What this calculator does
This tool estimates how much oxygen gas is still inside a medical or diving oxygen cylinder, and roughly how many minutes it can keep supplying gas at a chosen flow rate. It is a universal physics calculation and works for any cylinder; the default size options simply reflect common Japanese medical oxygen cylinder specifications. The maximum fill pressure for typical medical cylinders is about 14.7 MPa.
How to use it
Pick the cylinder's internal (water) capacity in liters, enter the value shown on the pressure gauge, choose the pressure unit (MPa or kgf/cm2), set a safety factor (usually 0.8), and select the oxygen flow rate in liters per minute. The result shows the remaining gas volume and the estimated usable time, color-coded green (over 60 min), yellow (30 to 60 min) or red (under 30 min).
The formula explained
A cylinder of internal water capacity C (liters) at gauge pressure P holds a gas volume of roughly \(C \times P\), when P is expressed in kgf/cm2 (since about 1 kgf/cm2 equals 1 atmosphere). Because 1 MPa is approximately 10 kgf/cm2, an MPa reading is multiplied by 10 (the multiplier M) before multiplying by capacity. So Remaining = \(C \times P \times M\). Usable time then equals the remaining volume times the safety factor, divided by the flow rate. Both results are truncated (floored) to whole numbers.
$$T = \left\lfloor \frac{V \cdot S}{F} \right\rfloor$$ $$\text{where}\quad \left\{ \begin{aligned} V &= \left\lfloor \text{Capacity (L)} \cdot \text{Pressure} \cdot \text{Unit factor} \right\rfloor \\ S &= \text{Safety factor} \\ F &= \text{Flow rate (L/min)} \end{aligned} \right.$$
Worked example
With a 3.4 L cylinder reading 14 MPa (M = 10), safety factor 0.8 and flow 5 L/min: Remaining = $$3.4 \times 14 \times 10 = 476 \text{ L}.$$ Usable time = $$476 \times 0.8 / 5 = 76.16,$$ floored to 76 minutes. Since 76 is over 60, the status is green.
FAQ
Why a safety factor? Gauges and flow are imprecise, and you should never run a cylinder fully empty. A factor of 0.8 reserves a margin.
What if I use kgf/cm2? Choose that unit and the multiplier becomes 1, so Remaining = \(C \times P\) directly.
Is this exact? No. It is a planning estimate only and not a substitute for direct monitoring or clinical judgment.
