What they don't teach you in Nitrox class...

[Warren_L]

I was thinking of getting a Nitrox specialty this winter. Is it as coplicated as it sounds?
Maybe i'll just use air.

Nope. It's a worthwhile specialty. Much of what is being discussed has to do with blending, which is another issue you won't get into unless you take a blender's course.

 

[bergersau]

Kevin, it's not that complicated.

Just a single (simple) equation you need to be able to transpose and solve three ways (Ie for Partial pressure, ideal mix and 02 fraction). You can just use tables in practice but I think you need to UNDERSTAND the relationship of these variables before just look up a table.

Then there is the theory behind why nitrox is usefull and the regulations/safety precautions (eg depth limits and the need for 02 clean tanks) you need to use the gas.

Oh - almost forgot you'll need nitrox decompression tables and/or an equivelant air depth conversion table for your dive planing.

Really its just a little more planning and disipline than an air dive - its worth it.

 

[Tamas]

The problem with "bad" nitrox fills is often related to how quickly air is added after the oxygen. If you look at the fill charts and want to fill your 3000psi cylinder to have EAN36 you first start off with 569 psi of oxygen and then top it off with air. However, if you think back to physics and Charles' Law you remember that with filling a cylinder (especially if you fill it too fast) you get a lot of heat which results in higher cylinder pressure. Once it cools, you have a lower cylinder pressure. Again, looking at the charts, if your final pressure after cooling is down to, say, 2700 psi that same amount of original oxygen (569 psi) will result in EAN39. You would need to then top it back off to where its 3000 psi when cool to get your EAN36.

There might be something to the heat affecting the final mix and reading the mix issue for sure, also with the mixing procedure and the material of the tanks.

When I get my Faber LP98's filled the fill is usually bang on (+/- 1%) and it takes a good hour or so do it, but when my buddy takes his PST HP120's to the same place, same blender, same time.....his always come out funky. The tanks heat up a lot quicker and cool down slower making the fill that much more difficult to get right, not just that the mix doesn't settle for at least 6-8 hours and changes and the gases cool inside the tank. When asking for a 32 mix, he often gets a 35.8% at the initial testing (despite using the predetermined charts) and later on that same mix will trickle down to a 33.6% a few hours later.......again the shop does the fills at the same rate yet the PST's behave and handle the gas differently.

 

[scubapolly]

First, it can take a long time to do a nitrox fill. So far I've experienced anywhere from 20 minutes for a single tank to more than 1.5 hours for two tanks.

P.P. blending does take a little longer, but shouldn't be that much longer. The only reason it should take 1.5 hours is if there is a "line" in front of you. I know when I went to my shop the other day, I was filling 2 sets of doubles, and there was someone already filling their own doubles plus a 100% deco bottle. If someone would have come into get their tanks filled it would have easily been over an hour until we got to them.

Third, apparently the %O2 reading will change (drop) as the tank cools. At least that's what they tell me. I'm assuming that it takes a while for the gases to mix if they did a partial pressure fill (pure O2 topped off with air), and that accounts for the change.

Nope, it has nothing to do with the mixing of the gas. As others have touched on, but no one has flat out answered, this has to do with the temperature of the tanks, and the fact that the O2 sensor in the analyzer is temperature sensitive.

 

[Don Burke]

Third, apparently the %O2 reading will change (drop) as the tank cools. At least that's what they tell me. I'm assuming that it takes a while for the gases to mix if they did a partial pressure fill (pure O2 topped off with air), and that accounts for the change.

The O2 sensor works by burning the gas coming through it and measuring the temperature. If the gas going in is hot, the reading is falsely high.

 

[Warren_L]

There might be something to the heat affecting the final mix and reading the mix issue for sure, also with the mixing procedure and the material of the tanks.

When I get my Faber LP98's filled the fill is usually bang on (+/- 1%) and it takes a good hour or so do it, but when my buddy takes his PST HP120's to the same place, same blender, same time.....his always come out funky. The tanks heat up a lot quicker and cool down slower making the fill that much more difficult to get right, not just that the mix doesn't settle for at least 6-8 hours and changes and the gases cool inside the tank. When asking for a 32 mix, he often gets a 35.8% at the initial testing (despite using the predetermined charts) and later on that same mix will trickle down to a 33.6% a few hours later.......again the shop does the fills at the same rate yet the PST's behave and handle the gas differently.

One would think that the tank shouldn't make much of a difference, given the same volume of gas being compressed. Different tanks may heat up faster than others due to different heat conductivities, but that would also imply that those that heat up faster also cool faster, as they will be more efficient radiators of heat. The amount of heat energy generated for the same volume of compressed gas should be equal and independent of the material of the tank that it is compressed into. The only other variance I can think of would be different values for frictional heat, which I don't believe would be a significant factor.

 

[Jeff Toorish]

I always drop my tanks off immediately after I dive. Just so my LDS has time to refill an neither of us is rushed.

I pretty much agree with the technical points written in this thread. The fact is, EAN/Nitrox is somewhat more tricky than regular O2 fills. By that token, Nitrox is also more difficult for the LDSs to deal with.

I like the benefits of Nitrox.

JT.

 

[Otter]

The O2 sensor works by burning the gas coming through it and measuring the temperature. If the gas going in is hot, the reading is falsely high.

You might want to check this out.

 

[Web Monkey]

That would be someone who doesn't want to bother doing a simple calculation. I'd also guess the guy doing the filling isn't the guy paying for the O2.

Either that, or he doesn't fill them at all and takes them somewhere else to get pre-banked mix.

Terry

Our only real local source insists on starting from an empty cylinder, bleeds them empty and starts from scratch. You have to leave them and come back the next day, yuck.
Pete

 

[Web Monkey]

I thought personal verification of gas mixes was required. Who doesn't do this?

I've been out on a number of dive boats in Florida and the Carribean where people were diving Nitrox and never checked the contents.

I've also been in shops where an employee hooks up an analyzer and shows it to the customer, but doesn't calibrate it. Maybe it was calibrated before, but you just never know.

Even saw one where the emplyee hooked up the analyzer and cranked the 0-adjust knob until the analyzer read the number she was looking for.

Terry