Nitrox Partial Pressure Blending

[jbisjim]

Partial Pressure is how our shop blends nitrox now. Does anyone have any trick as to getting the mix dead on in a shorter time than it takes me now? I had a couple of mixes i did come out much higher than anticipated. We use Dive rites Voyager program to get the right mix numbers, i cross checked it with another program and they were with in a few psi of each other. Those who partial pressure blend do you mix the o2 and then let the tank cool? or Load the compressed air in shortly after?

So do people have a system for partial pressure blenind that is sure fire for them?

Right now we are not banking due to needing some HP banks cascade o2 cleaned (which is a whole other question). We do have a stick, has anyone used some HP scuba scylinders to bank the HP. Or use a stick to fill each cylinder?

I am really looking for a better way to do the blending as it takes too long to partial pressure blend and do it efficiently. I can generally get the mixes with in a fraction of a percent, but every once in a while i get one that is way off.

Any and all thoughts would be helpfull. I am sure i missed some vital pieces of info.

 

[boydski]

I am really looking for a better way to do the blending as it takes too long to partial pressure blend and do it efficiently. I can generally get the mixes with in a fraction of a percent, but every once in a while i get one that is way off.

I use a blending stick for anything 40% or less and it works quite well, seems efficient and allows for topping off of half-filled cylinders. For deco gases I combine methods, for example for 50%, add 500 psi of oxygen, filled very, very slowly (so the cylider stays cool), then top with 40% yields 3000 psi of 50% every time. As slow as I go with the oxygen when partial pressure blending, I don't think it would be a very efficient setup in a shop.

YMMV

 

[jbisjim]

I use a blending stick for anything 40% or less and it works quite well, seems efficient and allows for topping off of half-filled cylinders...

Just so I am clear you use a stick for individual tanks under 40% ?

 

[Otter]

You must let it cool. In theory if the temperature was held constant across both fills, you would have the right blend...if warm/hot it would be a short fill, but have the right FO2. The easiest way to hold it constant is to let it cool after the first fill, which shouldn't take to long as you should be filling O2 VERY slowly with with a relatively low volume. Let it cool. Read the pressure and top off to get it 'exact'. Then fill with air and let it cool, read the pressure and top off. I am usually within .1-2 %

The other thing is that you need some turbulence when filling air to help it mix. If you are filling very slowly with air, you would need to wait for several hours -- overnight would guarantee it -- for molecular blending to occur before analyzing.

 

[wedivebc]

I usually put the O2 in slowly then when I top with air I fill to the pressure I know from experience will cool to my target pressure. The O2 is not likely to gain much heat since you are filling slow. The required air will come with practice. I can PP blend, using analogue gauges and still get within 1/4% of my target. It just takes practice.

 

[wedivebc]

-- for molecular blending to occur before analyzing.

What's that?

 

[boydski]

Just so I am clear you use a stick for individual tanks under 40% ?

Yes, for individual tanks we turn on the compressor, turn on the Oxygen to fill the tank with the desired blend, venting the filter if needed until the mix shows up after the filter.

 

[Uncle Pug]

Since you have cascade tanks already... though not O2 clean... just use your stick and continuous blend into the bank. From there you can fill scuba tanks and when the bank is too low to fill tanks completely top them off with the compressor using continuous blending.

Larry at ScubaToys would be a good person to talk to about continuous blending in a commercial setting.

 

[jbisjim]

With Under 40% blends is it safe to run through a compressor. The compressor at out shop had a burn out when this was tried previously. The perpatrators may have been running a higher blend through. Is 40% safe to pump through the compressor?

 

[triton94949]

...So do people have a system for partial pressure blenind that is sure fire for them?
...

It is called TRIAL AND ERROR. It is a part of the LEARNING PROCESS.

The main issues are exact pressure read-outs, cooling off, and order of gas blending.

Trial and error will teach you what works. Once you have gone through enough trials, you will develop a system that works for you. Take notes on every fill, and then play with the results. That will help you discover how to get exact O2 percentages.

Assuming that you have a good pressure gauge on the blender, and that you allow enough time between fills and before final top-off for the tank to cool off completely, and you always drain the tank first, and add the O2 first, then you should get O2 percentages that are right on.

The problems come when you try to partial pressure blend a partially filled tank. The law of physics that makes life difficult for you is called "gas compressibility." It will throw off your calculations.

Good luck, and have fun with your new blending toys!