What does the high-side gauge actually measure?

The high-side gauge reads the pressure inside the CO2 tank. As long as liquid CO2 remains, that pressure equals the vapour pressure of CO2 at the tank temperature and barely changes — around 850 psi at room temperature. So a full tank and a nearly empty one read almost the same until the liquid is gone.

Why does the high-side gauge stay constant then drop suddenly?

Liquid CO2 keeps the headspace saturated at its vapour pressure regardless of how much liquid is left. Only when the last liquid evaporates does the remaining gas-only pressure start to fall. A dropping high-side gauge means the tank is nearly empty, not half full.

What is the low-side gauge for?

The low-side gauge shows the regulated serving or carbonation pressure that you set with the adjustment screw. For most ales served cold this is around 8 to 12 psi; for force carbonation it may be set to 20 to 30 psi temporarily. It is independent of how full the tank is.

How can I tell how much CO2 is left?

The only reliable method is to weigh the tank. Subtract the tare weight stamped on the cylinder from the current weight to get the CO2 charge remaining. This tool estimates from pressure as a rough guide, but a scale is definitive.

What serving pressure should I use?

Serving pressure depends on temperature and desired carbonation volume, typically 8 to 14 psi for beer at cellar or fridge temperature. Set it on the low-side gauge with the regulator screw; the high-side gauge plays no part in serving.

CO2 Regulator Pressure Guide

A CO2 regulator has two gauges that confuse a lot of new kegging brewers. This guide explains exactly what each reads and uses the physics of CO2 vapour pressure to estimate how much gas you have left.

How it works

High-side gauge — tank pressure. This reads the pressure inside the cylinder. CO2 in a tank is stored as a liquid with gas above it. As long as any liquid remains, the gas above it sits at the vapour pressure of CO2, which depends only on temperature — about 850 psi at 21°C. This is why a full tank and a one-eighth-full tank read almost identically. The gauge only starts dropping once the last liquid has boiled off, which is your warning that the tank is nearly empty.

The vapour pressure rises steeply with temperature (roughly):

Temperature	CO2 vapour pressure
0°C / 32°F	~500 psi
10°C / 50°F	~640 psi
21°C / 70°F	~850 psi
31°C / 88°F	~1070 psi (near critical point)

Low-side gauge — serving pressure. This shows the regulated output you dial in with the adjustment screw, typically 8 to 14 psi for serving. It is completely independent of the tank’s fill level.

Estimating what’s left

Because pressure alone can’t tell you the remaining liquid, the only definitive method is to weigh the tank and subtract the tare weight stamped on the collar:

CO2 remaining = current weight − tare weight

This tool reads your high-side pressure against the expected vapour pressure at your temperature: if the gauge is at or above the vapour pressure, liquid still remains (treat as full); if it has dropped well below, you are on gas only and the cylinder is nearly empty.

Tips and notes

Keep your CO2 tank at a stable temperature; a cold garage drops the high-side reading without meaning the tank is empty.
A sudden drop on the high-side gauge during a session means you have minutes, not days, of gas left — swap or refill promptly.
Never judge “half full” from the high-side gauge; only a scale is reliable.
The low-side gauge is your serving control; leave the high-side gauge purely as a low-gas warning light.