Gettiing to 32%

[chillyinCanada]

I'm having trouble swallowing the position "if it is 28% freshly mixed it will settle out at 32%+/".

So one goes to the shop but refuses to take it unless it analyses as 32% at the shop and thenre-analyses at dive site, and what then? Gets 36%??

 

[mala]

partial pressure mixing always seems to take a while to get a correct reading.usually it will read low 02 and then gradually settle unless you roll the tank around.

I have a theory

the last gas in is at the highest temperature because its at the highest pressure.
its hotter so it will migrate to the top of the cylinder.
there is bound to be some mixing during filling but at the end the hot air will be at the top and the colder 02 at the bottom.

murphy's mixing law.

?

 

[RJP]

the last gas in is at the highest temperature because its at the highest pressure.

All the gas in the tank is at the same pressure, therefore all the gas in the tank is at the same temperature.

 

[mala]

All the gas in the tank is at the same pressure, therefore all the gas in the tank is at the same temperature.

how can gas enter from the gas line to the tank if it is at the same pressure.?

yes when the fill is finished the all the gas is at the same pressure but the last gas in is hotter and so the mix will not be uniform until the temps equalise.

 

[RJP]

Bill, the reason you were told not to put air in a Nitrox tank is that shops that don't pump Nitrox may not have the same standards of purity for their air as a shop does when it partial pressure blends. If the shop does partial pressure blending (putting O2 in and topping with air) they will have O2 clean air, so it doesn't matter what mix the tank has in it.

Still a good idea to be careful, as many shops that do partial-pressure filling have "two sided" fill stations. We have a nitrox/blending side and a "regular air" side as well. If you bring a tank in to a place that has both regular air and O2 clean air best to specify what you want, especially if your tanks aren't prominently marked.

---------- Post added November 30th, 2013 at 11:27 AM ----------

how can gas enter from the gas line to the tank if it is at the same pressure.?

yes when the fill is finished the all the gas is at the same pressure but the last gas in is hotter and so the mix will not be uniform until the temps equalise.

Re-read my post. I was talking about the gas in the tank, not the gas in the fill whip.

All the gas IN THE TANK is at the same pressure (once it's IN THE TANK) at any and all time points during the filling process. Therefore all the gas molecules IN THE TANK are at the same temperature at every point in time during the filling process. There is no "temperature equalization" between gasses in the tank necessary, or even possible.

 

[mala]

Still need to be careful, as many shops have "two sided" fill stations. We have a nitrox/blending side and a "regular air" side as well. If you bring a tank in to a place that has both regular air and O2 clean air best to specify what you want, especially if your tanks aren't prominently marked.

---------- Post added November 30th, 2013 at 11:27 AM ----------



The gas IN THE TANK is all at the same pressure once it's IN THE TANK. Therefore all the gas molecules IN THE TANK are at the same temperature.

the gas going into the tank is at a higher temp than the gas already there.
that's why the tanks get warmer during the fill.
the hot high pressure gas warms the lower temp gas eventually so that all is well after a few minutes.

the 02 that first goes into an empty tank usually cools the tank because of expansion.
that's because the tank is at 1 bar and the fill is higher.

 

[Dive-aholic]

They are not my tanks. I am renting them from the shop that is mixing the gas. I was wondering whether the low O2 % might be due to them using them for air, then Nitrox, and if so, is that bad? IIRC, during my Nitrox training they said that should not be done.

- Bill

Not at all. As long as cylinders are only filled with OCA (Oxygen Compatible Air) then you can put oxygen in them at anytime. Since these are the shops rental tanks that are not likely being filled anywhere else they are likely only getting OCA. I've got 3 filters on my set up and all my air is OCA regardless of whether it's nitrox, air, or trimix. I will partial pressure blend in any of my tanks. However, I will not partial pressure blend in tanks that are brought to me because I don't know the history of the tank. If it was filled at a shop that doesn't have OCA then you can pretty much guarantee contaminants have been introduced. I like my shop too much to risk that. Of course, I can continuous blend those tanks so it's not an issue getting the fills done.

