Permanent liquefied and dissolved gases: why an oxygen cylinder is sold by cubic metre, a CO₂ cylinder by kilogram and an acetylene cylinder by net weight

Gases go into cylinders in three physical forms, and the form decides how the contents are measured, sold and checked. Permanent gases such as oxygen, nitrogen and argon stay gaseous at any pressure at room temperature, so the gauge tells you the contents. Liquefied gases such as carbon dioxide and nitrous oxide are liquid in the cylinder, so the gauge reads the same until the liquid is gone and they are sold by weight. Acetylene is a dissolved gas, held in acetone or DMF inside a porous mass, and it is also measured by weight.

Critical temperatures are from the NIST Chemistry WebBook, except acetylene (Ambrose, Trans. Faraday Soc. 1956) and air (pseudo-critical, Lemmon and co-workers, 2000); ammonia's value differs slightly between equations of state and is rounded. The full set of constants is in critical temperature and pressure of gases.

Liquid carbon dioxide in a closed cylinder sits under its own vapour. The pressure of that vapour depends only on the temperature of the liquid, not on how much liquid there is. NIST saturation data give 34.9 bar absolute at 0 °C, 50.9 bar at 15 °C, 57.3 bar at 20 °C and 64.3 bar at 25 °C.

Draw gas off, and some liquid evaporates to replace it. The pressure holds. A full cylinder and a quarter-full cylinder at 20 °C both read about 56 bar on a gauge.

The pressure starts to fall only when the last liquid has evaporated. At that moment the cylinder is full of saturated vapour at 194.2 kg/m³ (NIST, 20 °C), so a 46.7 litre cylinder still holds about 9 kg of CO₂. For a cylinder charged with 30 kg, roughly 30 % of the gas is still inside when the needle first moves. From then on the gas behaves like a compressed gas and the pressure falls steadily.

Two practical effects follow.

• Heavy draw cools the liquid. Each kilogram that evaporates at 20 °C absorbs about 152 kJ (NIST latent heat), so a high flow chills the cylinder, the vapour pressure falls with the temperature, and the outside frosts. The gauge drops though liquid remains. This is why CO₂ regulators on high-flow duty carry heaters.
• Temperature moves the gauge more than contents do. The same cylinder reads about 34 bar on a cold morning near 0 °C and about 63 bar at 25 °C. Neither reading says anything about how much is inside.

For permanent gases the trade unit is the cubic metre. IS 309:2023 cl 6.2 and IS 1747:2023 cl 6.2 require the quantity of oxygen and nitrogen in a cylinder to be stated in cubic metres at 27 °C and 760 mm Hg. A filler calculates it from water capacity, settled pressure, temperature and the gas's compressibility, as shown in gas cylinder sizes and contents in India and gas laws for cylinder users.

For liquefied and dissolved gases the trade unit is the kilogram. The cylinder is weighed on a calibrated scale and the stamped tare is subtracted. Converting to volume needs the gas density at a stated temperature:

Full tables for ten gases are in Nm³ to kg conversion.

Acetylene's critical temperature, 35.18 °C, would make it a high-pressure liquefiable gas, but liquid acetylene and acetylene under high pressure can decompose explosively. Indian law therefore does not allow free acetylene to be liquefied or held at cylinder pressure like CO₂. Instead it is dissolved in acetone (or DMF for high-purity grades) held in a porous mass that fills the cylinder. Rule 37(vi) of the Gas Cylinders Rules 2016 caps the pressure at 16 kgf/cm² at 15 °C, and IS 14814:2023 cl 7.2.2 gives a maximum working pressure of 15.495 atm at 15 °C with acetone as the solvent.

The pressure of a DA cylinder depends on temperature and on the acetone content as well as on the acetylene, so the gauge is a poor guide to contents. Rule 37(ix) requires each cylinder to be stamped with its tare weight (inclusive of valve), its porous mass identity and its maximum gas capacity. The procedure is in how to check acetylene cylinder contents by weight, and the reason the gauge misleads in acetylene cylinder pressure does not show contents. KIGL fills dissolved acetylene in acetone and ultra-high-purity acetylene in DMF.

Liquid oxygen, nitrogen and argon are permanent gases cooled below their boiling points (−182.96 °C, −195.80 °C and −185.85 °C at 1 atm). They are held at low pressure in vacuum-insulated containers rather than high-pressure cylinders. Rule 2(xvi) defines a cryogenic container as a double-walled insulated vessel of more than 500 ml and not more than 1,000 litres; larger static tanks fall under the SMPV(U) Rules 2016. They are sold by weight or by litres of liquid. See cryogenic cylinders, dewars and tanks.