Gas cylinder autonomy calculator
V_usable = V_cylinder × (P − P_reserve) / P_atm
Enter the cylinder volume, the gauge pressure, your reserve pressure and the withdrawn flow: the tool calculates the gas volume actually usable once expanded (Boyle’s law, P_atm = 1.01325 bar) and the autonomy in minutes, hours or days. For compressed gases (nitrogen, oxygen, argon, helium, air) never for liquefied gases, whose pressure does not measure the content.
986.9 min
- Autonomy
- that is 16 h 27 min
- Calculation
- V_usable = V × (P − P_res) ÷ 1.01325 = 50 × (200 − 0) ÷ 1.01325 = 9869 L
9869 L of expanded gas, withdrawn at 10 L/min: 986.9 minutes of autonomy.
Scientific dossier
What the tool computes, what it assumes, where it stops being valid, and where its data comes from.
Method & formulasV_usable = V_cylinder × (P − P_reserve) / P_atm
V_usable = V_cylinder × (P − P_reserve) / P_atm
autonomy = V_usable / flow
P_atm = 1.01325 bar (standard atmosphere, a defined value)
This is Boyle’s law (P·V = constant at constant temperature) applied to expansion: the cylinder’s “water capacity” litres, held at 200 bar, become roughly 200/1.013 times more litres once brought back to atmospheric pressure. The reserve pressure subtracts the share you refuse to use; pressures are gauge readings, which keeps the formula consistent end to end.
- Water capacity
- · the internal geometric volume of the cylinder, stamped on the shoulder (a standard large cylinder holds 50 L). That is what enters the formula, not the expanded gas volume. Which is the result.
- Reserve pressure
- · the pressure you choose not to go below, to preserve gas quality. Avoid air ingress, or keep a margin. It is yours to set: the tool imposes no value.
- Flow at atmospheric pressure
- · the L/min shown by a flowmeter or rotameter are litres of expanded gas, exactly the volume the autonomy refers to.
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, in a direction that depends on the gas. Nitrogen holds slightly less than calculated, helium slightly more; this refinement is not modelled, it is stated. Temperature is assumed constant: fast withdrawal cools the expansion and transiently lowers the reading. Finally, the autonomy assumes a constant flow: it is a planning estimate, not a safety contract: cylinder replacement is decided at the gauge, not the stopwatch.
Major pitfall: liquefied gases have no pressure gauge for contentIn a CO₂, N₂O or propane cylinder.
In a CO₂, N₂O or propane cylinder. The gas is liquid at the bottom and the reading is the saturation vapour pressure: it stays nearly constant as long as liquid remains, then collapses at once. The gauge therefore does not measure the content, a nearly empty CO₂ cylinder shows the same pressure as a full one at equal temperature. For those gases, only weighing (tare stamped on the cylinder) gives the content, and this tool does not apply.