How do you determine how much gas remains in a cylinder?

For cylinders containing compressed (non-liquefied) gas, gauge pressure falls as gas is used; Boyle’s law can be used to calculate how much gas a cylinder will deliver at ambient pressure. Oxygen cylinders are an example.

Liquefied gas cylinders such as nitrous oxide and carbon dioxide contain liquid and vapour. In this instance, the gauge pressure only displays the vapour pressure, which does not relate to the amount of substance remaining in the cylinder. To determine how much remains, weight is used. Tare weight is the weight of the empty cylinder. The difference between the filled cylinder weight and tare weight indicates the amount of substance present. Taking nitrous oxide as an example, its molecular weight is 44 g. According to Avagadro’s hypothesis, one mole of nitrous oxide weighs 44 g and occupies 22.4 l at STP. A cylinder containing 2000 g of nitrous oxide will therefore contain 1018 l ([2000 g/44 g] × 22.4 l).

The gauge pressure of a nitrous oxide cylinder normally reads 4400 kPa (the SVP of nitrous oxide at 20°C). During use, vaporisation of liquid requires energy for the latent heat of vaporisation. This causes the temperature of the remaining liquid to fall, resulting in a decrease in vapour pressure within the cylinder. The gauge pressure falls to reflect this, which can be misinterpreted as a cylinder that no longer contains liquid. However, when cylinder use is ceased, temperature and vapour pressure increase to their original value.

The filling ratio is the weight of fluid in the cylinder divided by the weight of the water required to fill it. Liquefied gas within cylinders will exert a greater pressure when warm. To avoid cylinder pressures exceeding safety limits, cylinders are not completely filled, having a filling ratio of 0.75 in the UK and 0.67 in hotter climates.