Mixing gases and gas laws

I recently had a discussion as to why you sometimes see divers rolling their Enrich Air (aka Nitrox, for this thread) tanks after they have been filled to "help them mix." I have also seen divers tap their tanks with a large rubber mallet to "settle the mix."

Some people swear by the need to do this. Others scoff at this saying it is in direct conflict of the laws of gases. Some say the analyzer readings may be off or different during a second reading at a later time due to the temperature or humidity changes in the external air thus affecting your analyzer, but not the mix in the tank.

Add waiting for the gases to cool to analyze the mix.

What do you say? Urban myth, snipe hunt, or factual back-up exists?

Side topic response: Excellent timing to a recent discussion we were having about analyzing. I have heard/read form several sources that heat, cold and movement do not change the mix. That analyzer readings may be off or different during a second reading at a later time due to the temperature or humidity changes in the external air thus affecting your analyzer, but not the mix in the tank.

I am a little in the fog here with some of this (although I have read up on stratifying gas in tanks) and would be interested in hearing from others in this regard. New thread perhaps?

Why would an analyzer that has not been recalculated, read two different readings on a tank, which is a closed system and not susceptible to changes in humidity or temperature. Heat does not cause a change in molecular contents, nor does humidity. Moisture at the sensor from humidity and temperature changes is dry long before the mix percentage settles down when doing a check.

People are free to believe what they want about stratifying of gas in tanks, but for my own purposes, I check my tanks for verification, prior to my dives. To be honest, I don't do this all the time, but I do if I will be within 20ft of my MOD with my suspected mix. When I have supposed 32% in the tanks (from my mix check after filling) and I am jumping into 50ft of water, I usually don't re-check unless the tanks have been out of my hands. If I haven't had the tanks in my hands from the time they were filled, to the time I am jumping in the water, I put an analyzer on them EVERY time.

I recently had a discussion as to why you sometimes see divers rolling their Enrich Air (aka Nitrox, for this thread) tanks after they have been filled to "help them mix." I have also seen divers tap their tanks with a large rubber mallet to "settle the mix."

Some people swear by the need to do this. Others scoff at this saying it is in direct conflict of the laws of gases. Some say the analyzer readings may be off or different during a second reading at a later time due to the temperature or humidity changes in the external air thus affecting your analyzer, but not the mix in the tank.

Add waiting for the gases to cool to analyze the mix.

What do you say? Urban myth, snipe hunt, or factual back-up exists?

Scientifically speaking, temperature changes won't change the molecular mix (the number of each gas molecule type in your tank). But, temperature changes will affect the partial pressure of each gas component in the tank, which is what your analyzer is actually measuring, and what actually matters when you breathe it. Each gas component responds to temperature changes a little differently. It all has to do with how fast the different molecules in the tank are moving on average. Amazingly, they are bouncing around in there at thousands of miles per hour. They all slow down when they get colder, and speed up when they get hotter, but, the different molecule types don't respond exactly the same way. They follow different pressure/temperature curves. The heavier molecules with higher melting points, like O2, will slow down and lose partial pressure faster with declining temperature than lighter molecules like Helium. For small temperature changes, you won't notice much change in partial pressure readings. But, when you get a hot fill and then your tank cools by 50 deg F or more, you will.


&%$#, I think I just talked myself into buying an analyzer. There goes another 250 bucks.

actually it does need time to stabilize and mix together. The shop that is hosting me was mixing up a tank of 80% that a customer requested and i watched the blender mix it up (o2 first then air) and about 15mins later we analyzed it and it only came up to 48% so thinking something was wrong we got a different analyzer and got the same result. Thinking maybe someone back filled one of the o2 tanks (doing Partial Pressure fills), but the o2 tank came back at 99.999 so scratching our collective heads we just left the tank and came back to it the next day, and sure enough it came up to 80.4%. It was explained to me that the tank needs time to sit as a it will stratify in the tank intially till it throughly mixes and cools (this maybe why some roll them)

as for beating on them or rolling them I have no clue but i do think that the tank needs time to sit and cool before analyzing it as the everything might need some time to throughly mix and for it to cool which will effect the pressure in the tank. beyond that I have no clue

as for Trimix i will let DiverBrian comment on PP mixing of mix and see if it needs time to throughly mix because the HE is lighter then some of the other components in the gas and cause the gas to stratify.

