Nitrox PPO2 blending waiting period for homogenisation of mix

[JBRES1]

Why in the world would you do that? Once filled with a closed isolator, they won't mix at all between tanks, you know... If you want to assure the same mix in both tanks, you need to leave that isolator open during all stages of the fill.
Or, looking at it another way, why is the isolator there? It ain't for filling the tanks.
Rick

Hi Rick,

I first fill the o2 to lets say 500 psi , then close off the isolator.
I then top off with compressed air from the bank to 2650 +/-psi into each cylinder thru the left and right post. I let sit , test each cylinder for mix and final pressure, then open the isolator.

I look at like this, I have the same amount of o2 in each cylinder because I fill with the isolator open. I get very close to the same amount of compressed air in each cylinder when I top off. They will equalize when I open the isolator, and the mix in each cylinder should be very close.

I just don't like to fill the doubles with the isolator open.

Jim breslin
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[DA Aquamaster]

Yeah, a bit of OCD. But if you can, why not right?

But how would you know it's "right"? It's an issue of significant figures. If you are measuring with a ruler calibrated in 1/16ths of an inch, you can't go on to later multiply and divide to produce a figure with an accuracy to .0001 inch. You will always be limited by significant figures of your least accurate measurment or piece of data.

In terms of mixng gas +/-1%, the O2 sensor is not accurate to that level. If your sensor is only accurate to 1.0% or even to 0.5%, then making a mix accurate to 0.1% is futile. At best you can say your mix is perhaps "consistent", if and only if the sensor error is consistent, but you can never say with authority that your mix is any more accurate than the guy who is just fine being .5% off on the mix.

 

[Trace Malinowski]

Just late to the party and was reading about the fills being +/- .1% or whatever. Which leads me to the question, other than because you might have Obsessive/Compulsive Disorder, why does it REALLY matter? How close are you really diving to the MOD on your backgas?

I am late for the party, too, and I was wondering the same thing.

Since decompression, oxygen toxicity, instrument calibration, gas stratification, and volume/pressure/temperature relationships change and all are inexact sciences, as long as you are within a safe range of percentage for the depth and time you hope to dive, you should be safe.

ALWAYS analyze on the day you will be diving and analyze out to the decimal point to avoid confusion between contents wanted and final analysis of contents.

If you do not trust the analysis at time of fill, leave the contents label blank and analyze later.

I'm fairly confident in my analysis at the time of fill and anything between 30% and 34% will allow me to dive the mix to 100 feet and use ratio deco, and deco software, pretty accurately as if using Nitrox 32.

It's important to limit one's PPO2 at depth to a safe level. I have toxed at a ppO2 of 1.4 ATA in a heavy current on Nitrox 32. (112 feet & 31.9 mix). Since exertion vs. pressure is a most inexact science, it is the one we must concentrate on controlling during the dive itself.

 

[fppf]

Yes, I'm very consistent.
I take pride in my work, I don't see doing the best job I can as a "futile" effort.

Yes, the O2 sensor I use is factory rated at +/- 1% error. The instrument I use to measure the output of my sensor is 0.03% device. If you have access to a high accuracy gas calibrator you can characterize your sensor and get higher accuracy out of it for a limited amount of time. The mfgr of O2 sensors must take into account the long term stability and drift of the senor. They are much more accurate in the short term.

Now, if you REALLY want to split hairs the total accuracy of a system is the square root of the sum of the squares of all errors. Seeing that the sensor is 1% and most analyzers are using a 1% voltage measurement device the true total accuracy is really about 1.4%. Now, most places want a minimum TUR of 4:1 meaning they want the device measuring to be 4 times more accurate than what is being measured. 10:1 TUR is really the preferred in high accuracy stuff, but 4:1 is acceptable. So really we can't mix anything more than +/- 5.6%. But alass the scuba industry is more than happy with 1:1 TURs or even less.

Don't even get me started on regulator adjusting and rebuilding. Most dive shops never get there instruments calibrated or even checked on a regular baises. Let alone having a guage accurate enough to hold the typical +/- 3 PSI spec the mfgrs want held on first stages. To correctly measure that pressure to industry standards you need a 0.375% accurate gauge.

 

[Rick Murchison]

...I just don't like to fill the doubles with the isolator open....

Why not?
Rick

 

[RJP]

All I know is that when I fill my doubles I start off with the isolator open during o2 filling and closed during top off with air.

I just don't like to fill the doubles with the isolator open.

Boy, if there's ever a solution to a problem that doesn't exist...this is it. And one which increases the likelihood of creating a different problem further down the line:

There was a post a year or so ago in "Near Misses" from a guy who filled his tanks in this fashion, and ended up almost diving 100% 0₂ to a planned depth of 100+ feet because he didn't "check his work" correctly.

