Partial pressure mixing doubles

[wmspdi]

I am ready to double up a set of E7 100s for use with NITROX. The only local dive shops near me that fill NITROX use the partial pressure method (the closest LDS with premix 32% is 90 miles away).

My Question: When filling 2 tanks connected with a manifold is there an issue with matching the blends in the 2 tanks. Also what about "topping off" the the tanks with hyper pure air between refills. I sometimes do this when I don't need a specific blend, then analyze and dive the resulting mixture. Thanks.

 

[Doppler]

I am ready to double up a set of E7 100s for use with NITROX. The only local dive shops near me that fill NITROX use the partial pressure method (the closest LDS with premix 32% is 90 miles away).

My Question: When filling 2 tanks connected with a manifold is there an issue with matching the blends in the 2 tanks. Also what about "topping off" the the tanks with hyper pure air between refills. I sometimes do this when I don't need a specific blend, then analyze and dive the resulting mixture. Thanks.

Answers to questions one and two: Don't worry... thousands; no tens of thousands of technical divers do this every day.

Q1. When your tanks are filled, simply make sure the isolator is wide open... the gas will flow into both cylinders at "the same" rate and blending of gases will take place automatically...a gas will expand uniformly to fill the vessel that contains it, so the blends in the two tanks, as you put it, will match.

The only issue and something that seems to be generally misunderstood is the geometry of the manifold itself and that means that it sometimes takes a few minutes for the gases to homogenize. Rolling tanks does little to speed up this process and simply puts strain on the manifold and other hardware holding the tanks together.

Q2. Just make sure you reanalyse and adjust your dive plans accordingly.


N.B. Your gas storage system (these tanks and manifold) must be prepared for oxygen service.

 

[wmspdi]

Thanks for the quick reply. I wasn't sure if (when filling using the PP method) if each tank was bended as a single unit (with the isolation valve closed) or blended as one connected unit. My tanks are already in NITROX service, and the Thermo DIN/K manifold is factory new. I plan on having the conversion done when the tanks are due for VIP.

 

[Walter]

Factory new doesn't mean they are O2 clean. Rubber O-rings and silicone cannot be used for oxygen service. Check to make sure the mainfold doesn't contain any substances that can cause a fire in a high pressure oxygen atmosphere. If they do, they can be cleaned and those materials can be replace with oxygen compatible materials.

 

[mania]

It's not the question whether your manifold is brand new or not - it's the question whether it's suitable to work with pure oxygen.
If your tanks were filled with nitrox with the continuos mix method - then they didn't have to be oxygen clean. But with the partial preassure every single part has to be cleaned and has to qualify to work with pure O2.
Mania
Ups. Walter was a bit faster in sending than I...

 

[bgi]

Recalling college chemistry and physics, diffusion will mix gases in a closed container. If you start with a set of doubles with pure O2 in one cylinder and air in the other cylinder, opening the isolator will initiate diffusion mixing. How long would it take to mix? Hmmm... Any chemists here?

 

[Rick Murchison]

Recalling college chemistry and physics, diffusion will mix gases in a closed container. If you start with a set of doubles with pure O2 in one cylinder and air in the other cylinder, opening the isolator will initiate diffusion mixing. How long would it take to mix? Hmmm... Any chemists here?

First, let's be clear that this question has nothing to do with the initial question - partial pressure filling of doubles doesn't have a mixing problem when done right.
Now, for this question... a very, very long time.
Don't allow this situation to occur. If you do somehow screw up and manage to do it,
(1) close the isolator
(2) dump the air
(3) close the air tank valve
(4) open the isolator (very slowly), allow pressure to equalize in both tanks
(5) top with air for final mix
Rick

 

[wmspdi]

It's not the question whether your manifold is brand new or not - it's the question whether it's suitable to work with pure oxygen.
If your tanks were filled with nitrox with the continuos mix method - then they didn't have to be oxygen clean. But with the partial preassure every single part has to be cleaned and has to qualify to work with pure O2.
Mania
Ups. Walter was a bit faster in sending than I...

The new Thermo Pro modular valves, and Thermo manifold, are rated for Nitrox service from the factory. I will be replacing the Thermo valves that were installed on the PST E7 100 tanks.

My Nitrox tanks have only been filled by one LDS, and he uses the PP method. He is also the dealer selling, and installing, this new manifold. Since he will be the one placing pure O2 in the tank prior to blending, I would assume he would know if the product he is selling is O2 safe. The bands and manifold will be installed when the tanks are O2 cleaned on their next VIP.

My concern was with the migration of gases between the 2 tanks via the manifold and a consistant blend. I will usually get a 36% fill (since we don't have many site that exceed the MOD). If I have more than 1,500 PSI left after my dive I have the tank topped off with hyper pure air, analyze the resulting blend (usually around 28% to 30%) and dive it. I then take the tank(s) in for a 36% refill. If less than 1500 PSI I use the NITROX tank for pool work or have it drained (the LDS procedure) and refilled.

BTW- I noticed some strange looks from the other members of our sheriff department's underwater search and recovery team the first time I pulled out a NITROX tank for a pool training session. Hey, why waste 1,000 PSI of good air when I can use it in the pool? The looks should get worse if I show up with NITROX doubles. The team commander wants us to train the way we will dive. Since we are diving in "black water" with zero vis at times you are totally dependant on self rescue (you can't see your buddy). My options are a single Nitrox 100 cf and a 19 cf (air) filled pony tank (which I use currently) or NITROX doubles with twin regs which I will use for other types of diving as well. Thanks for your help.

 

[mania]

wmspdi
the question is - when filling with normal air - do you use filters? I presume you are using normal compresor for air - then to have your tanks oxygen compatible you need to use filters - otherwise lubricant used for normal compressor is not O2 compatible.
So you should really make sure that nothing that contains hydrocarbons is used for partial preassure mixing. It doesn't have to but in most of the cases will end up in O2 ignition.
Mania
PS. Not that I'm that clever but just finished the Trimix Gas Blender course

 

[wmspdi]

I am not a gas blender and I don't fill the tanks myself. The LDS topped them off with the same "hyper pure air" he used to blend with the O2. He charged me $4.50 to top off the NITROX tanks with air and did it on the spot. He charged $15.00 for a new blend, and I had to leave the tanks and pick them up days later. The top offs just got me all little more bang for my NITROX buck and saved me a trip. If I needed a specific blend I made two trips, spent the money and had it refilled.

The point may be moot now. I was surfing the web and I just found an LDS much closer to home (about 45 mins away) that banks NITROX 32%. I called and found they only charge for the gas used at $.19 per cf, and they can fill the tanks while I wait. The LDS I had been using required that I leave the tanks for 2 days and then drive back for pick-up (2.5 hours round trip). Since I don't need a 36% blend, and can live with 32%, I am making the switch and will have them convert my single tanks to doubles. End of problem!