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Thread: The Great Gas(bag) Debate

  1. #1
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    Default The Great Gas(bag) Debate

    I've noticed, on a couple of airgun forums now, that whenever someone asks questions about using gases other than compressed atmospheric air in a PCP rifle or pistol, the questions get shut down. Usually at speed. And with an alarming level of self-righteousness.

    I have to call "Halt!" and say: Why does anyone have the urge to shut down questions - even dangerous questions - that fast?
    Maybe out of a deep need to soothe their own insecurities, by telling other people what to do?

    Let me explain. I'm a technical writer by trade. I spend my working day waist-deep in highly detailed explanations that populate technical manuals hundreds of pages thick.
    If there's one thing I've learned about technical education, it's that the only effective way to explain a controversial subject is to start your cautionary tale with the 'What' - but then to give a meaningful follow-on by spending the remaining 90% of your effort on the 'Why'.
    Think about it. Basic human nature. If you simply tell someone "Don't do that", they won't get the point.
    If someone is set on a dangerous course of action, chances are they don't do blind obedience. That means they'll forge ahead anyway - if only out of resentment at being talked at. They'll likely hurt themselves, and your effort will have been wasted.

    In the face of that, how does it make sense to try to educate someone by witholding information from them?
    Isn't that stupidity masquerading as cleverness?
    If, on the other hand, you encourage discussion about WHY something is dangerous, that person is more likely to put two and two together and take your caution seriously.

    Let me start by adding my small pool of knowledge to the pot.
    I wasn't able to find specific PCP-related dangers related to Carbon dioxide or Liquefied petroleum gas (LPG) - can anyone add specific information on these to the knowledge base?

    1. WHY ARE PEOPLE TEMPTED TO USE GASES OTHER THAN COMPRESSED AIR?
    Because they're available to such people and/or their connected friends.
    And, chances are, because those gases are not under strict lock and key.

    2. WHAT OTHER COMPRESSED GASES ARE PEOPLE MOST LIKELY TO USE?
    Whatever's available and/or whatever their imagination, or their faulty knowledge, has convinced them is good to use.

    3. WHAT POSSIBLE DANGERS EXIST WHEN USING 'FOREIGN' GASES IN A PCP?

    - Cryogenic cooling: Sudden release of high gas pressure can cause violent reductions in temperature, leading to frostbite.
    - Asphyxiation: Any compressed gas contained in quantity that does not contain oxygen may be an asphyxiant. Most gases discussed here will readily dilute and displace breathable air. If you breathe them in, you will likely feel no discomfort. You will breathe out carbon dioxide, but breathing in, you will recieve no oxygen. You will gently and peacefully pass out in around 30 seconds, and be dead within three minutes. This happened in the early days of Space Shuttle development, and still happens once or twice a year in wine-bottling plants.
    - Reactivity: This can be divided into further sub-categories: exothermic (sudden release of heat) reactions, corrosion, sympathetic detonation, and instability under heat.
    - Oxidation: This hazard is most pronounced with the strong oxidisers, such as pure oxygen and chlorine. Oxidisers can corrode metals, and cause chemical burns.

    4. WHAT SPECIFIC DANGERS ARE APPLICABLE TO SPECIFIC GASES?
    I'm going to divide this up into a proposed, albeit controversial 'Safe' category, and 'Unsafe' category:

    SAFE GASES:

    - Nitrogen
    If the nitrogen is 'dry' (ie no moisture content), there is no danger whatsoever to the gun. Nitrogen is unreactive except at very high temperatures, and it actually comprises more than 70% of Earth's atmosphere. When you use straight compressed air in a PCP, the majority of it is, in fact, nitrogen.
    If dry nitrogen were cheaper and more readily available in small quantities, I would strongly encourage airgunners to use it exclusively.
    Much of nitrogen's danger stems from the fact that it's an asphyxiant.