Fortunately, he gave Darwin a miss that day...but only because his buddy had a gear problem and they ended up scrubbing the dive at about 25 feet. After the dive when breaking down his gear he noticed the isolator was closed. When he opened it he was surprised to hear the one side that was full of 32% transfilling over to the side that contained the 500 or so psi of 100% he had just been breathing off.

:shocked2:

 

[JanK]

I did some back-of-the-envelope calculations and I think that it's impossible for the gasses in one tank not to be mixed. Disclaimer: I am not a gas blender or supplier, just a physicist trying to make some sense of the figures.

Let's take a small portable compressor, e.g., a Bauer Junior II with 100 litre per minute (i.e., 3.5cfm) filling speed, which would fill my usual 15l 200bar tank in 30 minutes.

100 l/min is equal to 1.7 l/s. Now, one litre is 1000 cubic centimetres, so 1.7 l/s is 1700 cm^3/s, i.e., on average each second a 1700 cm^3 of air must be pushed into the tank. If one wants to squeeze 1700 cm^3 of a gas in one second through an opening of 1 square centimetre, the speed of the gas through that opening must be 1700 cm^3/s divided by 1 cm^2, i.e., 1700 cm/s, which is equal to 17 m/s (or 60 km/h or around 40 mph).

In other words the gas rushes into tank with such a speed that it takes only 50 milliseconds (one twentieth of a second) for it to travel from the valve to the bottom of a 0.8m long tank. I cannot imagine that flow under these circumstances would not be completely turbulent and that anything less than total blending of the gasses would occur.

And I assumed the cross section of the valve outlet into the tank to be 1 cm^2. I don't have a valve handy, but looking at, e.g., Standard 3000psi K-Valve - Sherwood - Scuba Tank/Cylinder Essentials, I see a much thinner tube that would protrude into the tank. The section is probably on the order of a mm^2, so the gas speeds would be correspondingly higher.

 

[RJP]

I did some back-of-the-envelope calculations and I think that it's impossible for the gasses in one tank not to be mixed. Disclaimer: I am not a gas blender or supplier, just a physicist trying to make some sense of the figures.

Never let the facts get in the way of a good story!

 

[fppf]

I did some back-of-the-envelope calculations and I think that it's impossible for the gasses in one tank not to be mixed. Disclaimer: I am not a gas blender or supplier, just a physicist trying to make some sense of the figures.

Let's take a small portable compressor, e.g., a Bauer Junior II with 100 litre per minute (i.e., 3.5cfm) filling speed, which would fill my usual 15l 200bar tank in 30 minutes.

100 l/min is equal to 1.7 l/s. Now, one litre is 1000 cubic centimetres, so 1.7 l/s is 1700 cm^3/s, i.e., on average each second a 1700 cm^3 of air must be pushed into the tank. If one wants to squeeze 1700 cm^3 of a gas in one second through an opening of 1 square centimetre, the speed of the gas through that opening must be 1700 cm^3/s divided by 1 cm^2, i.e., 1700 cm/s, which is equal to 17 m/s (or 60 km/h or around 40 mph).

In other words the gas rushes into tank with such a speed that it takes only 50 milliseconds (one twentieth of a second) for it to travel from the valve to the bottom of a 0.8m long tank. I cannot imagine that flow under these circumstances would not be completely turbulent and that anything less than total blending of the gasses would occur.

And I assumed the cross section of the valve outlet into the tank to be 1 cm^2. I don't have a valve handy, but looking at, e.g., Standard 3000psi K-Valve - Sherwood - Scuba Tank/Cylinder Essentials, I see a much thinner tube that would protrude into the tank. The section is probably on the order of a mm^2, so the gas speeds would be correspondingly higher.

Your leaving out 2 very important facts from your thought.
The gas pumped into the cylinder is not entering at 1 atmosphere, the standard pressure when specing volumes. If it enters the cylinder at 35 bar the speed using your example is now about 0.5 m/s. The second fact is as soon as the gas clears the orifice it will decelerate greatly.

 

[solowgregg]

Straight out of the DSAT gas blender manual (p.37) under the section entitled: Basic steps to partial pressure blending.

Introduce oxygen compatible air into the cylinder: slowly (6.9bar/100 psi per minute) to avoid potential ignition sources, until the mixture is diluted enough to increase it to a normal flow rate (approximately 40 percent oxygen).

The normal fill rate will create enough turbulence to provide a homogeneous mix. However, if the mix being produced is a 40 percent (or above) mix, then the fill rate never gets to the "normal" rate, and the mix will need to sit for six hours to homogeneously mix.

I'm not saying whether this is wrong or right, I'm simply taking what is in my manual and presenting it to you all for reference.