The Physical Laws Relating to Gas. Charles’s Law - Part 3

 This law is named after Jacques Charles (1746–1823), a French physicist who discovered that all

gases increase in volume by the same proportion if heated through the same temperature range, provided

that the pressure remained constant. This proportion is 1/273 of their volume at freezing point

(0◦C or 273K) for each 1K rise above 273K. Therefore, the temperature of a volume of gas would

need to be increased from 0◦C to 273◦C in order to double its volume. (Note: 1K rise = 1◦C rise.) 

Where the pressure remains constant, Charles’s law is expressed as:

                                                           Volume ÷ Temperature = Constant

In simple terms, if the temperature increases, so does the volume. As with Boyle’s law, the formula

can be redefined as:

                                                                   V1 ÷ T1 = V2 ÷ T2

Where V1 = original volume, T1 = original temperature, V2 = final volume and T2 = final temperature.

Practical example If 1 m3 of gas enters a building from outside where the temperature is 2◦C and

passes into a building where the temperature is 21◦C, the gas would increase in volume by:

                                                    V1 ÷ T1 = V2 ÷ T2, so (V1 × T2) ÷ T1 = V2

                                          ∴ 1 × (21 + 273) ÷ (2 + 273) = 1.07 m3, an increase of 7%

The Physical Laws Relating to Gas. Boyle’s Law - Part 2

 

This law is named after Robert Boyle (1627–1691), who discovered the relationship between volume

and pressure of a gas. He found that the absolute pressure of a given mass of gas is inversely

proportional to its volume provided that its temperature remains constant (Absolute pressure =

Atmospheric pressure (1013 mbar + gauge pressure). 

                                                                             So.....

To put it simply, if the absolute pressure (gauge pressure + atmospheric pressure) on a given quantity

of gas decreases, then its volume will increase. So if the absolute pressure is increased four-fold, the

volume would be reduced to one quarter. The formula can be redefined as:

                                                                    P1V1 = P2V2

Where P1 = original pressure, V1 = original volume, P2 = final pressure and V2 = final volume.

Practical example Suppose that the supply pressure to a building is 80 mbar and the total volume

is 1 m3, if the pressure was reduced to 20 mbar the new volume of the gas would be calculated as

follows.

                                                  P1V1 = P2V2, so P1V1 ÷ P2 = V2

                           ∴ (1013 + 80) × 1 ÷ (1013 + 20) = 1.06m3an increase of6%.

Conversely, if the supply pressure is increased to 800 mbar when a new medium pressure regulator

is fitted, supplying the same 1 m3 volume of gas, and also reduced to 20 mbar, there is an interesting

result:

                                                                   P1V1 ÷ P2 = V2

                                ∴ (1013 + 800) × 1 ÷ (1013 + 20) = 1.76m3, an increase of76%

This increase is probably the reason that the gas supplier installed the regulator prior to the meter.

The Physical Laws Relating to Gas. Graham's Law of Diffusion - Part 1

                                                            Graham’s Law of Diffusion

This law is named after Thomas Graham (1805–1869), who discovered that gases will mix with one another quite readily due to the continuous movement of the molecules. However, the rate at which they mix depends on the specific gravity or density of the gases. Graham made a container, consisting of two separate compartments with a small hole in their dividing wall. He placed a different gas in each of the compartments, one having a higher specific gravity than the other. He found that more faster, lighter molecules of the lighter gas passed through the hole rather than the slower, heavier molecules of the heavier gas. After many experiments he discovered that the rates of diffusion, or mixing, varied inversely to the square root of the density of the gas. Thus:

Diffusion rate ∝ 1 ÷

√

Density

Which basically means that a light gas will diffuse twice as fast as a gas four times its density.