FWIW and IMHO as this come from my observations of my hosts in PR

Tooth

The gases mix almost instantly. Temperature does not affect the mix, but it does affect the O2 sensor. Rolling tanks does not cause the gases to mix, but it does dissipate heat.

Scientifically speaking, temperature changes won't change the molecular mix (the number of each gas molecule type in your tank).

True.

But, temperature changes will affect the partial pressure of each gas component in the tank,

Nope.

which is what your analyzer is actually measuring,

Nope. It's measuring electrical current. The current is affected by % of O2, humidity and temperature.

But, temperature changes will affect the partial pressure of each gas component in the tank,

Nope.

Think about what happens in the tank at extreme temperatures. When you cool air way down near the boiling point of Oxygen, the O2 liquifies and the N2 stays gaseous. That is how they separate gases for comercial use. At that temperature, the O2 molecular content in the tank is still about 21%, but its partial pressure is nearly zero. The N2 molecular content is still about 79%, but, its partial pressure rises to almost 100%.

At diving temperatures, the effect is much less dramatic, but, the principle still applies. In adition to the number of each molecule type, partial pressure depends a great deal on molecular momentum, i.e., how big the molecules are and how fast they are moving. Their speed is highly dependent on their temperature. The ideal gas law says that the correlation between temperature and momentum is the same for all gasses, but, in reality they are slightly different.

Edited by BubbleBoy, 09 August 2007 - 05:36 AM.

I don't see how rolling the tank does anthing. If you take a bucket of water and twist the handle the bucket turns but very little of the water moves. If you let a tank sit the lighter gas goes to the top so the reading you get then is not true.
I think we should tell newbies that they need to do barrel rolls underwater to keep the mix even

I would love to hear a physics professor or something along those lines comment on this - I saw it for the first time here as well... People would have their stage filled and then rock them back and forth to "mix" them. My Tech instructor said it can help, but didn't really seem too convinced...

Think about what happens in the tank at extreme temperatures. When you cool air way down near the boiling point of Oxygen, the O2 liquifies and the N2 stays gaseous. That is how they separate gases for comercial use. At that temperature, the O2 molecular content in the tank is still about 21%, but its partial pressure is nearly zero. The N2 molecular content is still about 79%, but, its partial pressure rises to almost 100%.

Partial Pressure (PP) is determined by the GAS components in a mix. If you liquify O2 it is no longer a gas and PP no longer applies to it. However the PP of the remaining N2 does and the N2 will dissolve into the liquid O2 based on the N2 PP (Henry's Law).

partial pressure = total absolute pressure x volume fraction of gas component

So if you effectively remove the O2 from the gas mix then the N2 PP will rise accordingly. Temperature has no effect on PP according to Dalton's and Henry's laws as long as the static temperature of that mix maintains a gaseous state of all gaseous components. N2 and O2 in a gas mix at 12F has the same PP at 300F.

I recently had a discussion as to why you sometimes see divers rolling their Enrich Air (aka Nitrox, for this thread) tanks after they have been filled to "help them mix." I have also seen divers tap their tanks with a large rubber mallet to "settle the mix."

What do you say? Urban myth, snipe hunt, or factual back-up exists?

The gases in a tank are not oil and water. They will intermingle freely as their atomic weights are very close. How many times have you been strolling down the street only to walk into a bubble of gas that is 99% N2? It never happens because the movement of people, planes, the wind and weather keep the gases in the air you breathe moving. If you were able to isolate a SCUBA tank from all outside vibrations, movement and radiation, plus reduce the temperature to minimize the vibration of the gaseous atoms, then over time (a very, very looooooong time), the gases would separate in layers based on their atomic weights. Heavier gases at the bottom, lighter at the top. The motion of putting your tank on before a dive is enough agitation to mix things up.

