Nitrox Only

[Superlyte27]

You can use them for reg air fills but once you switch the tank to regular air you can't switch it back to nitrox unless its O2 cleaned. Both of my tanks had nitrox stickers on them when I bought them but I switched them over to regular air (just peeled the stickers off).

PS...The stickers are a PITA to get off if they are all scratched up.

HUH? That's not accurate.

---------- Post added November 30th, 2013 at 10:00 PM ----------

Wow, is there really this much bad information about air and nitrox out there?

-edit-
nevermind. I started to respond, but just too tired.

 

[redacted]

I don't understand why dive shop continue to take the risk involved in PP blending in tanks they do not have complete control over. It seems to me that technique should be reserved for DIY blenders.

 

[MXGratefulDiver]

Happy Thanksgiving,

Recently, I bought a couple of tanks that had stamped on the valve stem "Nitrox Only". Can anybody provide a good reason why I should only fill these tanks with Nitrox?

Debajo

Simple answer ... no. See more explanations that will follow ...

... Bob (Grateful Diver)

---------- Post added December 1st, 2013 at 08:13 AM ----------

You can use them for reg air fills but once you switch the tank to regular air you can't switch it back to nitrox unless its O2 cleaned. Both of my tanks had nitrox stickers on them when I bought them but I switched them over to regular air (just peeled the stickers off).

PS...The stickers are a PITA to get off if they are all scratched up.

Not true ... if you fill them with regular air that's been run through filters used for nitrox filling, there's absolutely no reason why you can't switch back and forth between air and nitrox.

... Bob (Grateful Diver)

---------- Post added December 1st, 2013 at 08:19 AM ----------

Is the reason that oxygen is added first because O2 tanks are only filled to 2000 psi?

It's because pressurizing cylinders produces heat ... so adding the oxygen first means that as air is introduced, increasing pressure and heat, it's producing a diluted nitrox mix at the source of that heat (more on that later).

And FWIW - deco bottles filled with oxygen are commonly filled to 3000 psi. The key is filling it very slowly, so that heat doesn't accumulate as it pressurizes.

... Bob (Grateful Diver)

---------- Post added December 1st, 2013 at 08:25 AM ----------

Now what if the Nitrox fill doesn't come from a partial pressure fill? If a membrane system was used for example, couldn't the air used still be "un-clean"?

Partial pressure blending is still the most common way to mix nitrox, because it's the cheapest method. But more and more dive ops are nowadays either using membrane systems, blending sticks, or banking nitrox ... so that what gets introduced into your cylinder is always 40% or less. But that wouldn't introduce "un-clean" air into your cylinder ... it would only mean that if your cylinder were already contaminated due to lower-grade air it wouldn't really be an issue as the cylinder is pressurized.

... Bob (Grateful Diver)

---------- Post added December 1st, 2013 at 08:29 AM ----------

The danger comes from the fact that many substances will ignite when they come into contact with a high enough percentage of oxygen. Most petroleum products like oil, grease, Vaseline, and even some hand lotions will ignite when exposed to a high enough percentage. Things like these coming into contact with pure oxygen is where the risk of fire or explosion comes from.

In a scuba cylinder the concern is accumulated hydrocarbons inside the cylinder or petroleum-based lubricants (o-ring grease) and sealants (O-rings) inside the valve. If the tank had been prepared for oxygen use, the valve must have also been cleaned, O-rings replaced with a non-butyl material (Viton, etc), and grease (crystalube, etc).

Hydrocarbons coming into contact with oxygen won't start a fire unless heat is introduced ... more on that later.

... Bob (Grateful Diver)

---------- Post added December 1st, 2013 at 08:31 AM ----------

No source of ignition is necessary. These substances can self-ignite. This may be even more likely under pressure.
https://www.google.com/search?as_q=&as_epq=Petroleum+based+products%2C+oils%2C+grease+and+lotions+can+self-ignite+in+the+presence+of+&as_oq=&as_eq=&as_nlo=&as_nhi=&lr=&cr=&as_qdr=all&as_sitesearch=&as_occt=any&safe=images&tbs=&as_filetype=&as_rights=#as_qdr=all&lr=&q=can+petroleum+and+pure+oxygen+self-ignite+

No they can't ... you're misunderstanding what that's telling you ... (more on that in a bit)

... Bob (Grateful Diver)

---------- Post added December 1st, 2013 at 08:33 AM ----------

http://www.usfa.fema.gov/citizens/home_fire_prev/hazmat.shtm

Quote from the above link: "Keep oil, grease, and similar petroleum-based products away from oxygen valves. They can cause a spontaneous explosion."



http://www.wakehealth.edu/Health-Central/NMR/Home-Medical-Oxygen-Safety-Tips/


Quote from the above link:
"Oils, grease and petroleum products can spontaneously ignite when exposed to high levels of oxygen. Also avoid using oil-based lotions, lip balm or aerosol sprays."

