LDS Filling Nitrox - Partial Pressure Blending

Partial pressure blending (pure oxygen is first put into the tank then topped off with air).

Let me elaborate. Let's start with a tank that has been O2 cleaned. Currently has 36% nitrox. By filling it first with pure oxygen. The mixture's percentage will increase possibly above 40% into the caution/danger zone. Then hydrocarbon free air is topped off to bring the mixture back down to 36%.



This could be a stupid question. However I would like to be educated for whatever it is worth. So here it goes.

For recreational purposes. Why even run the risk of making the mixture go above 40% during a fill? What is wrong with filling a tank with air first then topping it with pure oxygen?

Let me elaborate. Let's start with a tank that has been O2 cleaned. Currently has 36% nitrox. First fill it with hydrocarbon free air. That will drop the mixture percentage to somewhere below 36%. Next top it off with pure oxygen to bring the mixture back to 36%.

This way the mixture inside the tank will never reach 40%. Isn't this safer?



Why are PPB LDS choosing the "O2 first then air" method? I am all ears.

Topping off with O2 would require the shop to be able to deliver O2 at 3000psi. That would require them to have a compressor that can handle 100% O2, and at 3000psi or more.

The advantage of O2 1st is that it keeps pressures lower. That makes it so you can just take a bottle (or several hooked up as a cascade) of O2 and just let it bleed into the tank being filled rather than having to have a compressor that is O2 clean.

Charlie Allen

disclaimer: I'm not a gas blender, nor do I run a shop. But the answer is simple enough, and being a simple person, I can usually give a simple, understandable answer.

I would rather have LP o2 going into a bottle (Like 400psi), than try to boost o2 to 3000+psi, wouldn't you?

Besides, boosting is a PITA at higher pressures when the supply tank(s) are low.

Most handle 02 with respect, but are not scared to death of it for no reason.

Although I am sure I could explode something even with 400psi going into a scuba tank if I was sloppy enough, I don't believe it's a big risk and so it's just done slowly with a respect for what you're doing.

Don't bother re-inventing the wheel dude. It ain't really broke, but it's good you're thinking about it just the same.

Pure oxygen is also flammable, which is another consideration.

PPB is also cheaper to get started, where as a membrane system costs more.

Don't forget, tech divers use mixes of 50% or higher, right up to 100% ALL THE TIME. So if they can safely boost a bottle to 3000 psi of pure O2, putting 400 psi in it for all of 10 minutes before you start putting the air on top is a breeze.

Charlie99:

Topping off with O2 would require the shop to be able to deliver O2 at 3000psi. That would require them to have a compressor that can handle 100% O2, and at 3000psi or more.

The advantage of O2 1st is that it keeps pressures lower. That makes it so you can just take a bottle (or several hooked up as a cascade) of O2 and just let it bleed into the tank being filled rather than having to have a compressor that is O2 clean.

Charlie Allen

disclaimer: I'm not a gas blender, nor do I run a shop. But the answer is simple enough, and being a simple person, I can usually give a simple, understandable answer.

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Thanks for your 2cents. Now I'd understand. I knew there was a SAFER way. Just not economical.

Sounds like it is a business decision to put O2 first then top off with air. I could just imagine the initial cost plus operation cost of a 02 cleaned compressor capable of pushing 3500psi.

Jimmer:

Don't forget, tech divers use mixes of 50% or higher, right up to 100% ALL THE TIME. So if they can safely boost a bottle to 3000 psi of pure O2, putting 400 psi in it for all of 10 minutes before you start putting the air on top is a breeze.

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I would guess that most people don't boost pure O2 outside of Florida. I only get about 2000-2400 psi in my O2 bottle.

Playing with O2 is never a breeze- it might be routine, but it deserves respect. The first rule of gas blending is don't blow yourself up.

Scubastud16:

Pure oxygen is also flammable, which is another consideration.

PPB is also cheaper to get started, where as a membrane system costs more.

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Pure oxygen is not flammable, it will not burn in and of itself, it is 1/3 of the fire triangle which is composed of 1. oxygen, 2. fuel (something that will burn or react with oxygen) and 3. heat to start the process.

captain:

Pure oxygen is not flammable, it will not burn in and of itself, it is 1/3 of the fire triangle which is composed of 1. oxygen, 2. fuel(something that will burn or react with oxygen and 3. heat to start the process.

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My mistake, thanks for clearing it up.

do it easy:

I would guess that most people don't boost pure O2 outside of Florida. I only get about 2000-2400 psi in my O2 bottle.

Playing with O2 is never a breeze- it might be routine, but it deserves respect. The first rule of gas blending is don't blow yourself up.

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Yeah I don't see a lot of guys with 3000 PSI, but there are a few shops that will boost. Of the 4 shops I deal with a bit, 2 will boost, 2 cascade. Most guys just accept the 2000-2400 like you said. Either way the point still stands, lots of guys have dedicated O2 bottles, with 2000 PSI or more in them all the time. There I don't worry about PPB into a clean bottle with a blender that knows what he is doing (aka that respect thing you were talking about )