Measuring O2 in enriched air

[yle]

Your mix measurement won't change after 3 days with the tank standing, 24 hrs with the tank lying down...l

You're suggesting the orientation of the tank has a significant effect on the time it takes the gas inside to mix?

 

[yle]

I suggested that perhaps it was a result of him not calibrating on compressed air, and he dismissed me. Since I don't own an O2 measuring device of my own, I can't really test if this is true or not, but I'm just curious -- does it make a difference like PADI says, or is my instructor correct?

As has been already suggested, humidity and temperature have a significant effect on the O2 content of ambient air. My analyzer came with a small table that provided the %O2 in ambient air for a range of temperatures and humidity. At the extreme end (high temp and humidity... very useful for Florida summers) the %O2 is in the 18 to 19% range IIRC. Your ambient conditions probably weren't that extreme, but it is interesting that all of the instructors readings were off by about the same amount.

I am going to override your instructor and say that you were very likely correct. And your instructor was required to brush off your suggestion because it's an unwritten rule that the instructor always has to be right and if an instructor has his authority questioned he must instantly reassert his authority by making sure the student never questions his authority again. Because, as we all know, being an instructor means you know everything about diving.

The last two sentences, for the record, are sarcasm. Personally, I always find it refreshing when my students make suggestions like you did... because it reassures me that they (a) understand the material, (b) understand how to apply what they have learned and (c) should be better and safer divers for it. Which is always my main objective when teaching scuba.

 

[JohnnyC]

Altitude also matters. If I'm at home, without any correction applied, my rebreather thinks I'm gonna die.

 

[michael-fisch]

You're suggesting the orientation of the tank has a significant effect on the time it takes the gas inside to mix?

Exactly right!

In order to prove my point to yourself fill 3 empty AL80 tanks with 39Bar of O2 and top off slowly with air to 208Bar (3000psi). Should be a 36% Nitrox.
Leave 1 tank standing, lay the 2nd tank down and toss the 3rd tank in your car and go for a half hours drive.
Now bring that tank back into your mixing room and without moving the other 2 tanks measure and record O2 % of all 3 tanks.
Tomorrow without moving any of the tanks remeasure O2%, and continue to do that daily untill none of the tanks has a measurable change in O2%.

If you want get really wild, you can try to do the same on a set of doubles using the following method:
Empty the doubles, then close the isolator valve between the tanks and add 78 Bar of O2 to the left tank, and fill the left tank with Air to 208Bar. Now fill the right tank with Air to 208Bar. When you're done leave the tanks standing and open the isolator valve. A week later close the isolator valve and check O2 levels on both tanks.
Open the isolator valve and wait a month before you close it again and retest the individual tanks for O2.
I know that we all learned about Brownian motion in high school, and in any gas mixmaster course that we ever took, but I have never been able to get a set of doubles to have the same O2 levels in both tanks after doing the above experiment. Even driving the set of doubles around in a car for a day doesn't help much.

Michael

 

[WarrenZ]

If you want get really wild, you can try to do the same on a set of doubles using the following method:
Empty the doubles, then close the isolator valve between the tanks and add 78 Bar of O2 to the left tank, and fill the left tank with Air to 208Bar. Now fill the right tank with Air to 208Bar. When you're done leave the tanks standing and open the isolator valve. A week later close the isolator valve and check O2 levels on both tanks.
Open the isolator valve and wait a month before you close it again and retest the individual tanks for O2.
I know that we all learned about Brownian motion in high school, and in any gas mixmaster course that we ever took, but I have never been able to get a set of doubles to have the same O2 levels in both tanks after doing the above experiment. Even driving the set of doubles around in a car for a day doesn't help much.

Michael

Thats because even with the Isolator open between the tanks there is such a small area for any gas exchange to go across unless there is an outside force to push gas from one tank into the other there is no flow to do the mixing.

Thats why I always supervised whenever someone was filling my doubles, too many shops around here dont see enough doubles to figure out the isolator.

 

[michael-fisch]

Thats because even with the Isolator open between the tanks there is such a small area for any gas exchange to go across unless there is an outside force to push gas from one tank into the other there is no flow to do the mixing.

Thats why I always supervised whenever someone was filling my doubles, too many shops around here dont see enough doubles to figure out the isolator.

Exactly right!

Michael

 

[broncobowsher]

Use a better analyzer.

 

[michael-fisch]

Use a better analyzer.

I don't think that that makes much difference. Any analyser that was calibrated 15 minutes ago off a tank that has never had anything but air in it is good, as long as it also shows close to 100% off your O2 storage tanks. Any analyser that you didn't do that to, should be given to your son as something to play with instead of Lego blocks. Sure I now use the Divesoft analyser with the pro flow limiter and do a 3 point calibration on it, buts thats just me, so I can deliver repeatable results all the time.

Michael

 

[rjack321]

Shaking a tank of gas has no effect on how the molecules mix. Gas molecules move very fast. Any gas mix is effectively mixed, with the cylinder just sitting, rather quickly.

Not accurate.

It is easy to get gases to "layer", especially in a vertically oriented cylinder when topped off with a small compressor that generates little turbulence. They can layer based on temperature or based on molecular weight but usually temperature plays the biggest role. The surface area between the 2 layers will play a role in their mixing rate. A tall skinny vertically oriented cylinder like a 250cf storage bottle can take a very long time to completely mix if you partial pressure in 100% O2 (first) and top with air from a 3cfm compressor (which is slow). Lay that same cylinder on its side and the surface area between the cold O2 and the hot air is 6-8x bigger and it will mix far faster.

 

[TrimixToo]

Shaking a tank of gas has no effect on how the molecules mix. Gas molecules move very fast. Any gas mix is effectively mixed, with the cylinder just sitting, rather quickly. Although it would be rather amusing if you could get some video of your instructor shaking his scuba cylinders to mix the gas. Sounds like the kind of prank an instructor would play on his DM candidates.

Unfortunately, people tend to want to understand new things by trying to relate them to other things they already understand. Two liquids in a jar, like salad dressing, can separate due to their different densities (and the fact they are liquids.) And you can see them mix when you shake the jar.

But gases are different. Because they are not liquids.

You're not a gas blender, are you?