Learning to do maintenance on my own gear, Apeks

[Tanks A Lot]

He is right that halogenated polymers will emit harmful substances when they burn. Specifically, fluorinated ones, such as FKM, can emit carbonyl fluoride (COF₂), which is incredibly toxic. Its LC₅₀ is 360 ppm, making it ten times more deadly than carbon monoxide, which has an LC₅₀ of 3700 ppm.
There is a fallacy that often enters the conversation in SCUBA: “Well, if my regulator burns, the toxicity of the gas is the least of my problems!” This statement misses the mark entirely. It assumes that combustion is always catastrophic, which is not true at all. Anyone who has worked on enough oxygen equipment will eventually come across charred high-pressure seats, where partial combustion occurred. The user likely never noticed a thing. In such cases, trace amounts of toxic gases can be produced.

There have been high-profile cases in the medical industry where people died in hospitals. One such case was in France in 1993, where two people died after a PCTFE seat burned in a manifold without anyone noticing. PCTFE is even worse than fluorinated polymers alone, as it also contains chlorine. Its combustion products can include phosgene, which was responsible for over 85,000 deaths during World War I. France subsequently banned PCTFE in breathing oxygen equipment.

If you made it through the above, you might be wondering: “So, I either use something that doesn’t burn easily, but if it does, it emits deadly gases, or I use something that ignites rather quickly?!” And that, in essence, is the conundrum faced by oxygen gas engineers. There is no easy way out. The current consensus seems to be that FKM is the better choice, even with the added risk of toxic gases. The risk of EPDM or NBR catching fire outweighs it.

But the designer takes this into account, because designing for oxygen service is much more than just chucking in a few FKM O-rings and calling it a day. How does the gas flow? Where are the critical areas? Where do I see high flow rates? Where will adiabatic heating take place? What if the high-pressure seat ignites, have I designed the system so that O-rings, springs, and diaphragms are out of harm’s way?

I write all of the above not to put you off your stated goal, quite the contrary. But I would like you to approach the subject with the right mindset. The SCUBA industry is often a poor example in many areas. It’s a small, virtually unregulated industry. Look to other sectors, such as the medical field, and try to extrapolate from there.

A very basic approach to cleaning your SCUBA gear for oxygen would look as follows:

1. Wash the parts with dishwasher detergent. Any brand will do, as this is just the pre-clean. You might prefer non-scented types, but that’s a personal choice.
2. Rinse the parts generously.
3. Inspect the parts carefully. This will heavily influence the steps below. Are the parts coated with silicone grease? If so, that must be removed before the acid bath, as otherwise, the acid will just sit on top of the grease.
4. Alkaline bath. Specific formulas exist, but in a pinch, you can mix something with washing soda. Surfactants play a key role here. Dishwashing concentrates often contain anionic surfactants, which are excellent for alkaline cleaning. A little goes a long way.
5. Rinse again.
6. Acid bath, hopefully not with vinegar. Surfactants are again key. Look for products containing amphoteric or non-ionic surfactants. Some dishwashing liquids may have these. Don’t worry about shifting the pH with these additives, you’d need ludicrous amounts to make a difference.
7. Rinse yet again!
8. Neutralisation bath with baking soda.
9. Clean the parts with Simple Green or a suitable alternative.
10. Final rinse, preferably not with tap water. But don’t stress too much if distilled water isn’t available; although most hardware stores carry it.

Things to keep in mind:
In SCUBA, the worst offenders for hydrocarbon contamination are silicone grease and what it traps, along with compressor oil. Despite what many people claim, you cannot remove silicone grease with dishwashing liquid, Simple Green, acid baths, or most alkaline cleaners. Silicone grease contains PDMS oil, which has a siloxane backbone, a structure that is largely unaffected by these cleaners. You’ll need something like toluene or xylene to remove it. This is a major consideration when converting something for oxygen service that has previously been used.

Modern compressors run on synthetic oils, which alkaline cleaners are well suited to remove. However, if you’ve used gas from a compressor running on mineral oil, this is not the case. Mineral oils do not have ester links, so they do not saponify and are largely unaffected by alkaline cleaning.

In SCUBA, oxygen cleaning has often been conflated with “Just use Simple Green!”, but that is nonsense. You must approach the topic systematically, and if you suspect a certain impurity is present, use the appropriate removal method. Simple Green is great, but it is not the one-size-fits-all solution that many make it out to be.

Do not forget to test your parts for cleanliness. While none of the tests will detect trace amounts, water shake tests, beading tests, and visual inspection can provide peace of mind.

Again, I by no means want to put you off your stated goal. Quite the opposite. But it does mean you must approach the subject carefully and diligently. Oxygen cleaning isn’t some kind of magic that only certain people can do, but it does take patience and attention to detail.

