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

[mccabejc]

I've only been doing nitrox for a few months now, and I'm learning some stuff that might be useful to others who are considering it. Comments and insights welcome.

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. Presumably the long times are for partial pressure fills, not banked fills that are pre-mixed. Anyway, you may need to plan your fills a lot more than you're used to. The days of standing around for 10 or 15 minutes for a fill may be over if you go to nitrox. Now I have to find the right shop for the tanks I'm carrying (O2 clean or not), and plan to drop the tanks off and pick them up later, which mucks things up considerably.

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.

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.

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. Some don't require O2 clean tanks, and some do. I went the extra step of having both for max flexibility.

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. (I've got some unresolved issues in that department....)

 

[Warren_L]

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.

I've been told that too, but I honestly cannot say why that is. I mean, assuming a homogenous mixture, I wouldn't think that a temperature drop will affect the reading unless the sensor is heat sensistive, which I can perhaps see, but the actual % I would not think would change (though, as you say, the reading might).

Depending on the final mix you're going for, doing a pp blend fill with air top up, you should be able to introduce the air at a quick enough rate (once your mix falls below 40%) to pretty well homogenize your mix right away. It's when you fill slow (at O2 rates, for example when achieving a mix richer than 40%) you need to allow 6 hours or so for proper molecular migration in order to completely homogenize.

 

[my1ocean]

I've only been doing nitrox for a few months now, and I'm learning some stuff that might be useful to others who are considering it. Comments and insights welcome.

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. Presumably the long times are for partial pressure fills, not banked fills that are pre-mixed. Anyway, you may need to plan your fills a lot more than you're used to. The days of standing around for 10 or 15 minutes for a fill may be over if you go to nitrox. Now I have to find the right shop for the tanks I'm carrying (O2 clean or not), and plan to drop the tanks off and pick them up later, which mucks things up considerably.

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.

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.

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. Some don't require O2 clean tanks, and some do. I went the extra step of having both for max flexibility.

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. (I've got some unresolved issues in that department....)

Some one is yankin' your chain. +/- .5% should be expected. NOT +/- 5%. It shouldn't take that much longer to blend a tank than to fill a pre mix and your tank shouldn't be hot when you get it back. And no, the reading shouldn't change as you tank cools (unless it's really hot). Your tank needs to be O2 cleaned to blend, but not for a premix fill of less than 40%. O2 cleaning can be expensive, but if your dive lds isn't up front with the cost to get your tank back ready to go then you should look elsewhere.

 

[Aeolus]

1) Mixes in the US are usually a lot closer than the Caribbean.

2) ALWAYS check the fill yourself, preferably with your own analyzer.

3) It is MUCH better to set the CAL on your analyzer on an air tank rather than ambient.

4) Do not push your MOD (maximum operating depth).

DSDO

 

[Rick Inman]

I enjoy continuous blend fills. No waiting and no worrying about being O2 clean.

 

[Blackwood]

I enjoy continuous blend fills. No waiting and no worrying about being O2 clean.

Big time.

 

[Otter]

If they are dong PP, then it will take a while if done properly and within 1% of your desired mix. Why? Because you fill with O2 to a specified pressure and the top off with air. In theory, if you could keep the tank at the same temperature (warm or cold) during the initial fill and air top off, you would have the proper mix, albeit at a lower pressure (once the tank cooled).

So usually it something like, o2, cool, o2, cool, o2, cool, air, cool,air, cool,air,cool, analyze.

When calibrating the O2 analyzer with 'room air', one must consider both the ambient temperature and humidity. Analox has a chart that shows their analyzer reads something like 19.5% O2 at 90% humidity and 90F. Thats one of the reasons I always get close on ambient and then calibrate off of a cool, scuba cylinder known to have air.

 

[Whatgoesdown]

I agree with pretty much everything said so far! Sounds like the shop is just messing you around. Gas blending is not quite as straight forward as adding a to b and coming out with c, differing densities of gases have to be taken into account but there are tables to do this and really skilled blenders can get you within .1% with a PP fill. I once watched a guy hook up my tanks, open the O2 and start filling (I didn't see him opening up a valve on the air feed). I asked him if he knew what he was doing... a couple of times. I got a bit nervous when I saw my gauges pass 1000psi (with what I thought was pure O2) so I left the building for a bit. I reckoned if the gear held it I could sell the extra O2 to other divers. The guy had simultaneously filled the tanks and hit the mix on the nose!

Anyway, I digress. What I wanted to add was that if the temperature of the sensor changes between calibration and testing, you run the risk of throwing the result off.

 

[clarktenk]

I've only been doing nitrox for a few months now, and I'm learning some stuff that might be useful to others who are considering it. Comments and insights welcome.

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. Presumably the long times are for partial pressure fills, not banked fills that are pre-mixed. Anyway, you may need to plan your fills a lot more than you're used to. The days of standing around for 10 or 15 minutes for a fill may be over if you go to nitrox. Now I have to find the right shop for the tanks I'm carrying (O2 clean or not), and plan to drop the tanks off and pick them up later, which mucks things up considerably.

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.

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.

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. Some don't require O2 clean tanks, and some do. I went the extra step of having both for max flexibility.

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. (I've got some unresolved issues in that department....)

Hi I would have to agree with some of the earlier posts it shouldn't take 1.5 hrs to blen your tanks and they certainly shouldn't be off by as much as 4%. I blend my own tanks all the time and am a certified blender for what it's worth. All they have to do is as already explained put said amount of O2 in your tanks (slowly I might add no eed to stop repeatedly if you are blending at the proper rate anyway) and then put air on top. However you are correct in your statement that it is not a quick 10-15 mins.

I would then suggest waiting a few minutes and then checking the PO2 level. You should get a more accurate reading this way.

I would however suggest getting your tanks and valves cleaned before running pure O2 through them. Most shops here partial pressure blend so O2 cleaning is mandatory.
As for O2 cleaning prices vary from shop to shop and there shouldn't be any suprise charges they didn't tell you about up front. Nothing to do with diving but certainly bad business practice IMHO.

 

[CBulla]

I enjoy continuous blend fills. No waiting and no worrying about being O2 clean.

That must be nice, I don't know of one shop in the area that has that capability!