English | 繁體中文
A remaining-time estimator for portable medical oxygen cylinders, with switchable cylinder sizes. A single HTML file with no backend, suited to GitHub Pages deployment for nursing stations, patient transport or ambulances. The interface is in Traditional Chinese.
| Water volume | Type | Use case |
|---|---|---|
| 3.4 L (0.5 m³) | Small cylinder stocked at hospital nursing stations | Ward and OR transport |
| 2.8 L (0.4 m³) | Portable ambulance cylinder | Prehospital care, long-distance transport |
After switching, the result, reference-table values and formula coefficient all update instantly.
- Before transporting a patient, check whether the small cylinder will last until the destination
- Confirm onboard oxygen before an ambulance run
- Quickly check at the nursing station how long the current gauge pressure lasts at a given flow
- A digital replacement for laminated reference cards
All calculation happens locally in the browser; no data is sent to any server.
Available time (min) = ⌊ (gauge pressure − 200 psi) × (water volume ÷ 14.7) ÷ flow ⌋
| Parameter | 3.4 L version | 2.8 L version | Basis |
|---|---|---|---|
| Conversion factor | 3.4 ÷ 14.7 = 0.2313 L/psi | 2.8 ÷ 14.7 = 0.1905 L/psi | Boyle's law; 1 atm = 14.7 psi |
| Safety residual pressure | 200 psi (same for both) | 200 psi | See explanation below |
| Rounding | Floor (round down) | Floor (round down) | Conservative direction |
| "Please replace" threshold | Available time < 10 minutes | Available time < 10 minutes | Clinical safety margin |
| Swap warning line | ≤ 600 psi | ≤ 600 psi | Administrative swap threshold; doesn't affect calculation |
Note: At the same pressure, a 2.8 L cylinder lasts about 18% less than a 3.4 L one (volume ratio 2.8/3.4). For example, at 900 psi @ 15 L/min a 3.4 L cylinder still has 10 minutes, while a 2.8 L cylinder shows "please replace" straight away. This difference matters especially at the high flows used in ambulances.
The safety residual is subtracted from the gauge pressure before calculating, meaning the gas corresponding to that pressure is not counted as available time at all. This is deliberate, for the following reasons:
You can't swap cylinders mid-transport. If the cylinder runs out on the way, the patient faces immediate hypoxia. The residual must cover:
- Flowmeter reading error (typically ±5–10%)
- Regulator relief dead zone
- Staff reaction time from noticing low pressure to completing a swap
Some institutions use a constant discount method (multiplying the conversion factor by a fixed discount without subtracting a residual), for example:
Discount formula: available time = P × 0.185 ÷ flow
(i.e. 0.8 × 3.4/14.7, no fixed residual subtracted)
Back-calculating the equivalent residual implied by this formula:
P × 0.185 = (P − R) × 0.2313
R = P × 0.200 ← scales proportionally with pressure, not a fixed value
| Gauge pressure | Residual implied by discount method | This tool (fixed 200 psi) | More conservative |
|---|---|---|---|
| 1800 psi | 360 psi | 200 psi | Discount method |
| 1500 psi | 300 psi | 200 psi | Discount method |
| 1200 psi | 240 psi | 200 psi | Discount method |
| 1000 psi | 200 psi | 200 psi | Same |
| 900 psi | 180 psi | 200 psi | This tool |
| 600 psi | 120 psi | 200 psi | This tool |
The two methods cross at about 1000 psi. The discount method is more conservative at high pressure; this tool is more conservative at low pressure.
Because the most dangerous moment for a portable cylinder is exactly during transport at low pressure with no chance to swap, a fixed residual gives stronger protection in the low-pressure zone where clinical risk is highest, which is why this design was chosen.
A fixed residual of 150–200 psi (about 10–14 kg/cm²) is a commonly recommended minimum working pressure in the oxygen cylinder management literature.
Note: The 600 psi swap warning line shown in the interface is an administrative swap threshold (prompting nurses to arrange a replacement) and is unrelated to the formula; don't confuse the two.
- Volume switch: two buttons at the top (3.4 L ward / transport, 2.8 L ambulance); the whole page updates instantly
- Gauge pressure input: number field and slider stay in sync, with a three-color pressure bar below showing the current position
- Flow selection: 6 buttons (2 / 3 / 4 / 5 / 10 / 15 L/min), touch-friendly
- Three result states: green (normal) / orange (≤ 600 psi, swap recommended) / red (replace now)
- Standard reference table: embedded at the bottom, recalculated on volume switch, with the row matching the current input outlined; collapsible
- Works fully offline: a pure static single file, referencing only CDN Bootstrap and Google Fonts
git clone https://github.com/<your-username>/o2-cylinder-timer.git
cd o2-cylinder-timer
# Put index.html in the repo root
git add index.html README.md
git commit -m "feat: add 2.8L ambulance cylinder support"
git push origin mainThen go to GitHub repo → Settings → Pages → Branch: main / / (root);
once deployed it is available at https://<your-username>.github.io/o2-cylinder-timer/.
Shared policy parameters (residual, thresholds, pressure scale) are centralized in CONFIG; cylinder volumes are defined in TANKS, with coefficients derived automatically from the volume — managed in two layers, each a single source of truth.
// Shared policy parameters (independent of volume)
const CONFIG = Object.freeze({
SAFETY_RESIDUAL_PSI: 200, // safety residual pressure (psi)
UPDATE_THRESHOLD_MIN: 10, // "please replace" threshold (minutes)
WARN_SWAP_PSI: 600, // swap warning line (psi)
FULL_PSI: 1800, // full-cylinder reference pressure
MAX_INPUT_PSI: 2200, // input validation upper limit
PRESSURES: [1800,1500,1200,900,600,300,150],
FLOWS: [15, 10, 5, 4, 3, 2],
});
// Switchable cylinder volumes (to add a volume: add an entry + one HTML button)
const TANKS = Object.freeze({
'3.4': { vol: 3.4, label: '3.4 L', desc: '0.5 立方米（病房／轉送）' },
'2.8': { vol: 2.8, label: '2.8 L', desc: '救護車用' }
});Adding another volume: add an entry to TANKS and a vol-btn in the HTML; the calculation logic needs no changes.
Changing the residual policy: edit SAFETY_RESIDUAL_PSI and record the reason in the git commit message for auditability.
Tool output matches the standard paper reference table exactly (floor rounding, tolerance ±0 minutes):
3.4 L
| Gauge pressure | Flow | Paper | Tool |
|---|---|---|---|
| 1800 psi | 5 L/min | 74 min | 74 min ✓ |
| 1500 psi | 3 L/min | 100 min | 100 min ✓ |
| 900 psi | 15 L/min | 10 min | 10 min ✓ |
| 600 psi | 5 L/min | 18 min | 18 min ✓ |
2.8 L
| Gauge pressure | Flow | Calculated | Tool |
|---|---|---|---|
| 1800 psi | 5 L/min | 60 min | 60 min ✓ |
| 1500 psi | 2 L/min | 123 min | 123 min ✓ |
| 900 psi | 15 L/min | Please replace | Please replace ✓ |
| 600 psi | 5 L/min | 15 min | 15 min ✓ |
This tool is only a clinical reference aid; actual available time is affected by flowmeter error, regulator tolerance and environmental factors. It must not be the sole basis for patient-care decisions; follow your institution's medical gas management policies.
MIT © 2026 Che-chia Liang