Bottled gas

Bottled gas is a term used for substances which are gaseous at Standard temperature and pressure (STP) and have been compressed and stored in carbon steel, stainless steel, aluminum or composite bottles known as gas cylinders.

Gas state in cylinder
There are four cases: either the substance remains a gas at standard temperature but increased pressure, the substance liquefies at standard temperature but increased pressure, the substance is dissolved in a solvent, or the substance is liquefied at reduced temperature and increased pressure. In the last case the bottle is constructed with an inner and outer shell separated by a vacuum (dewar flask) so that the low temperature can be maintained by evaporative cooling.

Case I
The substance remains a gas at standard temperature and increased pressure. Examples include:
 * air
 * argon
 * helium
 * nitrogen
 * oxygen

Case II
The substance liquifies at standard temperature but increased pressure. Examples include:
 * butane
 * propane
 * carbon dioxide (also packaged as a cryogenic gas, Case IV)
 * nitrous oxide

Case III
The substance is dissolved at standard temperature in a solvent. Examples include:
 * acetylene


 * Note: Acetylene cylinders contain an inert packing material and are filled with a solvent such as acetone or dimethylformamide. The acetylene is pumped into the cylinder and it dissolves in the solvent.  When the cylinder is opened the acetylene comes back out of solution, much like a carbonated beverage bubbles when opened.

Case IV
The substance is liquified at reduced temperature and increased pressure. These are also referred to as cryogenic gases. Examples include:
 * liquid nitrogen (LN2)
 * liquid oxygen (LOX)
 * carbon dioxide (also packaged as a liquified gas, Case II)


 * Note: cryogenic gases are typically equipped with some type of 'bleed' device to prevent overpressure from rupturing the bottle and to allow evaporative cooling to continue.

Expansion and Volume
The general rule is that one unit volume of liquid will expand to approximately 800 unit volumes of gas at Standard temperature and pressure with some variation due to intermolecular force and molecule size compared to an ideal gas. Normal high pressure gas cylinders or bottles will hold from 200 to 400 atmosphere (unit)s. The atmosphere units pressure held by the bottle is equivalent to the number of volumes of standard temperature and pressure of the gas held by the bottle for an ideal gas.

Special Handling Considerations
Because the contents are under high pressure and are sometimes hazardous, there are special safety regulations for handling bottled gases. These include chaining bottles to prevent falling and breaking, proper ventilation to prevent injury or death in case of leaks and signage to indicate the potential hazards.

In the United States, the Compressed Gas Association sells a number of booklets and pamphlets on safe handling and use of bottled gases. (Members of the CGA can get the pamphlets for free.) The European Industrial Gases Association and the British Compressed Gas Association provide similar facilities in Europe and the United Kingdom.

Nomenclature Differences
In the United States, 'bottled gas' typically refers to liquified petroleum gas. 'Bottled gas' is sometimes used in medical supply, especially for portable oxygen tanks. Packaged industrial gases are frequently called 'cylinder gas', though 'bottled gas' is sometimes used.

The United Kingdom and other parts of Europe more commonly refer to 'bottled gas' when discussing any usage whether industrial, medical or liquified petroleum. However, in contrast, what the United States calls liquified petroleum gas is known generically in the United Kingdom as 'LPG'; and it may be ordered using by one of several Trade names, or specifically as butane or propane depending on the required heat output.

Colour coding
Different countries have different gas colour codes but attempts are being made to standardise the colours of cylinder shoulders.


 * Colours of cylinders for Medical gases are covered by an International Organization for Standardization (ISO) standard, ISO 32; but not all countries use this standard.


 * Within Europe gas cylinders colours are being standardised according to EN 1089-3, the standard colours applying to the cylinder shoulder only, i.e. the top of the cylinder close to the pillar valve.


 * In the United States, colour coding is not regulated by law. (Therefore, one should not rely on the colour of a cylinder to indicate what it contains.  The label or decal should always be checked for product identification.)

European cylinder colours
The colours below are specific shades, defined in the European Standard in terms of RAL coordinates:

Specific named gases

 * Acetylene cylinders are maroon.
 * Argon cylinders have a dark green shoulder.
 * Carbon dioxide cylinders have a grey shoulder.
 * Helium cylinders have a brown shoulder.
 * Nitrous oxide cylinders have a blue shoulder.
 * Nitrogen cylinders have a black shoulder. (Previously grey, in the United Kingdom).
 * Oxygen cylinders have a white shoulder. (Previously black, in the United Kingdom).

Based on gas properties

 * Toxic or corrosive gas cylinders have a yellow shoulder.
 * Flammable gas cylinders have a red shoulder.
 * Oxidising gas cylinders have a light blue shoulder.
 * Inert (non toxic, non flammable, non oxidising) gas cylinders have a bright green shoulder.
 * Toxic and flammable or toxic and corrosive gas cylinders have yellow and red shoulders (either two bands or quartered).
 * Toxic and oxidising or corrosive and oxidising gas cylinders have yellow and light blue shoulders (either two bands or quartered).

Gas mixtures, mostly for diving

 * Air cylinders have a white and black quartered shoulder.
 * Heliox cylinders, a diving mixture of helium and oxygen, have a white and brown quartered shoulder.
 * Nitrox cylinders, a diving mixture of nitrogen and oxygen, have a white and black quartered shoulder.
 * Trimix cylinders, a diving mixture of helium, nitrogen and oxygen, have a white, black and brown segmented shoulder.