Measuring O2 in enriched air

[broncobowsher]

And since nitrox cylinders are always labeled why would we need analyzers?

So you know what to write on the label

 

[BRT]

EAN28, EAN32, EAN36, etc., etc., etc.

MOD is a kinda important parameter.

Uh huh. But when they fill them they put a label on them. I trust that label every bit as much as I trust no label being air that is actually 21%. What do you think happens if the "air" tank you calibrate off of actually had some nitrox in it before it was refilled? The big outdoor air tank is the reliable one.

 

[BRT]

So you know what to write on the label

The fill station does that for you. And if they don't put a label on it it is always 21%, isn't it?

 

[tursiops]

The fill station does that for you. And if they don't put a label on it it is always 21%, isn't it?

No.
Even underpaid, undertrained, rushed and harried fill-station employees can make the occasional mistake.

 

[jgttrey]

Calibrating to ambient air is plenty good enough for recreational nitrox, especially if you adjust for humidity using the little chart. I would never calibrate of a cylinder I'm "assuming" is air based on markings or lack thereof. That's nuts. At least do a sanity check. Calibrate to ambient first. It should be very close on the air cylinder.

On the other hand, for reasons discussed above, if precision is required, then get some 100% O2 and do a two point calibration.

If you understand mv readings, the acceptable range for your cells, and linearity, etc., it will be pretty obvious if you've got a problem.

But for recreational use, calibration to ambient is more than good enough and idiot proof.

 

[Storker]

when they fill them they put a label on them.

If I haven't analyzed the tank and initialed that label, I'm not diving that tank. There's only one person I trust when it comes to gas analysis.

What do you think happens if the "air" tank you calibrate off of actually had some nitrox in it before it was refilled? The big outdoor air tank is the reliable one.

I seem to remember that I've said something similar upthread

 

[TrimixToo]

The fill station does that for you. And if they don't put a label on it it is always 21%, isn't it?

This is like taking someone else's word about whether a firearm is loaded. Just don't. Always calibrate the analyzer and then analyze your own gas, or at least watch it done.

 

[BRT]

No.
Even underpaid, undertrained, rushed and harried fill-station employees can make the occasional mistake.

Yes, that was my point. You cannot trust a tank of "air" to calibrate an O2 sensor.

 

[ScubaWithTurk]

Except there's no standard for this. Nitrox labels are a farce. I don't care if cylinders are pink with purple bands and a naked chick on them screaming "It's 32%!" There is neither a requirement nor a standard, nor should there be anything other than bare tanks unless you dive standard gases, then there should be a MOD label on it. Anything else and people start doing stupid things.

Analyze your gas and you never have to worry about what somebody put in a cylinder.

I never stated there was a standard for this and was speaking strictly about the center I have worked at the ones I have been diving with. I fully agree with having a MOD label on the cylinder. I also never said to NOT analyze.

I always analyze my own cylinders. It is my responsibility to make sure that I know the gas I am about to dive with.

And since nitrox cylinders are always labeled why would we need analyzers?

I stated some DCs have green and yellow labels on Nitrox cylinder. These labels do not state the percentage but state that it is for nitrox. Now lets say a blender fills the cylinder, measures it and determines it is 32.4% and labels it as such. Two days later I come to dive and get that cylinder. No way in hell I am trusting that it is 32.4% without verifying it for myself. Often times I have seen a cylinder labeled one way and be off at dive time by 2%.

Now what could be the cause of this? Could be it was not analyzed properly to begin with or it was analyzed too soon and had not yet homogenized properly. It could be the blender was doing multiple cylinders and put the sticker on the wrong cylinder. Humans are prone to error and if you do not check a cylinder prior to diving, it is a trust me dive.

 

[ScubaWithTurk]

Yes, that was my point. You cannot trust a tank of "air" to calibrate an O2 sensor.

Can you trust two cylinders? Say you calibrate on a cylinder thought to be air...then you analyze another cylinder that is supposed to be air. If you get 21% is that not evidence that you are in fact diving air? I always calibrate on an air cylinder and have never once had an issue with that air cylinder not being air