Charles’ Law
If you have read my first article in this series, An Introduction to Scuba Gas Laws – Part I: Boyle’s Law, you will already be familiar with Boyle’s Law which studies the relation between pressure and volume of a gas while temperature remains constant. Charles’ law also studies volume of gas however taking into consideration the effects of temperature on the volume and Pressure of the gas.
Jacques Alexander Charles was a French scientist, mathematician, inventor and a balloonist who first studied the effects of temperature on the volumes of a gas and formulated Charles’ Law in 1787. The law states that “At constant pressure, the volume of a given mass is of an ideal gas increases or decreases by the same factor as its temperature increases or decreases.”
Mathematically Charles’ Law is expressed as:
P1 x V1 P2 x V2
______ = ________
T1 T2
Where’ T’ is temperature,’ P’ is pressure and ‘V’ volume. ‘1’ indicates the initial temperature and volume while the’ 2’ indicates the final temperature and volume.
According to Charles’ Law, if we were to take a balloon filled with air and increase the temperature of the air inside, the volume of air would increase causing the balloon to expand. This is caused by the heating of the molecules of air inside the balloon causing them to move rapidly. In the same manner if we cooled the balloon in a freezer, the volume of air decrease, making the balloon look partially deflated.
In the same manner a Scuba tank filled to 3000 PSI if heated, would increase the pressure in the tank as the volume of the air inside the tank is at a constant.
So how is this applicable to Scuba Diving? For starters, Charles’ law helps divers understand the hazards of leaving scuba tanks out in the hot sun, or why we should never leave tanks in the trunk of a hot car. The gas under pressure subjected to heat can cause the tank to explode. A scuba tank filled to capacity with compressed air at 3000 psi could just as easily go up to 3400-3500 psi if heated. Proper storage of air tanks on the dive boat too is crucial to ensure the tanks aren’t left to bake in the sun.
The law explains why the pressure in a scuba tank recently filled may appear to drop or increase slightly depending on whether the tanks were filled outdoors, or indoors. The law also explains why scuba tanks when being filled with compressed air, get hot.
I’ll admit knowledge of Charles’ Law isn’t as useful to us as perhaps Boyle’s Law, nevertheless it helps us understand everyday phenomenon and helps us better understand the remaining gas laws as well.
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