Stratification of Nitrox Mixes

[WreckWriter]

SPEEDSTER once bubbled...


Tom,

how much of a difference have you seen and what method do you use to do your fills?

mike

We do partial pressure fills. Any other method likely would not see stratification...

I see an average difference of 1-3% O2 overnight, later analysis being higher in O2 percentage. Not much but enough to prove O2 stratification to me. I have no doubt that helium stratifies to an even greater degree but I don't have an He analyser to prove it.

Tom

 

[SPEEDSTER]

WreckWriter once bubbled...


I see an average difference of 1-3% O2 overnight, later analysis being higher in O2 percentage. Not much but enough to prove O2 stratification to me. I have no doubt that helium stratifies to an even greater degree but I don't have an He analyser to prove it.

Tom

hmm, perhaps this is more of a slow mixing of the gases than stratification? If the oxygen and nitrogen were seperating, the o2% would become lower over time, nitrogen is lighter than o2, so you would get a higher % of nitrogen in the reading. I'm assuming you store your tanks upright, by the way, otherwise this would be the opposite.

regardless, your experience supports the other thread cyklon posted.

-mike

 

[WreckWriter]

SPEEDSTER once bubbled...
hmm, perhaps this is more of a slow mixing of the gases than stratification?

Agreed, not permanent stratification, just slow mixing which creates a temporary stratification.

Tom

 

[spaz]

ummmm........no.


No combination of temperatures or pressures acheived by mixing SCUBA gasses, can cause the gasses used to achieved any physical state other than 'gas'. Gasses disperse infinitly into each other, completely irrespective of individual molecular weights, and depending on temperature they do it real fast (real a** fast).

Thank you, that is all I have.

Peace

 

[SPEEDSTER]

spaz once bubbled...
ummmm........no.


No combination of temperatures or pressures acheived by mixing SCUBA gasses, can cause the gasses used to achieved any physical state other than 'gas'. Gasses disperse infinitly into each other, completely irrespective of individual molecular weights, and depending on temperature they do it real fast (real a** fast).

spaz,

I see you are a chemist by day, and as such, I would have a hard time disagreeing with you, especially as I DO agree with you...however, what would lead to a change in o2% in Tom's tanks over time?

I initially thought that there would be an immediate mixing of gasses w/in the tank and would not change over time either, however I am not about to call Tom a liar as I believe him to be an all around good source for unbiased information. (scuba wise at least )

So, what do you think caused the variation in o2% readings?

-mike

 

[Friggincold]

Helium locks for your tanks valves get em wow you still have HE in your tanks don't let it drift away:bonk:

 

[ianr33]

Does the readout from an O2 analyser vary with temperature?. I would imagine that it does. If you calibrate the analyser at room temperature ,then pass hot gas from a just filled tank thru it,could that explain the difference.

 

[Walter]

Rolling tanks is effective, but it does not mix the gasses. Injection of gases at high pressure mixes them more than rolling ever could. Additionally, the gases would mix almost instantly even if they weren't mixed under pressure. Rolling cools a tank which will result in a more accurate reading.

 

[Genesis]

are indeed temperature sensitive.

They are, after all, just a fuel cell. And chemical reactions (which is how they work) are temperature dependant.

Most sensors contain internal temperature components (typically a calibrated thermistor) which attempts to compensate for ambient temperature differences. The problem is that the ambient temperature around the sensor is not the temperature of the gas impinging on the membrane, and immediately after a fill it could be QUITE HOT.

For example, its pretty common around here to get a fill of Nitrox in an AL80 that reads 3200 psi when the guy filling the tank is complete. A few hours later, with the valve closed, in 80F ambient temperatures, the gauge reads 3000 psi.

Simple application of Boyle's and Charle's law tells us how how the gas was - since we know how hot it IS.

To go from 3200 to 3000 psi there was a 6.6% reduction in temperature in KELVIN.

Since the endpoint was 299.66 K (~300K), the gas was at 318K, or 113F - at the completion of the tank fill.

Yes, that's almost 30F hotter.

This could easily account for an "off" reading if the sensor is not at gas temperature (and it won't be unless its immersed in the gas - the temperature compensating resistor is typically in the back of the shell, behind the membrane)

 

[WreckWriter]

so many fancy answers....

How many of you actually mix gases rather than just "know" gas "laws"?

Tom