Nitrox PPO2 blending waiting period for homogenisation of mix

[cool_hardware52]

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.

I don't get to excited about squirting in the last few 100 psi of OCA on top of the O2 fast even if it's EAN50.

There's no pure O2 in the whip, or adapter or valve at that point.

As mentioned before this does require a source of OCA that's at a pressure quite a bit higher than the final fill pressure.

Tobin

 

[JanK]

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.

The increased pressure indeed has an effect (I just logged on to write this post as I saw yours). In fact it corrects something that has been bothering me about the. Assuming a 1 mm^2 section the speed would be 100 times as high, 1700 m/s - in excess of the speed of sound at 1 bar. But increasing the pressure a 100 times to 100 bar decreases the speed to the same order of magnitude as before.

Does anyone have a valve and could estimate the section of the tube leading into the tank?

 

[solowgregg]

I don't get to excited about squirting in the last few 100 psi of OCA on top of the O2 fast even if it's EAN50.

There's no pure O2 in the whip, or adapter or valve at that point.

As mentioned before this does require a source of OCA that's at a pressure quite a bit higher than the final fill pressure.

Tobin

And with partial pressure blending your source of OCA will be a great deal higher than the pressure of O2 in the tank you're filling. And it tells you right in the quote to go slow until you dilute your mix until you get down to at least 40% O2.

 

[fppf]

The increased pressure indeed has an effect (I just logged on to write this post as I saw yours). In fact it corrects something that has been bothering me about the. Assuming a 1 mm^2 section the speed would be 100 times as high, 1700 m/s - in excess of the speed of sound at 1 bar. But increasing the pressure a 100 times to 100 bar decreases the speed to the same order of magnitude as before.

Does anyone have a valve and could estimate the section of the tube leading into the tank?

Read up on your fluid dynamics.

Super sonic flows through orifices is very common and actually a wanted effect in some cases. You can calculate the speed of the gas through the orifice based on the delta pressure, temperature, and weight of the gas.

The nice thing about super sonic flows is that once archived your flow rate will stay constant even if down stream pressure changes. Provided the delta pressure stays high enough to sustain super sonic flow.

 

[cool_hardware52]

And with partial pressure blending your source of OCA will be a great deal higher than the pressure of O2 in the tank you're filling.

Humm, not really.

Lets say I want 3500 psi of 50% and my compressor pumps to ~3500.

Just how fast do you think the last 100 psi of OCA gas will flow into the tank? As the pressure differential drops between the fill whip and the tank the rate of flow will slow and then.....it will stop.

OTOH if I have OCA banked at say 4500 psi I can allow the last 100-200 psi of OCA to flow rapidly into my 3500 psi cylinder.

And it tells you right in the quote to go slow until you dilute your mix until you get down to at least 40% O2.

Given that NO scuba tank valve is designed for O2 service, and that most valve seats are in fact "fuel" i.e. nylon a petroleum derived thermoplastic my main concerns are High pressure High content O2 behaving badly as it passes through the scuba tank valve.

How much O2 is in the valve when you are adding the last 100-200 psi of OCA to any mix? Looks like 20.9% to me.



Tobin

 

[solowgregg]

Humm, not really.

Lets say I want 3500 psi of 50% and my compressor pumps to ~3500.

Just how fast do you think the last 100 psi of OCA gas will flow into the tank? As the pressure differential drops between the fill whip and the tank the rate of flow will slow and then.....it will stop.

OTOH if I have OCA banked at say 4500 psi I can allow the last 100-200 psi of OCA to flow rapidly into my 3500 psi cylinder.



Given that NO scuba tank valve is designed for O2 service, and that most valve seats are in fact "fuel" i.e. nylon a petroleum derived thermoplastic my main concerns are High pressure High content O2 behaving badly as it passes through the scuba tank valve.

How much O2 is in the valve when you are adding the last 100-200 psi of OCA to any mix? Looks like 20.9% to me.



Tobin

Hahaha okay. Listen, all I'm saying is that is what's written in the DSAT manual. Write to them and tell them they're wrong, not me.

 

[cool_hardware52]

Hahaha okay. Listen, all I'm saying is that is what's written in the DSAT manual. Write to them and tell them they're wrong, not me.

Believe whomever you choose. This is not only matter on which Padi and I differ......