Rolling tanks does not cause the gases to mix, but it does dissipate heat.

The temperature of a gas is a measure of the motion of the individual molecules. If those molecules are agitated then heat increases. Motion is converted into heat simply because the external motion of the object is converted into the internal motion of the gas atoms. So in the example given of spinning a bucket of water, the movement of the metal in the bucket against the water will generate heat. It will be a lot less heat, minimal in fact, but an increase nonetheless.

Whenever two bodies of different temperature are placed in thermal contact, the warmer body loses heat to the cooler until the two of them are at the same temperature. So letting a newly filled tank sit will allow it to cool to the ambient temperature around it.

Rolling, hitting or moving tanks does nothing of consequence except give you an upper body workout...

Boyles Law, Charles Law, Daltons Law and several other laws are all based on ideal gas behavior (Kinetic Molecular Theory). They are all good approximations of how gases behave at room temperature and atmospheric pressure. Even so, I sometimes wish we didn’t have so many laws in science (I’m afraid if I discover something new I’m going to break one of them and get sued ).

What I’m talking about in my previous post is what happens when gases behave less than ideally. For those that aren’t as familiar with this, here are a few links to discussions of non-ideal gas behavior.

http://antoine.frost...nese/101/gases/

http://chemed.chem.p...deviation5.html

http://members.aol.c...hm/realgas.html


The two biggest contributors to non-ideal behavior of gas mixes are extreme temperatures (particularly near boiling points) and high pressure. We haven’t talked about the affect of total pressure much in this thread. However, when pressures are high, in the range of SCUBA Tank pressures, things start to get a little funny. The kinetic model of ideal gases starts to break down when the molecules get pushed closer together and bumping into each other more frequently. For example, the transition between solid, liquid, and gas phases isn’t as abrupt at high pressure. At something called the Triple Point (a specific combination of temperature and pressure that differs for each gas) the solid, liquid and gas phases all exist simultaneously.

For the most part, non-ideal gas behavior is very minor at the pressures and temperatures of the gases we breathe when diving. But, in some instances it can be noticeable. Helium in particular behaves somewhat differently than Oxygen and Nitrogen, because of its light molecular weight and extremely low boiling point (near absolute zero). So when you start putting Helium into your gas mixes, you might see some significant differences in partial pressures coinciding with changes in total pressure and temperature.

Edited by BubbleBoy, 09 August 2007 - 04:39 PM.

In my experience, air and oxygen fills mix pretty much instantly. There will perhaps be a couple of tenths change over a short time period after the fill. However, this is usually not enough to make any significant difference. On the other hand, I find that helium mixes, especially those with the higher helium contents (10/50, etc.), can take many hours to homogenize.

When mixing trimix using the partial pressure method, the air top component, if added slowly after the oxygen and helium, causes a need for several hours to pass before the mix will settle at the final fractions. By "hours," I mean up to 12 hours especially with high helium fractions (10/50, etc.). So, my experience has been that gases do not blend instantly at all.

On the other hand, if I put my tanks in the car and drive around on the road for a while, I find that the gas mixes much faster, and will be within .2 of the final mix within an hour or so. If I roll the tanks or tilt from side to side, I notice that the mixing occurs faster than letting them sit on their own but not nearly as quickly as when I drive them around in the car for a bit.

One thing that I do to mix faster is to use bursts of air during the air top process rather than filling at a steady rate. When I first turn on the air, I put in a good size burst for a few seconds. Then, I reduce the fill rate, and I occasionally put in some bursts along the way as I continue the fill. I find that this can get my tank fairly close to the final fraction pretty quickly.

Theory is one thing. I can only tell you that this is what I find after mixing quite a few batches of gas.