It's poorly worded ... and written by someone who clearly doesn't understand the laws of physics ...

... Bob (Grateful Diver)

 

[boulderjohn]

I don't understand why dive shop continue to take the risk involved in PP blending in tanks they do not have complete control over. It seems to me that technique should be reserved for DIY blenders.

I think a lot of people who live in areas with a lot of local diving appropriate for nitrox do not know what it is like to live in an area where that is not true. Here in Colorado, and I suspect in most of the states not adjoining an ocean or a Great lake, there is little call for nitrox. In our state, It's hard to believe anyone uses it for truly local diving. People who want tanks filled with nitrox or trimix are planning to take those tanks elsewhere, with that elsewhere requiring something like a 6 hour drive. Consequently, most shops do not offer nitrox or trimix fills at all. Those that do could never afford to keep it banked or purchase a system that pre-mixes it. Most of those cannot even justify the cost of a booster. So, if you want nitrox around here, you need to find a shop that will partial pressure blend it for you without a booster. The odds are that shop is doing it at a loss because it sees a local nitrox user as an avid diver they want to keep as a happy customer.

And FWIW - deco bottles filled with oxygen are commonly filled to 3000 psi. The key is filling it very slowly, so that heat doesn't accumulate as it pressurizes. )

This requires a booster. Otherwise you are limited to the pressure in your oxygen supply tank, which usually starts at 2,400 PSI. As that tank pressure drops from use, your ability to fill an oxygen tank drops with it.

 

[MXGratefulDiver]

Discontinuities in flow during cylinder filling (for example, at the edges on the end of the dip tube) can result in extrememly small bursts of high temperature which dissipate quickly but which can be enough to ignite nearby hydrocarbons. At any rate, this is what my advanced deco instructor told us. He said momentary spikes to one thousand F were possible.

Bryan

PS. No, I have no citation or Rubicon link to back this up. Anyone?

OK, now we're getting to the point. When pressurized oxygen is introduced into the cylinder, heat can be created, either through adiabatic process as the oxygen leaves the dip tube and enters the much larger volume of the cylinder, or by sharp edges inside the cylinder due to rust or pitting. This is why (a) oxygen cleaned tanks have lower tolerances for pitting and (b) you fill the tank very slowly when adding oxygen. This is what produces the heat ... there's nothing "spontaneous" about it.

... Bob (Grateful Diver)

---------- Post added December 1st, 2013 at 08:44 AM ----------

Analytical Chemist, Inc. http://www.airanalysis.com/Home_Page.html specializes in the analysis of compressed gases and life support systems for Scuba Diving, Fire and EMS workers and the Military. The following is copied from this page: http://www.airanalysis.com/Oxygen_Compatible_Air.html

　
"Adding compressed air directly to pure oxygen brings the danger of fire caused by ignition of the piping which becomes fuel for the oxygen. IGNITION COMES FROM hot spots inside the piping or SCUBA flask which can be caused by particle impingement AND OXIDATIVE HEATING OF ACCUMULATED FUGITIVE OIL. Sampling, analysis and specification protocols for this differ between laboratories; the differences are significant."

"The least expensive way to prepare oxygen enriched air (i.e. Nitrox) is to add compressed air directly to a cylinder which contains pure oxygen; this is called Partial Pressure Blending. Unfortunately, partial pressure blending is potentially dangerous. A fire in pure oxygen starts with ignition by particles hitting/scraping the piping walls as they rapidly shoot through the piping OR BY SPONTANEOUS COMBUSTION OF AN ACCUMULATED OIL SPOT. Therefore, air to be mixed with pure oxygen must meet purity criteria which are more stringent than for normal SCUBA air."

　
I don't see how it gets any plainer than that guys.

It's plain enough if you understand what it's really telling you. But "spontaneous" isn't a good choice of words. Generally, spontaneous combustion is caused by a chemical reaction due to mixing two substances that produces heat. In this case, heat is created due to pressurization, not chemical reaction ... and therefore it's not spontaneous ... it's caused. If you understand the cause ... which is due to the rate of flow, you can avoid the combustion by simply reducing the flow rate.

... Bob (Grateful Diver)

---------- Post added December 1st, 2013 at 08:45 AM ----------

Spontaneous combustion - Wikipedia, the free encyclopedia



Stuff can spontaneously self-ignite.

