NITROX CERTIFICATION PROTOCOLS - REAL WORLD

[BRT]

NO analyzer will calibrate correctly unless you know the percentage O2 in the gas you are using for calibration. Using air -- even ambient air -- and pure O2 are the usual calibration gases.
But SOME analyzers assume your calibration gas is air so default to assuming 20.9% O2 and there is no way to change that. If you use one of those analyzers with a bottle of pure O2, it will call the O2 "air" and if you then try and analyze your Nitrox, you'll get some ridiculously low number.

Now, what is the problem with recalibrating -- correctly -- in-between bottles of Nitrox?

Just that the sensor slowly drifts downward towards 21.9% and if you calibrate it as 21.9% when the sensor still has some high O2 gas in it you will not be correct..

 

[Wibble]

Just that the sensor slowly drifts downward towards 21.9% and if you calibrate it as 21.9% when the sensor still has some high O2 gas in it you will not be correct..

Hence you wave it around in the air to clear out the excess oxygen.

It's 20.9% by the way. OK, 21% to friends.

 

[BRT]

Hence you wave it around in the air to clear out the excess oxygen.

It's 20.9% by the way. OK, 21% to friends.

Yeah, I'm on a call. But you don't know when the excess has cleared out.

 

[Wibble]

Yeah, I'm on a call. But you don't know when the excess has cleared out.

If you've a flexible pipe connected to the analyser, you SUCK (not blow) the gas out of the pipe. This brings in fresh air over the sensor.

If it's an OEii, then it's just a matter of pulling out the flow restrictor and waving it around for a few seconds then putting the restrictor back in.

You know when the reading stops changing.

 

[BRT]

If you've a flexible pipe connected to the analyser, you SUCK (not blow) the gas out of the pipe. This brings in fresh air over the sensor.

If it's an OEii, then it's just a matter of pulling out the flow restrictor and waving it around for a few seconds then putting the restrictor back in.

You know when the reading stops changing.

All kinds of things you can try. But the best thing is to calibrate and then analyze all the tanks.

 

[boulderjohn]

The actual reality is that for recreational non-deco-limit nitrox, it doesn't need to be that accurate; a couple of percent either way probably won't matter.

Yikes! A tank loaded with 100% O2.

Just for shi** and giggles, play that scenario out for me. I’m ready for the dive. I enter the water. I place the regulator in my mouth and with my compadres, I begin my descent, unknown to me, that my tank contains 100% oxygen.

What happens next…

Okay, I am thoroughly confused.

Wibble says that "for recreational non-deco-limit nitrox, it doesn't need to be that accurate; a couple of percent either way probably won't matter."

In response, you produce a scenario in which you think you are breathing a normal nitrox mix (32%???) but are instead breathing 100%.

Are you saying that is an example of being "a couple percent either way"?

 

[Wibble]

Okay, I am thoroughly confused.

Wibble says that "for recreational non-deco-limit nitrox, it doesn't need to be that accurate; a couple of percent either way probably won't matter."

In response, you produce a scenario in which you think you are breathing a normal nitrox mix (32%???) but are instead breathing 100%.

Are you saying that is an example of being "a couple percent either way"?

Just to make doubly certain of the point I made earlier...
When you check your Nitrox mix one of the reasons for this is the rare possibility that the gas is actually 100% due to a cock-up by the gas monkey. This has happened -- I heard it on the intarwebs, so it must be true.

The other cock-up by the aforesaid gas monkey is issuing the wrong mix. Giving you a tank of 50% or 80% (if I'm around) instead of the 32% you ordered.

None of those examples are within a couple of percent of what you'd be expecting.


I did say that a couple of percent either way doesn't matter. For example, you've ordered 32% for a 30m/100ft dive. If it comes back as 30% or 34% it doesn't matter. Your MOD (maximum operating depth) for 34% would be...
dose / mix = 1.4 / 0.34 = 4.1ATA = 31m/103ft

However, if it had been hit with the cock-up hammer and was full of 50%, then the MOD would be:
dose / mix =1.4/0.5 = 2.8ATA = 18m/60ft
Worse still the PPO2 at 30m would be:
pressureATA X mix = 4 x 0.5 = 2.0 PPO2
That is full on danger of hyperoxia (too much oxygen causing you to fit, passing out, drowning...).


It's you lot who dive the same looking ali80 tanks! In Europe we use steel cylinders for backgas and ali80s for deco/stages.


TL;DR

Always analyse your gas in case there's a serious problem with the mix.