We say that a liquid is in equilibrium with its gas when the same mass is exchanged from the gas to liquid and from liquid to gas. This happens continuously, and this point is strongly dependent on pressure and temperature. Any change in temperature produces a change in the pressure. When temperature is increased, more liquid CO2 turns to gas, increasing pressure. When temperature is decreased more gaseous CO2 turns in liquid, decreasing the pressure.
The pressure cannot give any information about the liquid CO2 level in the reservoir, but the pressure in a CO2 tank will be the same — in the range of 0.1% to 100% of the liquid level inside! Therefore, a liquid level sensor is necessary to control the level. But the level is strongly dependent on the pressure and temperature inside the tank. So, to have good control of the CO2 liquid level, it is mandatory to have temperature control of the CO2 tank.
There are many points at which we can find equilibrium between gaseous CO2 and liquid CO2. All of these points are situated along the CO2 equilibrium curve. Choosing the best point is part of the system design procedure, because it defines the level of the liquid CO2 in the reservoir. As the CO2 condensing temperature at CO2 bottle pressure is about 14° C (57° F), we chose this value to control the pressure in the reservoir. The control is fully automatic.
