Gas cylinder autonomy calculator
Usable volume and autonomy of a compressed-gas cylinder at a given flow rate, reserve pressure included.
Result
986.9 min
- Autonomy
- that is 16 h 27 min
- Calculation
- Vusable = V × (P − Pres) ÷ 1.01325 = 50 × (200 − 0) ÷ 1.01325 = 9869 L
Note
9869 L of expanded gas, withdrawn at 10 L/min: 986.9 minutes of autonomy.
Calculation, assumptions and sources
OverviewUsable gas volume once expanded (Boyle’s law, P_atm = 1.01325 bar) and autonomy, from the cylinder volume, the gauge pressure, the reserve pressure and the flow.
Usable gas volume once expanded (Boyle’s law, P_atm = 1.01325 bar) and autonomy, from the cylinder volume, the gauge pressure, the reserve pressure and the flow. Compressed gases only (nitrogen, oxygen, argon, helium, air): for a liquefied gas, pressure does not measure the content.
Method & formulasV_usable = V_cylinder × (P − P_reserve) / P_atm
Boyle’s law (P·V constant at constant temperature) applies to the expansion: at 200 bar, one “water capacity” litre gives about 200/1.013 litres of gas at atmospheric pressure. The reserve pressure is subtracted from the reading. Pressures are gauge readings.
- Water capacity
- the internal geometric volume of the cylinder, stamped on the shoulder (a standard large cylinder holds 50 L).
- Reserve pressure
- the pressure you choose not to go below, to preserve gas quality or avoid air ingress.
- Flow at atmospheric pressure
- the L/min of a flowmeter or rotameter are litres of expanded gas.
Validity domainThe model is the ideal gas in isothermal expansion.
The model is the ideal gas in isothermal expansion. At 200 bar, the departure from real-gas behaviour (compressibility factor) reaches a few percent: nitrogen holds slightly less than calculated, helium slightly more. Fast withdrawal cools the expansion and temporarily lowers the reading. The autonomy assumes a constant flow.
Liquefied gasesIn a CO₂, N₂O or propane cylinder, the gas is liquid at the bottom and the reading is the saturation vapour pressure: nearly constant while liquid remains, it then drops abruptly.
In a CO₂, N₂O or propane cylinder, the gas is liquid at the bottom and the reading is the saturation vapour pressure: nearly constant while liquid remains, it then drops abruptly. At equal temperature, a nearly empty CO₂ cylinder shows the same pressure as a full one. The content is measured by weighing (tare stamped on the cylinder).