    - Helium
    Helium is completely chemically inert. It would therefore be safe to use in a PCP.
    It is much less expensive per weight than Argon, Xenon, Krypton and Neon, but also far more expensive per weight than dry Nitrogen. It is therefore impractical for use in a PCP.
    Helium is also an asphyxiant, but is less dangerous than the heavier noble gases because it is lighter than air.

    - Neon
    Neon is part of the Noble gases group in the periodic table. It is completely chemically inert and, like Helium, lighter than air, therefore presenting a reduced asphyxiation hazard. It would therefore be safe to use in a PCP.
    But it is up to 55 times more expensive per weight than Helium, and therefore completely impractical.

    - Argon, Xenon, Krypton
    These three are also part of the Noble gases group. All three are highly inert, and extremely unlikely to react with anything in your PCP or dive bottle, even under extremely elevated temperatures and pressures. Argon is fairly commonly available as a shielding gas for MIG and TIG welding, but it is substantially more expensive than dry Nitrogen.
    All three gases are also asphyxiants that are heavier than air, and will accumulate readily in low, unventilated spaces.
    Xenon and Krypton are also hard to find, and stupidly expensive. Filling a PCP with them would be like keeping your house warm by burning sheaves of R100 notes in your wood stove.

    UNSAFE GASES:

    - Cryogenic (liquid) nitrogen
    Highly dangerous due to its extreme cold (-193 degrees C).
    Spill even a little on yourself and you will suffer extreme frostbite or, in a worst-case scenario, lost limbs.
    If you somehow manage to get this stuff into a dive cylinder or PCP, you are most likely to damage that dive cylinder or PCP irreparably. The extreme cold will cause steel and aluminium to contract violently and destabilise its molecular structures, meaning whatever pressure is still in the dive cylinder or PCP will probably cause it to explode.

    - Hydrogen
    Hydrogen forms flammable, explosive mixtures with air in concentrations from 4% to 74%. A subsequent explosive reaction can be triggered by sparks, heat, or even sunlight. Even an invisible spark or static electricity from a person's clothing can cause ignition.
    Hydrogen easily leaks out from most standard connections and, once Hydrogen has leaked into air, the mixture may spontaneously ignite.
    Interestingly, Tritium (a radioactive isotope of Hydrogen also known as Hydrogen-3) is used as the reticle illumination source in Trijicon scopes and gunsights.

    - Oxygen
    Some materials that people normally regard as non-reactive, or that combust slowly in air, have a chemical makeup that renders them violently combustible or explosive in the presence of pure oxygen (think of the carbon-based and flourine-based formulations of most rubber O-rings).
    Combine that with the elevated temperatures that come from adiabatic heating, and you literally have a bomb waiting to go off.

    - Acetylene
    Acetylene is inherently unstable, and has shock-sensitive characteristics similar to sensitive explosives such as nitroglycerin.
    It does not need to mix with oxygen for its explosive effects to manifest. If it is subjected to pressures above approximately 1.9 Bar in conjunction with being subjected to shockwaves (caused, for example, by accidentally dropping your rifle), acetylene decomposes explosively into hydrogen and carbon.
    To reduce most of its explosive hazard, acetylene is supplied and stored dissolved in acetone or dimethylformamide, and is usually also contained in gas cylinders with a porous filling.
    More bad news for air rifle shooters: Copper and copper alloys (for example, brass) initiate the explosive decomposition of acetylene.

    - Chlorine
    By itself, Chlorine gas isn't combustible or explosive. But it is a powerful oxidising agent, meaning that it facilitates reactions between any oxygen and combustible materials that may be present.
    It's also quite corrosive, has a tendency to react with many metals to form chloride salts, can dangerously weaken certain plastics and stainless steel, and can also start fires in the presence of iron at high temperatures.
    Chlorine is also a poison that attacks the respiratory system and eyes, and can cause severe skin burns.
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  2. #2
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    Perhaps this should be made a sticky, for all the Darwin Award candidates out there?
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  3. #3
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    I could not agree more with what you shared...
    Very well written, and some very good facts and useful information!!
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  4. #4
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    Quote Originally Posted by Bludlust View Post
    Perhaps this should be made a sticky, for all the Darwin Award candidates out there?
    Indeed!!
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  5. #5
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    Thanks for the summary Nick.

