Why pressure does not tell you how much CO₂ is left — and why the gauge only drops near the end.

Many people have the misconception that the pressure gauge on our CO₂ cylinder tells us the amount of CO₂ left in the cylinder. However, that is not true. The gauge measures pressure, not volume or weight. The pressure reading will usually only start dropping significantly when the cylinder is almost empty. This post will explain why.

Many new users wonder why the gauge still reads “full” after weeks or months of use, then get a nasty surprise when the reading suddenly drops drastically, or even to 0, in a comparatively short period after the last check.

In an indirect way, the pressure gauge does give us a very rough indication of the amount of CO₂ in the cylinder. At a “full” reading, usually around 900 to 1000 psi, the cylinder could be anywhere from full to nearly empty. When the reading starts to drop significantly, the cylinder will usually be empty soon. This could be in a few days to a few weeks, depending on cylinder size and usage rate. When the reading is 0 psi, the cylinder is effectively empty for our purposes.

If you want to know the best way to estimate the amount of CO₂ left in your cylinder, please read my other article: Am I Getting a Full CO₂ Refill?

Now, why can’t the pressure gauge tell us the CO₂ level in our cylinder? First, we need to understand saturation pressure.

Saturation pressure is the pressure at which a liquid and its gas, can coexist in equilibrium at a specific temperature.

During refilling, as more CO₂ gas is added into the cylinder, the pressure rises. Once the pressure reaches the saturation pressure, adding more CO₂ does not mainly increase the pressure. Instead, the CO₂ gas condenses into liquid.

A properly filled CO₂ cylinder is therefore not filled only with gas. It contains mostly CO₂, liquid with CO₂ gas above it. The gas is there so that there is enough space left for the CO₂ to expand safely when temperature changes.

Starting from a full cylinder, as CO₂ gas is released for use, some of the CO₂ liquid evaporates into gas. This helps maintain the pressure at around the saturation pressure.

Essentially, as long as there is still CO₂ liquid in the cylinder, whether the cylinder is full or nearly empty, the pressure will remain roughly around the saturation pressure. The pressure only starts to drop significantly when little or no CO₂ liquid remains.

For CO₂, the saturation pressure at 25°C is roughly 930 psi, and at 29°C it is just above 1000 psi. Here in warm humid Singapore, we often see our CO₂ cylinders at around 1000 psi.

So, this is why the pressure gauge does not tell us the exact amount of CO₂ in our cylinder, and why the pressure reading does not drop steadily as the cylinder is used. The gauge can stay high for a long time, then fall quickly near the end.

I hope this article has helped you understand CO₂ cylinder gauges better.