Pseudo Partial Pressure Blending using HP 300 cuft rented tank?

[Basking Ridge Diver]

So this is one off and I could not find a thread that was similar.

Started looking at the some of the more popular DIY sites on how to create Nitrox. My question to me is straight forward but I am sure the answers coming back will be - so here goes...

I am trying to understand why in Pseudo Partial Pressure Blending - "SCUBA tanks and fill system need to be Oxygen Clean Certified".

It makes sense to me why the tank needs to be Oxygen Clean - you will be filling directly from the O2 to the Scuba Tank - slowly and cleanly.
What I don't understand is why the compressor or fill station if it is pumping Grade E air after the O2 has been transferred but is not O2 Clean is required... Is there a resource or a simple explanation for why the fill system to be O2 Clean?

So I am clear I am suggesting two separate steps - one to fill the scuba tank with O2 and the second step to top off the scuba tanks with air.

I am asking because I have access to a fire house compressor and 6 6000psi Cascade Air bottles - 3 banks of 2 bottles each. I know they are not "O2" clean but they produce Grade E air for our needs at the fire house.
I am thinking I could rent a cylinder of Industrial O2 - partially fill the scuba tanks with an O2 clean Whip and ensure the tanks are O2 clean. But then I would like to top the tanks off with the compressed air at the firehouse.

Obviously I don't want to make rookie mistakes and get hurt in the process. Can you point me to a source or give me an idea of - No this is crazy because the O2 will cause an explosion by not being O2 Clean with the fill system... If I rented a mix of less than 100% would that be better or "safer"?

Any comments or suggestions would be most welcome.

 

[TrimixToo]

The reason is simple. A non-OCA compressor will contaminate the tanks, and more importantly the valves, with oils and other hydrocarbons that should not be present in a PP blending system. These contaminants can lead to oxygen fires during the O2 fill blending phase, which can be very brief (contaminating the breathing gas) or not quite so brief (causing full-blown O2 fires that look a lot like explosions to the casual observer).

It could well be the case that the firehouse system makes OCA, but without testing you cannot be sure.

 

[tbone1004]

check the report on your scba compressor, it may well be OCA if there is no oil present *there isn't on the SCBA compressor that I use

 

[Basking Ridge Diver]

These contaminants can lead to oxygen fires during the O2 fill blending phase, which can be very brief (contaminating the breathing gas) or not quite so brief (causing full-blown O2 fires that look a lot like explosions to the casual observer).

Thank you and a question -

If I fill the scuba tank with O2 and that tank reads say 1000 psi. I fill from the cascade system which is about 6000psi for this example. It makes sense to me that the "air" would push into the O2 scuba tank due to opposing pressure differences. Where would the contamination come from?

My brief understanding is most fires have occurred while doing true PPBlending - the O2 is in the booster or compressor and is being put under pressure and heat. This is what seemed to cause most if not all the fires.

So my question would be is there a flaw in my thinking?
Thank you

 

[northernone]

It's the next fill that might catch you.

How I was explained it: Hypothetically, and technically, after your first non o2 safe air top up you now have a contaminated cylinder as you've blown air through a non o2 clean environment first (fill station bank) and filling the tank so that's no long o2 clean.

If this is real life significant or not I won't speculate upon.

Regards,
Cameron

 

[RayfromTX]

I think accumulation in the target tank from air sources that have traces of hydrocarbons in them are where you get your risk and it can come in the form of combustion on the subsequent fills. You may not know as it may just create trace combustion byproducts like CO if it doesn't create enough heat to combust the materials in the valve and tank. These are the thoughts that sent me in the direction of continuous blending with a Rix compressor. Now I bank 32%

 

[TrimixToo]

Thank you and a question -

If I fill the scuba tank with O2 and that tank reads say 1000 psi. I fill from the cascade system which is about 6000psi for this example. It makes sense to me that the "air" would push into the O2 scuba tank due to opposing pressure differences. Where would the contamination come from?

My brief understanding is most fires have occurred while doing true PPBlending - the O2 is in the booster or compressor and is being put under pressure and heat. This is what seemed to cause most if not all the fires.

So my question would be is there a flaw in my thinking?
Thank you

Someone already answered accurately, but let me offer a rephrasing of what was said.

Starting with an O2-clean tank, what you propose is perfectly safe. *Once.* Every successive non-O2-clean air fill increases your risk, starting with the second mix you blend through that same valve into that same tank, until they are O2 cleaned again. Then the risk step function starts anew. In addition to pressure-induced heat is the hazard created by particle impingment, particularly at sharp gas path angles, which abound in the typical scuba valve. Very small O2 fires that extinguish themselves nearly instantly can contaminate the breathing gas.

(If you were a trained gas blender, you'd have gotten an earful about this along with material to read. I'm happy to pay for certs that keep me alive, unlike the "underwater basket weaving diver" certs available from some agencies. This is a cert that keeps me whole, and was well worth the money.)

I have my own fill and blending station, and I fill everything from air to Trimix. I maintain it, have the air tested, and I keep up on filter changes. Every single tank I own that ever contains a breathing gas is O2 clean. This costs money and time. But it's *me* breathing the gases. And, I'm a technical diver, which multiplies the risk of any gas contamination. (200'-ish dives are the norm for the past few years, with deeper excursions.)

So, to paraphrase that hair dye commercial (was it "Preference" by Loreal?), "it's expensive, but I'm worth it."

If you're cutting corners, you might get away with it forever; I know people who cut corners and have skated for years. But a big part of diving is risk management. This is a risk that CAN be managed. Why take it? "You're worth it," too. Right?

My 2 PSI are, as usual, worth what you paid for them...

 

[Basking Ridge Diver]

Thanks all for your input I am in the beginning stages of trying to learn and understand - so my questions may seem basic and straightforward but I have a lot of catching up to do... I just bought the Oxygen Companions Guide but it is in route and has not hit my doorstop yet.

So then what about this Nitrox Stik I am reading about?

Mix the O2 and Air prior to getting to the compressor and fill with less than 40%? Seems safer - OCA is not needed or required and may fit my needs.
Are there cautions here too?

 

[RayfromTX]

There are always cautions but I would recommend reading the oxyhacker first and then ask questions you may have after that. It really will make most or all the questions you have now go away. I also visited the archives of perhaps 6 different forums going back about 15 years to get info that I later compared the oxyhacker and my common sense. My questions after that were much more specific. You may even get more meaning from the answers you have received here after that research. There will be some info that is IMO too risky and some that is fear based. There is a lot to learn and although forums can offer a lot of info, it is far to complex a subject to learn by asking a general question like "what about this Nitrox Stik I am reading about?" Chapters have been thoughtfully written to answer that question, while my answer would be that it is an expensive version of something you can build yourself. That is a very incomplete answer because it would be easy to build a continuous blending stick without the safety features I believe to be important. Welcome to the world of filling your own.

 

[tbone1004]

@Basking Ridge Diver no OCA required up to 40%, no O2 cleaning required on the tanks because they never see 100% O2. Much more cost effective to do it this way.

The biggest concern is having a cutoff solenoid or something similar tied from your compressor to your blend stick so the o2 bottle doesn't keep dumping O2 into the compressor while it is off. This would be bad