Yes ... but putting pressurized oxygen into a scuba cylinder isn't that stuff ...

... Bob (Grateful Diver)

---------- Post added December 1st, 2013 at 08:46 AM ----------

---------- Post added November 30th, 2013 at 10:00 PM ----------

Wow, is there really this much bad information about air and nitrox out there?

-edit-
nevermind. I started to respond, but just too tired.

I feel your pain, but it's morning here and I'm not tired enough yet ...

... Bob (Grateful Diver)

---------- Post added December 1st, 2013 at 08:47 AM ----------

I don't understand why dive shop continue to take the risk involved in PP blending in tanks they do not have complete control over. It seems to me that technique should be reserved for DIY blenders.

Two reasons ... (1) cost ... it's cheaper than the alternatives, and (2) inertia ... they've always done it that way

... Bob (Grateful Diver)

 

[101recon]

I guess I should just start over and more clearly express my point. I had no idea this debate would escalate to this point. My initial post was merely an attempt to add to boulderjohn's post and offer another reason why a tank might be considered "contaminated" by pointing out that there are dangers associated with pure oxygen coming into contact with certain types of grease or oils. My later posts were quick attempts to grab something off Google to explain what I was talking about. I was busy yesterday.Obviously I failed and will properly address that now.

I will admit that I do not know the correct terminology for the process. Spontaneous combustion or self-ignition is the best way I know how to describe it because this is how it has been described to me. I will do my best to find out the proper terminology. Until then I will just use terms like "combustion" or "ignite".

A little background....I was first warned of this danger over 30 years ago while I was in school learning to weld. In the 30 years since that time I have seen,read, and heard this warning countless times. I've seen it on OSHA posters, safety pamphlets from welding gas suppliers, and many other safety documents that pertain to welding and cutting. I have put in a few pipe lines for oxygen use and even discussed this issue of "combustion" with safety inspectors.

@ NWGratefulDiver.......Bob, ordinarily I would not even think of disputing you on Scubaboard. You are one of the most knowledgeable and experienced people here. Scuba is your area of expertise and I have learned much from reading your posts. My knowledge and experience of scuba is no where near yours. But, concerning the topic of pure oxygen and grease or oil causing "combustion", I must disagree on this matter. I believe my knowledge and experience to be correct. In fact, I believe we are discussing two different things.

First, YES, all combustion does need a source of ignition. I should have said pure oxygen and grease/oil does not need a SEPARATE source of ignition. And YES, flow rate can cause "combustion". But that's not what I'm talking about.

Pure oxygen and some types of grease/oil can create their own source of ignition. They can "ignite" without an external spark or source of heat. They create their own heat or "ignition source" through rapid oxidation. Pure oxygen causes some oils and greases (hydrocarbons) to oxidize rapidly, fast enough to reach kindling temp.

This Safetygram from Air Products ( A welding supplier) explains this in detail: http://www.airproducts.com/~/media/Files/PDF/company/safetygram-33.pdf

For those who don't care to read through a 5 page PDF document here are some relevant quotes followed by links to the pages or documents they were taken from:
"almost everything made up predominantly of carbon and hydrogen (coal, wood, petroleum products) has a "kindling temperature". Once that temperature is reached, "oxidation" suddenly becomes "burning", which then proceeds to produce enough heat to maintain the reaction until the supply of oxygen or fuel runs out" http://www.esabna.com/euweb/oxy_handbook/589oxy3_1.htm

"Oxygen can react explosively with oils and greases. People have been injured or even killed when pumps, engines, tyres and pressure equipment have been blown apart by the explosion. Oxygen can also cause other materials to ignite spontaneously." http://www.hse.gov.uk/pubns/hse8.pdf

"Even the smallest trace of grease or oil in an oxygen enriched atmosphere will have the
potential to self-ignite." http://www.wilhelmsen.com/services/maritime/companies/buss/BUSS_Pressroom/Documents/Understanding%20Oxygen.pdf

"Grease or oil that oxidizes very slowly in air will burst into flame in pure oxygen." http://www.niu.edu/ehs/chemical/compressedgas.pdf
"Any part of equipment used in operations with oxygen must be kept away from oil and grease. This equipment includes fittings, caps, valves, couplings, regulators, hoses and any other apparatus. Because pure oxygen can cause fire when it comes into contact with grease or oil " https://www.osha.gov/dte/grant_materials/fy09/sh-18796-09/sheetmetal.pdf

　
My apologies for the confusion and my failure to be more clear in my previous posts. (Most of you anyway.)