Measuring O2 in enriched air

[tarponchik]

...provided the flow rate is low enough that you don't get an increased pressure at the sensor.

An O2 sensor is an electrochemical cell. Its voltage output is proportional to the PO2 close to it, not the FO2. So if the flow is so high that the cell experiences a pressure significantly above ambient, it will give a too high reading.

In his case the readings were 2% too low.

 

[tarponchik]

its better to calibrate in air AND with 100 % o2..........just don't think having 2 units means 1 is right

The issue was discussed here in details previously, do your search.

 

[kelemvor]

Wow.. mind blown on the "shaking the tank" thing. When I get partial pressure fills at the LDS I've never seen them shake the tank or anything like that. However, I always test before leaving the shop and get roughly predicted values (MAYBE 1ppm off). I also test at the dive boat and don't recall ever seeing discrepancies. That said, I don't write down the value from the second test..it's just a confirmation of sorts.

Guess I've got some reading to do. Anyone happen to have a handy link to some interesting but fairly short details on the phenomenon?

 

[Basking Ridge Diver]

School physics tells us it should all be mixed pretty much instantly.

Actually this is inaccurate - it is called diffusion and depending on the space it takes a while for any mix to occur.

 

[Storker]

Anyone happen to have a handy link to some interesting but fairly short details on the phenomenon?

The only details I can give you is that the density of O2 is some 10% higher than that of air. (Gases - Densities).

But I can give you an illustration. Ever ordered, seen or heard of layered drinks? You make them by first adding the heaviest liqueur, then the others in successively lower densities. Just be careful to avoid mixing while pouring. Pour slowly, preferably over a spoon. Now, one of the more famous layered drinks, the B-52 is made from Kahlua (density: 1.13), Irish Cream (1.06) and Grand Marnier (1.03). Note that Kahlua is only 7% heavier than Irish Cream, and Irish Cream is only 3% heavier than Grand Marnier.

Since Mother Nature abhors differences, if you let that B-52 stand long enough, it'll end up as a dull mixture due to what we call diffusion. Kahlua moves into the Irish Cream and subsequently into the Grand Marnier layer, Irish Cream moves into the Kahlua and Grand Marnier layers, and Grand Marnier moves into the Irish Cream and subsequently into the Kahlua layer. And you're left with a shot glass of a mixture which lacks merit both in flavor (which it always lacked) and in appearance.

Same thing with a PP EAN mix if you top up with air slowly enough that you don't get much turbulence and mixing in the tank. If you top with air pretty fast, you can compare that with splashing the Irish Cream into the glass atop the Kahlua (it'll mix pretty well, and you won't get that cool layer effect), but if you top slowly - which you probably would want to do, since that reduces the temperature increase when you fill - you're basically doing the same thing as if you were pouring the Irish Cream slowly into the glass on top of a spoon, and you'll get a heavy O2 layer at the bottom and a lighter air layer at the top. Let it stand, and Mother Nature AKA diffusion will even out those differences, but you can speed up that process. Lay the tank on its side, and the cross-section of the diffusion front will be a lot larger, leading to shorter time for equilibrium. Throw the tank into your car's boot and drive a bit (preferably on bad roads; we don't have much else), and the tank will be shaken, which improves mixing. Roll the tank back and forth on the floor, and you'll mix its contents.

When I'm filling my tanks from our club's inline mixer, I fill, cool and top off with air. I never analyze for marking the tanks before I've driven home with my tanks, usually the day after or later. It's good practice if you believe in the physics, and it makes life easier for a lazy bastard like me.

 

[TrimixToo]

The only details I can give you is that the density of O2 is some 10% higher than that of air. (Gases - Densities).

But I can give you an illustration. Ever ordered, seen or heard of layered drinks? You make them by first adding the heaviest liqueur, then the others in successively lower densities. Just be careful to avoid mixing while pouring. Pour slowly, preferably over a spoon. Now, one of the more famous layered drinks, the B-52 is made from Kahlua (density: 1.13), Irish Cream (1.06) and Grand Marnier (1.03). Note that Kahlua is only 7% heavier than Irish Cream, and Irish Cream is only 3% heavier than Grand Marnier.

Since Mother Nature abhors differences, if you let that B-52 stand long enough, it'll end up as a dull mixture due to what we call diffusion. Kahlua moves into the Irish Cream and subsequently into the Grand Marnier layer, Irish Cream moves into the Kahlua and Grand Marnier layers, and Grand Marnier moves into the Irish Cream and subsequently into the Kahlua layer. And you're left with a shot glass of a mixture which lacks merit both in flavor (which it always lacked) and in appearance.

