Nitrox on boat with air refill

[sealark]

Gauge accuracy is what I have found to be the most problematic. Most gauges are off at some pressures and right on at other pressures. And always check the mix after mixing.

 

[Freewillow]

Thanks IVC, yes sealark, I was joking.

here we are not even mixing air with oxygen. We are mixing air with 20% of Nitrox.

To realise that people have to analyse the new mixture or need to make complicated calculation is just hilarious.

Lets say that you are left with 20% of nitrox 32. If you add 80 % of air, you are left with roughly Nitrox 22,5. What a scary risk compared to air :blinking:

 

[sealark]

What I do is keep a record of all my tanks 10 and for tanks with mix left in them. Hook the tank up take a pressure reading then look on the record for the recorded mix and calculate.

 

[dumpsterDiver]

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.

If you are unable to grasp the utility of having an independent reality check on the analyzer results... then I probably can't offer you much.

 

[IVC]

To realise that people have to analyse the new mixture or need to make complicated calculation is just hilarious.

I know, crazy stuff. When you're within 10-20% with O2 you are always good to go. Those silly technical divers worrying about PPO2 of 1.6 vs. 2.0 and alike doing calculations... children's play.

Oh, wait...

 

[Centrals]

I know, crazy stuff. When you're within 10-20% with O2 you are always good to go. Those silly technical divers worrying about PPO2 of 1.6 vs. 2.0 and alike doing calculations... children's play.

Oh, wait...

You are not serious, are you?

 

[KenGordon]

If you are unable to grasp the utility of having an independent reality check on the analyzer results... then I probably can't offer you much.

Since you chose a nasty tone....

You can't offer much if you expect the answer you calculated to be correct.

It is not an independent REALITY check. Several compounding sources of error are present. The pressure gauges, and the analysis of the two starting gasses. Also the assumption of linear gas behaviour.

Obviously if you mix 32 with air and analyse > 32 you should step back and figure it out.

 

[IVC]

You are not serious, are you?

The "oh, wait" part was supposed to convey sarcasm. My response was to a post ridiculing questioning of the O2 exposure by a new diver, i.e., me.

So to be explicit: "No, I am not serious; I was making the opposite statement."

 

[NAM001]

Knowing the correct mix in a tank is not all that important in this circumstance. You started with 32 percent, so you are good to what 120 or so for mod. every air top off is lower %age and a deeper mod. So an error is no consequence in the recreational depth limits. Now the under estimating of o2 is not a factor as it takes more time at depth than you have in your tanks to get to your cns or otu limits Once again no consequence. Proof to this position is: Common practice is to use nitrox and dive on air tables and not nitrox tables. Then there is the head math that you can do to get a close exstimate to your o2 content. Error either way is still a no consequence issue. IE you have 1000# of 32% and you surface and top off with air. thats roughly 300 o2 in the 1000 psi and then there is the 20% of 2k or 400 psi so 300+400 is 700 o2 in a 3000# tank. 750 is 25% of 3000 so you have about 25% nitrox in the tank. And that is darn near air. So you can imagine if you surfaced with 500 psi of 32% and topped off with air. Your first tank of nitrox is the most costly if not used correctly, after that it is gravy. Yes there are dangers in using any gas but like the fire triangle if you are missing one you dont have a fire. In this case it is O2% time and depth. Unless they are all there you don't have a problem because errprs are very forgiving.

 

[kelemvor]

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]


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He already said he didn't have an analyzer at the time in the first post. Most recreational divers I've encountered don't carry one on the boat. My guess is they don't have one at home either. I suspect it's more common for a nitrox diver to NOT own an analyzer themselves.