Temperature has a direct relationship to propane pressure.
A pot of water placed on a stove at a high temperature will eventually boil over.
In the same way, an increase in outside temperatures will cause an increase in the pressure in an LP gas container.
As the pressure increases, the volume of the liquid will also expand in the container.
For example, water expands 2.2%. At a temperature rise from 160° F to 600° F, liquid propane would expand 25% and steel would expand only 0.19%.
The fact that propane reacts more quickly to outside temperature changes and expands more rapidly emphasizes the importance of NEVER overfilling an LP gas container.
No LP container may be filled to more than 80% of its capacity without temperature correction.
This means that a container filled to the legal limit will propane stored inside as both a vapor and a liquid (as shown below), allowing for expansion within the container.