How much gas is in a cylinder? Water capacity filling pressure and the arithmetic for every industrial gas in India

Nobody in India can give you a single table of "cylinder contents" that is correct, because cylinder contents are not a property of the gas. They follow from four things: three figures that define the fill, namely the water capacity, the filling pressure and the reference temperature, and one property of the gas, its compressibility. This page shows the arithmetic, so a buyer can check any figure a supplier quotes, and it separates the two kinds of gas that must be treated differently: compressed gases, measured by pressure, and liquefied or dissolved gases, measured by weight.

For oxygen, nitrogen, argon, air, hydrogen and helium the gas stays gaseous at any pressure at room temperature, because each has a critical temperature far below ambient (oxygen −118.6 °C, argon −122.5 °C, nitrogen −147.0 °C). Contents follow from Boyle's law with a correction:

Contents (m³ at 15 °C, 1 atm) = WC (litres) × (filling pressure in bar gauge + 1.013) ÷ 1.013 ÷ 1,000 ÷ Z

where Z is the compressibility factor of the gas at the filling pressure and 15 °C. Z = 1 for an ideal gas. Below 1, the molecules attract each other and the cylinder holds more than ideal; above 1, they repel and it holds less.

Worked example, 46.7 litre cylinder filled to 150 bar gauge at 15 °C:

• Absolute pressure = 150 + 1.013 = 151.0 bar.
• Ideal content = 46.7 × 151.0 ÷ 1.013 ÷ 1,000 = 6.96 m³.
• Divide by Z for the gas.

Compressibility is from the NIST Chemistry WebBook fluid-property data, rounded; volumes at 15 °C and 1.013 bar. Nitrogen's Z crosses 1 between 150 and 200 bar, which is why its 200 bar content is slightly less than the pro-rata figure. Hydrogen and helium stay above 1 at every cylinder pressure, so a hydrogen cylinder always holds less than the same cylinder of oxygen.

Three conventions change the number without changing the gas.

1. Reference temperature. IS 309:2023 cl 6.2 and IS 1747:2023 cl 6.2 require oxygen and nitrogen contents to be stated in m³ at 27 °C and 760 mm Hg. Gas at 27 °C occupies 300.15 ÷ 288.15 = 1.042 times its volume at 15 °C, so the same oxygen cylinder is about 7.4 m³ at 15 °C and about 7.7 m³ at 27 °C. Some suppliers use 0 °C "normal" cubic metres, which gives a figure 5 % smaller than one at 15 °C.
2. Compressibility ignored. Many trade tables simply multiply WC by gauge pressure: 46.7 × 150 = 7,005 litres, "7 m³". That happens to land close for nitrogen and air but undercounts oxygen and argon by about 5 %.
3. Temperature at filling. A cylinder filled hot to 150 bar settles to a lower pressure, and therefore lower contents, when it cools. The legal limit is the pressure at 15 °C, which is why fillers correct for temperature.

At constant volume the pressure of a compressed gas rises in proportion to absolute temperature, about 0.4 % per °C near room temperature. An oxygen cylinder filled to 150 bar at 15 °C reads roughly 165 bar at 35 °C and 176 bar at 50 °C (NIST oxygen data). This is the basis of IS 8775:1978: fix the filling pressure at 15 °C so the developed pressure at 65 °C stays inside what the cylinder was tested for. Store cylinders shaded; a cylinder in Indian summer sun on a truck bed can exceed 50 °C at the shell.

Carbon dioxide and nitrous oxide are liquids in the cylinder at room temperature, with a vapour space above. The pressure is the vapour pressure of the liquid, about 57 bar for CO₂ at 20 °C, and it stays there until almost all the liquid has evaporated. The gauge therefore reads "full" until the cylinder is nearly empty. Contents are set and checked by weight.

The legal charge is defined by the filling ratio, the mass of gas divided by the mass of water the cylinder holds at 15 °C (GCR 2016 rule 2(xxv); IS 3710:1978 cl 2.1.2). Example, using an illustrative ratio of 0.667: a 46.7 litre cylinder may hold 46.7 × 0.667 = 31.1 kg of CO₂. At 15 °C and 1 atm, CO₂ has a density of about 1.87 kg/m³, so 31.1 kg is about 16.6 m³ of gas. To check a cylinder, weigh it and subtract the stamped tare.

The Indian filling ratio for CO₂ and N₂O is not in any current standard KIGL holds: IS 3710 covers only gases with a critical temperature above 70 °C, and IS 8866, which covered CO₂, is withdrawn. The figure now lives in IS 15975:2020 and the Gas Cylinders Rules. Overfilling is the danger: CO₂'s critical temperature is 31.0 °C, so above that a liquid-full cylinder becomes a single dense phase whose pressure climbs steeply with temperature. This is why GCR 2016 rule 4(2) requires a CO₂ cylinder valve to carry a bursting disc that opens between 200 and 220 kg/cm². The detail is on carbon dioxide technical data.

A DA cylinder is a third case. The acetylene is dissolved in acetone held in a porous mass, the pressure depends on temperature and on how much acetone is present, and the cylinder pressure is a poor guide to contents. GCR 2016 rule 37(ix) requires each cylinder to be stamped with its maximum gas capacity, its tare weight (inclusive of valve) and the porous mass identity, and rule 41 requires the filler to record the empty weight, the solvent added and the full weight at each charge. Contents are the gross weight minus the tare. Each kilogram of acetylene is about 0.92 m³ at 20 °C and 1 atm, from a gas density of 1.0896 g/l measured by Bottomley, Reeves and Seitlow. The procedure is on checking acetylene cylinder contents by weight, and why pressure misleads is on acetylene cylinder pressure does not show contents.