 

[Mobulai]

Whenever I see a notification for a post by @Tanks A Lot I know I’m in for a treat of a learning experience; Tank you Tank you for that awesomely educational „rant“ (and fixing a bunch of misconceptions on my side)
(More than I bargained for)

 

[DANDM]

This was very interesting and informative, thank you. I do have a couple of questions.

4. Alkaline bath. Specific formulas exist, but in a pinch one can mix something with washing soda. Surfactants play a key role, dishwashing concentrate often contains anionic ones, which are excellent for alkaline cleaning. A little surfactants go a long way.

Pardon my ignorance, but what is the goal of an alkaline bath? It doesn't seem like a common step for non-oxygen cleaning servicing. What pollutants is it aiming to clean? You mentioned compressor oils, but why wouldn't a thorough (extreme) simple green/detergent wash not achieve this?

With citric or phosphoric acid, what concentrations and time would you aim for?

 

[Tanks A Lot]

Alkaline cleaning is squarely aimed at removing many oils and organic residues. When cleaning something for oxygen, there are three main areas you should think about. Admittedly, these are rather vaguely defined, but broadly fall into opposite ends of the pH scale and the square middle.

1. Alkaline cleaning​

Alkaline cleaning works on many organic residues. The OH⁻ ions from the alkaline, which has a very high pH (many OH⁻ ions), are apt at removing a lot of organic residue. This works because the OH⁻ ions break down the triglycerides which are present in many oils, fats and greases. The products are glycerol and what we commonly call soap. These are water-soluble, which oils and greases are not, and it allows us to remove the converted oils and greases with the water.

People often conflate dishwashing liquid and other household soaps with alkaline cleaning, but that is incorrect. The early soaps we produced (already thousands of years ago) were truly alkaline. But today’s soaps are often virtually pH neutral. Imagine that wasn't the case. Every time you’d be doing the dishes or go into the shower, you would douse yourself in a strong alkali. Clearly, that isn't the case.

While some soaps and shampoos may be very, very slightly alkaline, the bulk of them aren’t at all. They often work with surfactants. These don’t just lower the liquid’s surface tension, which is essential for proper cleaning, but they also encapsulate dirt particles in something called micelles. These are tiny structures that have a hydrophobic head and a hydrophilic tail. In essence, the hydrophobic head buries into the grease particle, while the hydrophilic tail sticks into the water. Once enough of these have buried into a dirt particle to form a sphere around it, it’s called a micelle. This micelle can easily be suspended in water, as the outer layer is made up of the hydrophilic tails. Thus, grease and oils can be suspended within water, which they normally can’t. This is often referred to as emulsification.

The way surfactants clean is very distinct from how alkalines work. One chemically breaks down the oils and fats, while the other suspends them in solution.

If you think about alkaline cleaners, think about sodium hydroxide (lye), oven cleaner, and similar substances. These things truly cut through grease, unlike most household soaps. Something I have used when I wanted to mix something myself is sodium carbonate, often called washing soda or soda ash. You won’t reach extreme pH values, as it maxes out around 12-ish or so, but it is still a much better true degreaser than many soaps. True alkaline cleaning must be approached with a similar mindset to acid cleaning. This stuff is very harsh on your skin and causes similar burns to acids. Especially industrial oven cleaners can go up to close to 14 on the pH scale, which eats through skin.

So no, soaps generally do not count as alkaline cleaning at all, very few truly are. In a nutshell, alkaline cleaning aims to remove many greases, oils and fats. These often contain hydrocarbons, which easily ignite.

2. Acid cleaning​

Acid cleaning works on mineral residues. When you go diving in saltwater, or rarely very mineral-rich freshwater, mineral deposits will be left behind on your equipment. That white stuff you see on your regulator? That is calcium carbonate (CaCO₃). This stuff forms very differently from, say, common table salt (NaCl). While the latter more or less simply precipitates out of evaporating saltwater, CaCO₃ is formed by some easy-to-follow chemical formulas, which involve carbon dioxide, carbonic acid, carbonate and calcium. The thing about CaCO₃ is that, unlike many other salts that form from saltwater evaporating, this one isn’t water-soluble, not even a bit. Acid cleaning is almost exclusively used to remove the CaCO₃ salts from equipment. It has very little other use. It does so by introducing H⁺ ions, which essentially reverse the forming formulas (simplified a bit).

Now this also makes for the most satisfying part of any cleaning, because you can see it, it bubbles and makes the gear shiny. That is why it often has special status for technicians, it is something that they can see. But the truth is that, at least as far as oxygen cleanliness is concerned, this type of cleaning pales in importance compared to alkaline cleaning, which removes hydrocarbons.

