CAIRE HELiOS Plus Technical & Service Manual page 15

VII 1
Saturation Principles
If the pressure in a container of saturated liquid decreases, the temperature required for saturation to occur will decrease. This leaves
the liquid "super saturated" or too warm. When this occurs, rapid boiling and vaporizing of some of the liquid occurs. The rapid boil-
ing and evaporation of the liquid dissipates the excessive heat until the remaining liquid cools down to the new saturation temperature
associated with the decreased pressure (Figure 16).
Figure 16: Saturated (Boiling) Water at Lower Pressure
Oxygen, which is normally a gas at atmospheric pressure, changes into liquid form when it is cooled to about -297°F (-183°C) at
atmospheric pressure. It is saturated at this temperature (and pressure) which means it will remain a liquid as long as no additional
heat is added. However, the large quantity of heat present in the atmosphere constantly enters the liquid oxygen and causes it to boil
and vaporize back into a gas. Since it is virtually impossible to keep all of the heat in the atmosphere from entering the liquid oxygen,
constant boiling and vaporization occurs.
Now when liquid oxygen is placed in a closed container, the vaporizing gas is trapped nd begins to build pressure. As pressure in-
creases above atmospheric pressure, more heat is needed for boiling to occur at the higher pressure. The heat that is constantly avail-
able from the atmosphere warms the liquid to a higher temperature where boiling again occurs. The vaporizing gas builds pressure
and the process continues. As the pressure on liquid oxygen builds, the related saturation temperature of the liquid increases propor-
tionally (Figure 17).
Figure 17: Liquid Oxygen Saturation Curve
It is important to maintain liquid oxygen saturation (boiling) at the specified operating pressure of the HELiOS portable. As an oxy-
gen flow demand is put on the system, a slight decrease in pressure occurs due to oxygen withdrawal. The saturated liquid oxygen in
the system vaporizes enough gaseous oxygen to maintain system operating pressure. This ensures proper oxygen flow to the patient.
If the liquid oxygen saturation temperature is too low, the corresponding lower saturation pressure causes low oxygen flows to the
patient.
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HELiOS Portables Technical Service Manual • PN 20562190 Rev E