Anyone ever hear of tank rolling after mixed gas fill??

[paulwall]

From a nitrox course "study guide" (TDI)

Note: At times, it may be necessary to roll the tank on the floor, before measuring, to get a correct reading. In particular, Nitrox made by the blending process is sometimes stratified after the air is added to the Oxygen. This results in a low Oxygen reading. After a few hours, mixing takes place by itself. However, if in doubt, or to speed things up, roll the tank on the floor for 5 to 10 seconds. Once mixed, Nitrox stays mixed.

 

[Xanthro]

Thanks for taking my word because while I might not be able to eplain the science in this case I'm not telling lies. And...I've dealt with this many a night when trying to get packed to go diving. LOL

However, as an electronics guy (I should say former electronics guy) I've seen real life deviate from theory many times and dramatically at that. We calculate to get close and then go measure to find out what's going on with all the things that we didn't include in our model.

You have more than enough experience to have accounted for other factors that could give an invalid reading, hence your word is good enough.

 

[rat314]

How much will the reading be affected? If the temp is 30f higher, will air read as pure 02 or more like 25%. My guess is that even with the temperate drop, it will still be in the ballpark. Got any numbers to support the method?

I agree with Mike on this one. When I mix bottles of 50%, with slow flow rates, I get similar results.

My post was meant to support the statement that O2 Analyzers are affected by moisture, and not as much by Temperature.
Unless the temperature fluxuations cause internal tank condensation that is then introduced into the fuel cell.
In fact the fuel cells within can be damaged with very little moisture introduced. And even ultrasonic analyzers are affected in some manner by temperature and moisture as well.
The O2 percentage reading should remain accurate as long as the analyzer is calibrated properly. Some units require recalibration as often as every fill, Hudsons HCI every 8 hours. Our newest ones, (Sorry can't remember the brand name off hand.) Require calibration only as needed. (The unit actually reads recalibration needed when out of specs.)

The old ones we had at work were used to analyze purity of LOX and Gaseous O2 transfills. Even they usually began showing signs of damage in advance of their actual failure. Readings taking longer times to peak, unable or difficult to calibrate etc...
I was taught that temperatures effect on analyzers were more on line with slowing down the readings. This due to a gases tendency to expand as it warms.

 

[NYPI]

Most nitrox tanks are filled by partial pressure blending, meaning that oxygen is placed in the cylinder and then diluted with air. When analyzing nitrox mixes more than 50 percent it's important to remember that a properly filled tank will allow the gas to form layers. Oxygen is heavier than nitrogen and, because it is put into the tank first, it may lie on the bottom, giving an artificially low reading on initial analysis. For best results, have the blender fill your tank while it is lying down and wait at least 24 hours before doing your final analysis of the gas.

QUOTE Oct 2004 issue Scuba Diving

 

[Rec Diver]

My LDS does the floor stir as well. They also do partial blending of there Nitrox mixes. When we measured the perentage of O2 prior to stirring to those percentages after stirring, we found that there was a measureable difference. I also measured the percentage of O2 after the tank had sat for a while, about a week. The perceantage of change was negligible, if any at all. Since then I have requested that my Nitrox cocktails should be stirred and not shaken.

For those who think that gases can not seperate, I would suggest that you retake high school chemistry.

 

[ukwreckdiver]

Didn't believe myself when I first heard about this.

However, I have now seen this happen with rich nitrox mixes (ie 75-80%) and with trimix mixes.

The analysed mixes were different before and after agitating the cylinders.

 

[SDAnderson]

Has anyone actually analyzed a partial-pressure blended tank immediately after filling and then again a little while later to see if maybe this does actually make a difference?

Yes. Repeatedly.

I know it isn't supposed to matter but the plain simple fact of the matter is that it does. It doesn't seem to make much difference with recreational nitrox mixes but mixes with lots of oxygen or helium in them will definitely test more accurately if you give the tanks a spin.

I read the books and would have said it was hooey until I started mixing regularly. When I doubted, I was handed an analyzer and told to take a reading, then the tanks were spun and I took another reading. I don't remember what the difference was that day but, from experience, I can tell you that the discrepancy before and after a spin can easily be 2% to 5% on the O2, more on the He. From that point on, I have finished each technical mix that isn't going to sit at the fill station overnight by laying the tank(s) down on a lazy Susan and spinning them for two or three minutes before I analyze. It saves time, aggravation and money and provides a more accurate reading. It also makes me a few dollars every now and then when I convince the occasional whelp who is giving me flack about spinning tanks to put some money where his mouth is. I haven't been able to get anybody to go double or nothing on a deep trimix fill, yet, but I'm forever hopeful.

Anyone who mixes technical gases regularly knows that the reading you get right after a fill can be significantly different than the reading you get 24 hours later. Some may attribute this to a "mixing" effect, others to a "cooling" effect, but in the end it doesn't really matter why - what matters is you get a more reliable, more accurate reading if you take a couple of minutes to spin those bottles. Personally, I think it's probably a combination of both factors, but I can tell you that dropping the tanks in the pool for a few minutes doesn't have as much effect as a spin.

I know that there are those who disagree but the proof, as they say, lies in the pudding. For those of you with questions about the contradictory opinions you're seeing here, consider the sources carefully: all cyber-divers were not made equal. Like Mike and some of the others, I actually blend gases, including hypoxic trimix and technical nitrox and I do a lot of it. Heck, I even took the classes (TDI Advanced Gas Blender #117127) though I learned more by doing than reading about it. It seems that most of those who don't believe lack one critical component to having an opinion - they don't actually have much (if any) experience at a fill station.

Just sign me,
:mfight:

 

[Randy43068]

don't you have one of those long tubes in your nitrox tanks? Everyone has 'em!
:idea3:

In fact, I'm going to invent a stirrer blade accessory to fit inside of nitrox tanks! I'll be rich!

I'll tell ya what I'll play along with this silly game.

Since O2 is lighter than nitrogen then will all the nitrogen sink to the bottom of the tank leaving only a high percentage of o2 on the top of the tank? Gee wiz if thats the case could get a tox hit due to breathing to rich a mix since all the nitrogen molecules are on the bottom of the tank leaving only o2 at the top of the tank? But wait mayby in the process of setting up your tank, reg etc and jumping in the water will cause the mix to get all stired up again.

With all the worry of the mixed gasses becoming seperated I just don't know if it'll ever be safe to breath that nasty black air call nitrox. Why if the air becomes seperated between o2 and nitrogen how will I ever now what my EAD or MOD could be. Wait I'll just make sure to role over on my back several times during the dive to make sure that the gas is mixed up real good and never gets seperated.

 

[Randy43068]

if it were true, but I doubt it's needed, handling the tank from the fill station to the entry point or dive site would mix the gas enough.

 

[pants!]

Well, so much for the people who claimed that anyone who thinks agitation helps is "stupid"...