For a soaking solution, I would dilute phosphoric acid to around 10%. For an ultrasonic cleaner, aim at 5% or below. Citric acid does well in similar concentrations, although I might slightly increase them by a couple of percentage points. Citric acid is inherently weaker as an organic acid compared to the mineral phosphoric acid. Temperature is a key ingredient in any thoughtful cleaning process, think about the Sinner’s Circle, which I won’t expand on here. Aim for 50°C to 60°C, but avoid temperatures that would cause too much vapour. Acid vapour is nasty, even when it comes from "safe" household acids such as phosphoric or acetic acid.

You could use vinegar, it works as "well". But while phosphoric acid and citric form insoluble chromium compounds, acetic acid produces highly soluble ones. So no, phosphoric acid and citric acid will not leave the chrome untouched, they also attack it. The difference is that they generally attack the surface layer and then form a sort of protective barrier with the insoluble products. This isn’t a perfect protection layer, as it’s not a hard patina, but it gives at least some very crude protection.

All acids will attack chrome, but some do so more than others. And contrary to what many would like you to believe, it is not just down to the concentration of the acid, but often what products it forms with chromium oxide.

The time in the acid should be as little as possible, because like I mentioned above, they all attack chromium oxide to some extent, it’s just the nature of the H⁺ ions. With 10% phosphoric acid, it will not take more than a couple of minutes. Watch your gear and the white deposits of CaCO₃. If these are gone and the gear has stopped bubbling, you are done with acid cleaning. It does not clean anything else other than these mineral deposits.

3. Removal of anything that alkalines and acids don't touch​

Alkalines remove many greases and oils, acids remove minerals. But stuff like silicone grease needs another approach, such as toluene or xylene. Equally, old PFPE grease is impossible to remove without a specialized cleaner. The thing with PFPE grease is that it is fluorinated, and the chemical saying “like dissolves like” rings true here. Short of industrial substances like Tribolube® KO², Christo-Kleen 1, or Krytox MS-250 Re-Move™, I have not found something that reliably removes them. The trouble is that to remove such a halogenated grease, one likely needs a halogenated solvent and these are not exactly common in the household. It’s something I have not found an easy DIY solution for.

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You rightfully ask why alkaline cleaning might be in order when Simple Green or similar is used. There are two trains of thought here. The first would be: it isn’t necessary.

This might be true under most circumstances, especially if we just look at SCUBA. If you look at the MSDS of Simple Green Extreme, you’ll find that it has some very basic alkaline properties, often due to triethanolamine and potassium silicate. A lot of their cleaning comes from surfactants or propylene glycol butyl ether. These are very apt at removing oils and greases, but are not alkaline. When heavily diluted, it won’t leave you with much alkalinity. So yes, these cleaners are great at cutting through most greases and dirt, but more so due to the surfactants and propylene glycol butyl ether, and they often suffice. But there is the slight off-chance you’re working with something that is truly heavily contaminated, such as an unknowledgeable person using marine grease or some such thing. Admittedly, these are edge cases, but something to keep in mind. Simple Green is not a magic bullet.

Secondly, and far more important for me, the stuff isn’t cheap, at least not for me. Reusing it can have real value. If you remove pretty much all oils and greases and other contaminants before your gear touches the Simple Green, you can keep using it for many cycles. I like to use it as my last step, well, second-last if you count the final rinse. This has the added virtue of cleaning any residue that my earlier cleaning may have left. Silicone grease is terrible, but so are the residues of toluene or xylene I may have not fully removed. Think about the Simple Green (or whatever other compound in its place) as the final step to truly remove any leftover that you have missed earlier. A last step to give you peace of mind, not a magic bullet.

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None of what I have suggested above is what was used back in the old days. There are things that clean greases and oils far better than what I described, and there are acids which outshine phosphoric acid by a mile. Carbon tetrachloride and trichloroethylene were extensively used for degreasing. Nitric acid and chromic acid are apt at removing calcium carbonate and more importanly, they do not attack the chrome layer, or better said somewhat reform it.
But these are not used anymore, as they are either very toxic, super harmful to the environment, carcinogenic, or all three. They are truly nasty substances which I would steer well clear of.

 

[Tanks A Lot]

With regards to acids, I find the following pictures illuminating. These were chrome layered brass rings I cut into pieces and half immersed in acid. I have done this a few times, always with the same results.

Acetic acid

Citric acid

Phosphoric acid

It should be crystal clear to anyone that acetic acid should be the last choice for what we are doing. Unfortunately you will hear the vinegar suggestion ad nauseum, even explicitly stated in many service manuals. It has become somewhat religious for many people, god knows for what reason....
It is the worst choice that you have at your disposal. If you ever bothered to look at what the commercial cleaners, such as Cressi’s ALR 5000 or GSM’s Regulator Cleaner Solvent contain, you won't be surprised to find that they are always based on phosphoric acid.

