I believe, I believe!!!

[wedivebc]

I have been blending my own gas almost since I started diving nitrox. On the occasions where I have had to purchase it I have sometimes had to supress a snicker when the person blending my mix would roll the tank or (even funnier) shake it like a martini.
I was at a dive shop recently teaching a class. We were PP blending our mixes for the upcoming dives and we blended using analog gauges which although not the best would do for this job. The next day we analyzed and the mixes were way off the mark. No problem, readjust the dive profile for the leaner mix, and attempt to do better next time. I was still puzzled why they were so far off.
Next day I bring in digital gauge and booster. We blend our tanks for the following days dive. Next morning tanks are 10 - 20% off the mark. I am analyzing the oxygen tank, going over every possible scenario and scratching my head. So I call the store owner over tell him the situation and see if he had any solution. After going through the whole story, he says "did you shake them?" with which he picks up a tank shakes it and says "try that". I have a silly grin on my face as I hook up the analyzer fully expecting the same results I got the first time. To my amazement the reading shot up from 26% to 32% on the nose. The other tanks went from 66% to 80% on the nose after shaking.
To those gas blenders who caught me snickering and maybe a little eye-rolling, let me say sorry.
I still don't understand why that works though.

 

[ItsBruce]

I know a little about physics and gas and entrophy and the like. I'm absolutely amazed. And frightened.

This just goes to show the difference between theory and practice.

 

[The Kraken]

What you have is referred to as a colloidal suspension, I think.
Kind of like putting two liquids together that have different viscosities. Unless you shake it up, you'll just have a partial mixture and the different elements will not be distributed equally throughout the mixture.

the K

 

[WarmWaterDiver]

I think the relative void space between the gas molecules compared to the space actually occupied by the gas molecules has more to do with this.

A more liquid example of a colloidial suspension is mayonnaise - oil and water don't mix right? So what is in mayonnaise (or aioli for the more upscale folks)?

Hold the mayo for my tanks.

And, O2 & N2 have very, very similar bulk physical properties at the same temperature (such as viscosity) - the molecular weights are very similar.

 

[suthnbelle]

Kind of like putting two liquids together that have different viscosities. Unless you shake it up, you'll just have a partial mixture and the different elements will not be distributed equally throughout the mixture.

the K

Sort of like a black and tan?

 

[cmalinowski]

Sort of like a black and tan?

Ah, now it makes sense

 

[narcT]

He = 4g/mol
O2 = 32g/mol
N2 = 28g/mol
Ar = 40g/mol

 

[diversmarts]

Better believe it!

 

[wedivebc]

Further research today has revealed it makes a greater effect if you blow off some gas before analysis. We got decent readings today without shaking just by allowing an amount of gas to escape prior to hooking up the analyzer. This makes me think the gas in the dip tube needs to be purged.

 

[WarmWaterDiver]

wedivebc, how about conducting tests shaken and unshaken, and dip tube purged and dip tube not purged, a 2 X 2 matrix of combinations to see which is the predominant effect, and posting the results?