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  • Tracy Gunn

Understanding Charles`s Law in Physics IDC Dive Theory

Updated: Jan 11

Charles’ Law states that "the amount of change in either volume or pressure of a given gas volume is directly proportional to the change in the absolute temperature."

This means:

As the temperature increases, the volume of a flexible container will increase, OR the pressure inside a non-flexible container will increase.

The opposite happens when the temperature decreases. As the temperature decreases, the volume of a flexible container will decrease, OR the pressure inside a non-flexible container will decrease.

Jacques Charles

Jacques Alexandre Cèsar Charles, Born November 12, 1746, Beaugency, Orléanais.

He died on April 7, 1823 (aged 76) in Paris. He is known as a French inventor, scientist, mathematician, and balloonist. He married a woman 37 years younger than him, Julie Françoise Bouchaud des Hérettes, who became the muse of French poet Alphonse de Lamartine who had fallen in love with her.

By studying Boyle's Law, published 100 years earlier, Jacques Charles conceived using hydrogen as a suitable agent for lifting balloons. On August 27, 1783, he and the Roberts Brothers launched the world's first hydrogen-filled balloon on the site where the Eiffel Tower now stands, with Benjamin Franklin among the onlookers.

In 1787 he did the first experiments on pressure, volume, and temperature. Even though this was later published by Joseph Louis Gay-Lussac in 1802, Gay-Lussac credited the work to the unpublished work of Charles.

Charles's Law

Heat is the energy of molecular action. Therefore, if you heat gases, the molecules move faster and impact the interior of a container with more force.

In a flexible container, this movement is absorbed by increased volume, and the pressure is constant.

In a non-flexible container, this causes an increase in pressure, and the volume is constant.

Cooling down means the molecules lose heat and slow down, reducing their impact and, in turn, reducing either pressure OR volume.

Flexible Container

For a flexible container, if you increase the temperature, then the volume will increase but not the pressure (pressure is constant)

Non-flexible Container

For a nonflexible container, if you increase the temperature, the pressure will increase but not the volume (volume is constant). Also, if you increase the pressure, the temperature will increase.

Combined Law


BALLOON - The volume will increase if you leave a balloon outside in the sun.

SCUBA TANKS - If you leave a full tank outside in the sun, the pressure inside will increase, showing more gas supply.

  • If you take a full tank on a hot day but dive into very cold water, the pressure will decrease and, as a result, the gas supply.

  • You can also increase the temperature by filling tanks (adding gas to a fixed volume)

  • You decrease the temperature by releasing air from a tank (fixed volume). The tank cools when you open a tank valve and let the air out rapidly.

  • This also explains why, when ice diving, environmental seals are recommended in the regulator's first stages so that they do not freeze.

Understanding Charles's Law


For every 1ºC change (up OR down) in temperature, there is a 0.6 bar change in pressure (0.6 bar per 1ºC or 5 psi per 1ºF), although this rule of thumb is not precise but happens to work.

Example: A 12 litre tank is filled to 220 Bar at 28ºC. The tank is then used in 3ºC water; what is the pressure change inside the tank?

Volume does not change (inflexible)

28ºC - 3ºC = 25ºC change

25 × 0.6 bar = 15 Bar

220 Bar - 15 Bar = 205 Bar

For precise calculations, use the Combined Gas Law.

Combined Gas Law

P = Absolute Pressure

V = Volume

T = Absolute Temperature, such as Kelvin (+273ºC) or Rankin (+460ºF)

So, let's use the same example as before (in metric again)

Example: A 12 litre tank is filled to 220 Bar at 28ºC. The tank is then used in 3ºC water; what is the pressure change inside the tank?

P1 = 220 bar + 1 bar (atmosphere) = 221 bar absolute

V1 = Cylinder volume does not change (V1=V2), so we can cancel them out from both sides of the equation

T1 = 28ºC + 273ºC= 301 kelvins (K)

T2 = 3ºC + 273ºC = 276 kelvins (K)

P2 =??

So, the formula now reads (Gay-Lussacs Law)

P1 = P2

T1 T2

P2 = P1T2


P2 = 221 bar × 276K


P2 = 202.6 bar absolute

202.6 bar absolute - 1 bar = 201.6 bar gauge

* So, you can see that there is a difference between the "rule of thumb" and the Combined Gas Law, with the Combined Gas Law being more precise.

Now let's take a more challenging example.

Example: Imagine that you are supplying air from the surface to a commercial diver at 65 meters. The compressor has a capacity of 1000 litres per minute. The temperature on the surface is 32ºC, and the temperature of the water that the diver is in is 3ºC. How much air will the compressor be capable of supplying at that depth?

P1 1 bar absolute

V1 1000 litres

T1 305 K (32+273=305K)

P2 7.5 bar absolute (65÷10+1=7.5 ata)

T2 276 K (2+273=276K)

V2 unknown

So, the transposed equation would read as this.




V2= 1 bar × 1000 litres × 276K

305K × 7.5 bar

= 276000


=120.65 litres per min

Now you are ready for some testing on Charles's law.

Try the exam below.


Here are links to all the physics blogs

And to all the exams

Water, Heat, Light, Sound, and Gases Exam

Archimedes' Exam part 1

Archimedes' Exam Part 2

Under Pressure Exam

Boyle's Law Exam Part 1

Single-Level Depth Changes

Boyle's Law Exam Part 2

Multi-Level Depth Changes

Charles' Law Exam

Henry's Law Exam

Dalton's Law Exam

(1) The Encyclopedia of Recreational Diving (3rd ed.). (2008). PADI.

(2) Divemaster Course Instructor Guide (1999 edition). (2005). PADI

(3) Wikipedia contributors. (2021b, June 19). Jacques Charles. Wikipedia.

(4) Physics. (2019). NOAA Diving Manual - Physics of Diving. Published.

(5) An Explanation of Pressure and the Laws of Boyle, Charles, Dalton, and Henry. (1997). Scuba Diving Explained.


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