Vinegar is not the right choice and anyone who ever bothered with citric acid or phosphoric will never go back, ever.

 

[DANDM]

@Tanks A Lot brilliant response, thank you for taking the time to write such a detailed response. It's been a few years since I studied chemistry as part of my degree, and I was by no means talented at it, but your post truly juggled my memory and made a lot of sense. I even made soap during chemistry, so your explanation really made sense to me!

I'm adding citric acid to my shopping list. Will have to check if phosphoric acid is accessible and affordable down under in my corner of Australia.

It's great to read how oxygen cleaning works. I've been trying to explore whether that's something I want to do. I've learnt to service my gear from a Hog service clinic course and the RSlingers workshop, but oxygen cleaning is one step that I am a bit concerned about. I'd love to do it, but every manual just goes on about getting proper training. Unfortunately I can't find anyone offering an oxygen cleaning course in Australia. The only course is the TDI Oxygen cleaning course, but it requires the gas blending course as a prerequisite (paying for a course I don't intend on using?!) and having to work for a dive shop. That's just not something I'm interested in investing at the moment. Maybe with a bit more reading and preparation I'll take the challenge to O2 clean my regs some day, for now I'll just inform myself about the process.

 

[Tanks A Lot]

[...]
I'm adding citric acid to my shopping list. Will have to check if phosphoric acid is accessible and affordable down under in my corner of Australia.
[...]

Phosphoric acid really isn't hard to come by, it sounds scarier than it actually is. A very brief search turns up this example for Australia, where the price seems to be on par with what I would expect, maybe slightly on the higher end.

While A$18 might be pricier initially than citric or acetic acid, don't discount the fact that this is an 85% concentration and will last you quite some time, especially considering that you can reuse your solution. It probably ends up being cheaper than vinegar in the long run.

Your solution will lose strength over time as the H⁺ ions are consumed, but a great and inexpensive way to stay on top of this is to buy pH test strips. These are very cheap and will also last a long time. With them, you can periodically check whether your acid solution has lost too much potency and mix up a fresh batch if needed.

[...] but oxygen cleaning is one step that I am a bit concerned about. I'd love to do it, but every manual just goes on about getting proper training. [...]

The topic of oxygen cleaning has been approached by many in SCUBA with a certain degree of self-aggrandizement: “Only I know how to oxygen clean something—if you do it, you’ll die!” It’s a mentality that, unfortunately, far too many regulator technicians carry as well. Like it's some sort of exclusive club you don't belong to.

Don't be concerned; rather, approach the topic systematically. At its very core, oxygen cleaning means nothing more than the removal of all foreign substances. That is really all it entails.

If you dig deeper, there’s much more to it than just using Simple Green, yet that’s precisely what many so-called "oxygen technicians" do. Even with this lacklustre approach, they rarely cause issues, at least most of the time. Harlow’s book is a great starting point, but it should be read with an open mind.

I don’t mean to downplay the significance of oxygen cleaning either, but in the end, it is a simple, albeit slow, process if done correctly.

You should also keep in mind that cleaning for oxygen should never be confused with suitable for oxygen. Take this Z-valve, for example:

No matter how well you clean it, no matter what soft parts you use, PCTFE, FKM, you name it, it will never be suitable for oxygen. Why? Because the seat sits directly and frontally in the gas path, where adiabatic heating could occur if the valve is pressurised in the closed position. Oxygen clean doesn’t directly lead to oxygen suitable, and you'd do well to mentally trace the gas flow and identify critical areas. It's always these critical areas that deserve the most attention, especially when cleaning something that should be suitable for oxygen service.

A high-pressure seat situated directly in the gas path demands a lot of attention and diligence when cleaning and inspecting the surrounding area. An O-ring gland on the intermediate pressure outlet requires much less.

Oxygen technician courses often boil down to the cleaning steps I outlined earlier and go a bit more in-depth on how to verify cleanliness using certain tests. None of these tests are sufficient if one is being pedantic, nor are any of them difficult to perform. Water beading tests, water shake tests, visual inspection, and the like are fundamentally simple. Once again, they just require patience. The real tests like solvent tests or ferrocyanide paper tests are never done on a SCUBA technicians bench. The reason is the complexity, time involved or cost, yet stuff like this is what would be used by a space agency when verifying for oxygen cleanliness.

But don't let anyone tell you that you can't do this at home and require a perfectly isolated clean-room. Odds are that the very same people that spout this don't have one either and just chuck their stuff into Simple Green and you'd do a better job on your own.

 

[happy-diver]

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