A CO2 cylinder that reads a different pressure on a cold morning than a warm afternoon is not faulty — it is behaving exactly as physics demands. This chart shows the saturation pressure of liquid CO2 across the everyday temperature range and explains why the gauge tracks temperature, not how much gas is left.
How it works
Inside a CO2 tank, liquid and gas coexist. The gas sitting above the liquid is at the saturation pressure for the current temperature: the pressure at which liquid and vapour are in balance. That pressure depends only on temperature, not on how full the tank is.
Warm the tank and a little more liquid boils off, raising the pressure until a new, higher equilibrium is reached. Cool it and the reverse happens. The tool interpolates published CO2 saturation data to give the expected pressure at your temperature:
- 0 °C → about 490 psig
- 10 °C → about 610 psig
- 20 °C → about 755 psig
- 30 °C → about 930 psig
Gauge pressure (psig) is the absolute saturation pressure minus one atmosphere (about 14.7 psi), which is what a regulator shows.
Why the gauge cannot measure fill level
Because the pressure is fixed by temperature while any liquid remains, the gauge reads the same whether the tank is 90% full or 10% full. It only begins to drop once the last of the liquid has evaporated and just gas is left — by which point the tank is nearly empty. That is why a CO2 tank is weighed, not gauged, to judge its contents.
The critical point and safety
At 31.1 °C CO2 reaches its critical point (about 1071 psia). Above this temperature the liquid and gas phases merge into a single supercritical fluid, and pressure climbs steeply with both temperature and fill level. The simple chart no longer applies, and an overfilled tank left in a hot car or sunny room can develop dangerous pressure — store cylinders cool and shaded. All values are computed locally in your browser.