    Of the gases you listed and at room temperature, some will liquefy or condense at pressures way below what is useable in our PCPs designed for air
    Chlorine at 7bar (simply too toxic and corrosive anyway)
    LPG at under 3 bar
    CO2 at around 45 to 55 bar (air rifles designed for this lower pressure can use it but dont have the power of air driven pcps.)

    Others are too exotic
    Hydrogen is difficult to contain and can simply diffuse through many types of metals. Will pass through o-rings and plastics like butter

    The only ones that have potential to be viable alternatives to air are helium and nitrogen. The properties of helium make it possible to achieve higher projectile velocity at same conditions.
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  6. #6
    Sharp Shooter
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    Quote Originally Posted by JXV View Post
    CO2 at around 45 to 55 bar (air rifles designed for this lower pressure can use it but dont have the power of air driven pcps.)
    I have used an antique Giffard 8mm CO2 rifle that would challenge this comment.
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  7. #7
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    Quote Originally Posted by Dale View Post
    I have used an antique Giffard 8mm CO2 rifle that would challenge this comment.
    Grain mass and speed of projectile?
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  8. #8
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    JXV, thanks for contributing that info!

    When I first got the idea of putting all this 'on paper', I wanted to keep things as simple as possible, so the only distinction I made was between relatively 'Safe' and relatively 'Unsafe' gases.
    Rarity and expense of the gases in question didn't matter for purposes of the initial post, but as an example, yes - most Krypton reserves are held by universities and scientific research establishments, and aren't likely to be abused by PCP-wielding John E. Public.
    And unless you have high-security clearance from Armscor and NERSA, you're unlikely to have access to Uranium hexaflouride.

    An(other) important question to be asked is: Which dangerous gases are a) fairly cheap, b) fairly readily available, and c) therefore more likely to be abused by airgunners who are too clever for their own good?
    As you rightly pointed out, LPG and carbon dioxide are also high on the list.
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  9. #9
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    The main problem is that many of them are not stored or supplied at pressures useable for airgunning.
    The primary problems with gas as propellant are that :
    1) the gas itself has mass so in addition to accelerating the pellet down a barrel the gas must also accelerate itself and overcome its own frictional resistance to flow. I've seen estimates that up to 40% of the energy released during a shot is consumed by the air itself and never imparted to the pellet.
    2) gases have different sonic speeds and will also "choke" at different velocities when passing through an orifice like a transfer port. Choking flow is complex and we can get deep into thermodynamics on this one ... but maybe not here...

    Bottom line is that Helium is the only gas that possibly offers a significant benefit in terms of power and efficiency over air or nitrogen.
    It is inert, available, much less dense ( so it accelerates easily) and it also chokes at a higher speed than the others. The same pressure helium in your PCP would probably boost your power a lot but it would need tuning of the reg, hammer spring tension and possibly the transfer port to be efficient.
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  10. #10
    Sharp Shooter
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    Quote Originally Posted by Nick V View Post
    Grain mass and speed of projectile?
    8mm, i.e. .32 calibre lead ball approximately 49 grains.

    I didn't have the opportunity to chrono the rifle but estimated around 550-600fps based upon comparison with other known rifles.

    Not shabby for a late 19th Century rifle.

    CO2 whilst not as popular can give some pretty interesting performance and there are a few production and custom builds out there that will give a good account of themselves.

    British airgun builder John Bowkett has made some quite pokey larger calibre guns using this power source.
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  11. #11
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    Just for interest, "Green Gas" used in blowback airsoft pistols is a Propane/Butane mixture, i.e. lighter gas.

    I would rate the most dangerous gas as Oxygen. It is easily and commonly available, and can cause oils, orings, and even aluminium and magnesium alloys to combust. I used to have some photos of a Bam50 that was filled with oxygen and the valve burnt - it was not pretty, and caused serious injuries.
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