Educational blending experience

Skipbreather

Had an “educational” experience late last week when my dive buddy & I were mixing up gasses for a mini-expedition this past weekend to make several dives on the Lakeland (awesome wreck). Happened when placing an AL 80/ 11 liter bottle into stage bottle service. I’ve always been in the ‘gas molecules mix themselves rapidly’ camp, thinking all the stories of rolling cylinders on the dock were just so much wasted effort. Until now.

My technique such as it is has always been to first slowly fill O2, then the He just as slowly, blast in the air top up, allow everything cool & top the air again. I’d then analyze to cross check the math & move on. The bottle in question this time was to be an NX70. Bottle was slowly filled from empty w/ calculated O2 psi using a certified digital gauge. Here is where things differed: Bottle was then put onto an air bank that was regulated to prevent going over 3000 psi w/ a VERY slow bleed of air and left alone while we added O2 to the other bottles. We switched the booster over to He and again filled required amount into requisite bottles. We then returned to the NX70 bottle which was spot on at 3000 psi- again on the certified digital gauge- and I went to analyze while buddy started air topping the various other cylinders. It had been maybe 60 to 90 minutes since putting the NX70 bottle onto the air bank.

“What the hell is wrong here? We can’t be THAT sloppy?” The NX70 bottle was reading only 49%! I figured the analyzer cell was done even though it was only 1 1/2 years old, but checked it on a bottle of medical O2 to be sure. Quickly read 99.8% !?!?! Rolled NX70 on floor a couple of times and it now read 59%! Decided to let it sit & check later. Tossed it into the trunk & drove the hour home over lumpy roads and checked it again w/ same analyzer. Read 70.0%. I LOVE certified digital gauges.

Lesson learned.

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tony13

I did the same exact thing, went to analyze and was like what the hell. Live and learn, nice reminder.

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cool_hardware52

I might also add:

If you first decant O2, and then top with air, you need to "blast out" the gas in the valve before you analyze.

My analyzer features a nice DIN Adapter with built in flow restrictor, just screw it on, hook up the rubber tube and watch the digital display.

The flow rate is quite modest.

The valve body and dip tube will be full of nothing but air right after you top with air and this volume of gas is enough to greatly distort the analysis.

I open the valve and let a quick blast out prior to analyzing. This greatly improves the repeatability of the reading vs time.

Tobin

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Sutty

I gues Skipbreather knows to vent the air in the valve and the debris tube as from his story he's normally getting it right.
Sounds like the very slow top up with air allowed it not to mix properly. On my mix course I was taught to do the final part of the top-up with air rapidly specifically to cause turbulence in the cylinder to ensure mixing (and then still re-check it after its sat for a while).
However once gases are well mixed the laws of thermodynamics means they won't seperate out, but they DO need time or turbulence to mix evenly.

Nice story to illustrate this!

Neil

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cool_hardware52

I did'n mean to imply Skipbreather doesn't know about venting.

I was just recently surprised at how long it takes to get a stable reading with a low flow restrictor compared to higher flow sampling device, if you don't blow out the valve and dip tube.

This can "mask" the effects of "spin the bottle"

i.e.

1) Take reading and get a unexpected result.

2) Disconnect and Spin

3) Reconnect and get a reading closer to what you expect.

Was the 2nd reading due to spinning, time delay, or just more venting of the valve? All three could be playing a part.

Tobin

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cramerdn

Can't find my P-chem book just this instant but the mobility of molucules in a gas make rolling tanks pure bunk. Don't buy into the myth and stop looking for the real cause, so it does'nt lay dormant and bite you later.

Can't say for sure without being there but I would look for errors from Tobin's sugestion, temperature variation errors, or intermitant errors in test equipment.

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jaap

I'm not quite sure that I understand the word bunk, but its absolutely no myth that slow sequential filling of different gases to high pressures often requires either a long time or some induced motion (rolling the tanks) to get a good mix, especially if no He is involved in the mix.

With large size tall & narrow cylinders it can be a real problem if you can't pump premixed, "blast" the last gas, or roll the cylinders. The worst I experienced myself was an over week long waiting time before the mix was right when I mixed in some large stationary cylinders (being too large to cause turbulence with the flow we had and being stationary). When compressed the gas becomes really dense, sort of approaching liquid behaviour. So if one gas is slowly filled upon another at high pressures its kind of like very slowly letting a soluble dye into a tall glass of water. If the liquid is undisturbed it can take quite some waiting to get a homogeneous mix.

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Sutty

I don't know how fast molecular movement causes mixing of disparate gases, but would be interested to know, however gases of differing densities can "layer" eg hydrocarbon vapour in air, and mixing between layers doesn't always seem to be that rapid.
Also most dive-shop gas-mixers that I have met seem to think its a real phenomenon that mixing is not instantly complete, and if the experimental evidence disagrees with the theory they both need looking at again.

Neil

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tibby

We've seen the same phenomenon around here and it's not from trapped gas in the valve or analizer. Did plenty of flushes and waiting to stabilize. In my case I didn't even have to move the bottle, just let it sit there for several hours and then it read target value.
There's one thing happening that clues me in of slow mixing and that is having a hard time to get a stable reading from the analizer if mixing is not done. If the reading is low and the numbers keep "growing" I let the bottle settle for a while and come back to it later. Works well and I don't have to drive it around or anything like that.
Hope this helps.
Tibby

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Does the voice of reason change when you add helium to the mix?...hmm

teksimple

Yep, I had more than one experience like yours, Ken, as it took a while for me to believe it. Someone needs to do a study on the "preservative properties of breathing gases for deep diving." Perhaps tag the gas with radioactive isotopes and then take time lapse X-rays to see how long it indeed takes to mix the stuff. When I mix Trimix, now I don't even bother pulling out my helium analyzer until a few hours later.

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