It's already been mentioned that you should have your own analyzer so you can analyze at the dive site. Honestly, if the shop can't prefill the tanks early enough for you to get a 32% reading on them then I would stop using them. Find someone else who will give you the fills you request. In the meantime you might want to ask them if you can borrow their analyzer so when you get to the dive site you can confirm that the gas is truly at 32% like they claim it will be. The issue with their claim is that it can go up to 38% or higher without you knowing and that would put you beyond your MOD. Buy an O2 analyzer (not from that shop) but still ask for a loaner (bet they won't loan their analyzer out).

As for rolling tanks, I usually let my tanks sit for several hours before analyzing them. I taught a blender course a while back and we had to analyze right after doing the mixes. The blends were way off! I knew that was impossible because we did the calculations, confirmed they were correct, and I saw them put the proper amount of gasses in. We laid one of the tanks on it's side and the blend changed. No, we didn't even roll it. We just laid it on it's side. We stood it up while on the analyzer and the blend changed. We laid it back down and watched the numbers change. The gasses were moving in the tank as we went back and forth between standing the tank up and laying it down. We did this on all the tanks and got the same result! Prior to this I also thought rolling tanks was BS. I still don't think rolling tanks is worthwhile. I question the blend you'll get from rolling tanks. The best thing to do is give it time. Wait several hours from when the tank was filled before you analyze.

 

[dumpsterDiver]

All the gas in the tank is at the same pressure, therefore all the gas in the tank is at the same temperature.

Temperature and pressure are not necessarily the same. Our weather would be boring if this were true. Think of it this way, pressure must equalize, almost instantly (in a tank). Temperature of thick gases may not.

I had always assumed the discrepancy was associated with the density of the gases rather than temperature. Maybe it is both?

I am not sure, but it "seems' like if you fill a nitrox tank fast, the blasting of the air into the tank tends to mix it. Filling it slower, results in less mixing and more discrepancy in the reading of the mix right after the fill. So temperature sounds like it would be a causative factor to promote stratification, but filling it slower would keep the air cooler and reduce this effect, I think.

 

[RJP]

the gas going into the tank is at a higher temp than the gas already there.that's why the tanks get warmer during the fill.
the hot high pressure gas warms the lower temp gas eventually so that all is well after a few minutes.


the 02 that first goes into an empty tank usually cools the tank because of expansion.
that's because the tank is at 1 bar and the fill is higher

Oh, no. Not this again, please!

http://www.scubaboard.com/forums/ba...when-you-fill-them-higher-pressure-tanks.html

---------- Post added November 30th, 2013 at 12:12 PM ----------

Temperature and pressure are not necessarily the same.

Of course they are not the same, but they do vary directly. (And "weather" is a whole different pressure/temperature thing than gasses, so let's not go off the track.)

Gay-Lussac's law - Wikipedia, the free encyclopedia

As long as the O2 and N2 in question is gaseous (and not liquid) it must behave as a gas and adhere to Amontons' Law of Pressure-Temperature.

 

[mala]

Still a good idea to be careful, as many shops that do partial-pressure filling have "two sided" fill stations. We have a nitrox/blending side and a "regular air" side as well. If you bring a tank in to a place that has both regular air and O2 clean air best to specify what you want, especially if your tanks aren't prominently marked.

---------- Post added November 30th, 2013 at 11:27 AM ----------



Re-read my post. I was talking about the gas in the tank, not the gas in the fill whip.

All the gas IN THE TANK is at the same pressure (once it's IN THE TANK) at any and all time points during the filling process. Therefore all the gas molecules IN THE TANK are at the same temperature at every point in time during the filling process. There is no "temperature equalization" between gasses in the tank necessary, or even possible.

temperature equalization is not possible?

drysuits work because air does not conduct heat easily. diver warm-water cold.


next time you get a fill put your hand on the tank.
top hot
bottom not so hot
gas in top hot
gas at bottom not so hot.

work it out.