Need for air tank o2 analysis

[SailNaked]

assuming only 32% was being added by mistake then the worst case would be a full 32% O2 the MOD for this is 132 (1.6PPO2) for sport divers this would not be a problem and their computers would be set to air so also not a problem. I see no reason to check an air tank for O2. I am Nx cert so if there is Nx available I am diving it and would be checking anyway. no reason to scare the Air divers on a liveaboard or resort that has Nx into thinking they need to check for O2.

Edit:LOL we were posting the same thing at the same time.

 

[H2O 70]

Funny how that happens some times.

 

[diverdoug1]

Most of the liveaboards I dive don't pump pre bottled EAN. They produce it themselves, so the assumption of max. 32% can not be made. This is the very reason you must always analyze your nitrox tank contents. You can not assume the o2 content of your tank (therefore you can not assume the maximum o2 content that might be accidentaly pumped into an air tank). The only way I don't feel compelled to analyze my tank is if nitrox is not being pumped on that boat.

 

[SailNaked]

well I think it can be made, If they are custom blending the Eanx on each bottle then there is little chance that an air bottle would be filled improperly, not that I believe that any liveaboards are custom filling bottles except by specific request. they fill either from a bank that is premixed to 32% (or maybe it is 36% if you are only diving a shallow bottom) or they have a Nitrox unit that produces it on the fly. either way all the whips are at some % that will be known. I can see an operator with dozens of tanks to fill not noticing he is filling an air labeled tank, but I can not see there being more than the standard % in it. I would be more worried about CO than O2.

32% is standard because it coincides with the sport limit of 130 fsw. 36% has a mod of 113, still within 90% of all dives a sport diver will do. either way if you really feel a need to check your air tanks please do. i would spend the money on a CO detector first.

 

[H2O 70]

Most of the liveaboards I dive don't pump pre bottled EAN. They produce it themselves, so the assumption of max. 32% can not be made. This is the very reason you must always analyze your nitrox tank contents. You can not assume the o2 content of your tank (therefore you can not assume the maximum o2 content that might be accidentaly pumped into an air tank). The only way I don't feel compelled to analyze my tank is if nitrox is not being pumped on that boat.

The particular live aboard I cited in my post provided O₂ analyzers and required the divers to analyze their gas and sing off on a log.

I would question a live aboard option that offered EAN and didn't provide analyzers.

 

[ianr33]

Assuming we are only talking about 32%, accidentally filling tanks with nitrox is really unlikely to cause problems within recreational limits.
NDL for a 130 foot dive on air is only a few minutes,much shorter than the 45 minutes allowed at a pO2 of 1.6 (Which is what 32% gives at 130 feet)

Shallower than 110 feet (po2 of 1.4) there is effectively no risk. Between 110 and 130 some very small risk. Significantly deeper than 130 all bets are off.

 

[miketsp]

I do about 5 liveaboard trips per year, and observed a near miss during one of them last year. Busy dive deck scenario, tanks being filled between dives. I watched one of the crew hook up the nitrox fill whip to an air tank, and start to fill it. I called attention to it, and the DM sheepishly said "oops". This brings about the question, should divers be checking the o2 content of their tanks, even when not diving nitrox? Just to state the ovious for those divers who not aware of the possible lethal ramifications of this DM's error, having nitrox (with it's higher o2 content) in a tank thought to hold air, can cause a diver to have a seizure while diving if he exceeds a certain depth. So, when nitrox is being pumped in the vecinity of your air tank, should you be analyzing your "air" tank? I know that I have started doing this on all tanks I dive with.

In fact, what scares me even more is the possibility that a Nitrox diver could have chosen precisely that air cylinder to calibrate the sensor at 21%.
So if he's really got a richer mix like 36, 38 or 40 he's going to measure it as being much weaker. In percentage terms his MOD may be far more impacted than the diver getting EAN in the place of air putting him at a much higher risk.

 

[H2O 70]

Nitrox diver could have chosen precisely that air cylinder to calibrate the sensor at 21%.

Is there some argument to not use the ambient air to calibrate the sensor?

 

[diverdoug1]

Sailnaked, following your line of thought would indicate that you trust the person blending your mix with your life. That they could not possibly screw up and pump WAY higher then 32%. I propse that if they are enough of a screw-up to put the Nitrox whip to an air tank, they are also enough of a screw-up to accidently be pumpuming a high o2 mix at that time. Also, oxygen induced seizures are sometimes induced (atmittedly rarely) at relatively low partial pressures of oxygen. The lowest I have personally seen is 1.5 and 1.55 (both in female patients), but last year we had a diver fatality in Florida, attributed to oxygen toxicity and her ppo2 was only 1.4. I would love to see all the agencies go to 1.4 as their max. ppo2
Miketsp, I never use an air tank to calibrate my o2 sensor. Even at work I calibrate my o2 sensor with room air first, then 2 DIFFERENT 100% O2 sources.

 

[miketsp]

Is there some argument to not use the ambient air to calibrate the sensor?

..snip..
Miketsp, I never use an air tank to calibrate my o2 sensor. Even at work I calibrate my o2 sensor with room air first, then 2 DIFFERENT 100% O2 sources.

Many sensors manuals specifically instruct you to calibrate at the same flow rate as you will be measuring at. Sensors are basically a fuel cell so flow-rate has direct impact due to the cooling produced by the air-flow and the burning of more O2 molecules.

Also most sensors are sensitive to humidity so it is preferable to use dry air from a cylinder. Many sensors provide a humidity calibration curve, but then you need to know the humidity. Where I live swings of 80% over 24hours are common.

I only ever remember seeing one sensor manual that allowed calibration at ambient pressure.

The sensor I was using 2 weeks ago in Curação had a 4% difference between ambient air pressure and a cylinder. I personally have seen up to 8%.

Diverdoug1, if your particular instrument gives you good calibration at both extremes of the scale then fantastic but don't count on this all the time. In my experience this is rare and only while the sensor is new. For this reason you normally want to calibrate as close as possible to your intended measurement.
So if you're diving with 32 then a calibration at 21 is going to give a better precision than a calibration at 100.
If you're measuring below 80, you're still probably better off calibrating at 21 as the non-linearity is accentuated at the top end.
It is a fact that towards the end-of-life most sensors go non-linear at the high end of the scale. This is called end-of-life ceiling mode. To test for this you calibrate with near 100% O2 and then measure air. If the reading is over 22% then you are getting near end of life.

PS. EDIT- I added a curve from ANALOX showing ceiling effect and why calibration at 100% may not be a good idea.