How this calculator works
A compressed gas cylinder holds a fixed number of litres at atmospheric pressure for each psi of gauge pressure. That conversion number is the cylinder factor: about 0.28 litres per psi for a common E cylinder and 3.14 for the larger H and K cylinders. Multiplying the pressure above the residual by the factor gives the usable litres. Dividing by the flow in litres per minute and converting minutes to the requested unit gives the duration. The residual pressure is a safety margin so that the cylinder is not run to zero, where flow becomes unreliable and the gauge reads empty before the gas is gone. The estimate assumes a constant flow and a full cylinder at the stated pressure; a regulator, a nebuliser or a high-flow device can change the effective rate.
Formula
usable litres = (gauge pressure in psi - residual pressure in psi) x cylinder factor. duration in minutes = usable litres / flow in litres per minute. Multiply by 60 for seconds or divide by 60 for hours. The factors used are 0.16 for D, 0.28 for E, 1.56 for M, 2.41 for G and 3.14 for H and K cylinders.
Worked example
An E cylinder at 2,000 psi with a 200 psi residual and a 2 litre per minute flow has (2,000 - 200) x 0.28 = 504 litres of usable oxygen. At 2 litres per minute that is 252 minutes, or 4.2 hours. An H cylinder at 2,200 psi with the same residual and a 15 litre per minute flow has 2,000 x 3.14 = 6,280 litres, which lasts 418.7 minutes, or about 7 hours.
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Frequently asked questions
Can I use this for patient care?
No. It is a planning estimate. Cylinder factors vary between suppliers and cylinder designs, gauges have error, and a patient's actual flow may differ from the set rate. Use the label on the cylinder and your service's clinical protocol, and treat this page as a double-check rather than a decision tool.
Why subtract a residual pressure?
A cylinder that reads zero still contains some gas, but the flow becomes unreliable and the regulator may not deliver the set rate. Services reserve a margin, often 200 psi, so that the cylinder is changed before it is truly empty. Some guidance uses 500 psi or a fixed time margin instead, so enter the value your service uses.
Why do the cylinder factors differ?
Each cylinder size holds a different volume of gas at the same pressure. A larger cylinder has more internal volume, so each psi corresponds to more litres at atmospheric pressure. The factor is essentially the cylinder's water capacity divided by atmospheric pressure, rounded for practical use.
Does the flow rate stay constant?
Only if the regulator and the device hold it constant. A simple flowmeter set to 2 litres per minute delivers close to that rate until the pressure falls, while a high-flow device or a nebuliser can draw more. The duration estimate is only as good as the assumption of a steady flow.