Convert cryogenic liquids: litres liquid, kilograms and normal cubic metres of gas for LN2, LO2, LAr, LHe and LH2 - with cylinder equivalent and holding time.
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Liquid, kilograms or normal cubic metres? Converting cryogens correctly
Liquid nitrogen is filled in litres, billed in kilograms and consumed as gas in normal cubic metres – three units for the same quantity. Two property values bridge them: the liquid density at boiling point (LN₂: 0.808 kg/l at −195.8 °C) and the normal density of the gas (N₂: 1.250 kg/Nm³ at 0 °C, 1.013 bar).
Worked example: 100 litres of LN₂
100 l × 0.808 kg/l = 80.8 kg. As gas: 80.8 / 1.250 = 64.6 Nm³ – an expansion ratio of about 1:646. The same gas quantity in cylinders would be roughly 6.5 × 50 l/200 bar. This is exactly why liquid supply pays off from a few cylinders per week.
Expansion ratios
LN₂: 1:646 · LO₂: 1:798 · LAr: 1:782 · LHe: 1:700 · LH₂: 1:788 (referenced to 0 °C; figures referenced to 15 or 20 °C run 5–7 % higher – which is why published tables differ).
Tank holding time
Duration = gas stock / withdrawal rate. A 200 l LN₂ tank (129 Nm³) feeds a 2 Nm³/h consumer for 64 hours. Add boil-off losses in practice: even with the best insulation, 0.3–3 % of contents evaporate per day depending on tank size.
Safety note
The near-1:700 expansion makes trapped liquid dangerous: LN₂ locked between two valves builds enormous pressure on warming – such sections need relief valves. In enclosed spaces, oxygen displacement is an additional hazard.
