Hong Life CalculatorsUnderstanding the oxygen tank duration calculator — concept and principle
This calculator estimates how long a medical oxygen cylinder will last at a given flow rate before it reaches a safe reserve pressure. It covers common US cylinder sizes (D, E, M, G, H/K) and any cylinder with a known water capacity, in psi or bar. Paramedics, nurses and home oxygen users use it to plan transfers and refills.
How it is calculated
Duration (min) = (gauge pressure − reserve pressure) × cylinder factor ÷ flow (L/min)
- Cylinder factors (litres per psi): D 0.16, E 0.28, M 1.56, G 2.41, H/K 3.14.
- For other cylinders: litres ≈ water capacity (L) × usable pressure (bar); 1 bar = 14.5038 psi.
- A reserve, commonly 200 psi (about 14 bar), is kept so the cylinder is changed before it runs dry.
The principle
A full cylinder holds compressed oxygen at high pressure. The volume of gas released at atmospheric pressure is roughly proportional to the pressure drop, as Boyle's law describes. The cylinder factor converts each psi of pressure into litres of gas for that cylinder size. Dividing the usable litres by the flow gives minutes. The calculation ignores the small departure of oxygen from ideal-gas behaviour at high pressure.
Worked example
Default inputs: E cylinder, 2,000 psi, reserve 200 psi, flow 2 L/min.
- Usable pressure = 2,000 − 200 = 1,800 psi (÷ 14.5038 = 124.1 bar).
- Usable oxygen = 1,800 × 0.28 = 504 L.
- Duration = 504 ÷ 2 = 252 min = 4 h 12 min.
How to read the result
Plan to change or refill well before the time shown. Real duration is shorter with leaks, cold cylinders, inaccurate gauges or higher-flow devices, and longer with conserving devices that deliver oxygen only on inhalation. On transfers, carry a spare cylinder.
Common mistakes
- Using the wrong cylinder factor.
- Entering bar while the unit is set to psi.
- Forgetting the reserve and planning to empty the cylinder.
Frequently asked questions
How long does an E cylinder last at 2 L/min?
About 4 hours from 2,000 psi with a 200 psi reserve.
Why keep a reserve?
So there is time to change cylinders and the regulator still works reliably.
Does a conserving device double duration?
It can extend it considerably, but the gain depends on the device and breathing pattern.
Sources: Texas A&M University EMS, "Oxygen Duration" reference sheet · NIST Special Publication 811, Guide for the Use of the International System of Units (bar to psi conversion)