Same thing with a PP EAN mix if you top up with air slowly enough that you don't get much turbulence and mixing in the tank. If you top with air pretty fast, you can compare that with splashing the Irish Cream into the glass atop the Kahlua (it'll mix pretty well, and you won't get that cool layer effect), but if you top slowly - which you probably would want to do, since that reduces the temperature increase when you fill - you're basically doing the same thing as if you were pouring the Irish Cream slowly into the glass on top of a spoon, and you'll get a heavy O2 layer at the bottom and a lighter air layer at the top. Let it stand, and Mother Nature AKA diffusion will even out those differences, but you can speed up that process. Lay the tank on its side, and the cross-section of the diffusion front will be a lot larger, leading to shorter time for equilibrium. Throw the tank into your car's boot and drive a bit (preferably on bad roads; we don't have much else), and the tank will be shaken, which improves mixing. Roll the tank back and forth on the floor, and you'll mix its contents.

When I'm filling my tanks from our club's inline mixer, I fill, cool and top off with air. I never analyze for marking the tanks before I've driven home with my tanks, usually the day after or later. It's good practice if you believe in the physics, and it makes life easier for a lazy bastard like me.

This ^^^.

Some shops really slam the air in after the O2, which mixes the contents well. You can analyze mostly right away. However, some people believe that slower fill rates are safer (like 600 PSI/min max for air), and that generally does not stir the pot--er, tank--vigorously enough to get an accurate reading right away. My fill whips have flow restrictors, so it takes some time for the contents to settle unless the tanks are rolled, shaken, taken for a drive around the block, or whatever.

 

[TheGraveyardDiver]

"But I can give you an illustration. Ever ordered, seen or heard of layered drinks? You make them by first adding the heaviest liqueur, then the others in successively lower densities. "

LOL. You don't dump the tequila and syrup into the glass at 300 cubic feet per minute. LOL
When you try, you get a pink Tequlia sunrise, not a layered drink.
I've sent them back because the amateur bartender did just what you advised.
When I order a tequila sunrise, I want the sunrise on the bottom, not a tequila milkshake.

You're totally comparing apples to string beans there.

I admit I know know a whole lot about gas blending. But when it comes to Tequila, I'm a giant, dude.

 

[KenGordon]

The only details I can give you is that the density of O2 is some 10% higher than that of air. (Gases - Densities).

But I can give you an illustration. Ever ordered, seen or heard of layered drinks? You make them by first adding the heaviest liqueur, then the others in successively lower densities. Just be careful to avoid mixing while pouring. Pour slowly, preferably over a spoon. Now, one of the more famous layered drinks, the B-52 is made from Kahlua (density: 1.13), Irish Cream (1.06) and Grand Marnier (1.03). Note that Kahlua is only 7% heavier than Irish Cream, and Irish Cream is only 3% heavier than Grand Marnier.

Since Mother Nature abhors differences, if you let that B-52 stand long enough, it'll end up as a dull mixture due to what we call diffusion. Kahlua moves into the Irish Cream and subsequently into the Grand Marnier layer, Irish Cream moves into the Kahlua and Grand Marnier layers, and Grand Marnier moves into the Irish Cream and subsequently into the Kahlua layer. And you're left with a shot glass of a mixture which lacks merit both in flavor (which it always lacked) and in appearance.

Same thing with a PP EAN mix if you top up with air slowly enough that you don't get much turbulence and mixing in the tank. If you top with air pretty fast, you can compare that with splashing the Irish Cream into the glass atop the Kahlua (it'll mix pretty well, and you won't get that cool layer effect), but if you top slowly - which you probably would want to do, since that reduces the temperature increase when you fill - you're basically doing the same thing as if you were pouring the Irish Cream slowly into the glass on top of a spoon, and you'll get a heavy O2 layer at the bottom and a lighter air layer at the top. Let it stand, and Mother Nature AKA diffusion will even out those differences, but you can speed up that process. Lay the tank on its side, and the cross-section of the diffusion front will be a lot larger, leading to shorter time for equilibrium. Throw the tank into your car's boot and drive a bit (preferably on bad roads; we don't have much else), and the tank will be shaken, which improves mixing. Roll the tank back and forth on the floor, and you'll mix its contents.

When I'm filling my tanks from our club's inline mixer, I fill, cool and top off with air. I never analyze for marking the tanks before I've driven home with my tanks, usually the day after or later. It's good practice if you believe in the physics, and it makes life easier for a lazy bastard like me.

I wonder what’ “slowly enough” is. I have seen this at places where nobody is hanging about doing the fills, at inland sites and on liveaboards wher all the fills have to happen over lunch. Also the odd trimix fill. These days for rich deco fills I trust the maths of what went in more than an analyser initially. Perhaps you can only get so much gas velocity when adding air to o2 to make 80% that it is always slowly enough.

 

[Storker]

I wonder what’ “slowly enough” is.

When I fill EAN using my club's compressor/blender and top up with air, I tend to get somewhat lean analysis results if I analyze immediately after topping up. That compressor delivers 265 L/min.

 

[tarponchik]

Beautiful discussion, just beautiful. I need to point out, however, that whether Tequila molecules diffuse or not, it is useless to shake tanks. You can shake a tank but you can't shake the gas insude the tank. You have to put a little ball inside, like they do with Guinness beer cans.