NITROX CERTIFICATION PROTOCOLS - REAL WORLD

[tursiops]

A vast majority seem to calibrate on ambient air. But you can also use any air tank or known pure oxygen sources. For best results calibration gas should be metered at whatever rate your tester is designed for, and should be the same temperature and humidity as your testing gas.

Ambient air. I've heard that. OK. So, just open up my analyzer and wait until it hits 20.9%...

Note the caution about the ambient air should be the same temperature and humidity as the gas you are testing. Ambient air does not usually meet this requirement, especially the humidity. Here is a little cheat sheet from Analox about what you want to calibrate your analyzer to if you are going to try and remove the temp-humidity effect,

Example: If the air temp is 90F, and the air RH is 80% (not an unusual combination is a tropical location!), then if you are using that ambient air to calibrate your analyzer, you want to set the analyzer to 20.1%, NOT the 20.9% for "perfect" ambient air. Some will argue that 0.8% is negligible. Perhaps. But guessing and hoping the ambient air is 20.9% is hardly calibrating. It is guessing and hoping.

 

[scubadada]

The analyzer on the Den Laman/Sand Dollar Pier on Bonaire is set at 20.3 or 20.4. I have had several people ask me why it is not set at 20.9, @tursiops has nicely supplied the answer.

 

[AfterDark]

Did your instructor say right before the dive or before the dive?

If I check my tank at the LDS when it's filled on Monday and I dive on Sat, I don't check it on Sat right before the dive. The gas isn't going to change after the fill.

 

[Manatee Diver]

Something I need to get used to...and I've bitched about this before...and I've gotten criticized for it, and RIGHTLY SO -- On a dive trip, everybody is so rushed. So harried. So busy with tanks, equipment, fitting people for gear, registering guests, filling out paperwork, getting the boat ready...Everybody's runnin' around like a massive earthquake is about to strike and we've got to get off the mainland or we all perish!!!! hahahahaha...Right?" You know what I'm talkin' about. I love diving, but I hate the crazed environment you're thrust into on a trip...

I arrive early, and leisurely test and label all the tanks at the back of my car.

If I check my tank at the LDS when it's filled on Monday and I dive on Sat, I don't check it on Sat right before the dive. The gas isn't going to change after the fill.

Yes it can. If your tank was hot filled the reading will differ from what it will show when cold, as most shop analyzers are calibrated with a cold tank. Or in some cases just calibrated with ambient air which add humidity as an additional calibration air. Add in the fact that labels can and do get misplaced. As such in tech diving we test every tank the day of the dive.

 

[Wibble]

FWIW I've an Analox OEii which is a nice simple analyser. This came in its own Pelicase into which I've added a spare battery and a Sharpie marker. If travelling I'll also include some masking tape (i.e. the stuff you use for painting).

For cattle boats where you're renting a cylinder/tank, the label will be:

1. The analysed oxygen content to one decimal place with a % symbol -- so it's bleeding obvious
2. The date as in 25 Oct -- using the letters as well as numbers so it's bleeding obvious
3. The MOD in LARGE numbers with a m or ft, e.g. 35m -- so it's bleeding obvious
4. If the cylinder gets thrown into a pile of other cylinders, my name

Also put tape over the valve to show if it's been used or not.

For MOD calculation it's really easy: dose over mix = ATA. Dose is the PPO2 on the bottom which is always 1.4 (unless a deco cylinder, then it's 1.6)
e.g. mix is 32.2% then: dose=1.4 / mix=0.32 = 4.375 ATA, so the depth is 1 less, e.g. 3.3 = 33m (or if wedded to some unknown king's shoe size, then multiply by 33 (because 10m = 33ft), so it's 3.3 x 33 = 108ft)

Only you are responsible for what you breathe.

 

[boulderjohn]

One LDS I dive with does offer NITROX. It was the very LDS where I got certified. Since their trips are pretty expensive, I don't dive with them often, but I did use NITROX once. The analyzer worked flawlessly...I guess. LOL...One thing I didn't do was calibrate it first with an 02 tank. But, I asked for 32% and it registered 32%...I confess, I definitely want more practice, not only diving NITROX but analyzing as well...Just, not a lot of opportunity...

It is very common for an analyzer to be calibrated and then used repeatedly by everyone in the group diving with nitrox. There is nothing wrong with that. You might be the 10th person to use it, but that doesn't matter.

 

[boulderjohn]

If I check my tank at the LDS when it's filled on Monday and I dive on Sat, I don't check it on Sat right before the dive. The gas isn't going to change after the fill.

There is a range of situations. At one end of the range, you absolutely need to analyze before diving. At the other end of the range, there is no need. As suggested in this post, analyzing the tank, labeling it properly, and then diving it a few days later is pretty safe. If more time goes by, doubts arise, and it is best to be safe. I can fill out a long post with stories of people who were absolutely certain they knew what was in their tank but were wrong.

 

[360GUY]

"So it's bleeding obvious" What does that mean?

 

[Bearded Diver]

Thanks. Makes sense. Mind me asking what they charge for a nitrox dive?

 

[Bearded Diver]

Hat sounds chaotic AF as the kids say lol