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Thread: The beginners guide to compressed air.

  1. #16
    Sharp Shooter

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    Is it safe to leave the tank in your car when its hot?
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  2. #17
    Sharp Shooter
    ARF Member Of The Year 2011

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    Quote Originally Posted by Kat View Post
    Is it safe to leave the tank in your car when its hot?
    I don't, I make a point of finding shade when I plan on leaving the cylinder in the car. (I'm an ex Namibian so I drive a white car but still cannot monitor nor predict temperatures inside the closed car left in the sun.)
    I view a closed car in the sun as a heat source. Then there are other variables like when was the cylinder filled to capacity and what's the colour of the car, what time of year is it, what's the heat absorbing qualities of the parking area etc.
    Rather safe than sorry.
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  3. #18
    Sharp Shooter

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    Derek,

    Is there any formula for determining the pressure of air from a given weight?
    If i have a 10L bottle that contains 5kg of air, what would the pressure be? Lets assume a temp of 21'C...

    Ta,
    Ed.
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  4. #19
    Sharp Shooter
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    Quote Originally Posted by Edradour View Post
    Derek,

    Is there any formula for determining the pressure of air from a given weight?
    If i have a 10L bottle that contains 5kg of air, what would the pressure be? Lets assume a temp of 21'C...

    Ta,
    Ed.
    Ed, I'm not sure you'll be able to determine compressed air pressure by weighing a 10l cylinder on a normal scale. Since the air is not compressed to liquid state like LP gas for instance the difference in compressed mass will be too small to measure on conventional apparatus. Way to go here is a manometer/pressure gauge.
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  5. #20
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    Quote Originally Posted by Edradour View Post
    Is there any formula for determining the pressure of air from a given weight?
    The weight of air is slightly variable but it is generally said to be roughly 1.2g/L. So if we ignore the filters that remove moisture (weight) and different temperatures for a moment and you have 5kg of air, then you roughly have 5000Kg/1.2g Litres of air. Lets for argument sake say you have 3kg air, then that is 3000/1.2=2500 Litres. In a 10L bottle, that is theoretically 2500/10 ~250Bar.

    But as DvdM mentioned, it is probably not a good idea to use weight as an indicator of the pressure due to the many variables.

    Quote Originally Posted by DvdM View Post
    Since the air is not compressed to liquid state like LP gas
    And also because LP gas is almost pure while air could be anything that the inlet of the pump sucks in. The composition of air is too variable to make this an accurate calculation.
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  6. #21
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    Very informative post, thank you.
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  7. #22
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    This is scary to see what happens to those thanks!! Makes me think twice now on driving home with 2 of them filled up on the back seat!!
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  8. #23
    Dr. Jan Itor

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    Be careful, always de-pressurise the HPA cylinder before working on your airgun.



    And also: Canadian Airgun Forum | The AIR F/T Pressure Release Mishap

    PSA Reminder: Oxygen in a PCP air rifle, NOT EVEN ONCE!

    Oxygen : PCP Airguns : NOT EVEN ONCE!
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  9. #24
    Sharp Shooter
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    And that is why I send my rifles to the pros to have them serviced!
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  10. #25
    Sharp Shooter

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    Default The beginners guide to compressed air.

    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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