Tobin

 

[solowgregg]

Oh believe me, I'm not saying this is correct by any stretch of the imagination. I just thought I'd list what they claim as "Correct". I personally believe that the turbulence from air on top of O2 is sufficient mixing at almost any flow rate.

 

[NAM001]

Hello all

I'm new to Nitrox, having just completed my course.

I went for a 32 mix fill at my LDS (PPO2 blending) and was told that I had to wait for a few hours before they would analyze the mix and sign it off. Something about letting the mix homogenise. This kinda makes sense to me but this was not mentioned in the TDI/SSI course and I was wondering if anyone has heard of this.

Another question regarding this: Does the temperature of the cylinder (warmish after a fill) affect the PPO2 reading on the analyser? I'm thinking No because the fraction remains the same, right? Comments please.

Regards

Joe

NEVER HEARD OF THIS BEFORE ... PERHAPS THE BELOW IS WHAT THEY ARE TALKING ABOUT.

Another question regarding this: Does the temperature of the cylinder (warmish after a fill) affect the PPO2 reading on the analyser? I'm thinking No because the fraction remains the same, right? Comments please.

THIS I HAVE EXPERIENCED. I MAKE A 40% MIX APX 25% PURE O2 AND TOP OFF WITH AIR. FIRST I PUT 750 OF O2 IN THE TANK. LET IT SIT AND IT DROPS TO WEL UNDER 700. THE SLOWER YOU FILL THE LESS OF THIS EFFECT YOU HAVE. IF I DO NOT FILL SLOW AND THEN TOP OFF. I GET MY 40% ON THE SNIFFER LET IT SIT AND I LOOSE A FEW PERCENT. HENCE THE 25% AND NOT ACTUAL 24% FOR 40 MIX. O2 AND N2 HAVE DIFFERENT EXPASNSION RATES. YOU HAVE TO LET THE O2 COOL IF YOU ARE FILLING PP. LIKE WISE IF YOU ARE FILLING WITH PREMIX THEN THE PROBLEM FIXES IT SELFSINCE IT WAS RIGHT WHEN IT WENT IN. THE PREMIZX SAMPLE AND THE TANK SAMPLE WILL NOT EQUAL TILL THEY ARE THE SAME TEMP. YOU CAN TEST THIS BY TAKING A SAMPLE. INCREASE THE PREASURE ON THE SENSOR AND THE %O2 GOES UP. MY LDS HAD A TOUGH TIME WITH THIS MAKING THE % COME OUT OK. THE SLOWER THE FILLING OF AIR OR O2 THE CLOSER THE FINAL MATCHES YOU MIXING CALC'S. I THINK THE FILL RATE RECOMMENDED WAS LIKE 60 LBS/MIN FOR O2. THAT IS PAINSTAKINGLY SLOW. (HUMAN NATURE) IF YOU FILL THE O2 AT THE RATE OF AIR RECOMMENDED MAX 600/MIN YOU SHOULD LET IT COOL TO ROOM TEMP THEN TOP OFF WITH AIR TO 3000 AND LET IT SIT AND TOP OFF AGAIN AFTER THE PRESURE DROPS TO 2800 OR SO TO GET THE RIGHT MIX. . LET IT SIT AND CHECK AGAIN . AS LONG AS THERE IS A WHIP ON I FILL WITH TANK IN WATER. TO MANY, THIS IS A NO NO, I HAVNT HAD PROBLEMS WITH THIS.

 

[NAM001]

Yes, I suppose I'm imposing my own standards here, my appoligies.

Still, most cylinders are returned with 500 - 700 psi, and there's usually at least one O2 cylinder that can push in the extra 300 psi...it's not that hard.

My experience with shops that insist on emptying cylinders, is because they lack a blender program, or, don't care to do the math to top off a nitrox fill - hence the "lazy" comment.

Point taken, though - I've occasionally been down to the bottom of the last O2 supply cylinder, and been forced to drain the tank before blending.


All the best, James

YOU NAILED IT ON THE HEAD. VORTEX FLA IS LIKE THAT IF I GO TO DESTIN FLA THEY SAMPLE CONTENT AND TOP OFF WITH THE MIX NEEDED TO GET WHAT YOU WANT AND THEY CHARGE BY THE FT^3 OF O2 ADDED. I THINK AT THE TIME IT WAS 10c PER CUBE O2 PLUS 5.00 FOR THE FILL.