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

[Dr. Doug Ebersole]

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.

All that being said, it still only takes maybe 15-20 minutes to fill aluminum 80 nitrox tanks well -- definitely not 1 1/2 hours!

 

[MB]

First, it can take a long time to do a nitrox fill... Second, if you're expecting that you'll always get exactly a 36% fill, think again. ... Third, apparently the %O2 reading will change (drop) as the tank cools... Fourth, you should first find out what type of fills you'll get from the most convenient shop(s), and then decide if you want to get your tanks O2 cleaned... And fifth, if the shop tells you over the phone that it will cost $50 to O2 clean your tank, ask them how much additional it will cost for stickers, and viz, and anything else they might want to include.

Jim - Given the many threads from some members on SB about how little there is to learn from a nitrox class, and the often hostile posts in reaction to suggestions that we should hold a higher standard, its no surprise that this is "new information" to folks even after they've taken a nitrox class. But I have to tell you that there are many instructors who really do teach a complete class, and most of this information would absolutely be a part of it.

Example: when working with pure O2, the fill rate should never exceed 200 psi per minute. In fact, that rate would be considered "high pressure" and most folk who understand O2 would fill much more slowly. So partial pressure fills should take a considerable amount of time.

2d example: In the real world + or - 1% accuracy is a common standard, so asking for 34% mix and getting 35% is common, and considered acceptable (that is, IF the user actually understands how to adjust his or her plan to accomodate the mix).

3d example: Mix methods is a regular part of the content in any good nitrox class. Of course the canned course might neglect to link this to a student's real life use, but any instructor worth his or her salt should have students assessing where they'll typically get their fills, and planning accordingly. For example, the canned course teaches the difference between a membrane fill and pp fill; any respectable instructor should show the implications of that for how students prepare their cylinders, the types of stickers / bands to use, etc.

When folks expect a little more when they sign up for a nitrox class, they usually get it, even tho they might have to make a couple of calls to find an instructor who will actually teach a complete course.

 

[Web Monkey]

That's just somebody mixing nitrox who either doesn't know what they're doing or doesn't want to bother doing the calculations.

If you know what the current pressure and mix is, you can calculate what you have to add to get the desired pressure and mix.

Mine are always +/- .5% or thereabouts, but you should always personally analyze your tank anyway, because you just never know.

Terry

Second, if you're expecting that you'll always get exactly a 36% fill, think again. Apparently it's a lot more difficult than I expected. Recent case: asked for 36%, actual reading is almost 40%. Second attempt got it down to 38%. Finally gave up at 37%. I used to think, based on my nitrox instructor, that +/- 0.5% is pretty reasonable, but now I figure it's less hassle to take what they give me as long as it's within my planned depth limit.

 

[spectrum]

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

 

[archman]

Mine are always +/- .5% or thereabouts, but you should always personally analyze your tank anyway, because you just never know.

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

 

[scubak]

So,
Where do I buy my own analyzer...
I have a shop that can blend the mix and get it spot on....another shop, well we won't go there...and another shop that has to send it to their other shop so that takes a week....
Thanks,
K

 

[Tassie_Rohan]

Third, apparently the %O2 reading will change (drop) as the tank cools. At least that's what they tell me.

Ideally you should calibrate the meter with air at a similar temperature to the fill you're testing.

 

[CIBDiving]

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

If you don't the Nitrox Police come and drag you off to jail.

 

[Diesel298]

i was tought +/- 1%
99% of the time when i blend gas im within that 1%
if its not within that. then the person mixing your gas might not be qualified to do so....

 

[Kevin4diving]

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