

Tank 1: 800psi of Nitrox 36%
36*800psi/3500psi + 21* 2700psi/3500psi
8.22 + 16 = 24.42% when topped off with air

Tank 2: 1400psi of Nitrox 36% (perhaps I ended the dive early for some reason?)
36*1400/3500 + 21* 2100psi/3500psi
14.4 + 12.6 = 27% when topped off with air

Before I even pull out an analyzer I should at least have a ballpark idea of what is in my tanks and what they should analyze out to. If I don't it could mean a couple possible things, my analyzer is FUBAR'ed or perhaps I was not diving with a known gas to begin with (bad...)

There is a quote on here that I think is very appropriate in this situation which has to do with decompression algorithms. Nevertheless it still fits the scenario.

Decompression algorithms are akin to measuring with a micrometer, marking with chalk and cutting with an axe.

For our purposes these numbers could be "good enough". So now I am doing two dives with 24% and 27% in my tanks. I *might* as well treat 24% like air but what about 27%? (MOD 138ft at 1.4) It really depends on the dive profile I am doing. If I am diving to 140ft then yes by all means but if I am diving to 40-50ft I really don't care too much about tracking my O2 exposure based on these mixes.

Back to your original question on O2 exposure tracking. Would these mixes be significant enough to take into account of O2 exposure (CNS and OTU exposure)? Answer: It depends.

How deep, how long and how many dives, multiple days of diving, previous mixes used, etc. I think you'll find that in most recreational dive scenarios you will not be anywhere close to these limits which partly explains why the experienced divers on the boat didn't care or perhaps it was out of ignorance.

Either way, once you know what is in your tanks you can make an educated decision on if the mix should factor into your dive planning.

 

[IVC]

Wow I just don't understand all the fuss about a simple problem.

All the fuss is to determine it's a simple problem.

---------- Post added November 7th, 2015 at 11:44 AM ----------

Not a problem, I will top off with air and do some quick math.
...
Before I even pull out an analyzer I should at least have a ballpark idea of what is in my tanks and what they should analyze out to.

Thanks. This is the method I intend to start using - always do some quick calculation to get to the ballpark, then measure to confirm. If there are any discrepancies, slow down, rethink and don't proceed until it's resolved.

BTW, the quote you have is an old and well known quote used in physics and engineering.

 

[Freewillow]

"Wait 20 minutes for the gasses to equalize/mix."



:chuckle::eyes::yelclap:

 

[sealark]

"Wait 20 minutes for the gasses to equalize/mix."



:chuckle::eyes::yelclap:

O2 and air mix instantly. You can introduce o2 and analyze it as it goes by just before the tank valve and it will be comply mixed. I partial pressure mix. Any size empty tank for 32% at 3000 psi. 450 psi of o2 fill hot to 3300 it will cool to about 3000 and be within 1% of 32% I also Analyze every fill.

 

[IVC]

"Wait 20 minutes for the gasses to equalize/mix."



:chuckle::eyes::yelclap:

A truly "advanced Nitrox" instructor would teach his students an advanced mixing technique: "shake the tank for five minutes" - it saves 75% of time compared to the technique you are currently teaching.

---------- Post added November 7th, 2015 at 01:38 PM ----------

O2 and air mix instantly.

He was making fun of the question.

 

[KenGordon]

A minor issue but.. the advice to just buy an analyzer and follow it is not all that robust either.. Your analyzer should be used to CONFIRM your mix. You should have a good idea of what the mix is before you measure it and then use the analyzer to verify it. You should be able to do the fractions and percents in your head.. more or less.

It sounds like the OP has a healthy dose of skepticism about what people tell him. A good trait in my book.

I slightly disagree. What is a good idea of the mix?


The numbers you start with for the calculation of mix from previous mix, previous pressure and current pressure are not very reliable. In particular the pressure numbers may be off. The predicted mix is not something I would use to dive, so what is the value in confirming it?

I often top up a twinset using a 300 bar 12l cylinder of air or maybe 27 ish %. Before I do that I have a vague idea I got out with about X bar left, I expect the 300 has maybe 280 bar, but it might be 260 to 300 depending on who filled it, the weather etc. I could measure the pressure in the two sets, check the mix in each again and do some maths to get an expected mix, or I could just decant it over and then measure the mix. I have to measure the mix in any case. Will I choose to dive the predicted one and ignore the analyser? Or will I dive assuming the analysed o2 content?

I might expect 25%, but if I measure 24 or 26 I am diving it as 24 or 26.

Better to spend the time between dives talking about what you saw or what the plan is for the next one than obsessing on whether mixing 24l of 32% at 100 bar with 12l of 21% at 300 should give 29% or not.